Kliknij ten link, aby zobaczyć inne rodzaje publikacji na ten temat: Microstructural effects.
Rozprawy doktorskie na temat „Microstructural effects”
Utwórz poprawne odniesienie w stylach APA, MLA, Chicago, Harvard i wielu innych
Sprawdź 50 najlepszych rozpraw doktorskich naukowych na temat „Microstructural effects”.
Przycisk „Dodaj do bibliografii” jest dostępny obok każdej pracy w bibliografii. Użyj go – a my automatycznie utworzymy odniesienie bibliograficzne do wybranej pracy w stylu cytowania, którego potrzebujesz: APA, MLA, Harvard, Chicago, Vancouver itp.
Możesz również pobrać pełny tekst publikacji naukowej w formacie „.pdf” i przeczytać adnotację do pracy online, jeśli odpowiednie parametry są dostępne w metadanych.
Przeglądaj rozprawy doktorskie z różnych dziedzin i twórz odpowiednie bibliografie.
1
Portsmouth, Robert Lynton. "Microstructural effects in adsorptive separations". Thesis, University of Cambridge, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.260591.
Pełny tekst źródła
2
Rodriguez, Maria Remedios Carmona. "Small strain effects on microstructural evolution". Thesis, University of Sheffield, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.392716.
Pełny tekst źródła
3
Wohlschlögel, Markus Albin. "Microstructural effects on stress in thin films". [S.l. : s.n.], 2008. http://nbn-resolving.de/urn:nbn:de:bsz:93-opus-36733.
Pełny tekst źródła
4
Li, Ju 1975. "Modeling microstructural effects of deformation resistance and thermal conductivity". Thesis, Massachusetts Institute of Technology, 2000. http://hdl.handle.net/1721.1/46283.
Pełny tekst źródłaStreszczenie:
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2000.Includes bibliographical references (p. 344-360).
This is a study of the microstructural influences on thermo-mechanical behavior of selected metals and ceramics using computer simulation, with original contributions in both theoretical and applied aspects. There are three major thrusts. First, by constructing a many-body empirical potential for ZrCx and then carrying out MD simulations to calculate its lattice thermal conductivity, I obtain the first quantitative evidence ever that the vibrational contribution is only a small part of the total thermal conductivity of refractory carbides at realistic carbon vacancy concentrations. This is a long-standing problem which even the most recent review article on the subject give what I now believe is the wrong estimate. Second, ideal strengths are calculated for Ar,Cu,SiC crystals using both lattice and molecular dynamics methods. A set of homogeneous instability criteria are derived. Tension tests are performed on amorphous and nanocrystalline SiC at room temperature, based on which a grain size cutoff of ~20 nm is extrapolated for the Hall-Petch effect. Nano-indentation is performed on single-crystal and nanocrystalline Cu, and bursts of dislocation loops is observed at a local stress level consistent with recent experiments on Cu thin films. Third, an invariant loop summation similar to the J-integral is derived for the driving force on defect motion, but with the loop size now down to nanometers, and the summation now expressed in terms of interatomic forces instead of stress, a field concept which is hard to use in atomistic calculations and becomes ill-defined when defect separations approach the nanometer scale. It is shown first that the change in a system's total Helmholtz free energy due to a defect's move can be approximated by a local quantity involving only scores of atoms immediately surrounding the defect. Then, perturbation expansion is used to evaluate this local invariant for defect translation using only the current configuration. This driving force measure is then tested on a) self-interstitial diffusion near free surface in [alpha]-iron, b) crack-tip extension near a void in Si, c) screw dislocation translation in Si, with convincing results down to literally r = 1 nm, at a fraction of the cost of a full relaxation or free energy calculation for the whole system. This means that defect mobility can now be characterized by a universal and invariant standard, computable from a tiny atomistic calculation without relying on elasticity formulas or image summations. The standard is then used to determine the true Peierls-Nabarro stress in Si-like materials.
by Ju Li.
Ph.D.
5
Rezvanian, Omid. "GRAIN SUBDIVISION AND MICROSTRUCTURAL INTERFACIAL SCALE EFFECTS IN POLYCRYSTALLINE MATERIALS". NCSU, 2006. http://www.lib.ncsu.edu/theses/available/etd-01052006-204245/.
Pełny tekst źródłaStreszczenie:
The major objective of this research is to develop a unified physically-based representation of the microstructure in f.c.c. crystalline materials to investigate finite inelastic deformation and failure modes and scenarios at different physical scales that occur due to a myriad of factors, such as texture, grain size and shape, grain subdivision, heterogeneous microstructures, and grain boundary misorientations and distributions. The microstructurally-based formulation for inelastic deformation is based on coupling a multiple-slip crystal plasticity formulation to three distinct dislocation densities, which pertain to statistically stored dislocations (SSDs), geometrically necessary dislocations (GNDs), and grain boundary dislocations (GBDs). This dislocation density based multiple-slip crystal plasticity formulation is then coupled to specialized finite-element methods to predict the scale-dependent microstructural behavior, the evolving heterogeneous microstructure, and the localized phenomena that may contribute to failure initiation for large inelastic strains. The SSD densities provide a representation of cell-type dislocation microstructures and their related processes. The GND densities provide an understanding of the scale-dependent deformation behavior of crystalline materials as a function of grain and aggregate sizes. The GBD densities are formulated to represent the misfit dislocations that arise due to lattice misorientations across GBs, and to provide a framework to investigate the phenomena associated with the grain boundary orientations and distributions. This provides a local criterion of how GB interfaces, such as triple junctions are potential sites for failure initiation and localized behavior. The evolution of the GNDs is used to predict and understand how crystallographic and non-crystallographic microstructures relate to intragranular and intergranular deformation patterns and behavior. Furthermore, a clear understanding of how GB strength changes due to microstructural evolution is obtained as a function of microstructural heterogeneities that occur at different physical scales.
6
Morgan, Terence S. "Microstructural effects of neutron irradiation on ferritic/martensitic stainless steels". Thesis, Loughborough University, 1992. https://dspace.lboro.ac.uk/2134/13768.
Pełny tekst źródłaStreszczenie:
A commercial grade 12%CrMo VNb ferritic/martensitic stainless steel in the form of parent plate and high-nickel off-normal weld material has been fast neutron irradiated to equivalent damage levels of 33 and 50 dpa at 400 and 465°C respectively. The microstructural and microchemical changes induced in the irradiated material, together with as-tempered and thermal control material, have been determined to high resolution by conventional transmission electron microscopy and the use of a field emission gun scanning transmission electron microscope (FEGSTEM). Equilibrium (co )segregation of chromium, molybdenum and phosphorus was detected at boundary planes in thermally aged material, with greater enrichment at the higher ageing temperature. The relative magnitudes of apparent phosphorus segregation at the two temperatures were in accordance with McLean's model governing the kinetic approach to equilibrium. The electron probe I segregant interaction was modelled in an attempt to deconvolute true segregant concentrations from derived concentration profiles: these 'deconvoluted' concentrations approximated those predicted by McLean's model. The net effects of irradiation on parent plate interfacial microchemistry were found to be to: (i) inhibit the (co )segregation of chromium, molybdenum and phosphorus, (ii) cause chromium depletion from adjacent to boundary planes, (iii) cause enrichment of silicon at prior austenite and lath boundaries during irradiation at 400°C and (iv) cause enrichment of nickel at lath boundary planes only, at both temperatures. The radiationinduced precipitates ~C and G phase, both nickel- and silicon-rich, were observed. The fully martensitic off-normal weld metal transformed to a duplex austenite!ferrite structure during irradiation at 465°C; in contrast the thermal control was at least metastable. The transformation was thought to be a martensitic reversion, facilitated by radiation-generated dislocation loops acting as nucleation sites. The austenite was heavily voided (-15 vol.%); the ferrite was relatively void-free. Depletion of the oversized solutes chromium, manganese and molybdenum and enrichment of nickel, silicon, aluminium and traces of titanium were detected at void interfaces in the austenite: little segregation could be discerned at voids in the ferrite. Overall, the results within this work and in comparison to previous studies highlight the sensitivity to initial composition, microstructure and heat treatment that the 12%Cr ferritic/martensitic steels display in their response to irradiation.
7
Rohatgi, Aashish. "A microstructural investigation of shock-loading effects in FCC materials /". Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 1999. http://wwwlib.umi.com/cr/ucsd/fullcit?p9944211.
Pełny tekst źródła
8
Robinson, Michelle Christina. "Microstructural and geometric effects on the piezoelectric performance of PZT MEMS". Online access for everyone, 2007. http://www.dissertations.wsu.edu/Dissertations/Fall2007/m_robinson_091307.pdf.
Pełny tekst źródła
9
Booysen, Theo-Neal. "Microstructural effects on properties of additively manufactured Inconel 625 and 718". Thesis, Cape Peninsula University of Technology, 2019. http://hdl.handle.net/20.500.11838/3043.
Pełny tekst źródłaStreszczenie:
Thesis (MEng (Mechanical Engineering))--Cape Peninsula University of Technology, 2019Three Dimensional(3D) printing is known as additive manufacturing: it is a method of manufacturing parts or components form sheet, wire or powder in a manufacturing process. This method differs from traditional manufacturing techniques such as casting, moulding or subtracting materials which already exist. The type of material characterization is also very important in the development and improve or manufacturing of new materials for higher strength and various application. Selective Laser Melting(SLM) an additive manufacturing powder-based process has been adopted by automotive and aerospace industries. The reason for this is that it has many potential benefits, such as 3D designs of complex components in a shortened time frame, which offers financial savings. SLM process use metallic powders with different chemical composition to manufacture complex structures, which is an innovative material processing technology. In this research SLM, a typical additive manufacture process method, was used to manufacture additively manufactured Inconel 625 and 718. These sample specimens were investigated to determine their microstructural features and mechanical properties. The microstructural features were characterized using two different experimental surface microscopy methods: scanning electron microscope(SEM) and light optical microscope (LOM). The mechanical properties were determined by studying deformation and hardness characteristics using three-point bending and hardness tests. The relationship between processing, microstructure, grain sizes and mechanical properties was established. The understanding of SLM additive manufacturing of alloys is important as well for the adoption of the technology, and the possibility of replacing commercially produced cast and wrought alloys in the near future.
10
De, Jesús Aribet M. "Effects of mechanical stimulation on fibroblast-guided microstructural and compositional remodeling". Diss., University of Iowa, 2016. https://ir.uiowa.edu/etd/3068.
Pełny tekst źródłaStreszczenie:
Many physiological and pathological processes, such as wound healing and tissue remodeling, are heavily influenced by continuous mechanical cell-cell and cell-ECM communication. Abnormalities that may compromise the biomechanical communication between the cells and the ECM can have significant repercussions on these physiological and pathological processes. The state of the mechanical environment and the reciprocal communication of mechanical signals between the ECM and the cell during wound healing and aged dermal tissue regeneration may be key in controlling the quality of the structure and physical properties of regenerated tissue.
This dissertation encompasses a series of studies developed for characterizing the effects of mechanical cues on altering and controlling tissue remodeling, and regeneration in the context of controlling scar formation during wound healing, and the maintenance and regeneration of the dermal extracellular matrix (ECM) during aging. In order to achieve this goal, in vitro models that contained some features of the provisional ECM, and the ECM of the dermis were developed and subjected to an array of quantifiable mechanical cues. Wound models were studied with different mechanical boundary conditions, and found to exhibit differences in initial short-term structural remodeling that lead to significant differences in the long-term synthesis of collagen after four weeks in culture. Dermal models seeded with fibroblasts from individuals of different ages were treated with a hyaluronic acid (HA)-based dermal filler. Changes in the mechanical environment of the dermal models caused by swelling of the hydrophilc HA, resulted in changes in the expression of mechanosensitive, and ECM remodeling genes, essential for the maintenance and regeneration of dermal tissue. Taken together, these data provide new insights on the role of mechanical signals in directing tissue remodeling.
11
Wohlschlögel, Markus Albin [Verfasser]. "Microstructural effects on stress in thin films / vorgelegt von Markus Albin Wohlschlögel". Stuttgart : Max-Planck-Inst. für Metallforschung, 2008. http://d-nb.info/995391246/34.
Pełny tekst źródła
12
Pratapa, Suminar. "Diffraction-based modelling of microstructural size and strain effects in sintered ceramics". Thesis, Curtin University, 2003. http://hdl.handle.net/20.500.11937/1476.
Pełny tekst źródłaStreszczenie:
Crystallite (or grain) size and strain within a polycrystalline material may have a profound influence on its physical properties, eg. the fracture toughness, wear and thermal shock resistance. A diffraction pattern for a material conveys information about the strain through the strain-induced changes in the shapes of the Bragg peaks and also through peak shifts. Crystallite size effects also influence the peak shape. Therefore, it is possible, in principle, to extract descriptions of crystallite size and strain from the peak broadening of a diffraction pattern. Various methods for size and strain evaluations have been proposed for extraction of the size and strain information in metals and ceramic powders. However, there appear to be no detailed amounts in the literature to be on the development of models appropriate for sintered ceramic materials. The objectives for this study were to critically examine the existing models for crystallite size and strain assessments and then to develop a new physically-based model which might be appropriate for sintered ceramics. The principal steps for the research, designed to fulfill the study objectives, were (1) acquiring high-quality diffraction data with synchrotron radiation, laboratory x-ray and neutron diffraction techniques for model evaluation; (2) performing preliminary evaluation using the existing models; (3) developing a new model and the non-linear least-squares calculation software; and (4) performing peak profile analyses using the existing and new models to evaluate the effectiveness of the new model. A convolution model for crystallite size and strain determination from diffraction line broadening has been developed with particular reference to the characterisation of sintered ceramics.The size profile component function for the convolution model involves the modal size and the size distribution appropriate for `normal' crystallite growth according to the mean-field theory, as proposed recently in a seminal publication by Dr. Brian York of IBM. A Gaussian strain profile component function was considered in the study on the basis that it has been widely used for specimens which exhibit small microstrain (ca. 10-3 or less). The overall profile describing the diffraction pattern involves convolution of the instrument, size and strain effects. A non-linear least-squares refinement program entitled MOZAIX has been developed for profile fitting with the model. Data simulations were performed with the model, and non-linear least-squares optimisations for fitting the simulated data showed that the calculations were reasonable for low-strain sintered ceramics. The convolution model for size and strain assessments from diffraction line broadening has been evaluated with synchrotron and laboratory x-ray radiation diffraction data (SRD and XRD, respectively). The study made use of MgO ceramics with three different purity levels which had been sintered at a range of temperatures in order to provide diffraction data with a range of microstructural strain and size effects. The cubic symmetry of MgO provided isotropic size and strain effects as had been anticipated. The Voigt function, a convolution of the Gaussian and Lorentzian functions, is widely used to extract crystallite size and strain information from powder diffraction data using (1) Fourier transforms, (2) the Rietveld method and (3) integral breadth methods. Size and strain model evaluation carried out using the Voigt-based Rietveld and integral breadth methods assumes that the size effect contributes only to the Lorentzian component and the strain contributes only to the Gaussian component.Size and strain assessment using the Voigt integral breadth single-line and Rietveld methods has been examined in this study with diffraction data for MgO ceramics. Two major outcomes from the evaluation confirmed impressions gained from the literature that: 1. the integral-breadth single-line method can be used as a reliable technique for size and strain analysis; 2. analysis using the Voigt function has no physical basis, is inappropriate for profiles with 'super-Lorentzian' character and is inadequate for size-strain analysis since the function does not take into account the size distribution parameter. There has been a strong trend recently towards whole-pattern size and strain evaluations which are progressively replacing single-line methods. However, due to time constraints, this study was confined to single-line analysis with the focus being on the development of the model, and with an expectation that the single-line model would readily be extended in the future to use with whole-powder pattern data. The size-strain analysis results using the convolution model showed that sintering (1) promotes crystallite growth and (2) relieves residual strains in low density sintered ceramics and introduces strains in dense ceramics, presumably due to grain-grain shear interactions. The effect of sintering on the size distribution clearly depends on the crystallite growth behaviour. Comparing the SRD convolution size results with those from scanning electron microscopy (SEM) showed that (1) the "grains" imaged using SEM contain clusters of crystallites and (2) the SEM-derived and convolution size distributions are in a satisfactory agreement.In general, despite the larger uncertainties due to instrument resolution, the XRD results are in agreement with those from SRD. The size and strain values obtained with the convolution model were compared with those calculated using the Voigt single-line integral-breadth method. The comparison showed that size and strain results for both methods were dependent upon the character of the diffraction peak shapes. The convolution model improves the Voigt model in terms of (1) reliability of models from a physical point of view, (2) the additional size distribution parameter and (3) its applicability to `super-Lorentzian' profiles. Subsequent research is suggested to further improve the model in dealing with large microstrains and developing a whole powder fitting procedure.
13
Pratapa, Suminar. "Diffraction-based modelling of microstructural size and strain effects in sintered ceramics". Curtin University of Technology, Department of Applied Physics, 2003. http://espace.library.curtin.edu.au:80/R/?func=dbin-jump-full&object_id=14619.
Pełny tekst źródłaStreszczenie:
Crystallite (or grain) size and strain within a polycrystalline material may have a profound influence on its physical properties, eg. the fracture toughness, wear and thermal shock resistance. A diffraction pattern for a material conveys information about the strain through the strain-induced changes in the shapes of the Bragg peaks and also through peak shifts. Crystallite size effects also influence the peak shape. Therefore, it is possible, in principle, to extract descriptions of crystallite size and strain from the peak broadening of a diffraction pattern. Various methods for size and strain evaluations have been proposed for extraction of the size and strain information in metals and ceramic powders. However, there appear to be no detailed amounts in the literature to be on the development of models appropriate for sintered ceramic materials. The objectives for this study were to critically examine the existing models for crystallite size and strain assessments and then to develop a new physically-based model which might be appropriate for sintered ceramics. The principal steps for the research, designed to fulfill the study objectives, were (1) acquiring high-quality diffraction data with synchrotron radiation, laboratory x-ray and neutron diffraction techniques for model evaluation; (2) performing preliminary evaluation using the existing models; (3) developing a new model and the non-linear least-squares calculation software; and (4) performing peak profile analyses using the existing and new models to evaluate the effectiveness of the new model. A convolution model for crystallite size and strain determination from diffraction line broadening has been developed with particular reference to the characterisation of sintered ceramics.The size profile component function for the convolution model involves the modal size and the size distribution appropriate for `normal' crystallite growth according to the mean-field theory, as proposed recently in a seminal publication by Dr. Brian York of IBM. A Gaussian strain profile component function was considered in the study on the basis that it has been widely used for specimens which exhibit small microstrain (ca. 10-3 or less). The overall profile describing the diffraction pattern involves convolution of the instrument, size and strain effects. A non-linear least-squares refinement program entitled MOZAIX has been developed for profile fitting with the model. Data simulations were performed with the model, and non-linear least-squares optimisations for fitting the simulated data showed that the calculations were reasonable for low-strain sintered ceramics. The convolution model for size and strain assessments from diffraction line broadening has been evaluated with synchrotron and laboratory x-ray radiation diffraction data (SRD and XRD, respectively). The study made use of MgO ceramics with three different purity levels which had been sintered at a range of temperatures in order to provide diffraction data with a range of microstructural strain and size effects. The cubic symmetry of MgO provided isotropic size and strain effects as had been anticipated. The Voigt function, a convolution of the Gaussian and Lorentzian functions, is widely used to extract crystallite size and strain information from powder diffraction data using (1) Fourier transforms, (2) the Rietveld method and (3) integral breadth methods. Size and strain model evaluation carried out using the Voigt-based Rietveld and integral breadth methods assumes that the size effect contributes only to the Lorentzian component and the strain contributes only to the Gaussian component.
Size and strain assessment using the Voigt integral breadth single-line and Rietveld methods has been examined in this study with diffraction data for MgO ceramics. Two major outcomes from the evaluation confirmed impressions gained from the literature that: 1. the integral-breadth single-line method can be used as a reliable technique for size and strain analysis; 2. analysis using the Voigt function has no physical basis, is inappropriate for profiles with 'super-Lorentzian' character and is inadequate for size-strain analysis since the function does not take into account the size distribution parameter. There has been a strong trend recently towards whole-pattern size and strain evaluations which are progressively replacing single-line methods. However, due to time constraints, this study was confined to single-line analysis with the focus being on the development of the model, and with an expectation that the single-line model would readily be extended in the future to use with whole-powder pattern data. The size-strain analysis results using the convolution model showed that sintering (1) promotes crystallite growth and (2) relieves residual strains in low density sintered ceramics and introduces strains in dense ceramics, presumably due to grain-grain shear interactions. The effect of sintering on the size distribution clearly depends on the crystallite growth behaviour. Comparing the SRD convolution size results with those from scanning electron microscopy (SEM) showed that (1) the "grains" imaged using SEM contain clusters of crystallites and (2) the SEM-derived and convolution size distributions are in a satisfactory agreement.
In general, despite the larger uncertainties due to instrument resolution, the XRD results are in agreement with those from SRD. The size and strain values obtained with the convolution model were compared with those calculated using the Voigt single-line integral-breadth method. The comparison showed that size and strain results for both methods were dependent upon the character of the diffraction peak shapes. The convolution model improves the Voigt model in terms of (1) reliability of models from a physical point of view, (2) the additional size distribution parameter and (3) its applicability to `super-Lorentzian' profiles. Subsequent research is suggested to further improve the model in dealing with large microstrains and developing a whole powder fitting procedure.
14
Sauter, Linda Xenia. "Microstructural and film thickness effects on the thermomechanical behavior of thin Au films". Stuttgart : Max-Planck-Inst. für Metallforschung, 2007. http://d-nb.info/995370753/34.
Pełny tekst źródła
15
Abdeljawad, Fadi Faeq. "Microstructural Modeling of CSL Grain-Boundary Effects and Crack Growth in F.C.C. Polycrystals". NCSU, 2006. http://www.lib.ncsu.edu/theses/available/etd-01222006-115908/.
Pełny tekst źródłaStreszczenie:
A new multiple-slip rate-dependent crystalline constitutive formulation that is coupled to the evolutionary equations of mobile and immobile dislocation densities is developed. Dislocation densities were modeled as internal state variables that provide a more detailed microstructural description of the material?s inelastic deformation and interrelated physical mechanisms that control different failure modes. Specialized microstructurally-based finite-element schemes have been used to investigate the effects of crystallographic orientations of the grains and grain-boundaries (GBs), grain shape and size, (GB) misorientation and the dependency of GB strength and mechanical properties on specific CSL misorientations on the inelastic finite deformation and failure mode mechanisms in f.c.c. polycrystalline aggregates. A Voronoi algorithm was used to generate grains and GBs with random shapes and sizes. The combined effects of GB misorientation, structure and geometry, strain hardening, localized plastic shear slip, intensive regions of crystal lattice rotation and the evolution, interaction and accumulation of dislocation densities on the nucleation and growth of intergranular and transgranular failure modes in f.c.c. polycrystalline aggregates were studied. Results from this study are consistent with experimental observations that microstructures with desired material properties, such as resistivity to crack nucleation, can be achieved by the control of the crystallographic orientation distribution of the grain aggregate and CSL GB orientations. Results from this study show that transgranular failure modes are dominant in aggregates with a high frequency of Sigma-3 GBs, and intergranular fracture modes dominate the aggregate with a high frequency of Sigma-17b GBs.
16
Godha, Anshul. "Microstructural effects on fatigue damage evolution in advanced high strength sheet (AHSS) steels". Diss., Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/53510.
Pełny tekst źródłaStreszczenie:
An understanding of the damage evolution prior to crack initiation in advanced structural materials is of vital importance to the fatigue community in both academia and industry. Features known as the Persistent Slip Bands (PSBs) play an integral role in this damage evolution. Therefore, PSBs have been the focus of a lot of science-based investigations over the years. However, most existing studies in this area are restricted to analysis of PSBs in single crystal face centered cubic (FCC) materials. Moreover, these studies lack a quantitative analysis of the evolution of the fatigue damage (or PSBs) as a function of the material microstructure. This is especially true for relatively modern materials such as the Advanced High Strength Structural (AHSS) steels that are gaining a lot of importance in the automotive sector. Accordingly, the objective of this research is to quantitatively characterize evolution of PSBs in three AHSS steels having different microstructures as a function of number of fatigue cycles and strain amplitude. For this purpose strain controlled interrupted fatigue tests have been performed on two dual phase steels (DP-590 and DP-980) having different relative amounts of tempered martensite and a ferritic high strength low alloy steel (HR-590). Digital image analysis and Stereology have been used for unbiased quantitative characterization of the evolution of global geometry of the PSB colonies as function of number of fatigue cycles and strain amplitude. Evolution of PSB colonies has been couched in terms of variation of PSB colony volume fraction and total surface area unit volume, and total surface area of individual PSBs per unit volume and three-dimensional angular orientation distribution of the PSBs. For this purpose, new stereological techniques have been developed for estimation of the three-dimensional angular orientation distribution. The stereological data reveal that during strain controlled in these AHSS steels, volume fraction of the PSB colonies varies linearly with the their total surface area per unit volume. Detailed analysis of the stereological data leads to a simple geometric model for evolution of the PSB colonies in the three AHSS steels, which accounts for all observed data trends.
17
Mark, Alison Fiona Lockie. "Microstructural effects on the stability of retained austenite in transformation induced plasticity steels". Thesis, Kingston, Ont. : [s.n.], 2007. http://hdl.handle.net/1974/960.
Pełny tekst źródła
18
Sauter, Linda Xenia. "Microstructural and film thickness effects on the thermomechanical behavior of thin Au films". [S.l. : s.n.], 2006. http://nbn-resolving.de/urn:nbn:de:bsz:93-opus-31632.
Pełny tekst źródła
19
Nelson, Elizabeth A. "Microstructural effects of multiple passes during friction stir processing of nickel aluminum bronze". Thesis, Monterey, California : Naval Postgraduate School, 2009. http://edocs.nps.edu/npspubs/scholarly/theses/2009/Dec/09Dec%5FNelson.pdf.
Pełny tekst źródłaStreszczenie:
Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, December 2009.Thesis Advisor: McNelley, Terry R. Second Reader: Menon, Sarath. "December 2009." Description based on title screen as viewed on January 28, 2010. Author(s) subject terms: friction stir processing, nickel aluminum bronze, propellers, microstructural properties, grain refinement, volume fraction-temperature relationship. Includes bibliographical references (p. 45-46). Also available in print.
20
Kadri, Shabibahmed Jehangir. "Microstructural breakdown and scale-up effects in equal channel angular extrusion of cast copper". Texas A&M University, 2005. http://hdl.handle.net/1969.1/4341.
Pełny tekst źródłaStreszczenie:
The primary objectives of this study were: (1) to verify the effectiveness of ECAE
to induce equal amounts of strain and grain refinement in bars of different cross-sectional
areas, (2) to determine the effectiveness of ECAE in breaking down the as-cast
macrostructure in CDA 101 Cu and in producing a homogeneous material containing
micron-scale grains upon recrystallization, and (3) to determine a thermomechanical
processing (TMP) schedule (from the ones examined) that produces the best
microstructure in terms of grain size and uniformity. The effects of extrus ion route, levels
of strain and intermediate heat treatment were investigated.
To achieve the first objective, bars having square cross-sections of three different
sizes, 19 mm, 25 mm and 50 mm, were processed up to eight ECAE passes through
routes A, B, C and E. To achieve the second and third objectives, bars were processed up
to eight ECAE passes with and without intermediate heat treatments through routes Bc,
C, E and F. ECAE processing was carried out in a 90o extrusion die with sliding walls at
an extrusion speed of 2.5 mm/s. Recrystallization studies were carried out on the
processed material to evaluate the recrystallization behavior and thermal stability of the
material. The as-worked and recrystallized materials were characterized by Vickers
microhardness, optical microscopy (OM) and transmission electron microscopy (TEM).
Results indicate that similar hardness values, sub-grain morphology and
recrystallized grain size are generated in the three bars having different cross-sectional
sizes processed through ECAE. ECAE is shown to induce uniform strain in all three billet
sizes. ECAE is therefore shown to be effective in scale-up to a size of at least 50 mm,
with larger billets giving better load efficiency. Results from the later parts of this study indicate that eight extrusion passes via
route Bc produces the best microstructure in terms of grain size and microstructural
uniformity. The routes can be arranged in the sequence Bc> E, F> C for their ability to
produce a uniform recrystallized microstructure with small average grain size.
Macroscopic shear bands are sometimes generated during extrusion depending upon the
initial grain morphology and texture of the material.
21
Shi, Jibin. "Predictive Microstructural Modeling of Grain-boundary Interactions and Their Effects on Overall Crystalline Behavior". NCSU, 2009. http://www.lib.ncsu.edu/theses/available/etd-02132009-173832/.
Pełny tekst źródłaStreszczenie:
A dislocation-density grain boundary interaction scheme (DDGBI) has been developed to account for complex interrelated dislocation-density interactions of emission, absorption and transmission in grain-boundary (GB) regions for bicrystals and polycrystals with different random and coincident site lattice (CSL) GB arrangements. This scheme is coupled to a dislocation-density crystalline plasticity formulation and specialized finite-element scheme at different physical scales. The DDGBI scheme is based on slip-system compatibility, local resolved shear stresses, and immobile and mobile dislocation-density activities at GBs. A conservation law for dislocation-densities is used to balance dislocation-density absorption, transmission and emission in GB regions. It is shown that dislocation-density absorptions and pile-ups will increase immobile dislocation-densities in high angle CSL boundaries, such as Σ17b. Lower angle CSLs, such as Σ1, are characterized by high transmission rates and insignificant GB dislocation-density accumulations. The identification of how different material mechanisms dominate underscores that GB activities, such as dislocation-density absorption, transmission and emission are interrelated interactions. These GB processes can be potentially controlled for desired material behavior. This methodology, together with grain boundary sliding (GBS) scheme and a misorientation dependence on initial GB dislocation-densities, was extended to account for grain size effects on strength. The behavior of polycrystalline aggregates with random low angle and random high angle GBs was also investigated with different crack lengths. For aggregates with random low angle GBs, dislocation-density transmission dominates at the GBs, which indicates that the low angle GB will not significantly change crack growth orientations. For aggregates with random high angle GBs, extensive dislocation-density absorption and pile-ups occur. The high stresses along the GB regions can result in intergranular crack growth due to potential crack nucleation sites in the GB. It is also shown that GB sliding affects crack behavior by attenuating normal stresses and dislocation-density accumulation at critical GB interfaces.
22
Davis, Robin M. (Robin Manes). "Microstructural effects on capacity-rate performance of vanadium oxide cathodes in lithium-ion batteries". Thesis, Massachusetts Institute of Technology, 2005. http://hdl.handle.net/1721.1/32852.
Pełny tekst źródłaStreszczenie:
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.Includes bibliographical references (leaf 29).
Vanadium oxide thin film cathodes were analyzed to determine whether smaller average grain size and/or a narrower average grain size distribution affects the capacity-rate performance in lithium-ion batteries. Vanadium oxide thin films were prepared by sputtering onto ITO-coated glass substrates and crystallized in a refined annealing process to generate diverse microstructures. Average grain size and grain size distribution were determined in SEM analysis. No significant difference was observed in capacity rate behavior with changes in microstructure. However, it is speculated that further in situ analysis may show different relative diffusion rates into grains of differing sizes is related to different microstructures.
by Robin M. Davis.
S.B.
23
Rathore, Shambhu Singh. "Investigation of Microstructural and Carbon Deposition Effects in SOFC Anodes Through Modelling and Experiments". Thesis, Curtin University, 2017. http://hdl.handle.net/20.500.11937/69416.
Pełny tekst źródłaStreszczenie:
The investigation of the SOFC anode microstructural properties affected by microstructural parameters and degradation is the focus of this research. Imaging and image processing techniques are developed to achieve quantification of the anode microstructural information. The analytical and Computational Fluid Dynamics based modelling of the microstructure including the degradation effects developed in this work will enable the microstructure optimisation for achieving performance enhancements.
24
Takahashi, Keisuke. "Effects of mixing and pumping energy on technological and microstructural properties of cement-based mortars". Doctoral thesis, Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2014. http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-156776.
Pełny tekst źródłaStreszczenie:
Numerous recurrent situations following mixing and pumping of mortars and concretes cause degradation of fluidity and hardening characteristics. Which, in turn, lead to adverse effects on the quality of workmanship and structural defects.
Nonetheless, relatively little research on the mixing and pumping energies used for the onsite transport and preparation of mortar or concrete has been directed at the core reasons or mechanisms for changes in technological properties.
This dissertation describes and explains the effects of various mixing and pumping parameters on the mortar characteristics in a field trial and on a laboratory scale.
Observations using a rheograph revealed that shearing action does exhibit the most pronounced influence on the characteristics of mortars during the pumping. The performed investigations indicate that higher shearing actions, for example, excessive mixing duration and long-distance pumping lead to reduced flowability, accelerated and increased hydration rate, increased early compressive strength and early-age shrinkage.
From these findings, the underlying mechanism responsible for acceleration and increase of hydration rate is pinpointed as: the increased dissolution from the active surface area due to the destruction of the protective superficial layers of cement
grains, as well as a transition from flocculation to dispersion. The creation of new surfaces leads to further consumption of active super plasticizer in solution phase and to subsequent degrading changes in fluidity (decreasing flowability). The degradation of fluidity and densification of microstructure provoked by the hydration changes do increase the early age shrinkage of mortar.
25
Wieloch, Marcin Maciej. "The effects of freeze/thaw cycles on the microstructural features of air-entrained cementitious mortars". Thesis, Glasgow Caledonian University, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.422021.
Pełny tekst źródła
26
Colman, Charlotte. "Gypsum residues in fine recycled aggregates : effects on mechanical and microstructural properties of cementitious composites". Electronic Thesis or Diss., Ecole nationale supérieure Mines-Télécom Lille Douai, 2020. http://www.theses.fr/2020MTLD0012.
Pełny tekst źródłaStreszczenie:
Les sables recyclés représentent un flux important de déchets provenant de la démolition d’anciennes structures en béton. Ils ne sont jusqu’à présent pas valorisés à cause d’une absorption d’eau élevée, d’une densité plus faible et d’une teneur en fines plus importante que dans les granulats naturels. De plus, ils sont souvent contaminés par des sulfates. Ces sulfates pourraient provenir du plâtre du bâtiment démoli ou du ciment résiduel qui est couramment présent dans ces matériaux. Le taux de sulfate recommandé pour les granulats recyclés est inférieur à 0,2 % en masse selon la norme EN206, afin de limiter le risque d’attaque sulfatique : la réaction entre les composants du ciment, l’eau et les sulfates entraîne la formation de minéraux expansifs comme l’ettringite. Cette expansion peut éventuellement conduire à une fissuration du matériau et à une perte générale des performances mécaniques. Une étude systématique a permis de mettre en évidence que les sables recyclés provenant des centres de recyclage contenaient de 0,15 à 0,80 % de sulfates ce qui est plus ce qui est autorisé. Néanmoins, ces quantités n’ont pas engendré de gonflement ni de dégradation lorsque les granulats contaminés ont été incorporés dans des mortiers. Une teneur en sulfate très élevée de 3 % a été nécessaire pour observer une expansion significative ; toutefois l’expansion a pu être contrôlée en augmentant l’alcalinité d’un mélange. Des bétons fabriqués avec différents niveaux de sulfates ont présenté des gonflements ou non : l’ampleur d’expansion n’était pas proportionnelle à la teneur en sulfate. Le seuil qui a déclenché la réaction de gonflement dépendait aussi du type de granulat. Les résultats de cette étude fournissent des recommandations pour l’utilisation de sables recyclés contaminés par des résidus de gypse : une teneur en sulfate jusqu’à 0,3 % pourrait être possible, et même plus en fonction de certains paramètres et compositions de mélange. La limite actuelle est de 0,2 % semble trop stricte et freine la valorisation de sables recyclés dans des bétons durables et résistantsFine recycled aggregates are an important waste steam coming from the demolition of old concrete structures. They are up to now not valorized because of an increased water absorption, lower density and higher fines content compared to natural aggregates. Moreover, they often have a high sulfate concentration. These sulfates could have originated from plaster in a demolished building, or from the residual cement that is commonly present in these materials. The sulfate level of recycled aggregates is recommanded to stay under 0.2 mass% by standard EN206, to limit the risk on sulftate attack: the reaction between cement components, water and sulfates results in the formation of expansive minerals such as ettringite. This expansion can eventually lead to cracking of the material and a general loss in mechanical performances. Fine recycled aggregates from recycling plants did contain 0.15 to 0.80 % of sulfates which is more than allowed, but these amounts did not cause any swelling or degradation when the aggregates were incorporated into mortars. A very elevated sulfate content of 3 % was needed to observe significant expansion, and even this level of contamination could be mitigated by increasing the alkalinity of a mix. Concretes made with varying sulfate levels either expanded or they didn’t: the absolute swelling amount was not proportional to its sulfate content. The actual threshold concentration that started the swelling reaction was dependant on the type of coarse aggregate. The results of this study provide recommandations for the use of fine recycled aggregates contaminated with gypsum residues : a sulfate content of at least 0.3 % could be possible, and even more if certain parameters and mix compositions can be adjusted accordingly. The current limit of 0.2 % seems too strict and hinders the valorization of fine recycled aggregates in high quality construction applications
27
Zhang, Jinyong. "Mechanical Behavior and Microstructural Evolution in Metastable β Ti-Mo Based Alloys with TRIP and TWIP Effects". Thesis, Paris 6, 2014. http://www.theses.fr/2014PA066313/document.
Pełny tekst źródłaStreszczenie:
Dans ce travail, basé sur une approche semi empirique de conception d’alliages de titane à propriétés mécaniques contrôlées, un alliage modèle binaire Ti-12Mo (% massique) et des alliages ternaires sur la base du système Ti-Mo ont été élaborés, combinant des effets TRIP et TWIP lors de la déformation. (TRIP – Transformation Induced Plasticity : plasticité induite par transformation ; TWIP – Twinning Induced Plasticity : plasticité induite par maclage).Les résultats des essais mécaniques montrent que ces alliages présentent une haute résistance mécanique (1000-1200 MPa), une grande plasticité (entre 0,3 et 0,4) et un écrouissage amélioré grâce aux effets simultanés TRIP/TWIP. Différentes techniques de caractérisation telles que la diffraction de rayons X conventionnelle (XRD), la diffraction in-situ sur Synchrotron (SXRD), la diffraction d’électrons rétro-diffusés (EBSD), les mesures de résistivité électrique (ERM), la microscopie électronique en transmission (TEM) et les mesures et traitements automatiques des orientations cristallographiques associées (ACOM/TEM), ont été mis en œuvre pour étudier les mécanismes de déformation, les transformations de phases et l’évolution microstructurale.Différents mécanismes de déformation, tels que le maclage mécanique {332}<113> et la transformation martensitique induite sous contrainte α˝ ont été identifiés à l’issue des essais mécaniques, permettant d’expliquer l’excellente combinaison de propriétés mécaniques obtenue. L’optimisation microstructurale de ces alliages a été conduite à partir de recuits basses températures dans le domaine de précipitation de la phase ω avec pour objectif d’améliorer les propriétés mécaniques afin de contrôler la formation de la phase ω sans modifier de manière excessive la composition chimique de la matrice β, et conserver les effets combinés TRIP/TWIPIn this work, based on combination of the ‘d-electron alloy design method’ and controlling of electron/atom ratio (e/a), a model of binary Ti-12Mo (wt. %) and ternary Ti-Mo based alloys were designed, induced combined TRIP and TWIP effects (TRIP for Transformation Induced Plasticity and TWIP for Twinning Induced Plasticity). The tensile results show that so-designed alloys exhibit true stress-strain values at uniform plastic deformation, of about 1000-1200MPa and between 0.3 and 0.4 of strain, with a large strain-hardening rate. Several characterization techniques, such as conventional X-ray diffraction (XRD), In-situ Synchrotron X-ray diffraction (SXRD), electron backscatter diffraction (EBSD), electrical resistivity measurements (ERM), transmission electron microscopy (TEM) and automatic crystal orientation measurements (ACOM) TEM, were carried out to to investigate the deformation mechanisms and microstructure evolution sequence. Various deformation mechanisms, i.e. {332}<113> mechanical twinning, deformation induced ω phase and stress-induced α’’ martensite, were identified after mechanical testing, resulting in a combination of high strength, large ductility and improved strain-hardening rate. Furthermore, low temperature aging (LTA) treatments were performed on the Ti-12Mo alloy to improve the mechanical property through controlling the ω phase transformation without excessive modification of β matrix chemical composition, keeping the possibility for combined TRIP and TWIP effects to occur. The influence of LTA treatment on the mechanical behavior and microstructural evolution of Ti-12Mo alloy was discussed in detail
28
Reda, Hilal. "Modeling and computation of the effective static and dynamic properties of network materials accounting for microstructural effects and large deformations". Thesis, Université de Lorraine, 2017. http://www.theses.fr/2017LORR0007/document.
Pełny tekst źródłaStreszczenie:
Nous analysons les propriétés dynamiques de milieux architecturés périodiques et de réseaux fibreux aléatoires en petites et grandes déformations, à partie de méthodes d’homogénéisation afin de calculer leurs propriétés statiques et dynamiques. Des modèles effectifs de type micropolaire et du second gradient sont élaborés afin de prendre en compte l’impact de la microstructure sur le comportement effectif. L’influence des degrés de liberté en rotation additionnels et des gradients d’ordre supérieur du déplacement sur les relations de dispersion sont analysés pour des comportements élastique et viscoélastique du matériau constitutif. Les milieux continus généralisés ainsi construits conduisent à des effets dispersifs, en accord avec les observations. Dans la seconde partie du travail, nous analysons l’influence des grandes déformations sur la propagation des ondes élastiques dans des milieux architecturés périodiques. Des méthodes théoriques assortis de schémas numériques sont développés afin de prédire l’influence des déformations finies générées au sein des structures sur l’évolution de leur diagramme de bande. Un schéma incrémental d’évolution de la fréquence et de la vitesse de phase du milieu continu homogénéisé est établi, à partir d’une méthode de perturbation établie pour des structures 1D, 2D et 3D, en considérant plus particulièrement des structures auxétiques. Ce schéma montre un effet important de l’état de déformation appliquée et de la densité effective sur l’évolution de la fréquence et de la vitesse de phase des ondes. Une méthode de perturbation spécifique aux structures périodiques nonlinéaires est développée afin de généraliser le théorème de Bloch pour couvrir les non linéarités tant géométriques que matérielles. Des modèles hyperélastiques du premier et du second gradient de différentes structures sont identifiés par des tests virtuels reposant sur une méthode d’homogénéisation dédiée, qui permettent de formuler des équations d’onde spécifiques – équations de Burgers et de Boussinesq – dont les propriétés dispersives sont analyséesMicropolar and second gradient effective continua are constructed as two different strategies to account for microstructural effects. The influence of additional degrees of freedom or higher order displacement gradients on the dispersion relations is analyzed in both situations of elastic and viscoelastic behaviors of the material. Generalized effective continua lead to dispersive waves, as observed in experiments. In the second part of the thesis, we analyze the influence of large deformations on the propagation of acoustic waves in repetitive network materials. Both theoretical and numerical methods are developed in order to assess the influence of finite strains developing within the networks on the evolution of their band diagrams. An incremental scheme for the update of frequency and phase velocity of the computed homogenized medium is developed based on a perturbation method for 1D, 2D and 3D structures, considering with a special emphasis auxetic networks. This scheme shows an important effect of the applied finite deformation on the frequency and phase velocity of the propagating waves. A perturbation method for nonlinear periodic structures is developed to extend Bloch’s theorem to cover both geometrical and material nonlinearities. Hyperelastic first and second order gradient constitutive models of different network materials are identified based on dedicated homogenization methods, from which specific wave equations are formulated - Burgers and Boussinesq equations - the dispersion properties of which are analyzed
29
Ahmed, Mohammad Sharfuddin Aerospace Civil & Mechanical Engineering Australian Defence Force Academy UNSW. "Effects of systematic increase in pozzolanic materials on the mechanical, durability, and microstructural characteristics of concrete". Awarded by:University of New South Wales - Australian Defence Force Academy. School of Aerospace, Civil and Mechanical Engineering, 2007. http://handle.unsw.edu.au/1959.4/38645.
Pełny tekst źródłaStreszczenie:
The use of high performance concrete in construction has been enhanced by the use of pozzolanic materials. However, the use of these materials has not been optimized. Such optimization may be achieved by a systematic increase in the amount and combination of pozzolanic material additions, with accompanying studies of their effects on the mechanical, durability and microstructural characteristics of blended concrete. This work evaluated various concrete durability issues by studying systematic increases of pozzolanic materials such as fly ash and blast furnace slag (BFS) in the range of 25, 50 and 70%, and silica fume at 10% of total cementitious materials, forming various binary and ternary concrete blends. The concrete specimens were cured for a period of seven days after demoulding in line with widely practiced commercial curing procedures. The research explored the role and effectiveness of various binary and ternary blends of pozzolanic materials on the mechanical, durability and microstructural characteristics of concrete. Durability was evaluated by two independent rapid chloride permeability tests measured as charge passed and chloride conductivity from the RCPT and UCT tests respectively. These two rapid tests were coupled with long-term ponding tests to evaluate chloride ingress and the extent of corrosion for a period of two years. Further durability tests such as carbonation, drying shrinkage and porosity of these blends were also undertaken. This study also utilized micro-analytical techniques such as X-ray diffraction and Scanning Electron Microscopy to follow the hydration mechanism in various binary and ternary blends. Statistical significance testing was used to analyse and confirm all experimental results and conclusions. It is well known that a level of caution is exercised in the construction industry in the use of ternary blends. This study aims to evaluate the durability aspects of ternary concrete blends, in addition to binary blends, for resistance to chloride, corrosion, carbonation attacks and provide recommendations relating to the limits of blending level, as well as exposure conditions for blended concretes, based on the results of this study. It is expected that this will fill a major knowledge gap observed in the concrete industry. A comparison of two rapid chloride permeability tests such as UCT and RCPT indicates that the UCT test is easy and practicable, and does not contradict results obtained in the standard RCPT. However, the statistical significance of results obtained for some blends was only able to be established by using the RCPT. This effect can be attributed to the larger size specimens compared to UCT. The recommended blend to acquire both early-age and long-term strength development in fly ash is the ternary blends comprising 10% silica fume and 25% fly ash cast using lower w/b ratio. In addition, the same blend exhibited lower carbonation depth, lower charge passed from RCPT, lower chloride ingress and higher corrosion resistance characteristics from long-term ponding test compared to other blends of fly ash. In BFS blends, an increase in compressive strength was observed only in the specimens of 25% BFS compared to other higher percentage blends, while the higher addition of 50 and 70% replacement showed no significant difference in compressive strength between them and their corresponding ternary blends with addition of silica fume. The results of this study indicate that control (OPC) specimens cast with increased w/b ratio of 0.48 showed higher chloride ingress compared to both binary blends of 70% fly ash and 70% BFS specimens. This indicates that (OPC) cast using higher w/b ratio is to be avoided in chloride environments. On the other hand, though, the ternary blends of 10% silica fume and up to 50% fly ash exhibited lower chloride ingress compared to their respective binary blends of fly ash. However, these ternary blends exhibited lower compressive strength, more negative corrosion potential and higher corrosion rate, compared to the respective binary blends of 25% fly ash and its ternary blends. Therefore, the recommended blend observed in the long-term ponding test is the ternary blend of 25% fly ash and 10% silica fume. The recommended level of corrosion resistance in slag specimens is achieved by the use of ternary blends comprising silica fume at 10% added to the blend that contains up to 70% slag. However, the recommended level of slag for a lower carbonation effect is the use of a ternary blend comprising 50% slag and 10% silica fume (3B5S1) which showed a carbonation depth of 10.8 mm and a compressive strength of 53.2 MPa after 365 days of exposure. The drying shrinkage of concrete increased with the increase in fly ash and the same trend was observed in BFS specimens. However, the results were not significantly different between their respective blends. The extent of carbonation in fly ash specimens was higher compared to BFS blends specimens. This can be attributed to the formation of dusty and weak surfaces on the outer surface in addition to the excessive leaching of sodium chloride solution from the long-term ponding test in the former specimens compared to later. The high volume pozzolanic materials, irrespective of fly ash or BFS and addition of silica fume (70% fly ash and 10% silica fume, and 70% BFS and 10% silica fume), showed higher cumulative pore volume indicating that these blends with seven days of curing were not beneficial. These high volume ternary blends required prolonged curing to release portlandite from the hydration of cement to continue the pozzolanic reaction. This study has shown that 7-days curing of the pozzolanic concrete is inadequate if pozzolanic activity is to be invoked. This is particularly the case when it is expected that the concrete is likely to be subjected to a harsher than usual environment characterised by a dry atmosphere.
30
Smith, J. J. "Microstructural characterisation of duplex 316 weld metals : The effects on the mechanical and high temperature properties". Thesis, University of Southampton, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.384218.
Pełny tekst źródła
31
Yan, Miaolei. "Defect Analysis and Microstructural Effects on the Surface Exchange Properties of La0.7Sr0.3MnO3(LSM) Epitaxial Thin Films". Research Showcase @ CMU, 2015. http://repository.cmu.edu/dissertations/650.
Pełny tekst źródłaStreszczenie:
La0.7Sr0.3MnO3 (LSM) is a perovskite oxide material that possesses many interesting electromagnetic and electrochemical properties, making it desirable as magnetic tunnel junction (MTJ) and solid oxide fuel cell (SOFC) electrodes.
32
Jin, Yan. "THREE-DIMENSIONAL MICROSTRUCTURAL EFFECTS ON MULTI-SITE FATIGUE CRACK NUCLEATION BEHAVIORS OF HIGH STRENGTH ALUMINUM ALLOYS". UKnowledge, 2016. http://uknowledge.uky.edu/cme_etds/63.
Pełny tekst źródłaStreszczenie:
An experimental method was further developed to quantify the anisotropy of multi-site fatigue crack initiation behaviors in high strength Al alloys by four-point bend fatigue testing under stress control. In this method, fatigue crack initiation sites (fatigue weak-links, FWLs) were measured on the sample surface at different cyclic stress levels. The FWL density in an alloy could be best described using a three-parameter Weibull function of stress, though other types of sigmoidal functions might also be used to quantify the relationship between FWL density and stress. The strength distribution of the FWLs was derived from the Weibull function determined by fitting the FWLs vs. stress curve experimentally obtained. As materials properties, the FWL density and strength distribution could be used to evaluate the fatigue crack nucleation behaviors of engineering alloys quantitatively and the alloy quality in terms of FWL density and strength distribution. In this work, the effects of environment, types of microstructural heterogeneities and loading direction on FWLs were all studied in detail in AA7075-T651, AA2026-T3511, and A713 Al alloys, etc. It was also found that FWLs should be quantified as a Weibull-type function of strain instead of stress, when the applied maximum cyclic stress exceeded the yield strength of the tested alloys.
In this work, four-point bend fatigue tests were conducted on the L-T (Rolling-Transverse), L-S (Rolling-Short transverse) and T-S planes of an AA7075-T651 alloy plate, respectively, at room temperature, 20 Hz, R=0.1, in air. The FWL populations, measured on these surfaces, were a Weibull-type function of the applied maximum cyclic stress, from which FWL density and strength distribution could be determined. The alloy showed a significant anisotropy of FWLs with the weak-link density being 11 mm-2, 15 mm-2 and 4 mm-2 on the L-T, L-S and T-S planes, respectively. Fatigue cracks were predominantly initiated at Fe-containing particles on the L-T and L-S planes, but only at Si-bearing particles on the T-S plane, profoundly demonstrating that the pre-fractured Fe-containing particles were responsible for crack initiation on the L-T and L-S planes, since the pre-fracture of these particles due to extensive deformation in the L direction during the prior rolling operation could only promote crack initiation when the sample was cyclically stressed in the L direction on both the L-T and L-S planes. The fatigue strengths of the L-T, L-S and T-S planes of the AA7075 alloy were measured to be 243.6, 273.0 and 280.6 MPa, respectively. The differences in grain and particle structures between these planes were responsible for the anisotropy of fatigue strength and FWLs on these planes.
Three types of fatigue cracks from particles, type-I: the micro-cracks in the particles could not propagate into the matrix, i.e., type-II: the micro-cracks were fully arrested soon after they propagated into the matrix, and type-III: the micro-cracks became long cracks, were observed in the AA7075-T651 alloy after fatigue testing. By cross-sectioning these three-types of particles using Focused Ion Beam (FIB), it was found that the thickness of the particles was the dominant factor controlling fatigue crack initiation at the particles, namely, the thicker a pre-fractured Fe-containing particle, the easier it became a type-III crack on the L-T and L-S planes. On the T-S plane, there were only types-I and III Si-bearing particles at which crack were initiated. The type-I particles were less than 6.5 μm in thickness and type-III particles were thicker than 8.3 μm. Cross-sectioning of these particles using FIB revealed that these particles all contained gas pores which promoted crack initiation at the particles because of higher stress concentration at the pores in connection with the particles. It was also found that fatigue cracks did not always follow the any specific crystallographic planes within each grain, based on the Electron Backscatter Diffraction (EBSD) measurement. Also, the grain orientation did not show a strong influence on crack initiation at particles which were located within the grains. The topography measurements with an Atomic Force Microscope (AFM) revealed that Fe-containing particles were protruded on the mechanically polished surface, while the Si-bearing particles were intruded on the surface, which was consistent with hardness measurements showing that Si-bearing particles were softer (4.030.92 GPa) than Fe-containing ones (8.9 0.87 GPa) in the alloy.
To verify the 3-D effects of the pre-fractured particles on fatigue crack initiation in high strength Al alloys, rectangular micro-notches of three different types of dimensions were fabricated using FIB in the selected grains on the T-S planes of both AA2024-T351 and AA7075-T651 Al alloys, to mimic the three types of pre-fractured particles found in these alloys. Fatigue testing on these samples with the micro-notches verified that the wider and deeper the micro-notches, the easier fatigue cracks could be initiated at the notches. In the AA2024-T351 samples, cracks preferred to propagate along the {111} slip plane with the smallest twist angle and relatively large Schmid factor. These experimental data obtained in this work could pave a way to building a 3-D quantitative model for quantification of fatigue crack initiation behaviors by taking into account the driving force and resistance to short crack growth at the particles in the surface of these alloys.
33
Nafisi, Shahrooz. "Effects of grain refining and modification on the microstructural evolution of semi-solid 356 alloy = Effets de l'affinage des grains et de la modification sur l'évolution microstructurale de l'alliage 356 semi-solide /". Thèse, Chicoutimi : Université du Québec à Chicoutimi, 2006. http://theses.uqac.ca.
Pełny tekst źródła
34
Jamaly, M. A. M. Nasim. "The effects of microstructural features and process parameters on the hottearing in direct chill cast aluminum alloys". Thesis, University of British Columbia, 2012. http://hdl.handle.net/2429/43316.
Pełny tekst źródłaStreszczenie:
Hot tearing is an irreversible failure that occurs above the the solidus temperature of an alloy during casting, in the presence of a liquid phase. These cracks possess serious quality implications in industrial direct chill (DC) casting process. During solidification, thermal stresses are induced due to the heterogeneous temperature distribution, which causes variations in thermal strain and may result in cracks if these stresses are large enough. In order to investigate the occurrence of hot tears in DC casting, a DC casting finite element process model for round billets was incorporated with (1) a new semi-solid constitutive law for aluminum alloy AA5182 that takes into account cooling rate, grain size and porosity, and (2) a model for cooling rate induced grain size variation. A hot tearing index was calculated from the semi-solid strain predictions from the model. This hot tearing index, along with semi-solid stress predictions from the model, was used to link hot tearing with microstructural features (i.e. grain size and coalescence temperature) as well as process parameters (e.g. casting speed). It was found that grain refinement plays an important role in the formation of hot cracks. In addition, lower assumed coalescence temperature and slow casting speeds were found to improve hot tearing resistance. In addition to simulation of DC casting, experimental studies on an as cast AA5182 ingot (DC cast) were made in terms of grain size, chemical composition and solidification kinetics. Samples for these experiments were collected from the steady state region of the ingot. The results of these experimental investigations show that, (1) grain size increases from surface to centre of the ingot, (2) there is considerable macrosegregation of the alloying elements along the cross section of the ingot, and (3) the solidification kinetics vary as a function of both position and cooling rates. These experimental observations influence the hot tearing susceptibility of the DC cast product. Thus, for the process model to be more accurate in predicting hot tears, inclusion of these factors, along with an improved model for grain size variation is suggested.
35
Ding, Yi. "Effects of elevated temperature exposure on the microstructural evolution of Ni(Cr)-Cr3C2 coated 304 stainless steel". Thesis, University of Nottingham, 2009. http://eprints.nottingham.ac.uk/10917/.
Pełny tekst źródłaStreszczenie:
Agglomerated and sintered 25Ni(Cr)-75Cr3C2 powder was deposited onto 304 stainless steel substrates by HVOF thermal spraying. Post spraying thermal treatments were carried out on as-sprayed samples at different temperatures (700°C and 800°C) in air (oxidising). Samples were heat treated from 1hour up to 16 days and furnace cooled. The powder, coatings and substrate were characterised by a variety of techniques including optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), microhardness testing and thermogravimetric analysis (TGA). The results of agglomerated and sintered powder coating microstructural characterisation show that the as-sprayed coating comprised the carbide phase Cr3C2, a small amount of amorphous phase and a metallic phase which was Ni-rich. The spray process also led to the formation a small amount of Cr2O3 in the coatings. In the heat treated coatings, the carbides Cr7C3 and Cr23C6 were found at the coating substrate interface, the amorphous structure phase was eliminated. With heat treatment, matrix phase supersaturation was reduced, while widespread carbide nucleation and growth generated an expansive carbide skeletal network especially near the coating/substrate interface. An initial softening of the coatings occurred but subsequently the hardness increased again after 2 days was a function of carbide development. The oxidation kinetics of the 25Ni(Cr)-75Cr3C2 powder and coatings during elevated temperature oxidation in air at 650°C to 1000°C have also been studied. The oxidation behaviour was found to be governed by a parabolic rate law up to 800°C. The activation energy of the coating oxidation reaction, Q, was found to be 164 kJ/mol. The mechanism of high temperature oxidation of 25Ni (Cr)-75Cr3C2 coating on top surface has been discussed. Inter-diffusion between the coating and the 304 stainless steel substrate has been investigated. A distribution of the precipitate phase Cr23C6 within the 304 steel was found from etched substrate samples. The decrease of the microhardness measured with distance away from the interface also revealed that diffusion occurred between coating and substrate during the annealing period. Carbon diffusion distances at different annealing conditions in this work have been measured according to the appearance of the Cr23C6 precipitates and variation in microhardness away from the interface. A simple mathematical model has been used to calculate the theoretical carbon diffusion distance in substrate.
36
Bakare, Mayowa Sunday. "The effects of microstructural modifications on corrosion resistance of metallic corrosion resistant materials Inconel 625 and FeCrMoCB". Thesis, University of Nottingham, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.546474.
Pełny tekst źródła
37
Ahmed, Nauman. "Characterisation of different forms of Inconel 625 for determining the effects of microstructural modifications on electrochemical behaviour". Thesis, University of Nottingham, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.493327.
Pełny tekst źródłaStreszczenie:
Thermal spraying is widely used to produce application-based superior surfaces on relatively cheap bulk materials, thus saving materials and cost. The concept is based on the assumption that the coating offers the same superior properties as the bulk material of the same composition. However, the use of thermal spray coatings for corrosion protection requires an awareness regarding microstructural modification during coating deposition and due to which, a performance gap may exist between coating and the bulk material.
38
Nakos, James Spiros. "Effects of crystal growth process parameters on the microstructural optical and electrical properties of CdTe and CdMnTe". Thesis, Massachusetts Institute of Technology, 1988. http://hdl.handle.net/1721.1/14574.
Pełny tekst źródła
39
Pontikos, Nicholas Michael. "Laser induced microstructural and morphological alterations and their effects on electron transfer kinetics at glassy carbon electrodes /". The Ohio State University, 1992. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487780865407154.
Pełny tekst źródła
40
McCarthy, John M. "The microstructural effects of metallization and heat treatment on thin gate oxide for use in sub-micron MOSFETs /". Full text open access at:, 1996. http://content.ohsu.edu/u?/etd,13.
Pełny tekst źródła
41
Kitey, Rajesh Tippur Hareesh V. "Microstructural effects on fracture behavior of particulate composites investigation of toughening mechanisms using optical and boundary element methods /". Auburn, Ala., 2006. http://repo.lib.auburn.edu/2006%20Spring/doctoral/KITEY_RAJESH_50.pdf.
Pełny tekst źródła
42
Sauter, Linda Xenia [Verfasser]. "Microstructural and film thickness effects on the thermomechanical behavior of thin Au films / vorgelegt von Linda Xenia Sauter". Stuttgart : Max-Planck-Inst. für Metallforschung, 2007. http://d-nb.info/995370753/34.
Pełny tekst źródła
43
Tovee, John-Paul. "Microstructural influence on the effects of forward and reverse mechanical deformation in HSLA X65 and X80 linepipe steels". Thesis, University of Birmingham, 2014. http://etheses.bham.ac.uk//id/eprint/5171/.
Pełny tekst źródłaStreszczenie:
Five API grade steels designed for linepipe applications produced using different processing routes and with varying microstructures were studied against differences in work hardening and work softening behaviour obtained from mechanical data. The rolling history and wt % additions of alloying elements will determine how the microstructures perform under reverse deformation schedules commonly seen during large diameter linepipe fabrication as steels can undergo work softening in the reverse direction of deformation, otherwise known as the Bauschinger effect. The Bauschinger effect is known to be dependent on the initial forward pre-strain, volume fraction (VF) of carbo-nitride particles and initial dislocation density. The effects of grain size and solid solution strengthening are a matter of debate in the literature and the combined effects of all five strengthening mechanisms have rarely been quantified. TEM investigations determined the dislocation densities to be between 2.2 x1014 m-2 - 5.8 x1014 m-2 in the as received condition. Observed trends presented and discussed in this body of work have given a greater insight into the influence microstructure has on the mechanical properties across a wide range of HSLA steels of similar strength grades, which are of important consideration for future development of low carbon steels designed for the petrochemical industry.
44
Meroueh, Laureen. "Effects of doping and microstructural variables on hydrogen generated via aluminum-water reactions enabled by a liquid metal". Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/129067.
Pełny tekst źródłaStreszczenie:
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2020Cataloged from student-submitted PDF of thesis.
Includes bibliographical references (pages [110]-127).
Hydrogen has the potential to replace fossil fuels in numerous industrial sectors, considering its high energy density, ability to be used within our existing power or heating infrastructure, and lack of CO₂ emissions upon conversion of hydrogen's chemical energy into electricity. However, 96% of hydrogen is currently produced through steam methane reformation, which emits ~12 tons of CO₂ for every 1 ton of hydrogen produced. Consequently, hydrogen production accounts for roughly 830 million tons of annual global CO₂ emissions. Additionally, hydrogen storage can be impractical and expensive. The aluminum-water reaction presents itself as a hydrogen storage and generation solution. Without a passive oxide layer, aluminum will react with water to produce emission-free hydrogen, on-demand.
We enable the reaction by harnessing eutectic gallium-indium (eGaIn), an ambient temperature liquid metal that permeates through aluminum grain boundaries, disrupting its passive oxide layer and inhibiting passivation of its grain surfaces. The focus of this work is on the investigation of the underlying aluminum-water reaction mechanism in the presence of eGaIn and on understanding the effects of using scrap aluminum (i.e. doped aluminum) as feedstock. Surprisingly, experiments show that silicon doping has a tremendous accelerating effect on the aluminum-water reaction in the presence of eGaIn. In combination with grain size manipulation, Si-doping can increase hydrogen evolution rates by two orders of magnitude compared to pure aluminum. Doping with magnesium significantly retards the aluminum-water reaction, resulting in relatively steady hydrogen evolution rates. It is also shown that eGaIn permeates through aluminum as a line dislocation front.
These discoveries demonstrate that doping, grain refining and grain coarsening offer latitudes in the engineering of aluminum microstructures to tune hydrogen generation rates across three orders of magnitude and to tune the reaction efficiency. Using the results of this work, I provide a guide to the design/selection of aluminum for controllable hydrogen generation according to application. Lastly, while the corrosion of aluminum and its commercial alloys has been historically studied, results of this work show that the redox behavior of aluminum in the presence of eGaIn strays from what can be understood within the classical corrosion (galvanic theory) framework.
by Laureen Meroueh.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Mechanical Engineering
45
Love, Ryan. "A microstructural analysis of the effects of news on order flows and on price discovery in foreign exchange markets". Thesis, London School of Economics and Political Science (University of London), 2005. http://etheses.lse.ac.uk/1788/.
Pełny tekst źródłaStreszczenie:
This thesis brings together a number of studies using high frequency foreign exchange (FX) data. The first part examines the effects of scheduled, publicly released macroeconomic news, while the final chapter considers another, related, aspect of FX microstructure. Chapter 1 provides an introduction to the thesis and reviews the literature in high frequency empirical FX research. In Chapter 2, I use up to ten months of FX transactions and quote data to analyse foreign exchange activity around times of scheduled news releases. The effects of news on exchange rate levels are examined, as well as the effects on spreads, trading volume and volatihty. Chapter 3 extends this analysis, asking how public information enters prices. Under rational expectations and efficient markets hypotheses, the news contained in public information announcements should be impounded directly, with there being no role for trades in this process of information assimilation. However, the results suggest that up to two thirds of the price relevant information enters via trading (order flow in particular). Chapter 4 provides an explanation why order flow is so important around public news releases and also examines the effects of news on market depths. In Chapter 5 I examine how much information is carried in trades by looking at the price impact of order flow when feedback trading is allowed. The model that is often used in the literature is proved to be misspecified when temporally aggregated data are employed and Chapter 5 introduces a method to estimate the otherwise unidentified model. Using impulse response functions, I show that trades actually carry more information than previous estimates suggest.
46
Motaman, S. Amir H. [Verfasser], Wolfgang [Akademischer Betreuer] Bleck, Dierk [Akademischer Betreuer] Raabe i Ulrich [Akademischer Betreuer] Prahl. "Modeling of the microstructural effects on the mechanical response of polycrystals / Seyedamirhossein Motaman ; Wolfgang Bleck, Dierk Raabe, Ulrich Prahl". Aachen : Universitätsbibliothek der RWTH Aachen, 2021. http://d-nb.info/1229991328/34.
Pełny tekst źródła
47
Davidson, Laura Christine. "Microstructural Characterization of LENSTM Ti-6Al-4V: Investigating the Effects of Process Variables Across Multiple Deposit Geometries". Wright State University / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=wright154505910306871.
Pełny tekst źródła
48
Hecht, Matthew David. "Effects of Heat Treatments and Compositional Modification on Carbide Network and Matrix Microstructure in Ultrahigh Carbon Steels". Research Showcase @ CMU, 2017. http://repository.cmu.edu/dissertations/1023.
Pełny tekst źródłaStreszczenie:
This dissertation investigates microstructure/property relations in ultrahigh carbon steel (UHCS) with the aim of improving toughness while retaining high hardness. Due to high C contents (ranging from 1 to 2 wt%), UHCS exhibit high strength, hardness, and wear resistance. Despite this, applications for UHCS are currently limited because they typically contain a continuous network of proeutectoid cementite that greatly reduces ductility and toughness. In previous research, thermomechanic processing had seen considerable success in breaking up the network. However, the processing is difficult and has thus far seen very limited industrial application. Chemical modification of the steel composition has also seen some success in network break-up, but is still not well understood. There have been relatively few fundamental studies of microstructure evolution in UHCS; studies in the literature typically focused on lower C steels (up to 1 wt% C) or on cast irons (>2.1 wt% C). Thus, this work was undertaken to gain a better understanding of microstructural changes that occur during heat treatment and/or chemical modification of UHCS with a focus on the distribution of proeutectoid cementite within the microstructure. This dissertation is composed of eight chapters. The first chapter presents an introduction to phases found in UHCS, descriptions of research materials used in each chapter, and the hypotheses and objectives guiding the work. The second chapter describes a study of the microstructure found in a 2C-4Cr UHCS before and after an industrial-scale austenitizating heat treatment that increased hardness and toughness and also produced discrete carbide particles in the matrix. The third chapter establishes and demonstrates a metric for measuring connectivity in carbide networks. The fourth chapter describes a series of heat treatments designed to investigate kinetics of spheroidization and coarsening of carbide particles and denuded zones near cementite network branches in 2C-4Cr UHCS. The fifth chapter describes an additional series of heat treatments comparing coarsening kinetics in 2C-1Cr and 2C-4Cr UHCS. Lowering the Cr content caused clustering of cementite particles near grain boundaries, in contrast to the denuded zones observed in the higher Cr UHCS. The fifth chapter details four in situ confocal laser scanning microscopy heat treatments of 2C-4Cr UHCS. The seventh chapter investigates the effects of a 2wt% Nb addition on 2C-4Cr UHCS. The eighth and final chapter summarizes the findings of all the experiments of the previous chapters and revisits the objectives and conclusions.
49
Dini, Hoda. "As-cast AZ91D magnesium alloy properties : Effects of microstructure and temperature". Doctoral thesis, Tekniska Högskolan, Högskolan i Jönköping, JTH, Material och tillverkning, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:hj:diva-38148.
Pełny tekst źródłaStreszczenie:
Today, there is an essential need for lightweight, energy-efficient, environmentally benign engineering systems, and this is the driving force behind the development of a wide range of structural and functional materials for energy generation, energy storage, propulsion, and transportation. These challenges have motivated the use of magnesium alloys for lightweight structural systems. Magnesium has a density of 1.74 g/cm3, which is almost 30% less than that of aluminium, one quarter of steel, and almost identicalto polymers. The ease of recycling magnesium alloys as compared to polymers makes them environmentally attractive, but their poor mechanical performance is the primary reason for the limited adoption of these alloys for structural applications. The Mg-Al-Zn alloy AZ91D exhibits an excellent combination of strength, die-castability, and corrosion resistance. However, its mechanical performance with regard to creep strength, for example, at evaluated temperatures is poor. Moreover, very little is known about the correlation between its mechanical properties and microstructural features. This thesis aims to provide new knowledge regarding the role played by microstructure in the mechanical performance of the magnesium alloy. The properties/performance of the material in relation to process parameters became of great interest during the investigation. An exhaustive characterisation of the grain size, secondary dendrite arm spacing (SDAS) distribution, and fraction of Mg17Al12 was performed using optical and electron backscatter diffraction (EBSD). These microstructural parameters were correlated to the offset yield point (Rp0.2), fracture strength, and elongation to failure of the material. It was proposed that the intermetallic phase, Mg17Al12, plays an important role in determining the mechanical and physical properties of the alloy in a temperature range of room temperature to 190°C by forming a rigid network of intermetallic. The presence of this network was confirmed by studying the thermal expansion behaviour of samples of the alloy containing different amounts of Mg17Al12. A physically based constitutive model with a wide validity range was successfully adapted to describe the flow stress behaviour of AZ91D with various microstructures. The temperature-dependent variables of the model correlated quite well with the underlying physics of the material. The model was validated through comparison with dislocation densities obtained using EBSD. The influence of high-pressure die-cast parameters on the distortion and residual stress of the cast components was studied, as were distortion and residual stress in components after shot peening and painting. Interestingly, it was found that intensification pressure has a major effect on distortion and residual stresses, and that the temperature of the fixed half of the die had a slight influence on the component's distortion and residual stress.Numera finns det ett väsentligt behov av lätta, energieffektiva och miljövänliga tekniksystem. Detta behov är drivkraften för utveckling av ett brett utbud av material för energigenerering, energilagring, framdrivning och transport. Dessa utmaningar motiverade användningen av magnesiumlegeringar för lättviktskonstruktioner. Magnesium har en densitet på 1,74 g/cm3, vilket är ca 30% lägre än för aluminium, en fjärdedel av densiteten för stål och nästan i nivå med många polymerer. Då magnesiumlegeringar dessutom är lätta att återvinna, jämfört med polymerer, gör det dem miljömässigt attraktiva. Låga mekaniska egenskaper är den främsta orsaken till begränsad användning av dessa legeringar för lastbärande tillämpningar. Mg-Al-Zn-legeringen AZ91D uppvisar en utmärkt kombination av styrka, gjutbarhet och korrosionsbeständighet. Dess mekaniska egenskaper vid förhöjd temperatur, som tex kryphållfasthet, är låga. Dessutom är korrelationen mellan mikrostruktur och mekaniska egenskaper oklar. Denna avhandling syftade till att ge ny kunskap om mikrostrukturens roll för magnesiumlegeringars mekaniska egenskaper. Slutligen var materialets egenskaper i förhållande till processparametrar vid tillverkningen av stort intresse. En omfattande karaktärisering av kornstorleks-, sekundära dendritarmavstånds (SDAS)-fördelning och fraktion av Mg17Al12 utfördes med hjälp av optisk mikroskopering och diffraktion av bakåtspridda elektroner (EBSD). Mikrostrukturen korrelerades till sträckgränsen (Rp0.2), brottstyrkan och brottförlängningen. Det föreslogs att den intermetalliska fasen, Mg17Al12, spelar en viktig roll vid bestämning av legeringens mekaniska och fysikaliska egenskaper vid temperaturintervall från rumstemperatur upp till 190°C genom att bilda ett styvt nätverk av intermetaller. Uppkomsten av ett sådant nätverk stöddes genom en studie av den termiska expansionen av legeringen för olika fraktioner av Mg17Al12. En fysikalisk konstitutiv modell med ett brett giltighetsområde användes framgångsrikt för att beskriva det plastiska flytbeteendet hos AZ91D för olika mikrostrukturer. De temperaturberoende variablerna i modellen korrelerade ganska väl med materialets underliggande fysik. Modellen validerades genom att jämföra dislokationstätheten som predikterades av modellen och den med EBSD uppmätta dislokationstätheten. Påverkan av pressgjutningsparametrar på geometrisk tolerans och restspänning hos de gjutna komponenterna studerades. Vidare studerades geometrisk tolerans och restspänning av komponenter efter pening och målning. Intressant nog hade eftermatningsfasen en stor effekt på geometrisk tolerans och restspänningar. Dessutom hade temperaturen på den fasta formhalvan av verktyget även ett visst inflytande på komponentens geometriska tolerans och restspänning.
50
Mattiello, Adriana. "Visco-plasticity and damage modeling of single crystal superalloys at high temperatures : a tensorial microstructure-sensitive approach". Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLN004/document.
Pełny tekst źródłaStreszczenie:
Un modèle phénoménologique 3D de visco-plasticité couplée avec les évolutions microstructurales et l'endommagement est proposé pour les superalliages monogranulaires base Nickel, les matériaux des aubes de turbine à haute pression de moteurs d'hélicoptères. L'anisotropie de la mise en radeaux, la croissance et la dissolution de la phase durcissante sont modélisés. Une variable tensorielle et sa loi d'évolution permettent la description de la variation de la largeur des couloirs de matrice. Ce travail s'appuie sur la décomposition en modes de Kelvin du tenseur d'élasticité. Cette décomposition conduit également à une description multi-critère mésoscopique de la visco-plasticité cubique. Une formulation en (visco-)plasticité cristalline a été également proposée. Une loi d'endommagement avec seuil de type dD/dt=... est formulée pour la modélisation du fluage tertiaire et pour la prévision de la ductilité en traction. Une expression originale du seuil d'endommagement rend compte des effets de vitesse sur l'amorçage de l'endommagement par visco-plasticité. Une étude expérimentale a été conduite sur le CMSX-4, l'alliage au centre de cette étude, parallèlement aux travaux de modélisation. Des essais de dissolution ont été réalisés afin de mesurer la variation de la fraction volumique des précipités avec la température. Les mécanismes de déformation du matériau ont été observés en fluage isotherme à 850°C et 1050°C selon les principales directions cristallines du triangle stéréographique standard et constituent une base d'identification pour le modèle, la réponse mécanique du matériau obtenue. Trois essais de traction ont été réalisés selon la direction <111>, deux à vitesse de chargement constante, le troisième à vitesse variable. Des essais cyclés thermiquement de type 150h-moteur ont été réalisées sur le banc MAATRE. Des analyses EBSD et MET ont été réalisés sur les échantillons orientés selon les directions cristallines <011>, <111> et <112> et testés à 850°C. Ces analyses ont montré que le mâclage est le principal mécanisme de déformation pour des déformations supérieures à $1-2%$ selon ces orientations cristallines en fluage à cette température et à haute contrainte (>400 MPa). Enfin, le modèle a été implanté dans le code à élément Finis ZéBuLon, sans ou avec endommagement, et des calculs de structures ont été réalisésA 3D phenomenological model coupling viscoplasticity, microstructural evolutions and damage is proposed for Ni-based single crystal superalloys, which are widely used materials for high pressure turbine blade in helicopter engines. The anisotropy of the gamma'-rafting, the gamma'-coarsening and the dissolution of the hardening gamma' phase are modeled. A tensorial variable and its evolution law allow to describe the variation of the gamma channels. The modeling is based on the Kelvin decomposition of the elasticity tensor. This decomposition leads to a phenomenological multi-criterion description of the cubic visco-plasticity. A formulation based on the single crystal (visco-)plasticity framework is also proposed. A damage law of type dD/dt=… is introduced in order to model the tertiary creep stage and the ductility. A novel rate sensitive damage threshold is introduced in order to account for the rate sensitivity of the damage onset by visco-plasticity. An experimental study has been carried out on the CMSX-4 alloy, which is the material of main interest in this study, in parallel to the modeling work. Dissolution tests have been carried out to measure the gamma'-volume fraction variation with temperature. The deformation mechanisms of the CMSX-4 alloy have been observed by performing tensile creep tests at 1050°C and 850°C. These tests have also constituted a database for the model identification. Three tensile tests have been realized along the <111> crystal direction, two at constant strain rate and the third by varying the strain rate. Non-isothermal creep tests reproducing the 150h-type engine test have been performed on the MAATRE bench. EBSD and TEM analysis have been realized on the specimens oriented along the <011>, <011> and <112> crystal directions and crept at 850° C. These analysis have shown that micro-twinning governs the deformation along these crystal directions during creep at this temperature and at high stresses (> 400 MPa) for deformation in excess of 1-2%. Finally, the model (with and without damage) has been encoded in the ZéBuLoN Finite Element solver and structure computations have been performed