Academic literature on the topic 'Kinetics of non-isothermal crystallization'

Abstract The non-isothermal crystallization behavior of polyamide 5,6 (PA56) was investigated by differential scanning calorimeter (DSC), and the non-isothermal crystallization kinetics were analyzed using the modified Avrami equation, the Ozawa model, and the method combining the Avrami and Ozawa equations. It was found that the Avrami method modified by Jeziorny could only describe the primary stage of non-isothermal crystallization kinetics of PA56, the Ozawa model failed to describe the non-isothermal crystallization of PA56, while the combined approach could successfully describe the non-isothermal crystallization process much more effectively. Kinetic parameters, such as the Avrami exponent, kinetic crystallization rate constant, relative degree of crystallinity, the crystallization enthalpy, and activation energy, were also determined for PA56.

Isothermal and non-isothermal crystallization kinetics of polycyclohexylene dimethylene cyclohexanedicarboxylate (PCCE) were investigated via differential scanning calorimetry (DSC). Isothermal melt crystallization kinetics were analyzed using the traditional Avrami equation. Non-isothermal melt crystallization kinetics data obtained from DSC were analyzed using the extended Avrami relation and a combination of the Avrami equation and the Ozawa relationship. The glass transition temperature, equilibrium melting point, isothermal crystallization activation energy, and non-isothermal crystallization activation energy were determined. Furthermore, a predictive method based on the Nakamura model was proposed and was used to describe the non-isothermal crystallization kinetics based on the isothermal experimental data. The results suggested that the original Nakamura equation was not successful in describing the non-isothermal crystallization of PCCE over a wide range of cooling rates. It was found that the non-isothermal crystallization kinetics of PCCE, over a wide range of cooling rates, could best be described by modifying the differential Nakamura equation to include a varied Avrami index.

Abstract The non-isothermal crystallization kinetics with heating rates ranging from 10 K s-1to 80 K s-1and the isothermal crystallization kinetics during annealing from the glass transition temperature to the crystallization onset temperature of a Zr57Cu15.4Ni12.6Al10Nb5 amorphous alloy were studied in detail using X-ray diffraction and differential scanning calorimetry. During non-isothermal crystallization, it is more difficult to nucleate than to grow, and the crystallization resistance increases first and then decreases. During isothermal crystallization of the alloy from 713- 728 K, there are two exothermic peaks corresponding to a diffusion-controlled growth process with decreasing nucleation rate and increasing nucleation rate. From 733- 748 K, only one exothermic peak appears, and the growth process is controlled by the interface with decreasing nucleation rate. Isothermal crystallization is a process in which the crystallization resistance increases. The resistance of isothermal crystallization is less than that of non-isothermal crystallization.

Isothermal crystallization kinetics of amorphous Ga–Sb–Te films was studied by means of a time-resolved optical transmission method. Thin films with compositions along the pseudo-binary tie-lines Sb7Te3–GaSb and Sb2Te3–GaSb in the ternary phase diagram were prepared by the co-sputtering method. Crystallization of GaSbTe films reveals a two-stage process: an initial surface nucleation and coarsening (Stage 1) followed by the one-dimensional grain growth (Stage 2). The kinetic exponent (n) value in Stage 1 shows strong dependence on film compositions, while that of Stage 2 is less dependent. The activation energy in Stage 1 increases with increasing GaSb content and reaches the maximum values at compositions close to GaSb, but a decreasing trend was observed in Stage 2. Kinetics parameters between isothermal crystallization of thin films and non-isothermal crystallization of powder samples analyzed by differential scanning colorimetry [J. Mater. Res. 19, 2929 (2004)] are compared. The kinetic parameters in Stage 1 show much correspondence with those of non-isothermal cases in comparable kinetic exponents but with lower activation energies. The discrepancies between nonisothermal and isothermal kinetics are attributed to the sample morphology and the constraint effects.

Polyvinlyl alcohol (PVA)/graphene oxide (GO) composites are prepared by solution blending method. And the non-isothermal crystallization kinetics of as-prepared composites is evaluated by differential scanning calorimetry (DSC). The results indicate the graphene oxide can significantly modify the non-isothermal crystallization behavior of the PVA, for instance improved crystallization temperature and prolonged crystallization time. Enhanced crystallization temperature illustrates that GO can act as effective nucleating agent. However, prolonged crystallization time means that GO can retard the whole crystallization. Further kinetics analysis indicates that both the crystallization kinetics of neat PVA and PVA/GO match the Mo model very well. According to the Mo model, during the whole crystallization process, graphene oxide perform as a retardant. In conclusion, graphene oxide can act as effective nucleating agent due to strong interaction bewteen graphene oxide and PVA matrix. On the other hand, graphene oxide loaded may lead to other side effects. This side effects may lead to the retarded crystallization speed finally.

This work has mainly focused on the poly (L-lactide) (PLLA) which is a material for multiple applications with performances comparable to those of petrochemical polymers (PP, PS, PET, etc. ...), readily recyclable and also compostable. However, PLLA has certain shortcomings that limit its applications. It is a brittle, hard polymer with a very low elongation at break, hydrophobic, exhibits low crystallization kinetics and takes a long time to degrade. The properties of PLLA may be modified by copolymerization (random, block, and graft) of L-lactide monomers with other co-monomers. In this thesis it has been studied the crystallization and morphology of random copolymers poly (L-lactide-ran-ε-caprolactone) with different compositions of the two monomers since the physical, mechanical, optical and chemical properties of a material depend on this behavior. Thermal analyses were performed by differential scanning calorimetry (DSC) and thermogravimetry (TGA) to observe behaviors due to the different compositions of the copolymers. The crystallization kinetics and morphology of poly (L-lactide-ran-ε-caprolactone) was investigated by polarized light optical microscopy (PLOM) and differential scanning calorimetry (DSC). Their thermal behavior was observed with crystallization from melt. It was observed that with increasing amounts of PCL in the copolymer, there is a decrease of the thermal degradation. Studies on the crystallization kinetics have shown that small quantities of PCL in the copolymer increase the overall crystallization kinetics and the crystal growth rate which decreases with higher quantities of PCL.

L'objectiu de la tesi és estudiar la cinètica de les cristal·litzacions primàries en vidres metàl·lics mitjançant simulacions de tipus phase field. Una cristal·lització primària és una transició de fase sòlid-sòlid on la fase que cristal·litza (fase transformada o fase secundaria) té una composició química diferent de la fase precursora (fase no transformada o fase primària).
Les dades experimentals obtingudes a partir de l'estudi calorimètric de cristal·litzacions primàries s'analitzen generalment en el marc del model KJMA (Kolmogorov, Johnson & Mehl, Avrami). Aquest model proporciona l'evolució temporal de la fracció transformada basant-se en tres hipòtesis:
- Els nuclis de la fase secundaria estan distribuïts aleatòriament en tot l'espai.
- El creixement d'aquests nuclis és isotròpic.
- El creixement s'atura únicament per xoc directe (hard impingement).

En la cristal·lizació de vidres metàl·lics s'ha observat experimentalment un alentiment de la cinètica respecte del comportament calculat emprant la citada cinètica KJMA. Aquest alentiment s'explica a la literatura en base a que en aquest tipus de transformacions, controlades per difusió, la interacció entre els cristalls no és directa sinó que es produeix a través dels perfils de concentració (soft impingement) i, a més, l'evolució d'aquests perfils de concentració causa canvis en la concentració de la matriu amorfa, estabilitzant la i per tant fent que la nucleació de nous cristalls esdevingui no aleatòria. Diversos autors han proposat modificacions del model KJMA per tal d'intentar superar aquestes limitacions, basats bé en consideracions geomètriques, bé en aproximacions de camp mitjà. A pesar de tot, cap d'aquests models és capaç d'explicar satisfactòriament la cinètica observada en cristal·litzacions primàries. L'objectiu d'aquest treball ha estat la simulació realista de la cinètica de les transformacions primàries per trobar una explicació consistent a les diferències observades entre les dades experimentals i els models teòrics disponibles.
Per tal de poder descriure de forma realista el procés de cristal·lització primària s'ha d'estudiar el procés de nucleació i creixement de la fase secundaria alhora que es resol l'equació de difusió en la fase primària. En aquest treball s'ha emprat un model de simulació phase field que permet estudiar aquest sistema introduint una nova variable lligada al camp de concentració que pren dos valors diferents segons es tracti de fase transformada o no transformada. Amb aquest tipus de models també es poden introduir diferents protocols de nucleació i per tant estudiar independentment els efectes de la nucleació en la cinètica. D'aquesta manera s'han realitzat simulacions en 2 i 3 dimensions de cristal·litzacions primàries amb diferents graus de fracció transformada final). Els resultats de les simulacions s'ha comparat amb el model KJMA i, contra el que es preveia, s'ha obtingut un bon acord entre les fraccions transformades del model KJMA i de les simulacions. Donat que el model KJMA no reprodueix satisfactòriament el comportament experimental d'aquest resultat es dedueix que ni el soft impingement ni la nucleació no aleatòria son les responsables de l'alentiment de la cinètica obtingut en cristal·litzacions primàries.
Per tal de trobar una explicació físicament convincent del comportament observat experimentalment s'ha aprofundit en l'estudi teòric de les cristali·litzaciones primàries, incloent-hi l'efecte dels canvis composicionals que tenen lloc en la matriu a mesura que la transformació es produeix. Aquest fet, tot i ser conegut a la bibliografia, ha estat sistemàticament ignorat en l'elaboració de models cinètics. En concret, s'ha fet palès que canvis en la composició química de la fase primària han d'afectar de forma radical a la viscositat, que varia fortament a prop de la transició vitrea, i han de produir canvis en les propietats de transport atòmic. Això s'ha modelat a través de l'assumpció d'un coeficient de difusió depenent de la concentració, en base a la relació modificada d'Stokes-Einstein entre la viscositat i el coeficient de difusió. Les simulacions phase-field amb un coeficient de difusió d'aquest tipus donen lloc a una cinètica més lenta i que mostra un acord excel·lent amb la cinètica experimentalment observada en cristal·litzacions primàries de vidres metàl·lics. Per tant, les simulacions phase field confirmen que la cinètica de les cristal·litzacions primàries està controlada fonamentalment pel canvi en les propietats de transport atòmic, mentre que els efectes de soft impingement i nucleació no aleatoria, tot i estar presents, son secundaris.
El objetivo de la tesi es estudiar la cinética de las cristalizaciones primarias en vidrios metálicos mediante simulaciones de tipo phase field. Una cristalización primaria es una transición de fase sólido-sólido donde la fase que cristaliza (fase transformada o fase secundaria) tiene una composición química diferente a la fase precursora (fase no transformada o fase primaria).
Los datos experimentales obtenidos a partir del estudio calorimétrico de cristalizaciones primarias se analizan generalmente en el marco del modelo KJMA (Kolmogorov, Johnson & Mehl, Avrami). Este modelo proporciona la evolución temporal de la fracción transformada basándose en tres hipótesis:
- Los núcleos de la fase secundaria están distribuidos aleatoriamente en todo el espacio
- El crecimiento de estos núcleos es isotrópico
- El crecimiento se detiene únicamente por choque directo (hard impingement).

En la cristalización de vidrios metálicos se ha observado experimentalmente un retardo de la cinética respecto del comportamiento calculado usando la cinética KJMA. Este retardo se explica en la literatura en base a que en este tipo de transformaciones, controladas por difusión, la interacción entre los cristales no es directa sino que se produce a través de los perfiles de concentración (soft impingement) y, además, la evolución de estos perfiles de concentración causa cambios en la concentración de la matriz amorfa, estabilizándola y por tanto haciendo que la nucleación de nuevos cristales sea no aleatoria. Varios autores han propuesto modificaciones del modelo KJMA para intentar superar estas limitaciones, basados bien en consideraciones geométricas, bien en aproximaciones de campo medio. A pesar de todo, ninguno de estos modelos es capaz de explicar satisfactoriamente la cinética observada en cristalizaciones primarias. El objetivo de este trabajo ha sido la simulación realista de la cinética de las transformaciones primarias para hallar una explicación consistente a las diferencias entre los datos experimentales y los modelos teóricos disponibles.
Para describir de manera realista el proceso de cristalización primaria se tiene que estudiar el proceso de nucleación y crecimiento de la fase secundaria a la vez que se resuelve la ecuación de difusión en la fase primaria. En este trabajo se ha usado un modelo de simulación phase-field que permite estudiar este sistema introduciendo una nueva variable ligada al campo de concentración que toma dos valores diferentes según se trate de fase transformada o no transformada. Con este tipo de modelos también se pueden introducir diferentes protocolos de nucleación y por tanto estudiar independientemente los efectos de la nucleación en la cinética. De esta manera se han realizado simulaciones en 2 y 3 dimensiones de cristalizaciones primarias con diferentes grados de fracción transformada final. Los resultados de la simulaciones se han comparado con el modelo KJMA y, en contra de lo que se preveía, se ha obtenido un buen acuerdo entre las fracciones transformadas del modelo KJMA y de las simulaciones. Dado que el modelo KJMA no reproduce satisfactoriamente el comportamiento experimental, de este resultado se deduce que ni el soft impingement ni la nucleación no aleatoria son las responsables del retardo en la cinética obtenido en cristalizaciones primarias.
Para encontrar una explicación físicamente convincente del comportamiento observado experimentalmente se ha profundizado en el estudio teórico de las cristalizaciones primarias, incluyendo el efecto de los cambios composicionales que tienen lugar en la matriz a medida que la transformación se produce. Este hecho, aún y ser conocido en la bibliografía, ha sido sistemáticamente ignorado en la elaboración de modelos cinéticos. En concreto, se ha hecho patente que cambios en la composición química de la fase primaria tienen que afectar de forma radical a la viscosidad, que varía fuertemente cerca de la transición vítrea, y tienen que producirse cambios en las propiedades de transporte atómico. Esto se ha modelado a través de la asunción de un coeficiente de difusión dependiente de la concentración, en base a la relación de Stokes-Einstein modificada entre la viscosidad y el coeficiente de difusión. Las simulaciones phsae-field con un coeficiente de difusión de este tipo dan lugar a una cinética más lenta y que muestra un acuerdo excelente con la cinética experimentalmente observada en cristalizaciones primarias de vidrios metálicos. Por tanto, las simulaciones phase-field confirman que la cinética de las cristalizaciones primarias está controlada fundamentalmente por los cambios en las propiedades de transporte atómico, mientras que los efectos de soft-impingement y nucleación no aleatoria, aún y estar presentes, son secundarios.
The aim of this thesis is to study the kinetics of primary crystallization in metallic glasses by means of phase-field simulations. A primary crystallization is a solid-solid phase transformation where the crystallized phase (transformed phase or secondary phase) has a chemical composition different than the precursor phase (untransformed phase or primary phase).
Experimental data from calorimetric studies of primary crystallization are usually studied in the framework of the KJMA model (Kolmogorov, Johnson & Mehl, Avrami). This model yields the temporal evolution of the transformed fraction on the basis of three main assumptions:
- A random distribution of particle nuclei of the secondary phase
- The growth of these nuclei is isotropic
- The growth is only halted by direct collisions (hard impingement).

In the crystallization of metallic glasses, a slowing down of the kinetics respect the behavior calculated with the KJMA kinetics has been observed. This delay is explained in the literature by the fact that in this kind of transformations, that are diffusion controlled, the interaction between the crystals is not direct but through the concentration profiles (soft impingement) and moreover, the evolution of these profiles causes changes in the concentration of the amorphous matrix, stabilizing it and thus, the nucleation of new nuclei become non random. Several authors had proposed modifications to the KJMA model to try to overcome these limitations, based either on geometrical considerations or in mean field approaches. However, none of these models is able to explain the observed kinetics in primary crystallizations. The aim of this work has been the realistic simulation of the kinetics of primary crystallization to find a explanation to the differences between the experimental data and the available theoretical models.
In order to describe in a realistic way the process of a primary crystallization, the nucleation and growth process of the secondary phase has to be studied at the same time that the diffusion equation is solved in the primary phase. In this work, it has been used a phase field model for the simulations that allows to study this system introducing a new variable, coupled to the concentration field, that takes two different values in each of the existing phases. With these kinds of models, different nucleation protocols can also be introduced and thus, independently study the effects of the nucleation in the kinetics. Therefore, 2 and 3 dimensional simulations of primary crystallization have been performed with several degrees of final transformed fraction. The simulation results have been compared with the KJMA model and, unexpectedly, a good agreement between the simulations and the KJMA model has been obtained. As the KJMA model does not reproduce satisfactorily the experimental behavior, from this result can be deduced that neither the soft impingement nor the non random nucleation are the responsible of the slowing down observed in the kinetics of primary crystallization.
In order to find a physical convincing explanation of the observed experimental behavior, the theoretical study of primary crystallization has been extended, including the effects of the compositional changes that take place in the matrix as the transformation proceed. This fact, notwithstanding being known in the literature, has been systematically ignored in the development of the kinetics models. In particular, it has become clear that changes in the chemical composition of the primary phase have to radically affect the viscosity, that strongly varies near the glass transition, and some changes in the atomic transport properties must occur. This has been modeled through the assumption of a compositional dependent diffusion coefficient, on the basis of a modified Stokes-Einstein relation between viscosity and diffusion coefficient. Phase field simulations with a diffusion coefficient of this type yield a slower kinetics and show an excellent agreement with the kinetics experimentally observed in primary crystallization of metallic glasses. Thus, phase field simulations confirm that the kinetics of primary crystallization is fundamentally controlled by the changes in the atomic transport properties, while the soft impingement and non random effects, although being present, are secondary.

The aim of submitted diploma thesis is the study of non-isothermal crystallization kinetics of polylactide (PLA) with selected agents (1 %) and observation of the emerging crystalline structure under polarizing optical microscope. The agents were talc, a mixture of organic salts with the addition of amorphous SiO2 (HPN 68L) and zinc stearate (HPN 20E) and LAK-301 (potassium salt of 5-dimethylsulfoisophtalate), which is a nucleating agent developer for PLA. The PLA matrix served as a reference. Non-isothermal crystallization took place on a differential scanning calorimeter at cooling rates () 0,3; 0,5; 0,7; 1; 1,5; 2 °C/min After non-isothermal crystallization, the crystalline fraction (Xc) od PLA was evaluated from X-ray diffraction analysis, and the supramolecular structure was observed after chemical degradative etching using confocal laser scanning microscope. The crystallization kinetics were evaluated by the methods of Jeziorny and Mo and the activation energy of the crystallization was determined according to the Friedmann method. All prepared materials were amorphous (Xc 40 % for up to 1,5 °C/min). However, for LAK-301, Xc decreased to 30 % already at the = 2 °C/min and it can be assumed that with increasing its nucleation activity will decrease. A spherulitic structure was observed in all samples, but the number and size of spherulites decreased with increasing and the appearance varied according to the type of agent. Both kinetic models proved to be unsuitable for materials with low Xc and the highest because the rate of crystallization did not change. With the Jeziorny method, it was possible to evaluate the kinetics only for the relative crystallinity Xt = 29–50 % and with the Mo method it was not possible to evaluate the data for the highest for PLA matrix and sample with HPN 68L. The samples with LAK-301 and HPN 68L showed the lowest activation energy.

No presente trabalho, foram processados compósitos de polietileno de alta densidade (PEAD) com hidroxiapatita deficiente de cálcio (HA), com o objetivo de obter materiais com melhores propriedades mecânicas e bioatividade. A adição da HA deficiente de cálcio proporcionou um aumento no módulo de elasticidade (maior rigidez), menor resistência ao impacto e decréscimo do grau de cristalinidade do PEAD, proporcionando uma maior bioatividade ao material. A análise térmica exploratória (sistema não isotérmico) foi realizada por meio da técnica de calorimetria exploratória diferencial (DSC) e foram avaliados os teores de fosfato de cálcio e a velocidade de rotação da rosca no processamento dos materiais. No estudo da cristalização não-isotérmica observou-se uma diminuição da temperatura de cristalização com o aumento da taxa de resfriamento para todos os materiais sintetizados. A energia de ativação (Ea) da cristalização dos materiais foi avaliada por meio dos métodos Kissinger e Ozawa. A amostra com 5% de HA deficiente de cálcio e velocidade de processamento de 200 rpm foi a que apresentou menor valor de energia de ativação, 262 kJ/mol, menor desvio da linearidade e a que mais se assemelhou à matriz de PEAD sem HA. O teor de hidroxiapatita deficiente de cálcio não favorece o processo de cristalização devido à alta energia de ativação determinada pelos métodos descritos. Provavelmente, a velocidade de rotação, favorece a dispersão da carga na matriz de PEAD, dificultando o processo de cristalização. Na aplicação do método de Osawa-Avrami, os coeficientes de correlação indicaram perda na correlação linear. Estas perdas podem estar associadas a uma pequena percentagem de cristalização secundária e/ou à escolha das temperaturas utilizadas para determinar a velocidade de cristalização. Na determinação dos parâmetros pelo método de Mo, as menores percentagens de cristalização apresentaram um grande desvio da linearidade, com coeficiente de correlação bem menor que 1 e com o aumento da percentagem de cristalização, o desvio da linearidade diminui, ficando próximo de 1. Os resultados obtidos mostraram que o modelo de Mo e de Osawa-Avrami não foram capazes de definir o comportamento cinético dos materiais produzidos neste trabalho.
In this work, composites of high density polyethylene HDPE with calciumdeficient hydroxyapatite were synthesized in order to obtain materials with good mechanical properties and bioactivity. The addition of calcium-deficient hydroxyapatite resulted in an increase in elastic modulus (high rigidity), lower impact resistance and lower HDPE crystallinity degree, promoting, in these materials, a higher bioactivity. Scanning thermal analysis (non-isothermal system) was carried out by differential scanning calorimetry (DSC), and it was evaluated the calcium phosphate content added and the screw speed in the processing. In non-isothermal crystallization studies it was observed a decrease in crystallization temperature as the cooling rate was increased for all produced materials. The activation energy of crystallization was evaluated by Kissinger and Ozawa methods. The sample with 5 wt.% of calcium-deficient hydroxyapatite and processed at 200 rpm screw speed showed the lower value of activation energy (262 kJ/mol) and the lower deviation from linearity. Calcium-deficient hydroxyapatite does not promote the crystallization process due to the high activation energy determined by the described methods. Probably the screw speed promotes the dispersion of the filler in the HDPE matrix and hinders the crystallization process. Correlation coefficients in Osawa-Avrami method indicated loss in the linear correlation. These losses might be associated with a small percentage of secundary crystallization and/or the temperatures chosen to determine the crystallization rate. The parameters obteined from Mo method, the lower percentages of crystallization showed a great deviation from linearity, with correlation coefficient much smaller than 1, when increasing the percentage of crystallization, the deviation from linearity decreases, getting closer to 1.The results of Mo and Osawa-Avrami models were not able to set the kinetic behavior of the materials produced in this study.

Metallic glasses possess attractive properties, such as high strength, good corrosion resistance, and superior soft magnetic performance. They also serve as precursors for synthesizing nanocrystalline materials. In addition, a new class of composites having crystalline phases embedded in amorphous matrix is evolving based on selective crystallization of metallic glasses. Therefore, crystallization of metallic glasses and its effects on properties has been a subject of interest. Previous investigations from our research group related to laser assisted crystallization of Fe-Si-B metallic glass (an excellent soft magnetic material by itself) showed a further improvement in soft magnetic performance. However, a fundamental understanding of crystallization and mechanical performance of laser treated metallic glass was essential from application point of view. In light of this, the current work employed an integrated experimental and computational approach to understand crystallization and its effects on tensile behavior of laser treated Fe-Si-B metallic glass. The time temperature cycles during laser treatments were predicted using a finite element thermal model. Structural changes in laser treated Fe-Si-B metallic glass including crystallization and phase evolution were investigated with the aid of X-ray diffraction, differential scanning calorimetry, resistivity measurements, and transmission electron microscopy. The mechanical behavior was evaluated by uniaxial tensile tests with an InstronTM universal testing machine. Fracture surfaces of the metallic glass were observed using scanning electron microscopy and site specific transmission electron microscopy. Fe-Si-B metallic glass samples treated with lower laser fluence (<0.49 J/mm2) underwent structural relaxation while higher laser flounces led to partial crystallization. The crystallization temperature experienced an upward shift due to rapid heating rates of the order of 104 K/s during laser treatments. The heating cycle was followed by termination of laser upon treatment attainment of peak temperature and rapid cooling of the similar order. Such dynamic effects resulted in premature arrest of the crystallite growth leading to formation of fine crystallites/grain (~32 nm) of α-(Fe,Si) as the major component and Fe2B as the minor component. The structural relaxation, crystallization fractions of 5.6–8.6 Vol% with α-(Fe,Si) as the main component, and crystallite/grain size of the order of 12 nm obtained in laser fluence range of 0.39-0.49 J/mm2 had minimal/no influence on tensile behavior of the laser treated Fe-Si-B metallic glass foils. An increase in laser fluence led to progressive increase in crystallization fractions with considerable amounts of Fe2B (2-6 Vol%) and increase in grain size to ~30 nm. Such a microstructural evolution severely reduced the strength of Fe-Si-B metallic glass. Moreover, there was a transition in fracture surface morphology of laser treated Fe-Si-B metallic glass from vein pattern to chevron pattern. Tensile loading lacked any marked influence on the crystallization behavior of as-cast and structurally relaxed laser-treated metallic glass foils. However, a significant crystallite/grain growth/coarsening of the order of two and half times was observed in the fractured region compared to the region around it for the laser-treated partially crystallized metallic glass foils. The simultaneous effects of stress generation and temperature rise during tensile loading were considered to play a key role in crystallite/grain growth/coarsening.

This thesis is concerned with the development of methods and models to aid in optimization and development of new materials for Fused Filament Fabrication (FFF). We demonstrate a novel FFF nozzle design to enable the first measurements of in-situ rheology inside FFF nozzles, which is critical for part performance by ensuring that the polymer extrudate is flowing at an appropriate temperature and flow rate during the part build process. Testing was performed using Acrylonitrile butadiene styrene filament and a modified Monoprice Maker Select 3D printer. Tests using the default temperature control settings of the printer showed an 11 °C drop in temperature and significant fluctuations in pressure, during printing and while idle, of ± 2 °C and +/-14 kPa. These deviations were eliminated at lower flow rates with a properly calibrated proportional–integral–derivative (PID) system. At high flow rates, drops in temperature as high as 6.5 °C were observed even with a properly calibrated PID, providing critical input to the impact of flow rate and PID calibration on polymer melt temperature inside FFF nozzles. Pressure readings ranging from 140-6900 kPa were measured over the range of filament feed rates and corresponding extrusion flow rates. Theoretical predictions of pressure profiles, assuming a powerlaw fluid model, matched well with experimental results. Our nozzle prototype succeeded in measuring internal conditions of FFF nozzles for the first time, thereby providing several important insights into the printing process which are vital for monitoring and improving FFF printed parts. Furthermore, finite difference simulations based on first principles analysis are presented which are capable of quantifying the effect of processing conditions on the properties of semicrystalline parts made by FFF. Each layer was modelled as a rectangular cross section which was broken down into smaller elements for modelling. Crystallinity of each element was calculated using a parallel Avrami model which accounts for changes in crystallization rate due to temperature and multiple crystallization mechanisms. The amount of polymer diffusion, also referred to as the degree of healing, was calculated using a novel incremental diffusion model which accounted for not only changes in reptation time due to temperature but also restrictions to healing due to crystallinity. To the authors knowledge, this is the first healing model capable of accounting for the effect of crystallinity on healing and is relevant to any process involving healing of crystalline interfaces; not just FFF. Cumulative shear stresses between each layer and at the bottom of the part were also calculated for the first time using a force balance model by assuming constant shear strain throughout each layer. Simulations were performed using typical printing conditions for polyether ether ketone. In the first layer of a 24 layer part, the average degree of crystallinity, healing, and shear stress were 25.0%, 53.8% and 19.4 MPa respectively. The degree of crystallinity and healing at layer 22 (which represented the steady state values) were 18.4-25.0% and 51.4% respectively. When crystallinity was not accounted for, varying the printing parameters and material properties supported the use of high temperatures and specific heat in addition to a low printing speed, heat transfer coefficient, and thermal conductivity to maximize part properties. These conditions also supported crystallization, however, which led to a simultaneous reduction in the part properties when crystallinity was taken into account. These contradictory effects will need to be considered when optimizing the printing parameters, though the optimal balance will be highly dependent on the material used and the limitations of the printer. Experimental validation of the accuracy of the heat transfer and polymer diffusion models was performed using an amorphous polymer (polyether imide). Single road wide parts were printed at various nozzle temperatures, bed temperatures, and printing speeds and the results were compared to the simulated results. The predicted shear stress in the bottom of the part ranged from 2.3-3.8 MPa and correlated to warpages at the corners of each part of 1.2-2.4 mm. A linear increase in warpage with predicted shear stress was observed supporting the shear stress model. Predicted degrees of healing ranged from 2-90% but the experimental results ranged from 15-36%. Results of the healing model underpredicted strength at low printing speeds and over predicted strength at high printing speeds. The experimental validations showed the capabilities of the models, but the effect of printing speed will need to be investigated further to improve the accuracy of the healing model.
MS

In industrial forming processes such as extrusion or injection molding, polymeric materials experience severe thermomechanical conditions: high pressure, high deformation rates, very fast cooling kinetics and important temperature gradients. In semi-crystalline thermoplastics, such as polypropylene, these phenomena have a major influence on the crystallization occurring during cooling, which determines the final microstructure. Predicting the solidified part properties by numerical simulation requires the implementation of a crystallization kinetics model including both the thermally and flow induced effects. In this work, a numerical model simulating polymer crystallization under non-isothermal flows is developed. The model is based on the assumption that the polymer melt elasticity, quantified by the first normal stress difference, is the driving force of flow-induced extra nucleation. Two sets of Schneider equations are used to describe the growth of thermally and flow induced nuclei. The model is then coupled with the momentum equations and the energy equation. As an application, a simple shear flow configuration between two plates (Couette flow) is simulated. The relative influence of the mechanical and thermal phenomena on the crystallization development as well as the final morphology distribution is finally analyzed as a function of the shearing intensity, in terms of nucleation density and crystallite mean sizes.

PLA (Poly Lactic Acid) is a bio-sourced and a biodegradable polymer. It represents an interesting substitute for some petrochemical based polymers, especially because of its wide range of applications in the biomedical, agriculture and packaging fields. Unfortunately, PLA exhibits slow crystallization kinetics, limiting the amount of crystallinity in the final product, which is a handicap in order to extend its use. Many authors have investigated the crystallization of polymers; nevertheless several physical mechanisms remain not yet understood. This work aims a complete characterization of PLA in order to improve the understanding of its crystallization kinetics. The quiescent crystallization was investigated using Differential Scanning Calorimetry (DSC) measurements in isothermal and non-isothermal conditions for PLA and PLA with 5wt % talc. The flow effect on crystallization was studied using a thermocontrolled hot-stage shearing device (Linkam) coupled with an optical microscope. The number of activated nuclei and the growth rate were measured as functions of temperature. In addition, the linear viscoelastic properties were obtained from a rheometer with plate-plate geometry. The enhancement of the crystallization was quantified and analyzed in terms of the half crystallization time t1/2. This characteristic time t1/2 is found to be drastically decreased by both the talc and the flow which promote supplementary nucleation leading to various crystalline microstructures. The flow is known to orient and stretch molecules leading to an extra nucleation. An original description of this phenomenon is proposed using two characteristic Weissenberg numbers; based on the definition of Rousse and reptation times. Finally, we have proposed a semi-empirical model to quantify the thermal and flow contributions on the crystallization.