Rozprawy doktorskie na temat „Mg AZ31B”

Magnesium and its alloys are considered as the potential biomaterials due to their biocompatibility and biodegradable characteristics but suffer from poor corrosion performance. Various surface modification techniques are employed to improve their corrosion resistance. In present case, laser surface melting was carried out on AZ31B Mg alloy with various laser energy densities using a continuous wave ytterbium laser. Effect of laser treatment on phase and microstructure evolution was evaluated by X ray diffraction and scanning electron microscopy. Multi-physics thermal model predicted time temperature evolution along the depth of the laser treatment zone. Additionally, electrochemical method and bio-immersion test were employed to evaluate the corrosion behavior in simulated body fluid medium. Microstructure revealed grain refinement and even distribution of Mg17Al12 phase along the grain boundary for laser treated samples leading to substantial enhancement in the corrosion resistance of the laser treated samples compared to the untreated alloy. The laser processed samples also possessed a superior wettability in SBF solution than the untreated sample. This was further reflected in enhanced bio-integration behavior of laser processed samples. By changing the parameters of laser processing such as power, scanning speed, and fill spacing, a controllable corrosion resistance and bioactivity/biocompatibility of the implant material was achieved.

Laser surface modification of AZ31B Magnesium alloy changes surface composition and roughness to provide improved surface bio-wettability. Laser processing resulted in phase transformation and grain refinement due to rapid quenching effect. Furthermore, instantaneous heating and vaporization resulted in removal of material, leading the textured surface generation. A study was conducted on a continuum-wave diode-pumped ytterbium laser to create multiple tracks for determining the resulting bio-wettability. Five different laser input powers were processed on Mg alloy, and then examined by XRD, SEM, optical profilometer, and contact angle measurement. A finite element based heat transfer model was developed using COMSOL multi-physics package to predict the temperature evolution during laser processing. The thermal histories predicted by the model are used to evaluate the cooling rates and solidification rate and the associated changes in the microstructure. The surface energy of laser surface modification samples can be calculated by measuring the contact angle with 3 different standard liquid (D.I water, Formamide, and 1-Bromonaphthalen). The bio-wettability of the laser surface modification samples can be conducted by simulated body fluid contact angle measurement. The results of SEM, 3D morphology, XRD, and contact angle measurement show that the grain size and roughness play role for wetting behavior of laser processing Mg samples. Surface with low roughness and large grain size performs as hydrophilicity. On the contrast, surface with high roughness and small grain size performs as hydrophobicity.

This thesis presents results from the application of cryogenic cooling on multiple-pass friction stir processing and the subsequent orthogonal machining on friction stir processed and as-received Mg AZ31B-O disks, and shows their combined effects on microstructure and microhardness values. A simple friction stir tool, a specially designed fixture and liquid nitrogen are used to perform multiple-pass friction stir processing experiments on Mg AZ31B-O alloy. The friction stir processed and as-received sheets are then made into disks for the orthogonal machining experiments. This study analyzes the microhardness, microstructure changes by cryogenic friction stir processing and the effect of machining conditions such as dry, MQL and cryogenic and cutting parameters on the Mg AZ31B-O alloy. Four different speeds and three different feed rates are used for the orthogonal machining experiments. The effects of stirring parameters such as the translational feed, rotational speed, cooling conditions and the machining parameters are studied. The resulting microstructure and microhardness from these processes hold a key to the mechanical properties of the alloy. This analysis would help to understand and evaluate the specific aspects of grain size and microhardness that influence the fatigue life of a component.

Magnesium and its alloys have been considered for load-bearing implant materials due to their similar mechanical properties to the natural bone, excellent biocompatibility, good bioactivity, and biodegradation. Nevertheless, the uncontrollable corrosion rate in biological environment restrains their application. Hydroxyapatite (HA, Ca10(PO4)6(OH)2) is a widely used bio-ceramic which has bone-like mineral structure for bone fixation. Poor fracture toughness of HA makes it not suitable for load-bearing application as a bulk. Thus, HA is introduced into metallic surface in various forms for improving biocompatibility. Recently friction stir processing (FSP) has emerged as a surface modification tool for surface/substrate grain refinement and homogenization of microstructure in biomaterial. In the pressent efforts, Mg-nHA composite surface on with 5-20 wt% HA on Mg substrate were fabricated by FSP for biodegradation and bioactivity study. The results of electrochemical measurement indicated that lower amount (~5% wt%) of Ca in Mg matrix can enhance surface localized corrosion resistance. The effects of microstructure,the presence of HA particle and Mg-Ca intermetallic phase precipitates on in vitro behavior of Mg alloy were investigated by TEM, SEM, EDX,XRD ,and XPS. The detailed observations will be discussed during presentation.

As ligas de magnésio atraíram a atenção novamente nos últimos anos por causa de suas propriedades de baixa densidade, resistência à tração e rigidez específica. Por outro lado, a maior limitação para o uso de ligas trabalhadas é a baixa conformabilidade em temperatura ambiente devido à estrutura hexagonal compacta (HCP) das ligas. O presente trabalho de pesquisa teve como objetivo estudar o encruamento, recristalização e crescimento de grãos durante a laminação de liga magnésio AZ31B em alta e baixa temperatura, analisando a evolução da microestrutura, da textura e a variação das propriedades sensíveis à microestrutura. A liga AZ31B é sensível à taxa de deformação em alta temperatura, entretanto, a anisotropia é negativamente afetada na laminação a frio, portanto, apresenta uma melhor laminação na faixa de temperaturas de 200 a 300ºC, devido ao refinamento de grãos, causado pela recuperação e recristalização dinâmica. O estudo foi realizado em amostras de uma chapa de liga de magnésio AZ31B recristalizada (2 mm de espessura). Amostras foram deformadas por laminação em temperaturas diferentes (25, 100, 200, 250 e 300ºC) e com diferentes taxas de deformação. A caracterização microestrutural foi realizada com auxílio de várias técnicas complementares de análise microestrutural, tais como microscopia óptica, microscopia eletrônica de varredura, análise de raios-X por dispersão de energia, difração de raios X e microdureza Vickers. A deformabilidade e a ocorrência de recristalização dinâmica e crescimento de grãos mostraram forte dependência com as condições de laminação. Na laminação a frio, o refinamento de grão foi mais efetivo com baixas taxas de deformação (1,6 s-1) do que na laminação a quente. Entretanto, a intensa textura basal foi enfraquecida em temperaturas próximas a 300ºC e com taxas de deformação próximas a 3,5 s-1. A ductilidade das ligas pode ser melhorada em altas temperaturas de deformação, pelo refinamento dos grãos que produz a diminuição da fração volumétrica das regiões macladas e pelo aumento do número de sistemas de deslizamento, além do enfraquecimento da intensa textura basal, característica das ligas de magnésio.
Magnesium alloys have attracted the attention again in recent years because of their low density, their specific tensile strength and rigidity. However, the greatest limitation for the usage of wrought magnesium alloys is their poor formability at room temperature due to the hexagonal closed packed (HCP) crystal structure. The present research focused on study the work-hardening, recrystallization and grain growth during rolling of AZ31B magnesium alloy at low and high rolling temperature. It was made through the analysis of microstructure and texture evolution and variations of microstructure-sensitive properties. The AZ31 magnesium alloy is sensitive to strain rate at high temperature, meanwhile, the anisotropy is adversely impacted in cold rolling sheets. Thus, AZ31B magnesium alloy exhibits better workability in 200-300°C temperature range due to the grain refinement caused by dynamic recovery and dynamic recrystallization. This research was carried out on samples of recrystallized sheet (2 mm in thickness). Samples were deformed by rolling at different temperatures (25, 100, 200, 250 and 300°C), using different strain rates. Microstructural characterization was done by using several complementary techniques of microstructural analysis, such as optical microscopy, scanning electron microscopy, X-ray analysis by energy dispersive, X-ray diffraction and Vickers microhardness tests. A competition between dynamic recrystallization and grain growth depends on rolling conditions. Low strain rate (1,6 s-1) at cold rolling improved more effective in refining grains than warm rolling. Meanwhile, the intense basal texture was weakened at 300°C with a high strain rate of 3,5 s-1. The ductility of magnesium alloys can be greatly improved at high temperature, with a fine grain structure that causes the reduced volume fraction of twins, and an increase the number of slip systems, weakening the intense basal texture, rather characteristic for magnesium alloys.

A recent study on the Post-Formed properties of Superplastically Formed Magnesium AZ31B has shown that the heating time prior to testing has a major effect on the Post Forming properties of the superplastically material. To this point, there has been very little examination into the effect of pre-heating or annealing on superplastic forming (SPF) properties. In this work, the effects of annealing prior to the SPF of Mg AZ31 alloy were examined. Both high temperature SPF tensile and bulge specimens were formed after annealing. Multiple annealing temperatures were examined to produce specimens with grain sizes ranging from 8 andamp;igrave;m to 15 andamp;igrave;m for comparison with traditional SPF results. The results show that the effect of annealing can be suitable for the improvement of thinning and possibly the forming time of superplastically formed Magnesium alloys through the control of the microstructure.

Refill Friction Stir Spot Welding (RFSSW) is a new solid-state joining technology, which is suitable for joining similar and dissimilar overlap sheets connections, particularly in aluminium and magnesium alloys. This welding method is expected to have wide applications in joining of body parts in the automotive industry. In the present study, RFSSW has been used to join 1.0 mm gauge sheets of two material combinations: similar AA6111-T4 automotive aluminium alloy joints and a dissimilar aluminium AA6111-T4 to magnesium AZ31-H24 alloy combinations. The performance of the joints was investigated in terms of the effect of the welding parameters (including tool rotation rate, sleeve plunge depth, and welding time etc.) to improve current understanding and allow optimisation of the process for short welding-cycles when joining similar and dissimilar light alloys. The results of the investigations on similar AA6111 welds showed the ability to use a wide window of process parameters that resulted in joints with a successfully refilled keyhole and flat weld surface, even when using a welding time as short as 0.5 s. The joints in the as-welded condition showed strengths as high as 4.2 kN, when using welding parameters of 1500 rpm, 1.0 mm with a range of welding times from 0.55 to 2.0 s. All joints showed a nugget pull-out failure mode when using a sleeve plunge depth of 0.8 mm or more, as a result of increasing the joint area. The strength of the joints further improved and reached peak loads of 5.15 and 6.43 kN after natural and artificial ageing, respectively, for welds produced using optimised welding parameters of a 2500 rpm tool rotation rate, a 1.5 s welding time and a 1.0 mm plunge. This improvement in strength resulted from the improvement in the local mechanical properties in the HAZ and other regions, which results from a minimal HAZ due to the rapid weld cycle and the re-precipitation of GPZs and clustering on natural ageing, or β on artificial ageing. A modification to the RFSSW process was developed in this project to solve the problems faced when dissimilar welding Mg to Al. This modified process involved adding a final brief pin plunge stage to consolidate refill defects and it was successful in producing nearly defect-free joints with improved mechanical properties, using a wide range of the process parameters. The average peak load of the joints increased with increasing tool rotation rate, to reach a maximum value at 2500 rpm due to eliminating the weld defects by increasing the material plasticity. However, increasing the tool rotation rate further to 2800 rpm led to a decrease in the average peak failure load due to eutectic melting at the weld interface. The optimum welding condition was thus found to be: 2500 rpm, 1.0 s, and 1.0 mm, which gave an average peak failure load of 2.4 kN and average fracture energy of 1.3 kN.mm. These values represent an improvement of about 10 % and 27 %, respectively, compared to welds produced with the conventional RFSSW process, and about 112 % and 78 % of the Mg-Mg similar joints produced using the same welding conditions. A FE model developed in this project was successful in increasing understanding of the behaviour of the RFSSW joints when subjected to lap tensile-shear loading. The stress and strain distribution in the modelled samples showed that the highest concentration occurring in the region of the confluence of the SZ with the two sheets. With increasing extension, these regions of highest stress and strain propagated to the outer surfaces of the two sheets and then annularly around the weld nugget. This annular ring of high strain concentration agreed well with the failure path and results in the full plug pull-out fracture mode shown by the experimentally tested samples. The predicted force-extension curves showed high agreement with the experimental results, especially when including the effect of the hook defect and correction of compliance in the experimental results.

Soldagem a ponto por fricção é um processo que ocorre no estado sólido com alta eficiência energética, baixo custo de produção além de ser um processo ambientalmente limpo. Estes processos por fricção são uma alternativa a processos convencionais tais como solda ponto por resistência, rebitamento e "clinching". A Solda Ponto por Ficção e Mistura Mecânica, ou SPFMM, é uma excelente alternativa aos processos tradicionais de união, desde que é eficiente na união de ligas de alta resistência da indústria automotiva e aeroespacial como alumínio e magnésio, que são materiais que apresentam dificuldades na soldagem com a utilização de métodos convencionais através da fusão dos materiais envolvidos. A ausência de uma fase de fusão nesses processos elimina defeitos como porosidades, bolhas, rebaixos, inclusões e microestruturas indesejáveis, que freqüentemente aparecem na solda e zonas afetadas pelo calor em processos de soldagem por fusão. Estes materiais são interessantes, especialmente na indústria automotiva, devido a sua excelente relação de resistência/peso, onde a técnica já é utilizada apresentando redução drástica de custos de operação e aumento a resistência das juntas formadas. O processo de SPFMM consiste em uma ferramenta, que é uma combinação de um pino e um ombro que em rotação, penetra nas chapas formando uma junta sobreposta. Esta ferramenta rotativa ao penetrar utiliza elevadas cargas axiais. O contato do pino com regiões adjacentes e do ombro com a superfície superior da junta geram calor de fricção. Este calor de fricção promove a plastificação do material da junta, que ao mesmo tempo é misturado pelo pino consolidando, assim, a formação da junta. Após a remoção da ferramenta e fim do processo, um furo remanescente permanece no centro da solda. O objetivo do presente trabalho é avaliar a influência da utilização de diferentes perfis de ferramenta e diferentes velocidades de rotação sobre a soldagem e o comportamento mecânico de ligas de alumínio e magnésio soldadas pelo processo de SPFMM. Também buscou-se observar a influência da ferramenta sobre a variação do fluxo de material. As soldas foram realizadas com duas diferentes velocidades de rotação para o magnésio e três diferentes velocidades de rotação para as ligas de alumínio. E um total de doze combinações de quatro diferentes perfis de pino e três diferentes perfis de ombro foram utilizadas para a produção destas soldas. A taxa de penetração, profundidade de penetração e tempo de mistura foram mantidos constantes. A caracterização metalúrgica foi feita através de microscopia ótica, lupa e eletrônica de varredura. A caracterização mecânica das juntas foi feita através de ensaios de cisalhamento, perfis de microdureza e monitoramento de torque e força durante o processo de soldagem. Ainda foram realizadas análises do fluxo de material durante o processo. Foi possível observar uma tendência de aumentar a resistência mecânica da junta com a utilização de velocidades de rotação mais elevadas em ambos os materiais. A utilização de ferramentas com diferentes perfis apresentou diferenças representativas, porém com muita dependência da combinação de velocidade de rotação, pino e ombro utilizados.
Friction based spot welding are processes that occurs in solid state with high energy efficiency, low costs and environmentally friend. These processes provide an alternative to conventional spot joining methods as resistance spot welding, riveting and clinching. Friction Stir Spot Welding is a good alternative to traditional spot joining processes since it is able to join high strength aerospace and automotive alloys like aluminum and magnesium, which are difficult to join by conventional fusion. The absence of melting during welding avoids defects like porosity, inclusions, and undesired microstructures, that are frequently observed in fusion weldings. These materials are interesting especially regarding the automotive industry, because of its mechanical strength/weight relationship, where the technique is already in use with drastic reducing of operating costs and increasing in joint mechanical strength. The SPFMM process consists in a rotating tool comprised of a pin and a shoulder that penetrates into two overlap positioned sheets. The tool displaces and plasticize the adjacent material and provide the joint consolidation leaving a keyhole after tool removal and the process is finished. The aim of this work was to evaluate the influence of different tool profiles on the metallurgical and mechanical behavior of friction stir spot welded joints of aluminum and magnesium alloys. The joints were performed with two different rotation speeds for the magnesium AZ31 samples and with three different rotation speeds for the aluminum AA6181-T4 samples and twelve combinations of three different shoulders and four different pins. The plunge rate, plunge depth and dwell time were kept constant. The metallurgical characterization was performed using optical and scanning electron microscopy. The mechanical performance of the joints was evaluated in terms of microhardness profiles and shear test. It was possible to observe a tendency to increase the mechanical strength with the usage of higher rotational speeds for both materials. The usage of different tool profiles also presented different mechanical performance, but it’s strongly influenced by the combination of rotational speed, pin and shoulder.

碩士
中華大學
機械工程學系碩士班
96
Abstract Formability is used to describe the ease with which a metal can be shaped by plastic deformation. The evaluation of the formability of a metal involves both measurements of resistance to deformation (strength) and determination of the extent of plastic deformation that is possible before fracture (ductility). Properties of a metal, strain or stress state during deformation, temperature, strain rate and thickness would affect the formability of a metal. This study examined the mechanical property and formability of the AZ31B-O Mg alloy thin sheet. Uniaxial tension tests and press-forming tests were carried out at two different temperatures. The influences of anisotropy and temperature on deformation characteristics were investigated. Formability parameters such as average plastic strain ratio, planar anisotropy, and work hardening exponent were determined by tensile test results. The forming limit diagrams have been experimentally evaluated at various temperatures. Anisotropic behaviors were observed in the mechanical properties at all test temperatures. The tensile properties and formability parameters were correlated with the forming limit diagrams. Keywords: Formability, Formability parameter, Forming limit diagram, Anisotropy.

Magnesium and its alloys are extensively used for various industries such as aerospace, automobile and electronics due to their excellent properties such as low density, high strength and stiffness and electromagnetic shielding. However, the wide spread applications of these alloys are limited due to the undesirable properties such as poor corrosion, wear and creep resistance and high chemical reactivity. These alloys are highly susceptible to galvanic corrosion in sea water environment due to their high negative potential (-2.37 V vs SHE). The effective way of preventing corrosion is through the formation of a protective coating, which acts as a barrier between the corrosive medium and the substrate. Many surface modification methods such as electro/ electroless plating, conversion coating, physical and chemical vapour depositions, thermal spray coating etc., are available currently to improve the corrosion resistance of Mg alloys. Of these methods, the electroless nickel plating has gained considerable importance because of its excellent properties such as high hardness, good wear and corrosion resistance. The properties of binary electroless nickel coating have been further improved by the addition of a third element such as cobalt, tungsten, tin and copper etc. It has been reported that the addition of tungsten as the third element in the Ni-P improves the properties such as hardness, wear and corrosion resistance, thermal stability and electrical resistance. Magnesium alloys are categorized as a “difficult to plate metal”, because of their high reactivity in the aqueous solution. They react vigorously with atmospheric oxygen and water, resulting in the formation of the porous oxide/ hydroxide film which does not provide any protection in the corrosive environment. Further, the presence of this oxide film prevents the formation of a good adhesive bond between the coating and the substrate. The surface treatment process for removal of the oxide layer is very much essential before plating the Mg alloy. Currently two processes such as zinc immersion and direct electroless nickel plating are adopted to plate Mg alloys. Etching in a solution of chromate and nitric acid followed by immersion in HF solution to form a conversion film is necessary for direct electroless nickel (EN) plating of Mg alloy. However, strict environmental regulations restrict their usage because of hazardous nature. Expensive palladous activation treatment is a well-known process as a replacement for chromate-HF pretreatments for Mg alloys. It has been reported that EN plating has been carried out over Mg alloys by using conversion coating followed by HF treatment. Formation of an intermediate oxide layer by electrolytic methods is also one of the ways these toxic pretreatments can be avoided. Microarc oxidation (MAO) is an environment friendly surface treatment technique which provides high hardness, better corrosion and wear resistance properties for the Mg alloys. EN coating has been prepared on MAO layer for improving the corrosion resistance. These MAO/EN composite coatings have been prepared using chromic acid and HF pretreatment process. As the replacement for the chromate-HF pretreatment, SnCl2 and PdCl2 sensitization and activation procedures respectively were adopted over MAO layer for the deposition of Ni-P coating. From the above reported literature, it can be inferred that for the activation of inert MAO layer to deposit electroless nickel coating, the hazardous chromate/HF and highly expensive PdCl2 activation processes were followed. Therefore, there is a need for identifying an alternative simple and cost effective pretreatment process for the deposition of electroless nickel. It is well known that borohydride is a strong reducing agent that has been used for the deposition of Ni-B coatings. In the present study, an attempt has been made to utilize borohydride in the pretreatment process for the reduction of Ni2+ ions over the MAO interlayer, which provides the nucleation sites for the deposition of Ni-P coating. Ni-P and Ni-P/Ni-W-P duplex coatings were deposited from stabilizer free carbonate bath on AZ31B Mg alloy to improve the corrosion resistance of the base substrate. The conventional chromate and HF pretreatment processes were followed for the deposition of electroless nickel coating. In order to improve the corrosion resistance of the duplex coating, post treatments such as heat treatment (4 h at 150°C) and chromate passivation were adopted. EDX analysis of AZ31B Mg alloy showed the presence of 2.8 wt.% of Al and 1.2 wt. % Zn with the balance of Mg for AZ31B Mg alloy. After the chromic acid and HF treatment, the magnesium content was reduced from 90.0 wt % to 54.9 wt%, which could be due to the incorporation of chromium on the surface layer. The surface showed about 17.8 wt. % of F. The alloy exhibited the roughness of about 0.29± 0.01µm after mechanical polishing. The roughness value was significantly changed after the chromic acid treatment processes. The maximum roughness of about 1.28±0.06 µm was obtained after the HF activation. XPS analysis confirmed the existence of chromium in +3 oxidation state after the chromic acid treatment. The Ni-P coating thickness of about 25 microns was obtained in 1 h and 15 min. In the case of duplex coatings, Ni-P plating was done for 45 min. to obtain approx. 17 microns thickness and Ni-W-P plating was done for 1.15 h to obtain a thickness of approx. 10 microns, resulting in a total thickness of 25 ± 5 microns. Ni–P coating exhibited nodular morphology with porosity. The size of these cluster nodules were of about 10 µm in diameter. On the other hand, the duplex coating exhibited a less nodular, dense and smooth appearance. From the compositional analysis it was found that Ni–P coating contained about 6 wt. % P. In the case of duplex coating, the P content was reduced to 3 wt % due to the incorporation of about 2 wt% of tungsten. In corrosion studies, the potentiodynamic polarization data obtained for bare Ni-P coating in 0.15 M NaCl solution exhibited a higher current of about 218 μA/cm2 as compared to the substrate due to the porosity of the coating. However, the Ni-P/Ni-W-P duplex showed 55 times improvement in corrosion resistance, vis-a-vis Ni-P due to the dense nature of the coating. The corrosion resistance of the coatings increased in the following order: Ni-P < bare alloy < duplex < duplex-passivated < duplex-heat treated passivated. In EIS study, the Nyquist plot obtained for the bare substrate and Ni–P coating showed the presence of inductance behavior at the lower frequency region due to the adsorption of electroactive species over the substrate through the porous oxide layer. However, the passivated and duplex passivated coatings exhibited only capacitive behavior due to their compact nature. From the above, it can be concluded that, direct deposition of Ni-P coating over the chosen Mg alloy using chromic acid and HF pretreatment process resulted in porous morphology, which affected the corrosion resistance of the coating. As an alternative strategy, the microarc oxidation conversion coating was developed on Mg alloy and characterized. The MAO coating was developed using silicate electrolyte at three different current densities (0.026, 0.046 and 0.067 A/cm2) for about 15 min. With respect to the MAO coating, an increase in the current density increased the pore diameter and decreased the pore density. The surface of the coating became coarser and rough. The cross-sectional morphology of the coating showed two district layers namely the dense and thin inner layer and a porous thick outer layer. The thickness of the coating increased with increase in current density. MAO coating prepared at an intermediate current density of 0.046 A/cm2 exhibited a higher thickness of about 12 µm and a further increase in current density showed a decrease in thickness, due to the greater rate of dissolution of Mg, relative to the rate of deposition. The surface roughness of the MAO coatings also increased with increase in current density. The Ra value increased from 1.39±0.06 to 3.52±0.17 µm with increase in current density. XRD peaks obtained for the Mg substrates corresponded predominately to magnesium. However, the coated specimens showed the presence of peaks corresponding to Mg2SiO4 along with Mg and MgO. The corrosion measurements for the bare substrate and MAO coatings were carried out in 3.5% NaCl medium (0.6 M). Based on potentiodynamic polarization studies, the MAO coating prepared at 0.046 A/cm2 exhibited a lower corrosion current density with a higher Rp value, which was about five orders of magnitude higher than the bare substrate, due to the dense nature of the coating. In EIS study, MAO coatings were fitted with the two time constants equivalent circuit containing outer porous layer and inner barrier layer. The barrier layer resistance values were higher than that of porous layer resistance, which indicated that the resistance offered by barrier layer was higher than the porous layer. The total resistance value obtained for the coating prepared at 0.046 A/cm2 were higher compared to the other coatings, which attested to its better corrosion resistance. The electrochemical noise measurement was carried out for longer immersion durations upto 336 h in 3.5% NaCl solution. The noise resistance value obtained for the base Mg alloy was about 100 Ω at 1h immersion, whereas for the MAO coating prepared at 0.04 A/cm2 a maximum value of about 34.8 MΩ was achieved and it was retained even after 96 h of immersion. Mott–Schottky analysis showed that the oxide layer on magnesium substrate acted as a n-type semiconductor, whereas the MAO coatings exhibited p-type semiconductor behavior. The MAO coating obtained at an intermediate current density showed a higher acceptor density and the flat band potential, which resulted in the better performance of the coating in corrosive environment. In another set of investigations, the Ni-P and Ni-P/Ni-W-P coatings were deposited on AZ31B Mg alloy with MAO coating as an interlayer. The MAO layer was activated by a simple borohydride pretreatment process. During the pretreatment process, the MAO coating was subjected to mild alkali treatment, immersion in the Ni-P plating solution and finally immersion in borohydride solution. During each pretreatment step, the sample was characterized for their surface morphology and composition. The surface morphology showed the distribution of spherical particles over the surface of MAO coating after immersion in the Ni-P plating solution. EDX analysis showed the presence of 2.4 wt. % of Ni, which confirmed that Ni ions were adsorbed over the surface of the MAO coating during the pretreatment process. XPS analysis carried out after immersion in the Ni-P plating solution indicated that Ni existed in +2 oxidation state. The surface became smooth and uniform with flake- like morphology after the borohydride treatment, which indicated that the surface was etched by the borohydride solution. EDX analysis showed the presence of 1.8 wt.% of Ni after borohydride reduction. XPS analysis confirmed the reduction of nickel to the zero oxidation state. Additionally, MAO/Ni-P and MAO/Ni-P/Ni-W-P duplex coatings were developed on MAO coating after a simple borohydride pretreatment. Ni-P and duplex coatings showed uniform and dense nodular morphology without any defects, which clearly indicated that the borohydride treatment provided a uniform and homogeneous active surface for the deposition of electroless nickel based coatings. Borohydride pretreatment process resulted in excellent bonding between MAO/Ni-P layers in the cross section. Based on potentiodynamic polarization studies, the corrosion current values obtained for MAO/ Ni-P and MAO/Ni-P/Ni-W-P duplex coatings were about 1.44 and 1.42 µA/cm2, respectively. The coating showed about 97 times improvement in corrosion resistance compared to the bare substrate, attesting to the dense nature of the coating. In EIS study, the single time constant equivalent circuit was used for fitting the spectra, which pertained to the coating /electrolyte interface. The single time constant could be attributed to the pore-free dense, uniform coatings developed over the MAO interlayer. For the MAO/Ni-P and MAO/Ni-P-Ni-W-P duplex coatings, the charge transfer resistance of about 15 and 11 kΩcm2 were obtained for duplex and Ni-P coatings, which reinforce the better corrosion protective ability of the coating. The above investigation confirms that MAO coatings have good corrosion resistance in the aggressive chloride medium. Consequently, they can serve as an ideal interlayer for the deposition of the electroless nickel coating. Even if the electroless nickel coating is found to fail in harsh environments, the MAO interlayer can protect the base substrate due to its higher corrosion resistance. It is also noteworthy that the borohydride treatment provides better adhesion between the MAO/Ni-P interlayer.

Magnesium and its alloys are extensively used for various industries such as aerospace, automobile and electronics due to their excellent properties such as low density, high strength and stiffness and electromagnetic shielding. However, the wide spread applications of these alloys are limited due to the undesirable properties such as poor corrosion, wear and creep resistance and high chemical reactivity. These alloys are highly susceptible to galvanic corrosion in sea water environment due to their high negative potential (-2.37 V vs SHE). The effective way of preventing corrosion is through the formation of a protective coating, which acts as a barrier between the corrosive medium and the substrate. Many surface modification methods such as electro/ electroless plating, conversion coating, physical and chemical vapour depositions, thermal spray coating etc., are available currently to improve the corrosion resistance of Mg alloys. Of these methods, the electroless nickel plating has gained considerable importance because of its excellent properties such as high hardness, good wear and corrosion resistance. The properties of binary electroless nickel coating have been further improved by the addition of a third element such as cobalt, tungsten, tin and copper etc. It has been reported that the addition of tungsten as the third element in the Ni-P improves the properties such as hardness, wear and corrosion resistance, thermal stability and electrical resistance. Magnesium alloys are categorized as a “difficult to plate metal”, because of their high reactivity in the aqueous solution. They react vigorously with atmospheric oxygen and water, resulting in the formation of the porous oxide/ hydroxide film which does not provide any protection in the corrosive environment. Further, the presence of this oxide film prevents the formation of a good adhesive bond between the coating and the substrate. The surface treatment process for removal of the oxide layer is very much essential before plating the Mg alloy. Currently two processes such as zinc immersion and direct electroless nickel plating are adopted to plate Mg alloys. Etching in a solution of chromate and nitric acid followed by immersion in HF solution to form a conversion film is necessary for direct electroless nickel (EN) plating of Mg alloy. However, strict environmental regulations restrict their usage because of hazardous nature. Expensive palladous activation treatment is a well-known process as a replacement for chromate-HF pretreatments for Mg alloys. It has been reported that EN plating has been carried out over Mg alloys by using conversion coating followed by HF treatment. Formation of an intermediate oxide layer by electrolytic methods is also one of the ways these toxic pretreatments can be avoided. Microarc oxidation (MAO) is an environment friendly surface treatment technique which provides high hardness, better corrosion and wear resistance properties for the Mg alloys. EN coating has been prepared on MAO layer for improving the corrosion resistance. These MAO/EN composite coatings have been prepared using chromic acid and HF pretreatment process. As the replacement for the chromate-HF pretreatment, SnCl2 and PdCl2 sensitization and activation procedures respectively were adopted over MAO layer for the deposition of Ni-P coating. From the above reported literature, it can be inferred that for the activation of inert MAO layer to deposit electroless nickel coating, the hazardous chromate/HF and highly expensive PdCl2 activation processes were followed. Therefore, there is a need for identifying an alternative simple and cost effective pretreatment process for the deposition of electroless nickel. It is well known that borohydride is a strong reducing agent that has been used for the deposition of Ni-B coatings. In the present study, an attempt has been made to utilize borohydride in the pretreatment process for the reduction of Ni2+ ions over the MAO interlayer, which provides the nucleation sites for the deposition of Ni-P coating. Ni-P and Ni-P/Ni-W-P duplex coatings were deposited from stabilizer free carbonate bath on AZ31B Mg alloy to improve the corrosion resistance of the base substrate. The conventional chromate and HF pretreatment processes were followed for the deposition of electroless nickel coating. In order to improve the corrosion resistance of the duplex coating, post treatments such as heat treatment (4 h at 150°C) and chromate passivation were adopted. EDX analysis of AZ31B Mg alloy showed the presence of 2.8 wt.% of Al and 1.2 wt. % Zn with the balance of Mg for AZ31B Mg alloy. After the chromic acid and HF treatment, the magnesium content was reduced from 90.0 wt % to 54.9 wt%, which could be due to the incorporation of chromium on the surface layer. The surface showed about 17.8 wt. % of F. The alloy exhibited the roughness of about 0.29± 0.01µm after mechanical polishing. The roughness value was significantly changed after the chromic acid treatment processes. The maximum roughness of about 1.28±0.06 µm was obtained after the HF activation. XPS analysis confirmed the existence of chromium in +3 oxidation state after the chromic acid treatment. The Ni-P coating thickness of about 25 microns was obtained in 1 h and 15 min. In the case of duplex coatings, Ni-P plating was done for 45 min. to obtain approx. 17 microns thickness and Ni-W-P plating was done for 1.15 h to obtain a thickness of approx. 10 microns, resulting in a total thickness of 25 ± 5 microns. Ni–P coating exhibited nodular morphology with porosity. The size of these cluster nodules were of about 10 µm in diameter. On the other hand, the duplex coating exhibited a less nodular, dense and smooth appearance. From the compositional analysis it was found that Ni–P coating contained about 6 wt. % P. In the case of duplex coating, the P content was reduced to 3 wt % due to the incorporation of about 2 wt% of tungsten. In corrosion studies, the potentiodynamic polarization data obtained for bare Ni-P coating in 0.15 M NaCl solution exhibited a higher current of about 218 μA/cm2 as compared to the substrate due to the porosity of the coating. However, the Ni-P/Ni-W-P duplex showed 55 times improvement in corrosion resistance, vis-a-vis Ni-P due to the dense nature of the coating. The corrosion resistance of the coatings increased in the following order: Ni-P < bare alloy < duplex < duplex-passivated < duplex-heat treated passivated. In EIS study, the Nyquist plot obtained for the bare substrate and Ni–P coating showed the presence of inductance behavior at the lower frequency region due to the adsorption of electroactive species over the substrate through the porous oxide layer. However, the passivated and duplex passivated coatings exhibited only capacitive behavior due to their compact nature. From the above, it can be concluded that, direct deposition of Ni-P coating over the chosen Mg alloy using chromic acid and HF pretreatment process resulted in porous morphology, which affected the corrosion resistance of the coating. As an alternative strategy, the microarc oxidation conversion coating was developed on Mg alloy and characterized. The MAO coating was developed using silicate electrolyte at three different current densities (0.026, 0.046 and 0.067 A/cm2) for about 15 min. With respect to the MAO coating, an increase in the current density increased the pore diameter and decreased the pore density. The surface of the coating became coarser and rough. The cross-sectional morphology of the coating showed two district layers namely the dense and thin inner layer and a porous thick outer layer. The thickness of the coating increased with increase in current density. MAO coating prepared at an intermediate current density of 0.046 A/cm2 exhibited a higher thickness of about 12 µm and a further increase in current density showed a decrease in thickness, due to the greater rate of dissolution of Mg, relative to the rate of deposition. The surface roughness of the MAO coatings also increased with increase in current density. The Ra value increased from 1.39±0.06 to 3.52±0.17 µm with increase in current density. XRD peaks obtained for the Mg substrates corresponded predominately to magnesium. However, the coated specimens showed the presence of peaks corresponding to Mg2SiO4 along with Mg and MgO. The corrosion measurements for the bare substrate and MAO coatings were carried out in 3.5% NaCl medium (0.6 M). Based on potentiodynamic polarization studies, the MAO coating prepared at 0.046 A/cm2 exhibited a lower corrosion current density with a higher Rp value, which was about five orders of magnitude higher than the bare substrate, due to the dense nature of the coating. In EIS study, MAO coatings were fitted with the two time constants equivalent circuit containing outer porous layer and inner barrier layer. The barrier layer resistance values were higher than that of porous layer resistance, which indicated that the resistance offered by barrier layer was higher than the porous layer. The total resistance value obtained for the coating prepared at 0.046 A/cm2 were higher compared to the other coatings, which attested to its better corrosion resistance. The electrochemical noise measurement was carried out for longer immersion durations upto 336 h in 3.5% NaCl solution. The noise resistance value obtained for the base Mg alloy was about 100 Ω at 1h immersion, whereas for the MAO coating prepared at 0.04 A/cm2 a maximum value of about 34.8 MΩ was achieved and it was retained even after 96 h of immersion. Mott–Schottky analysis showed that the oxide layer on magnesium substrate acted as a n-type semiconductor, whereas the MAO coatings exhibited p-type semiconductor behavior. The MAO coating obtained at an intermediate current density showed a higher acceptor density and the flat band potential, which resulted in the better performance of the coating in corrosive environment. In another set of investigations, the Ni-P and Ni-P/Ni-W-P coatings were deposited on AZ31B Mg alloy with MAO coating as an interlayer. The MAO layer was activated by a simple borohydride pretreatment process. During the pretreatment process, the MAO coating was subjected to mild alkali treatment, immersion in the Ni-P plating solution and finally immersion in borohydride solution. During each pretreatment step, the sample was characterized for their surface morphology and composition. The surface morphology showed the distribution of spherical particles over the surface of MAO coating after immersion in the Ni-P plating solution. EDX analysis showed the presence of 2.4 wt. % of Ni, which confirmed that Ni ions were adsorbed over the surface of the MAO coating during the pretreatment process. XPS analysis carried out after immersion in the Ni-P plating solution indicated that Ni existed in +2 oxidation state. The surface became smooth and uniform with flake- like morphology after the borohydride treatment, which indicated that the surface was etched by the borohydride solution. EDX analysis showed the presence of 1.8 wt.% of Ni after borohydride reduction. XPS analysis confirmed the reduction of nickel to the zero oxidation state. Additionally, MAO/Ni-P and MAO/Ni-P/Ni-W-P duplex coatings were developed on MAO coating after a simple borohydride pretreatment. Ni-P and duplex coatings showed uniform and dense nodular morphology without any defects, which clearly indicated that the borohydride treatment provided a uniform and homogeneous active surface for the deposition of electroless nickel based coatings. Borohydride pretreatment process resulted in excellent bonding between MAO/Ni-P layers in the cross section. Based on potentiodynamic polarization studies, the corrosion current values obtained for MAO/ Ni-P and MAO/Ni-P/Ni-W-P duplex coatings were about 1.44 and 1.42 µA/cm2, respectively. The coating showed about 97 times improvement in corrosion resistance compared to the bare substrate, attesting to the dense nature of the coating. In EIS study, the single time constant equivalent circuit was used for fitting the spectra, which pertained to the coating /electrolyte interface. The single time constant could be attributed to the pore-free dense, uniform coatings developed over the MAO interlayer. For the MAO/Ni-P and MAO/Ni-P-Ni-W-P duplex coatings, the charge transfer resistance of about 15 and 11 kΩcm2 were obtained for duplex and Ni-P coatings, which reinforce the better corrosion protective ability of the coating. The above investigation confirms that MAO coatings have good corrosion resistance in the aggressive chloride medium. Consequently, they can serve as an ideal interlayer for the deposition of the electroless nickel coating. Even if the electroless nickel coating is found to fail in harsh environments, the MAO interlayer can protect the base substrate due to its higher corrosion resistance. It is also noteworthy that the borohydride treatment provides better adhesion between the MAO/Ni-P interlayer.

The ability to effectively join magnesium alloys to steel will facilitate increased application and use of Mg alloys in the automotive and aerospace industries where joining Mg alloys to steel in order to achieve light weight, versatile and tailored properties in one composite part is highly desirable. The current thesis details (i) the development of a laser brazing technology for joining Mg alloy-interlayer-steel dissimilar metal combinations, (ii) thermochemical analysis of phases formed at the interface of a Mg alloy-steel joint during laser brazing, (iii) the bonding mechanisms in the Mg alloy-interlayer-steel joints using Al-12Si, Ni, and Sn interlayers, and (iv) the mechanism responsible for wetting of steel by molten Mg alloy during the laser brazing. Firstly, a diode laser brazing procedure has been developed for joining AZ31B-H24 Mg alloy sheet to aluminum coated steel sheet using a AZ92 Mg alloy filler wire. The results of this study suggest that feasibility of this process depends strongly on the pre-existing Al-12Si coating layer on the steel sheet that promotes wetting of the AZ92 Mg alloy filler alloy as well as formation of a layer of θ-Fe(Al,Si)3 interetallic compound along the fusion zone-steel interface. The average joint efficiency was 29% with respect to the AZ31B-H24 Mg alloy base metal. Failure occurred when cracks propagated along the intermetallic layer. Secondly, to predict early stage phase formation in the Mg alloy-interlayer-steel system during the laser brazing process, the thermodynamic stability of precipitated phases at the Mg alloy-Ni-steel interface during laser brazing has been evaluated using FactSage thermochemical software. Assuming local chemical equilibrium at the interface, the chemical activity-temperature-composition relationships of intermetallic compounds that might form in the AZ92 magnesium alloy-Ni-steel system in the temperature range of 600-1100 °C were estimated. The addition of a Ni interlayer between the steel and the Mg brazing alloy was predicted to result in the formation of the AlNi, Mg2Ni, and Al3Ni2 intermetallic compounds at the interface depending on the local maximum temperature. This was confirmed experimentally by laser brazing of AZ31B-H24 magnesium alloy and steel sheet with a micro-layer of electro-deposited Ni using AZ92 magnesium alloy filler wire. Bonding between the magnesium alloy and the steel was facilitated by the formation of a transition layer composed of a solid solution of Ni in Fe on the steel followed by a layer of α-Mg + Mg2Ni eutectic. A band of AlNi with different morphologies also formed along the fusion zone-steel interface, but was not directly responsible for bonding. The average joint efficiency was 56.5% with respect to the AZ31B-H24 Mg alloy base metal and 94.8% higher than that of laser brazed joint using Al-12Si interlayer. Thirdly, to study a low melting point temperature interlayer element, the brazeability of AZ31B-H24 magnesium alloy sheet to Sn-coated steel sheet has been investigated. All tensile-shear specimens fractured in the steel base metal well away from the brazed joint. The results showed that while the Sn coating promoted good wetting between the molten filler alloy and the steel sheet, it did not play a role in forming the final bond. Its primary function appeared to be in maintaining a clean, oxide-free steel surface until the molten Mg filler alloy could come in direct contact with the steel surface. Bonding between the magnesium alloy and the steel was facilitated by the formation of two nano-scale transition layers composed of Fe(Al) solid solution on the steel followed by a layer of Al8Mn5 phase on top of Fe(Al) in the fusion zone along the interface. High resolution-TEM analysis showed that an orientation relationships (OR) with low angle of rotation of the matching planes and low interplanar mismatch existed at the Fe(Al)-Al8(Mn,Fe)5 interface. This was found to be responsible for the low interfacial energy density, good wetting and strong interfacial bond observed in this complex dissimilar metal system. Finally, wetting has been characterized by measuring the contact angles of AZ92 Mg alloy on Ni electro-plated steel as a function of measured peak temperature reached during laser heating. Reactions between molten Mg and Ni led to a contact angle of about 86º in the peak temperature range of 618-750 ºC (denoted as Mode I) and a dramatic decrease to about 46º in the temperature range of 824-1020 ºC (denoted as Mode II). Scanning and transmission electron microscopy (SEM and TEM) indicated that AlNi + Mg2Ni reaction products were produced between Mg and steel (Mg-AlNi-Mg2Ni-Ni-Fe) in Mode I, and just AlNi between Mg and steel (Mg-AlNi-Fe) in Mode II. From high resolution TEM analysis, the measured interplanar mismatches for different formed interfaces in Modes I and II were 17% {Mg-AlNi}-104% {AlNi-Mg2Ni}-114% {Mg2Ni-Ni} and 18% {Mg-AlNi}-5% {AlNi-Fe}, respectively. Therefore, it is suggested that the poor wettability in Mode I was caused by the existence of Mg2Ni since AlNi was the immediate layer contacting molten Mg in both Modes I and II and the presence of Mg2Ni increases the interfacial strain energy of the system. This study has clearly demonstrated that the lattice mismatching at the interfaces between reaction product(s) and substrate, which are not in direct contact with the liquid, can greatly influence the wetting of the liquid.

博士
國立中山大學
材料科學研究所
92
The microstructures and mechanical properties of the AZ31 Mg tubes fabricated by one-pass forward piercing tube extrusion operated at 250-400oC and 10-2-100 s-1 are examined. The grain size is refined from the initial ~75 �慆 grain size down to ~1.5 �慆. The room temperature tensile elongation along the extrusion direction also increases from ~13% for the as-received billet up to 51%. The highest superplastic elongation of 610% was obtained as tensile loaded at 300oC and 2x10-4 s-1, and high strain rate superplasticity of 406% and 502% was achieved at 300oC and 400oC with a high strain rate of 1x10-2 s-1. Preliminary hydroforming or tube bulging at room temperature has demonstrated the feasibility. Hydrofoming at elevated temperature of 200oC or above should exhibit much more promising results, utilizing the capability of LTSP and HSRSP of the extruded tubes.

Mg-alloys, despite being the lightest structural metallic materials, find limited applications due to their poor workability, which is due to the hcp structure that does not provide sufficient number of independent slip systems for compatible deformation. Workability improves with the increase in the deformation temperature, when non-basal slip starts playing a larger role in deformation. Efforts were made to improve the workability through control of texture, grain refinement and alloying. Alloying activates non-basal slip by decreasing the critical resolved shear stress (CRSS) on non-basal planes or by promoting cross slip through an increase in the stacking fault energy (SFE) on basal planes. In this thesis, the effect of Li addition to the most widely used wrought Mg-alloy AZ31 on its workability is examined. Plastic deformation behaviour of a series of AZ31-Li alloys with temperature, T, and strain rates, ε , as variables was studied, so as to identify the optimum Li content that results in highly workable alloy. The T and ε combinations that are best suited for hot deformation of these alloys were also identified through processing maps and microstructural analysis. First, deformation behaviour of the base AZ31 is examined in detail. Compression tests were carried out, with T ranging between 150 and 400 °C and at ranging from 10-3 to 102 s-1, covering entire hot working range of the alloy. The results suggest that the deformation behaviour of AZ31 could be partitioned into three temperature regimes. In low T regime, twinning played an important role. It changes the orientation and increases hardening rate, θ (given by dσ/dε where σ and ε are true stress and strain respectively); material exhibits macroscopic flow localization and cracking along twin boundaries. The onset of twinning was examined in detail by examining the local maxima before ϵpeak strain in plot between d2σ/dε2 vs. ε. Twinning was found to occur at all the deformation conditions. Dynamic recrystallization (DRX) was observed at temperatures above 250 °C whereas deformation at low T (< 250 °C) led to extensive twinning at all . ε . At intermediate T of 250-300 °C, plastic strains tend to localize near grain/twin boundaries, confining DRX only to these regions. Increase in T promotes non-basal slip, which, in turn, leads to uniform deformation; DRX too becomes uniform. The dependence of critical stress (σc) for the onset of DRX and peak flow stress (σp) on Zener-Hollomon parameter (Z) indicates that these stresses increase with Z. Activation energy (Q) for the deformation of AZ31 was estimated at peak stress and steady state conditions. High values of Q (150-200 kJ/mol) indicate cross slip as the rate controlling mechanism, at the peak, in the stress-strain responses. For steady state, Q corresponds to lattice/grain boundary diffusion (90-150 kJ/mol). Next, the effect of Li on deformation behaviour of AZ31 was examined. In addition to AZ31 without any Li (0Li), three alloys 1 (1Li), 3 (3Li) and 5 (5Li) wt% Li were prepared with the aid of a specially designed set-up for melting and casting of Li containing alloys. Experimental results on homogenized alloys show that 1Li alloy’s overall response is similar to that of 0Li alloy, but 3Li and 5Li alloys exhibit distinctly different deformation behaviour. Li addition facilitates cross slip by increasing SFE on basal planes, thus leading to change in the deformation mechanism of the alloy. Increased softening due to cross slip decreases θ and also the twin density at low ϵ (<10-2 s-1). During deformation at low ϵ and low T, high Li alloys reveal cavities along the grain boundaries in contrast to cracking along twin boundaries that was observed in AZ31. In the intermediate T range, high Li alloys reveal the presence of a small mantle, which can be attributed to the increased cross slip with increasing Li. In fact, Li addition was found to restrict DRX and promote dynamic recovery (DRY). As ϵ increases in this T regime deformation becomes more homogeneous and twinning occurs extensively in high Li alloys. This results in remarkable increase in dσ/dε (θ) in these alloys and DRX was predominantly seen at twinned regions. At high ϵ -T regime, where non-basal slip and twinning occur uniformly, DRX is observed throughout the samples. On the basis of d2σ/dε2 – ε plots, it was found that twinning occurs at almost all -T combinations examined in present study for 0Li and 1Li alloys. In high Li alloys, twinning activity was found to be insignificant at low ε , resulting in low twin density than low Li alloys. Twinning occurs at very early stages of deformation. In the low T and high ε regime, extensive twinning in high Li alloys is noted. In high T regime, presence of twins was not prominent due to the preferential occurrence of DRX at twin boundaries. Estimated values of Q in high Li alloys were found to be very low and correspond to lattice/grain boundary diffusion of Li in Mg, indicating that cross slip is no longer the rate controlling mechanism. Instead, unpinning of kinks from Li atoms appears to control the deformation. Cross slip is promoted by Li through increase in SFE at basal planes. Onset of the DRX was predicted and it was observed that high Li alloys posses lower σc at low ε , but at high ε , σc was either comparable to or higher than low Li alloys. Processing maps were generated for all the alloys using Prasad's as well as Murty's models. Instability predictions of Prasad’s and Murty’s models are similar, except that isoefficiency contours in the latter are slightly shifted to higher ε . These maps indicate to an increase in the workability with the addition of Li to AZ31. Instability predicted by processing maps in the low ε regime in high Li alloys is attributed to underestimation of stress values due to spline interpolation. High sensitivity observed for high Li alloy at intermediate ε (10-1 – 100 s-1) is attributed to the change in the deformation mode i.e. from slip to twinning. Deformation at high T leads to dissolution of Li containing precipitates, which in turn increases the solid solution strengthening in the alloy. Hence, increase in flow stress is observed with increase in T in high Li alloys. This structural change too causes instability predictions in the high -T regime. The 0 Li alloy exhibits peak efficiency of 45% in T = 250-400 °C and ε = 10-1.25 - 100.25 s-1 regime. DRX is observed in this regime and optimum conditions for deformation predicted for this alloys are T = 350 °C and ε = 10-1 s-1. These alloys can be worked at low ε regime too (T = 250-400 °C and ε = 10-2.5 – 10-1 s-1) where the softening mechanism is DRY. Accordingly, it is concluded that the intrinsic workability of AZ31Mg-alloy increases with the addition of 3% and 5% Li.

Mg-alloys, despite being the lightest structural metallic materials, find limited applications due to their poor workability, which is due to the hcp structure that does not provide sufficient number of independent slip systems for compatible deformation. Workability improves with the increase in the deformation temperature, when non-basal slip starts playing a larger role in deformation. Efforts were made to improve the workability through control of texture, grain refinement and alloying. Alloying activates non-basal slip by decreasing the critical resolved shear stress (CRSS) on non-basal planes or by promoting cross slip through an increase in the stacking fault energy (SFE) on basal planes. In this thesis, the effect of Li addition to the most widely used wrought Mg-alloy AZ31 on its workability is examined. Plastic deformation behaviour of a series of AZ31-Li alloys with temperature, T, and strain rates, ε , as variables was studied, so as to identify the optimum Li content that results in highly workable alloy. The T and ε combinations that are best suited for hot deformation of these alloys were also identified through processing maps and microstructural analysis. First, deformation behaviour of the base AZ31 is examined in detail. Compression tests were carried out, with T ranging between 150 and 400 °C and at ranging from 10-3 to 102 s-1, covering entire hot working range of the alloy. The results suggest that the deformation behaviour of AZ31 could be partitioned into three temperature regimes. In low T regime, twinning played an important role. It changes the orientation and increases hardening rate, θ (given by dσ/dε where σ and ε are true stress and strain respectively); material exhibits macroscopic flow localization and cracking along twin boundaries. The onset of twinning was examined in detail by examining the local maxima before ϵpeak strain in plot between d2σ/dε2 vs. ε. Twinning was found to occur at all the deformation conditions. Dynamic recrystallization (DRX) was observed at temperatures above 250 °C whereas deformation at low T (< 250 °C) led to extensive twinning at all . ε . At intermediate T of 250-300 °C, plastic strains tend to localize near grain/twin boundaries, confining DRX only to these regions. Increase in T promotes non-basal slip, which, in turn, leads to uniform deformation; DRX too becomes uniform. The dependence of critical stress (σc) for the onset of DRX and peak flow stress (σp) on Zener-Hollomon parameter (Z) indicates that these stresses increase with Z. Activation energy (Q) for the deformation of AZ31 was estimated at peak stress and steady state conditions. High values of Q (150-200 kJ/mol) indicate cross slip as the rate controlling mechanism, at the peak, in the stress-strain responses. For steady state, Q corresponds to lattice/grain boundary diffusion (90-150 kJ/mol). Next, the effect of Li on deformation behaviour of AZ31 was examined. In addition to AZ31 without any Li (0Li), three alloys 1 (1Li), 3 (3Li) and 5 (5Li) wt% Li were prepared with the aid of a specially designed set-up for melting and casting of Li containing alloys. Experimental results on homogenized alloys show that 1Li alloy’s overall response is similar to that of 0Li alloy, but 3Li and 5Li alloys exhibit distinctly different deformation behaviour. Li addition facilitates cross slip by increasing SFE on basal planes, thus leading to change in the deformation mechanism of the alloy. Increased softening due to cross slip decreases θ and also the twin density at low ϵ (<10-2 s-1). During deformation at low ϵ and low T, high Li alloys reveal cavities along the grain boundaries in contrast to cracking along twin boundaries that was observed in AZ31. In the intermediate T range, high Li alloys reveal the presence of a small mantle, which can be attributed to the increased cross slip with increasing Li. In fact, Li addition was found to restrict DRX and promote dynamic recovery (DRY). As ϵ increases in this T regime deformation becomes more homogeneous and twinning occurs extensively in high Li alloys. This results in remarkable increase in dσ/dε (θ) in these alloys and DRX was predominantly seen at twinned regions. At high ϵ -T regime, where non-basal slip and twinning occur uniformly, DRX is observed throughout the samples. On the basis of d2σ/dε2 – ε plots, it was found that twinning occurs at almost all -T combinations examined in present study for 0Li and 1Li alloys. In high Li alloys, twinning activity was found to be insignificant at low ε , resulting in low twin density than low Li alloys. Twinning occurs at very early stages of deformation. In the low T and high ε regime, extensive twinning in high Li alloys is noted. In high T regime, presence of twins was not prominent due to the preferential occurrence of DRX at twin boundaries. Estimated values of Q in high Li alloys were found to be very low and correspond to lattice/grain boundary diffusion of Li in Mg, indicating that cross slip is no longer the rate controlling mechanism. Instead, unpinning of kinks from Li atoms appears to control the deformation. Cross slip is promoted by Li through increase in SFE at basal planes. Onset of the DRX was predicted and it was observed that high Li alloys posses lower σc at low ε , but at high ε , σc was either comparable to or higher than low Li alloys. Processing maps were generated for all the alloys using Prasad's as well as Murty's models. Instability predictions of Prasad’s and Murty’s models are similar, except that isoefficiency contours in the latter are slightly shifted to higher ε . These maps indicate to an increase in the workability with the addition of Li to AZ31. Instability predicted by processing maps in the low ε regime in high Li alloys is attributed to underestimation of stress values due to spline interpolation. High sensitivity observed for high Li alloy at intermediate ε (10-1 – 100 s-1) is attributed to the change in the deformation mode i.e. from slip to twinning. Deformation at high T leads to dissolution of Li containing precipitates, which in turn increases the solid solution strengthening in the alloy. Hence, increase in flow stress is observed with increase in T in high Li alloys. This structural change too causes instability predictions in the high -T regime. The 0 Li alloy exhibits peak efficiency of 45% in T = 250-400 °C and ε = 10-1.25 - 100.25 s-1 regime. DRX is observed in this regime and optimum conditions for deformation predicted for this alloys are T = 350 °C and ε = 10-1 s-1. These alloys can be worked at low ε regime too (T = 250-400 °C and ε = 10-2.5 – 10-1 s-1) where the softening mechanism is DRY. Accordingly, it is concluded that the intrinsic workability of AZ31Mg-alloy increases with the addition of 3% and 5% Li.

博士
國立中山大學
材料與光電科學學系研究所
99
Ultrafine-grained(UFG) AZ31 Mg alloy was obtained by equal-channel angular extrusion(ECAE) and subsequent annealing at elevated temperatures. The basal texture component for ECAEed material is located on the Z plane of the ECAEed billets. Tensile tests were performed at temperatures between room temperature and 125℃, and strain rates used ranging from 3*10-5 to 6*10-2 s-1. The experimental results showed that a high tensile yield stress of 394 MPa was obtained at room temperature under a strain rate of 3*10-3 s-1. Strengths of UFG AZ31 specimens were greatly improved due to grain refinement. It was found that strain rate sensitivity of UFG AZ31 alloy increased significantly from 0.024 to 0.321 with increasing temperature. The constant k of Hall-Petch equation, σ=σ0 +kd-1/2, decreased with increasing temperature, and decreasing strain rate. Negative k values were ontained at 75℃ and 100℃ under a strain rate 3*10-5 s-1. When compressed along X, Y and X45Z billet orientations, strain localization within shear bands was found in UFG AZ31 specimens. Shear bands are formed inclined near 45 to the compression axis. The smaller the grain size, the thinner the shear band. Different Hall-Petch constant k were found in specimens deformed along different orientations, which is caused by different deformation mechanisms. The formation of tension twins is the primary deformation mechanism for compressed X and Y samples, and basal slip is responsible for the deformation of X45Z sample. tension twins were found in 0.46 μm grain size specimens.

碩士
逢甲大學
機械工程學所
91
The study is mainly focused on the improvement of roll mill and the alteration of mechanical property after magnesium alloy Extruded sheet rolling. On the improvement of the roll mill, as the magnesium sheet hot rolling, the magnesium sheet would make the temperature to cold fast because of the lower temperature of the roller if the temperature of the roller has been under 200℃. To add keeping temperature apparatus in the intake of the roller, it could compensate effectively the thermal loss of magnesium sheet handed out from the heating furnace oven to roll and increase the precision of experiment. The design of roll mill should have more rotational speed of the roller and then decrease A.C frequency using frequency inverter to decrease rotational speed of roller with decreasing rotational speed of the motor. In the aspect of source material of mechanical property, the directional of magnesium alloy is less effect in UTS（Ultimate Tensile Strength）and elongation, but more different in YS（Yield Strength）.To extrude magnesium alloy vertically is well for YS, but the direction is less effect for UTS, YS and elongation after Stress-Relieving Anneal. In the aspect of magnesium alloy rolling, the result of experiment is found that as using accumulative rolling in 300℃, 350℃ and 400℃, the UTS is best in 300℃ and the YS is less different in 300℃and 350℃, and the elongation is best in 400℃. In addition, it is the result of work hardening in rolling that the more accumulative rolling rate increases, the more UTS and YS increase, the more elongation decrease. The more accumulative rolling rate increase, the more sheet width increase continually. The augmentation of the sheet width is more large in 400℃, it is next in 350℃, and it is smallest in 300℃.

碩士
逢甲大學
材料科學與工程學系
105
Micro-arc oxidation (MAO) treatment provides oxide layers with good adhesion, high hardness and well corrosion resistance on in valve metals and alloys. In addition, MAO treatment is relatively simple and easy to operate equipments. Most of the electrolyte applied in MAO process friendly to the environment, which is also an important advantage. This study emphasizes on the influence of power source and electrolytes on MAO process for corrosion resistant improvement of Mg-MAO coatings, such as unipolar and bipolar current mode, introduce phytic acid, sodium hydroxide and sodium fluoride with different additives as electrolytes. In the results, it is observed that varying of electrolyte composition (eg: cobalt sulfate and potassium permanganate)－can impact the outlook color and micro-morphology of Mg-MAO coatings. The optimum condition with best corrosion resistant is the MAO process under pulsed bipolar mode with adding tri-sodium phosphate and sodium aluminate in the electrolyte. The best reached corrosion current density is 6.17 × 10-9 A / cm2 which corresponds to corrosion resistance 1.242 × 108 Ω cm2. It is also find out that the difference in applied power mode results the coating characteristics significantly.

Superplastic deformation behaviour of AZ31 magnesium alloy having initial grain sizes 8, 11 and 17μm alloy was investigated at 673 K with initial strain rates ranging from 1x10-2 to 1x10-4 s-1. Mechanical data on fine grained AZ31 alloy with grain sizes 8 and 11 μm indicated a transition in deformation mechanisms. The strain rate sensitivity, m ~ 0.5 at low strain rates and m ~ 0.2 at high strain rates which suggest GBS and dislocation slip as the corresponding deformation mechanism. For coarse grained alloy having grain size 17 μm, m < 0.4 at low strain rates and ~ 0.2 at high strain rates, suggesting dislocation slip as the deformation mechanism. A superplastic maximum elongation of ~ 475% was observed for 8 μm alloy at low rate of deformation. At high strain rates, the deformation was non-superplastic for fine and coarse grained alloys. The contribution of GBS to total strain, ξ in the low strain rate regime was evaluated to be 50 – 60%, for both low and high elongation. EBSD studies indicated the maintenance of high fraction of high angle boundaries up to true strain of ~ 0.88 and a reduction in texture intensity. These observations show GBS as the dominant deformation mechanism for fine grained alloy. At higher strain rate, ξ was estimated to be 30%. Fraction of high angle boundaries was reduced and [0001] direction of grains was found to be rotated towards the tensile direction, suggesting dislocation slip. Based on mechanical data, flow localization and cavitation studies; the failure of the material during high rates of deformation was mainly due to flow localization. Extensive cavitation along with more uniform flow at a lower strain rate regime suggests the failure due to the cavity interlinkage and coalescence. The present GBS data are consistent with the previous relevant data in fine grained Mg based alloys in the low strain rate regime. The GBS data obtained in the dislocation regime in the present study are also in agreement with that of the previous investigations in fine grained Mg alloys.

Superplastic deformation behaviour of AZ31 magnesium alloy having initial grain sizes 8, 11 and 17μm alloy was investigated at 673 K with initial strain rates ranging from 1x10-2 to 1x10-4 s-1. Mechanical data on fine grained AZ31 alloy with grain sizes 8 and 11 μm indicated a transition in deformation mechanisms. The strain rate sensitivity, m ~ 0.5 at low strain rates and m ~ 0.2 at high strain rates which suggest GBS and dislocation slip as the corresponding deformation mechanism. For coarse grained alloy having grain size 17 μm, m < 0.4 at low strain rates and ~ 0.2 at high strain rates, suggesting dislocation slip as the deformation mechanism. A superplastic maximum elongation of ~ 475% was observed for 8 μm alloy at low rate of deformation. At high strain rates, the deformation was non-superplastic for fine and coarse grained alloys. The contribution of GBS to total strain, ξ in the low strain rate regime was evaluated to be 50 – 60%, for both low and high elongation. EBSD studies indicated the maintenance of high fraction of high angle boundaries up to true strain of ~ 0.88 and a reduction in texture intensity. These observations show GBS as the dominant deformation mechanism for fine grained alloy. At higher strain rate, ξ was estimated to be 30%. Fraction of high angle boundaries was reduced and [0001] direction of grains was found to be rotated towards the tensile direction, suggesting dislocation slip. Based on mechanical data, flow localization and cavitation studies; the failure of the material during high rates of deformation was mainly due to flow localization. Extensive cavitation along with more uniform flow at a lower strain rate regime suggests the failure due to the cavity interlinkage and coalescence. The present GBS data are consistent with the previous relevant data in fine grained Mg based alloys in the low strain rate regime. The GBS data obtained in the dislocation regime in the present study are also in agreement with that of the previous investigations in fine grained Mg alloys.

Yes
In this research work, the transient liquid phase (TLP) bonding process was utilized to fabricate joints using thin (20μm) nickel and copper foils placed between two bonding surfaces to help facilitate joint formation. Two joint configurations were investigated, first, Ti- 6Al-4V/CuNi/Mg-AZ31 and second, Ti-6Al-4V/NiCu/Mg-AZ31. The effect of bonding time on microstructural developments across the joint and the changes in mechanical properties were studied as a function of bonding temperature and pressure. The bonded specimens were examined by metallographic analysis, scanning electron microscopy (SEM), and X-ray diffraction (XRD). In both cases, intermetallic phase of CuMg2 and Mg3AlNi2 was observed inside the joint region. The results show that joint shear strengths for the Ti-6Al-4V/CuNi/Mg- AZ31 setup produce joints with shear strength of 57 MPa compared to 27MPa for joints made using the Ti-6Al-4V/NiCu/Mg-AZ31 layer arrangement.
NSERC (Canada)

博士
國立成功大學
材料科學及工程學系碩博士班
100
Owing to the advantages such as low mass density and high specific strength, AZ31 Mg-Al-Zn alloy has been used in the sheet forming. Since the alloy contains few second phase particles, it is suitable for the investigation in mechanical properties. Friction stir process (FSP) was developed as a powerful approach to grain refinement for ductility improvement. This thesis adopted FSP in the surface modification of the extruded AZ31 Mg alloy. The aim of this study was to investigate the effects of FSP on the texture and the grain structure of AZ31 Mg alloy, and the influence of microstructural characteristics resulting from FSP on the tensile properties was discussed. Experimental results showed that grain refinement can be acquired by FSP. The friction stir processed (FSPed) alloy had comparatively homogeneous grain size distribution than the base mental does. However, banded structures combining grains with different grain size can be recognized in the stir zone. X-ray diffraction patterns revealed the rotation of the basal planes tilted from the process direction in the stir zone; meanwhile, the basal planes of the extruded sample aligned parallel to the extrusion direction. Because of the above-mentioned basal texture in the FSPed alloy, the tensile stress of the FSPed specimen loaded along the process direction was obviously lower than that of the extruded specimen in tension along the extrusion direction, and the yield stress of the FSPed specimen had no obvious temperature dependence. Coupled with the sections of specific orientations, the yield stress of the FSPed specimen was similar to that of the region with the minimum deformation resistance. Loaded perpendicular to process direction, the weak regions were close to the two sides of the stir zone, but the weak regions located close to the center of the stir zone in tension along the process direction. A better uniform elongation can be acquired by reducing yield stress and increasing work hardening rate, which is responsible for the better tensile ductility of the FSPed specimen at room temperature and 100°C in comparison with that of the extruded specimen. According to the deformed microstructure, the increase of work hardening rate in the FSPed specimen could be referred to the occurrence of the tension twins in the tensile deformation. At 200°C and 300°C, the uniform elongation of the FSPed alloy reduced due to lacking the work hardening contributed by the tension twins. Furthermore, dynamic recovery and dynamic recrystallization were active at 200°C and 300°C, which also caused the reduction of uniform elongation of the tensile specimen. On the other hand, the formation of voids on the grain boundaries was observed in the deformed microstructure of the FSPed specimens. The total elongation of the FSPed specimens decreased with increasing temperature to 200°C and 300°C. Besides the effect of work hardening rate on the uniform elongation, the existence of banded structures in the stir zone strongly affected the tensile ductility of the FSPed specimens. Loaded along the process direction, the FSPed specimens tended to fracture along an arc shape. From the observation of their fracturing surface, there were some regions with low fracture resistance in the stir zone, and these regions may become the source of fracturing in the tensile deformation of the FSPed specimen. The effect of the banded structure on the fracturing also affected the reducing ductility of FSPed loaded at 200°C to 300°C since the arc-shaped fracturing morphology could be recognized at all test temperatures. In tension perpendicular to the process direction, fracturing developed easily in regions which had low plastic deformation resistance or inhomogeneous grain structure.

碩士
國立臺灣科技大學
機械工程系
105
Magnesium alloy has good mechanic properties and is considered the green material of the era. However, poor corrosion and wear properties limit its application. Micro-arc oxidation is one of the latest surface treatment technologies forming ceramic-like oxide coating on valve meta such as magnesium, aluminum, and titanium. This coating enhances anti-corrosion and wear ability remarkably. This study is focusing on the effects of various nanoparticles and the duration time in silicon electrolyte of MAO coatings and of AZ31 magnesium alloy when evaluated. The SEM, XRD, EDS, and polarization curve to exam and analyze the MAO surface morphologies, coating layer, and corrosion properties. Thickness and pore size of oxidation film increase as the duration extends. The mechanical properties enhanced when added nanoparticles into electrolyte bound with oxidation film and filled up pores on the surface. The results have shown that oxidation film with IF-WS2 has better mechanical properties (hardness), however, compared against oxidation film with Al2O3 nanoparticles it has poor anti-corrosion abilities. Thickness, surface appearance, density, structure and composition of the oxidation layer may affect its anti-corrosion ability.

No
The transient liquid phase (TLP) bonding of Ti-6Al-4V alloy to a Mg-AZ31 alloy was performed using an electrodeposited Ni coating containing a dispersion of Ni and Cu nanoparticles. Bond formation was attributed to two mechanisms; first, solid-state diffusion of Ni and Mg, followed by liquid eutectic formation at the Mg-AZ31 interface. Second, the solid-state diffusion of Ni and Ti at the Ti-6Al-4V interface resulted in a metallurgical joint. The joint interface was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction analysis. Microhardness and shear strength tests were used to investigate the mechanical properties of the bonds. The use of Cu nanoparticles as a dispersion produced the maximum joint shear strength of 69 MPa. This shear strength value corresponded to a 15 % enhancement in joint strength compared to TLP bonds made without the use of nanoparticles dispersion.
The authors would like to acknowledge The German Jordanian University (GJU), and NSERC Canada for the financial support for this research.

碩士
國立交通大學
材料科學與工程系
89
In this research, the fine grain, mechanical properties, annealing and superplasticity of AZ31 magnesium alloy through ECAE ( Route C ) process were studied. Most of grains were fined by six extrusion passes. The initial grains were 12.6mm in size and the final grains were 1.4mm after six extrusion passes. The UTS of orignal specimen were promoted from 251 MPa up to 350 MPa by six extrusion passes, and the result was in agreement with Hall Petch Theory. For the grain growth, the grains were recrystallized stage at annealing temperature below 210 ℃ for 1 hour. However, the grains remarkably grew up to 6 mm in size, as the annealing temperature higher than 220 ℃. Comparatively, the hardness was decreased from HV 83 to HV 60with the grain growth. The optimum elongation was 317 % by superplastic tensile test at the operating conditions, temperature 400 ℃ and strain rate 10-3 s-1. The fracture mechanism was voids formed at grain boundaries, and an increase in voids and void coalescences were raised with elongation till to fracture.

碩士
明志科技大學
材料工程系碩士班
107
Plasma electrolytic oxidation (PEO) or so called Micro arc oxidation (MAO) process has been widely studied and applied in industries due to its ability to create functional oxide layers on Mg, Al, Ti and Zr alloys. In this work, we used plasma electrolytic oxidation processing parameters on the characteristics of oxide layer on AZ31 Mg alloy. The process was carried out in different power modes (unipolar, bipolar mode) and different concentrations of KOH were added to the electrolyte to observe the effect of different KOH additions on the properties of the oxide layer. Then use the Taguchi method L9 array to find the deposition parameters that can deposit the best adhesion and corrosion resistance oxide layers. The oxide layer deposited parameters were change the anode-on time, anode current setting, cathode current setting and the cathode-on time. The surface and cross-sectional morphologies of the oxide layers were investigated by scanning electron microscope (SEM) and α-step profilometer, it can be found that the thickness of the oxide layer increases with increasing the KOH addition. The oxide layer formed by adding 7 g/L KOH in the bipolar mode has the flattest surface, and adding 5 g/L KOH has the thickest oxide layer. Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) techniques were employed to determine the chemical composition and crystallography, it can be found that the oxide layers formed via the unipolar and bipolar modes have MgAl2O4 crystal structure in all samples. Afterwards, the adhesion, wear resistance and hardness of the oxide layer were analyzed by scratch test, Ball-on-disk wear test and a nanoindentation test. It showed that the oxide layers formed by the bipolar mode have better adhesion, wear resistance and hardness than the unipolar mode. In addition, the corrosion resistances of PEO treated AZ31 by different power sources parameters were higher than that of untreated ones. After analyzing the properties and the Taguchi method signal-to-noise ratio (S/N ratio) and variance analysis (ANOVA), it was found that the anode-on time was the biggest influence factor affecting on the adhesion and corrosion resistance properties of the oxide layers. Finally, the maximum signal-to-noise ratio was used to process the oxide layer, and successfully deposited oxide layers with high adhesion and high corrosion resistance.

博士
國立中山大學
材料科學研究所
93
There have been numerous efforts in processing metallic alloys into fine-grained materials, so as to exhibit high strain rate superplasticity (HSRSP) and/or low temperature superplasticity (LTSP). The current study is to apply the most simple and feasible one-step extrusion method on the commercial AZ31 magnesium billet to result in low temperature and high strain rate superplasticity (LT&HSRSP). The one-step extrusion was undertaken using a high extrusion ratio at 250-350oC, and the grain size after one-step extrusion became ~1-4 mm. The processed AZ31 plate exhibited high room temperature tensile elongation up to 50%, as well as superior LTSP and/or HSRSP up to 1000%. Meanwhile, the AZ31 alloy was also conducted by equal-channel angular pressing (ECAP). It is demonstrated that an elongation of 461% may be attained at a temperature of 150oC, equivalent to 0.46 Tm where Tm is the absolute melting temperature. This result clearly demonstrates the potential for achieving low temperature superplasticity. A detailed investigation, using x-ray diffraction (XRD), electron back scattering diffraction (EBSD), and transmission electron microscopy / selected area diffraction (TEM/SAD), revealed different textures in the as-extruded and as-ECAP bars. These dominant textures were characteristic of <10 0>//ED in the extruded bars and < 76>//ED in the ECAP condition, where ED is the extrusion direction. The results show that the basal planes tend to lie parallel to the extrusion axis in the extruded bars but there is a rearrangement during ECAP and the basal planes become reasonably aligned with the theoretical shearing plane. As to the extruded plates, the {0002} planes tended to lie on the plane that contains the extrusion axis. At different tensile temperatures, different deformation mechanisms would be dominant. Over the lower loading temperatures within 150-200oC, the true strain rate sensitivity, mt, after extracting the threshold stress is determined to be 0.28, suggesting that power-law dislocation creep but the Qt value is not related to any creep mechanism. It should be partly due to thermal activated dislocation slip mechanism. However, more data need to be tested systematically this part in the future study in order to define the correct deformation mechanism. As to the loading temperatures over 250-300oC, the mt value and the true activation energy for the extruded specimens are calculated to be ~0.4-0.5 and ~90-100 kJ/mol, implying that the major deformation mechanism is grain boundary sliding plus minor solute drag creep, with the rate controlling diffusion step being the magnesium grain boundary diffusion.

No
Transient liquid phase (TLP) brazing of Mg–AZ31 alloy and Ti–6Al–4V alloy was performed using double Ni and Cu sandwich foils. Two configurations were tested; first, Mg–AZ31/Cu–Ni/Ti–6Al–4V and second, Mg–AZ31/Ni–Cu/Ti–6Al–4V. The effect of set-up configuration of the foils on microstructural developments, mechanical properties and mechanism of joint formation was examined. The results showed that different reaction layers formed inside the joint region depending on the configuration chosen. The formation of e phase (Mg), r (CuMg2), d (Mg2Ni) and Mg3AlNi2 was observed in both configurations. Maximum shear strength obtained was 57 MPa for Mg–AZ31/Ni–Cu/Ti–6Al–4V configuration and in both configurations, the increase in bonding time resulted in a decrease in joint strength to 13 MPa. The mechanism of joint formation includes three stages; solid state diffusion, dissolution and widening of the joint, and isothermal solidification.
The authors would like to acknowledge The German Jordanian University (GJU), and NSERC Canada for the financial support for this research.

Strips of magnesium alloy AZ31-B were cast on a single belt casting simulator with moving-mold system. A mixture of carbon dioxide (CO 2 ) and sulfur hexafluoride (SF 6 ) gases was used as a protective gas during melting and casting of the magnesium alloy. The castability of the magnesium alloy strips was investigated for different types of substrates: smooth low carbon steel substrate, copper with six different roughnesses, graphite-coated smooth steel and graphite-coated copper with different roughnesses. Moreover, the effect of strip thickness on the casting properties were investigated. The heat flux through the copper substrate was higher than that through the steel substrate, while coated substrates showed lower heat fluxes than the uncoated substrates. The highest heat flux was recorded for the grooved (rough) substrate with 0.15 mm depth, while further increasing the surface roughness (depth) resulted in a decrease in the heat flux. Decreasing the heat flux reduced the quality of the top surface of the strips. However, as the thickness of the strip decreased, although the heat flux decreased, the quality of the top surface increased. As the heat flux increased, the grain sizes of the strips were reduced while the secondary arm spacing (SDAS) decreased. The mechanical properties, TS, YS and HV increased when the heat flux increased. Although magnesium oxide normally appears in white, the black color of the outer MgO layer of the strips formed under unprotected conditions is due to the presence of sub-micron MgO particles.

碩士
大同大學
材料工程研究所
91
Severe deformation processing is an emerging method for refining the grain structures of conventional alloys to submicro levels. As such, this study aims to explore the microstructural evolution and property change in two Mg alloy, namely the AZ31 and AZ31-0.2Zr, by two different types of mechanical processings including rolling and ECAE. The effects of dynamic recovery, dynamic recrystallization and dynamic grain growth on the high temperature tensile ductility of the Mg alloys were also examined in this study. The results showed that the dynamic recrystallization occurred easily in the AZ31 and AZ31-0.2Zr Mg alloys, even when they were not pre-strained, provided that the tensile test conditions are adequate. The occurrence of dynamic recrystallization in the Mg alloys can be assisted by an increase in testing temperature and a decrease in the strain rate. The faster and more efficient rerystallization reaction found in the AZ31-0.2Zr alloy is believed due to the presence of Zr-containing compounds or particles, which serve as easy nucleation sites for recrystallization. The small amount of Zr addition in AZ31 Mg alloy not only assisted the recrystallization from happening, it also retarded the grain growth effectively. The tensile elongations of AZ31-0.2Zr alloy were generally greater than those of AZ31 alloy at 300℃ and 400℃ under strain rates of 1x10-3 s-1 and 1x10-4 s-1. The tensile elongation of warm rolled, ε = 0.75, AZ31-0.2Zr Mg alloys tested at 400℃ under an initial strain rate of 1x10-4 s-1 exhibited a maximum tensile elongation of 160%. The tensile elongation of AZ31-0.2Zr Mg alloy subjected to 200℃ ECAE for 6 passes exhibited a maximum elongation of 200% under the same tensile test condition. Like static annealing the dynamic annealing can be divided into three parts, namely the dynamic recovery (DRV), dynamic recrystallization (DRX) and dynamic grain growth (DGG). The ranking of their contribution in assisting the tensile ductility in the AZ31-0.2Zr alloy in a descending order was found to be DGG, DRV and DRX. The extent of individual contribution to the tensile ductility by DRV, DRX and DGG may change significantly depending to the microstructural state of the Mg alloy, the amount of stored strain energy, and the test conditions, including temperature and the strain rate.