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ELEMENT LOSS OF AZ91D MAGNESIUM ALLOY DURING SELECTIVE LASER MELTING PROCESS
Kaiwen WEI,Zemin WANG,Xiaoyan ZENG
Acta Metall Sin    2016, 52 (2): 184-190.   doi:10.11900/0412.1961.2015.00212
Abstract97)   HTML5)    PDF (3463KB)(419)      

Magnesium alloys have attracted more attentions due to their low densities and excellent specific strengths. However, proper manufacturing methods are still needed to promote further applications of magnesium alloys due to the shortcomings of conventional processing methods. As one of the most promising additive manufacturing technologies, selective laser melting (SLM) was utilized to process the most commonly-used AZ91D magnesium alloy in this work. Element vaporization mechanism during the forming process and the influence of element vaporization on chemical composition, microstructure, and mechanical properties of the final products were investigated using OM, SEM, EDS, XRF and XRD. The results show that the relative content of Mg in the SLM-processed samples (86.61%~88.68%) was lower than that in the original AZ91D powders (90.63%) , whereas the relative content of Al in the former ones (10.40%~12.56%) was higher than its counterpart in the latter ones (8.97%). This variation matches well with the calculation by Langmuir model, demonstrating that element vaporization of AZ91D mainly targets at Mg. With the increase of laser energy density (EV), weight ratio of Mg to Al (η) in the SLM-processed samples first increased, then decreased and finally tended to be constant. η of the sample prepared at 55.6 J/mm3 (sample No.8) presented a smallest difference with that of the original powders. A model illustrating analytic relationship between η and EV was established by mathematical regression with the fitting index R2 being 0.858. The sample processed at 166.7 J/mm3 (sample No.1) underwent one of the most remarkable compositional variation and exhibited a typical solidified microstructure similar to the die-cast AZ91D in which net-like β-Mg17Al12 precipitates were distributed around the α-Mg matrix. However, β-Mg17Al12 content as well as solid solubility of Al in α-Mg matrix was much higher in sample No.1. The enhanced tensile strength and micro-hardness as well as the deteriorated elongation of sample No.1 could be attributed to the composition variation during SLM process.

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RESEARCH ON THE TENSION-COMPRESSION ASYM-METRY OF AS-EXTRUDED ZK60 MAGNESIUM ALLOYS AT ROOM TEMPERATURE
Jinbao LIN,Weijie REN,Xinyi WANG
Acta Metall Sin    2016, 52 (3): 264-270.   doi:10.11900/0412.1961.2015.00324
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Most wrought magnesium alloys exhibit a significant tension-compression asymmetry in yield and work hardening behaviors. To some extent, the widespread implementation of wrought magnesium alloys is hindered due to this disadvantage in some special conditions. In this work, in order to quantitatively analyze the effects of the deformation mechanisms on the tension-compression asymmetry of wrought magnesium alloys, the plastic deformation behavior of the as-extruded ZK60 magnesium alloy under uniaxial tension and compression at room temperature is investigated by the crystal plasticity simulation and experimental methods. The crystal plasticity constitutive model including slip and twinning mechanism is established by modifying the viscoplastic self-consistent (VPSC) model. The activation and evolution of basal slip, prismatic slip, pyramidal slip, {1012}<1011> tensile twinning and {1011}<1012> compression twinning are quantitatively studied during the process of uniaxial tension and compression deformation. Tensile-compression asymmetry of the as-extruded ZK60 alloy with fiber texture is analyzed based on the microscopic plastic deformation mechanism. The results show that the tension and compression twinning in the axial tension-compression process are difficult to active, basal slip is the main deformation mode in the early stage of deformation, but the orientation factor of basal slip is low and has a hard orientation resulting in higher yield stress. With the rotation of grains, the critical shear stress of basal slip reduces, stress continues increasing and prismatic slip becomes the main deformation mechanism, moreover, pyramidal <c+a> slip also has a high activity. At this stage, the strain hardening rate is low and the stress-strain curve is smooth. In the early stage of compression, the tensile twinning has a high activity due to its low critical shear stress (CRSS), leading to a lower yield stress. The tensile twinning gradually saturated after the strain reaches 6.0%. And then, the relative activity decreases rapidly and the hardening rate increases at the same time. Since a large number of twin boundaries hindered the movement of dislocations, slip is no longer the major mechanism. In the later stage, the compression twinning startes activation and its relative activity rises rapidly, the accumulated stress during plastic deformation could be released and then the hardening rate decreases. It can be seen that, the variation in the relative activity of each deformation mode during compression deformation is much more complex than that during tension. The yield asymmetry and different work hardening behavior could be attributed to the combined effects of the strong fiber texture and the polar nature of twinning.

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EFFECTS OF HIGH TEMPERATURE SHORT TIMEAGEING TREATMENT ON THE MICROSTRUC-TURES AND MECHANICAL PROPERTIES OFMg-3Nd-1Zn ALLOY
Wenhui WANG,Di WU,Rongshi CHEN,Changsheng LOU
Acta Metall Sin    2016, 52 (5): 567-574.   doi:10.11900/0412.1961.2015.00333
Accepted: 23 February 2016

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Among the traditional cast magnesium alloys system, Mg-3Nd-1Zn alloy with high strength and heat resistance, has been widely applied in aeronautics such as in engine box and wing rib of airplane. In present research, the as-cast Mg-2.7Nd-0.6Zn-0.5Zr (NZ31) alloy was solution treated and then aged at temperatures ranging from 200 ℃ to 300 ℃. The microstructures and mechanical properties of the aged specimens especially at relatively high temperature (225~300 ℃) were systematically characterized by OM, SEM and TEM. A new kind of precipitates distributed in line was clearly found in the specimens aged at high temperature (225~275 ℃) for a short time (15~30 min), which corresponded to a significant enhancement of hardness and tensile strength at room temperature. The TEM results showed that the precipitates distributed in line had a composition of Mg12Nd and a granular shape, and mainly formed along the (0001)Mg basal plane. The special distribution of the granular Mg12Nd precipitates was effective barriers to the basal slips and may probably restrain the intergranular coordination during tensile deformation, leading to a large strengthening effect, that is, the yield strength and ultimate tensile strength, of the NZ31 alloy aged at 250~275 ℃ for 20~30 min, increased by about 70% and 29% respectively, in comparison with only the solutionized one.

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EFFECTS OF ROLLING DEFORMATION ON TEXTURE AND LPSO PHASE OF SPRAY-DEPOSITED MAGNESIUM ALLOYS CONTAINING Nd
Zhenliang LI,Fei LIU,Aiping YUAN,Baoyu DUAN,Xiaowei LI,Yiming LI
Acta Metall Sin    2016, 52 (8): 938-944.   doi:10.11900/0412.1961.2015.00592
Accepted: 20 April 2016

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Mg alloys have been applied widely as structural materials over the past decades, with low density, high specific strength, stiffness, specific elastic modulus, and high recycling rate. However, their features of poor ductility and formability at room temperature have limited their application due to hexagonal close-packed crystal structure with less independent slip systems. Grain refinement and texture randomization are two means to activate other slip systems. In this work, the billets of Mg-9Al-3Zn-1Mn-6Ca-2Nd alloy produced by spraying deposition method (the Osprey process) were studied in order to analyze the effect of rolling deformation at 350 ℃ and pass reduction ε =20%, 25% and 30% on texture and microstructure evolution of an extruded size-asymmetry Mg alloy by SEM, TEM and XRD. The results show that under the condition of reduction of ε =20% at 350 ℃, a long-period stacking ordered phase with 24R structure was formed in (Ca, Nd)Al2 phase (C15 Laves phase ). All of basal texture (0002), prismatic texture {100}<0001>, and pyramidal texture {102} were activated, with pole density level weakened while pass reduction increased (ε =20%, 25% and 30%), namely, texture randomization achieved in Mg alloy, with main causes of nanometer-sized dispersed C15 phase impeding dislocation movement and sub-cells inducing the process of recrystallization.

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MICROSTRUCTURE, MECHANICAL PROPERTIES AND SOLIDIFICATION BEHAVIOR OF AM50-x(Zn, Y) MAGNESIUM ALLOYS
Feng WANG,Dezhi MA,Zhi WANG,Pingli MAO,Zheng LIU
Acta Metall Sin    2016, 52 (9): 1115-1122.   doi:10.11900/0412.1961.2016.00048
Accepted: 27 May 2016

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As the lightest metallic structural material, magnesium alloys were widely used in automotive, aerospace, electronic equipment and other fields. Among commercial magnesium alloys, AM series were commonly used due to excellent ductility and energy absorption. However, their relatively poor strength greatly restricted their extended use. In order to improve mechanical properties of AM50 alloy, the Zn and Y elements were added into the AM50 alloy in the form of atomic ratio of 6∶1 by the permanent mold casting. The microstructure, solidification behavior and mechanical properties of AM50-x(Zn, Y) (x=0, 2, 3, 4, 5, mass fraction, %) alloys were investigated by OM, SEM, EDS, XRD, thermal analysis and tensile tests. The results indicated that addition of Zn and Y elements with an atomic ratio of 6∶1 to AM50 alloy, the microstructures were obviously refined, and the quasicrystal I-phase(Mg3Zn6Y) cannot form. In addition, the granular Al6YMn6 phase and fine Al2Y phase were formed in the microstructure, and the size of Al6YMn6 phase increased with increasing the Zn and Y content. The Φ-Mg21(Zn, Al)17 phase with lamellar structure was formed around β phase when x≥3, and its amount increased with increasing the Zn and Y addition. Thermal analysis results show that the Φ-Mg21(Zn, Al)17 phase was formed at 354 ℃ by the peritectic reaction, in which the precipitation temperatures of α-Mg and β phase were decreased with the increase of x content. Due to the formation of Al6YMn6, Al2Y and Φ-Mg21(Zn, Al)17 phases, the size and amount of the β phase was decreased. For AM50-4(Zn, Y) alloy, the microstructure was greatly refined, and the ultimate tensile strength, yield strength and elongation of the alloy reached to the maximum, 206.63 MPa, 92.50 MPa and 10.04%, respectively.

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STUDY ON THE DYNAMIC RECRYSTALLIZATION SOFTENING BEHAVIOR OF AZ80 MAGNESIUM ALLOY
Yun CAI,Chaoyang SUN,Li WAN,Daijun YANG,Qingjun ZHOU,Zexing SU
Acta Metall Sin    2016, 52 (9): 1123-1132.   doi:10.11900/0412.1961.2016.00051
Accepted: 27 May 2016

Abstract103)   HTML2)    PDF (1457KB)(484)      

Magnesium alloys are considered as one of the lightest structural metallic materials with excellent properties such as high specific strength, superior damping characteristics and electromagnetic shielding performance. In order to improve the mechanical properties of magnesium alloys, the hot rolling, hot extrusion and other hot forming processes are often introduced to produce the high performance parts. Both of the two softening mechanisms, dynamic recovery and dynamic recrystallization (DRX), occur during the hot deformation. As an important softening mechanism in hot processing, DRX is beneficial to obtaining fine grains structure, eliminating defects and improving mechanical properties for magnesium alloys. In this work, isothermal compression tests of AZ80 magnesium alloy were conducted on Gleeble thermo-mechanical simulator in the temperature range of 200 to 400 ℃ and strain rate range of 0.001 to 1 s-1. In view of the dynamic hardening and softening mechanisms, the softening behavior of AZ80 magnesium alloy, dominated by dynamic recrystallization, was depicted. Dynamic recrystallization volume fraction was introduced to reveal the power dissipation during the microstructural evolution which was indicated by the strain rate sensitivity value based on the dynamic material model. To quantify the dynamic recrystallization softening behavior by the strain rate sensitivity (SRS) value, the SRS value distribution maps were constructed depending on various temperatures and strain rates. Therefore, the critical conditions and evolution process were studied in terms of temperatures and strain rates, while features of the SRS value distribution maps at different strains were deeply investigated. It can be concluded that the value of dynamic recrystallization critical condition decreases and dynamic recrystallization volume fraction increases when the temperature increases and strain rate decreases during the deformation. The strain rate sensitivity was positive correlated with the dynamic recrystallization volume fraction. It has been verified effectively by the analysis of microstructure that the region in which the strain rate sensitivity value is above 0.21 corresponds to the higher dynamic recrystallization volume fraction and lower strain rate.

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RESEARCH PROGRESS ON 3D DENDRITE MORPHO-LOGY AND ORIENTATION SELECTION DURING THE SOLIDIFICATION OF Mg ALLOYS: 3D EXPERIMENTAL CHARACTERIZATION AND PHASE FIELD MODELING
Tao JING, Sansan SHUAI, Mingyue WANG, Qiwei ZHENG
Acta Metall Sin    2016, 52 (10): 1279-1296.   doi:10.11900/0412.1961.2016.00323
Accepted: 12 September 2016

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As a typical hexagonal close-packed structure metal, the dendritic morphology and preferential orientation of Mg would be influenced by many factors. Current investigations still fall short on the thorough description of the diversity and complexity of dendrites growth patterns and their origination, therefore, this paper re viewed recent research progress of this group on 3D characterization of microstructure in solidified magnesium alloys. Using synchrotron X-ray tomography and phase-field modeling, the formation mechanism of the diverse α-Mg (X) dendrites and the affections of alloying element (such as Al, Ca, Zn, and Sn), solute concentration on the growth selection and evolution of α-Mg dendrites during solidification were studied. The results indicate that the alloying elements and solute concentration would impose a significant influence on the morphology and orientation selection of the primary α-Mg dendrites. In Mg-Ca and Mg-Al (hcp-fcc) alloys, dendrites tend to grow with preferred orientation of <112?0> or <224?5> which is in good agreement with the traditional expected direction. The equiaxed growth dendrites in Mg-Sn (hcp-bct) alloys evolve as a structure with 18 branches, six of which grow on the basal plane along <112?0> and the remaining 12 along <112?X> (X≈2) off the basal plane. For the case in Mg-Zn alloys, an orientation transition from <112?0> on the basal plane to <112?1> off the basal plane are observed with the increasing addition of Zn alloying element, a hyperbranched seaweed structure is also revealed with an interim composition. A probable explanation is that the addition of high anisotropy Zn would slightly alter the anisotropy of interfacial free energy in front of the growth interface which results in a dendrite orientation transition (DOT). These findings partially reveal the underlying formation mechanism and origination of the diversity dendritic morphologies and branching structures of α-Mg dendrites in Mg alloys. Furthermore, with the fast X-ray imaging facility, in situ observations of the 3D microstructure evolution in Mg alloys during solidification are also carried out and the evolution of α-Mg dendrites are obtained for further analysis.

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Study on the Microstructural Evolution and Mechanical Properties Control of a Strong Textured AZ31 Magnesium Alloy Sheet During Cryorolling
Yaqiong YAN,Jinru LUO,Jishan ZHANG,Linzhong ZHUANG
Acta Metall Sin    2017, 53 (1): 107-113.   doi:10.11900/0412.1961.2016.00134
Accepted: 28 October 2016

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A strongly basal textured AZ31 magnesium alloy sheet with the normal direction (ND) perpendicular to the c-axis has been cryorolled at the liquid-nitrogen temperature to the strain of different amount to analyze the influence of cryogenic rolling temperature. The microstructure and texture of the cryorolled samples have been investigated by using SEM, EBSD and XRD. And the mechanical properties of the cryorolled sheets have also been tested under quasi-static uni-axial tension at the ambient temperature along the rolling direction (RD) and transverse direction (TD) respectively. The microstructural and textual evolutions of the strongly basal textured AZ31 magnesium alloy sheets during cryorolling and the relationship between mechanical properties and the microstructural and textural evolutions of cryorolled samples has also been discussed in this work. The results show that a lot of twins have been observed in cryorolled sheets, and they were found to be {101?2} tension twins. {101?2} tension twins were the dominant twinning type of the AZ31 magnesium alloy sheet during cryogenic rolling. With the increase of cryogenic rolling pass, new texture component with the c-axis paralleled to the normal direction (ND) was strengthened and the breadth of {101?2} tension twins was also increased. Grains were separated by the twin grain boundaries after cryorolling. The mechanical test results show that the strength of the sheets increased while the ductility decreased after cryogenic rolling. The strength of the sheets along RD was higher than that along TD.

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Research Progress in Bioresorbable Magnesium Scaffolds
Tingfei XI, Lina WEI, Jing LIU, Xiaoli LIU, Zhen ZHEN, Yufeng ZHENG
Acta Metall Sin    2017, 53 (10): 1153-1167.   doi:10.11900/0412.1961.2017.00319
Accepted: 29 August 2017

Abstract123)   HTML31)    PDF (9003KB)(1773)      

Because the bioresorbable scaffold (BRS) could overcome the difficulties caused by traditional nondegradable stents including chronic inflammation, late stent thrombosis, and long-term antiplatelet therapy, BRS is the research focus of interventional medical engineering. Because of both the high supporting strength and bioresorbable feature, the bioresorbable magnesium scaffold (BMS) is the research focus of BRS. In this paper, development process of Biotronik serial magnesium stents along with research progress of our domestic AZ31, JDBM and MgZnYNd stents is reviewed. According to the results of extensive in vitro and in vivo studies, BMS is safe and effective in vivo although its degradation rate is faster than our expectation. Through developing novel alloy system and improving stents' structure, the performance of BMS will be better and it will play more important role on the therapy of cardiovascular disease.

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Research Progress of Biodegradable Magnesium Alloys for Orthopedic Applications
Guangyin YUAN, Jialin NIU
Acta Metall Sin    2017, 53 (10): 1168-1180.   doi:10.11900/0412.1961.2017.00247
Accepted: 16 August 2017

Abstract183)   HTML21)    PDF (6632KB)(1068)      

Magnesium and its alloys exhibit high mechanical strength and good biocompatibility, and their modulus is similar to natural cortical bone, which could help to avoid the stress shielding effect. These advantages make them promising candidates for bone repair applications. This paper summarizes the advantages, history, challenges, and the recent research progress of biodegradable Mg alloys for orthopedic application. At last, it gives a detailed introduction of the latest researches of Shanghai Jiao Tong University on biodegradable Mg alloys, and related work to promote their clinical applications.

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Development in Biocompatibility of Biodegradable Magnesium-Based Metals
Ying ZHAO, Lilan ZENG, Tao LIANG
Acta Metall Sin    2017, 53 (10): 1181-1196.   doi:10.11900/0412.1961.2017.00259
Accepted: 14 August 2017

Abstract79)   HTML16)    PDF (4307KB)(619)      

Magnesium-based metals become novel biodegradable implanting material and present good clinical application prospect due to their good biocompatibility, mechanical properties matching with bone tissue as well as absorbable and biodegradable properties in human body. They are expected to replace traditional medical metals such as stainless steel and titanium alloy in the area of orthopaedics and cardiovascular stent. In this paper, the current research status about the biocompatibility of magnesium based metals both at home and abroad in recent years has been reviewed. In vitro and in vivo cytocompatibility, hemocompatibility and histocompatibility have been mentioned from aspects of alloying and surface modification. The clinical application and development tendencies for magnesium based metals are also proposed.

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Research of Biodegradable Mg-Based Metals as Bone Graft Substitutes
Jiahui DONG, Lili TAN, Ke YANG
Acta Metall Sin    2017, 53 (10): 1197-1206.   doi:10.11900/0412.1961.2017.00279
Accepted: 15 August 2017

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Bone defects are very challenging in orthopedic practice due to a variety of reasons. Bone repair requires four critical elements, biocompatibility, osteoconduction, osteoinduction and osteogenesis. The autografts still exist some problems for applications such as the limitation of available autogenous bones and post-operative complications, although they are considered as the “gold standard” in bony defect repairs. Generally the synthetic bone substitutes do not possess osteoinductive and osteogenic activities. Therefore, the clinical bone grafts and bone-graft substitutes have their own shortcomings in the repair of bone defects. Biodegradable magnesium-based metals, including pure magnesium and magnesium alloys, have been concerned and studied recently due to their biodegradation, good biocompatibility and similar elastic modulus and density with bone tissue. This paper summarizes the biological behavior of magnesium-based metals for bone defects repair application, including ability of promoting osteogenesis, osteoconduction and potential osteoinduction, as well as some particular biofunctions such as antibacterial and antitumor properties. The great advantages and potentials of magnesium in bone defects repair can not be denied as a promising class of bone substitutes, although further researches are still needed for clinical applications.

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Recent Advances on Biodegradable MgYREZrMagnesium Alloy
Lili TAN, Junxiu CHEN, Xiaoming YU, Ke YANG
Acta Metall Sin    2017, 53 (10): 1207-1214.   doi:10.11900/0412.1961.2017.00266
Accepted: 29 August 2017

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In recent years, magnesium and its alloys as biodegradable materials have attracted much attention. Biodegradable MgYREZr, mainly WE43 alloy, with good integrated properties has been studied in favor. In this paper, the microstructure, mechanical properties, biodegradable property and biocompatibility of biodegradable MgYREZr alloy were reviewed, as well as the clinical results of the bone fixation screws developed in Germany using the alloy with similar composition to WE43. MgYREZr alloy presents uniform microstructure and higher mechanical properties after large plastic deformation with the grain size of less than 1 μm. The RE elements can be dissolved and stabilize the corrosion layer, which can decrease the degradation rate of the alloy accompanying with optimized heat treatment. The animal tests showed biocompatibility and good bioactivity. Clinical tests showed the MgYREZr alloy screws presented equivalent to titanium screws for the treatment of mild hallux valgus deformities, however resorption cysts was revealed by X-rays when the acute scaphoid fractures were treated with a double-threaded screw made of MgYREZr, and it was only after 6 months that the fractures were consolidated enough to allow physical work. So for different clinical cases, the degradation and biological behaviors of MgYREZr alloys need to be further studied in vitro and in vivo. To control the degradation rates to meet the different clinical requirements is still a major obstacle for biodegradable MgYREZr alloys to enlarge their clinical application.

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Corrosion Behavior of AZ31 Magnesium Alloy in Dynamic Conditions
Linyuan HAN, Xuan LI, Chenglin CHU, Jing BAI, Feng XUE
Acta Metall Sin    2017, 53 (10): 1347-1356.   doi:10.11900/0412.1961.2017.00248
Accepted: 03 August 2017

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Magnesium alloy is now a promising bio-absorbable material. The previous researches on the corrosion and degradation behavior of biomedical magnesium alloy are mainly carried out in static conditions in vitro. However, considering the real physiological flow field of different flow states in vivo, the static degradation experiments can not effectively simulate the real situation. It is important to study the effect of flow field on the corrosion behavior of magnesium alloy and establish the relationship between flow rate and corrosion rate for the research and development of biomedical magnesium alloy. The corrosion behavior of AZ31 magnesium alloy in the flow field was studied using a self-designed dynamic test bench in vitro by electrochemical measurement, tensile method, pH value test of simulated body fluid (SBF) and SEM observation in this work. The relationship between the corrosion rate of magnesium alloy and the flow rate of the flow field was investigated from the perspective of corrosion electrochemistry. The influence of flow state and flow-induced shear stress (FISS) on corrosion behaviors at different positions of magnesium alloy was also studied by ANSYS finite element analysis. The results show that the flow field will accelerate the corrosion of AZ31 magnesium alloy and the corrosion rate increases with the increase of the flow rate. There is a relationship between the corrosion current density icorr of magnesium alloy and the average flow rate ν during the early corrosion stage, namely icorr-1=ic-1+Aν-1/2, where ic is the corrosion current density ignoring the influence of diffusion, and A is a constant. With the corrosion time extended, due to the influence of the corrosion products, the experimental results gradually deviate from the calculated linear relationship of icorr-1-1/2. Also, there are significant differences in the fluid flow state and FISS distributions at different positions of the sample. The mass transfer coefficient at the edge of the sample under different flow rates is 4~5 times bigger than that at the middle position. The localized corrosion morphology corresponds well to the FISS distribution and the difference of flow state.

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Characterization and Modeling Study on Interfacial Heat Transfer Behavior and Solidified Microstructure of Die Cast Magnesium Alloys
Shoumei XIONG, Jinglian DU, Zhipeng GUO, Manhong YANG, Mengwu WU, Cheng BI, Yongyou CAO
Acta Metall Sin    2018, 54 (2): 174-192.   doi:10.11900/0412.1961.2017.00418
Accepted: 01 December 2017

Abstract101)   HTML5)    PDF (16999KB)(666)      

Magnesium alloys are widely used in various fields because of their outstanding properties. High-pressure die casting (HPDC) is one of the primary manufacturing methods of magnesium alloys. During the HPDC process, the solidification manner of casting is highly dependent on the heat transfer behavior at metal-die interface, which directly affects the solidified microstructure evolution, defect distribution and mechanical properties of the cast products. As common solidified microstructures of die cast magnesium alloys, the externally solidified crystals (ESCs), divorced eutectics and primary dendrites have important influences on the final performance of castings. Therefore, investigations on the interfacial heat transfer behavior and the solidified microstructures of magnesium alloys have considerable significance on the optimization of die-casting process and the prediction of casting quality. In this paper, recent research progress on theoretical simulation and experimental characterization of the heat transfer behaviors and the solidified microstructures of die cast magnesium alloys was systematically presented. The contents include:(1) A boundary-condition model developed based on the interfacial heat transfer coefficients (IHTCs), which could precisely simulate the boundary condition at the metal-die interface during solidification process. Accordingly, the IHTCs can be divided into four stages, namely the initial increasing stage, the high value maintaining stage, the fast decreasing stage and the low value maintaining stage. (2) A numerical model developed to simulate and predict the flow patterns of the externally solidified crystals (ESCs) in the shot sleeve during mold filling process, together with discussion on the influence of the ESCs distribution on the defect bands of die cast magnesium alloys. (3) Nucleation and growth models of the primary α-Mg phases developed by considering the ESCs in the shot sleeve. (4) Nucleation and growth models of the divorced eutectic phase, which can be used to simulate the microstructure evolution of die cast magnesium alloys. (5) The 3D morphology and orientation selection of magnesium alloy dendrite. It was found that magnesium alloy dendrite exhibits an eighteen-primary branch pattern in 3D, with six growing along <112?0> in the basal plane and the other twelve along <112?3> in non-basal planes. Accordingly, an anisotropy growth function was developed and coupled into the phase field model to achieve the 3D simulation of magnesium alloy dendrite.

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Anisotropy and Deformation Mechanisms ofAs-Extruded Mg-3Zn-1Y Magnesium AlloyUnder High Strain Rates
Xudong LI, Pingli MAO, Yanyu LIU, Zheng LIU, Zhi WANG, Feng WANG
Acta Metall Sin    2018, 54 (4): 557-565.   doi:10.11900/0412.1961.2017.00147
Accepted: 12 October 2017

Abstract67)   HTML4)    PDF (5031KB)(799)      

As a very important design principle, the dynamic properties of materials attracted extensive attention in resent years and a bunch of works have been done concerning with the materials deformation behaviors under high strain rates. However, the dynamic behaviors of magnesium alloys are not through understood, especially the rare earth based magnesium alloys. In order to investigate the dynamic and anisotropic behavior under high strain rates deformation of as-extruded Mg-3Zn-1Y magnesium alloy, the split Hopkinson pressure bar (SHPB) apparatus was used to testing the true stress-true strain curves under the high strain rates of 1000, 1500 and 2200 s-1 of as-extruded Mg-3Zn-1Y magnesium alloy. The OM and SEM were used to analysis the micorstructure evolution and fracture surface morphology of the alloy. The true reason behind the anisotropic phenomenon was revealed based on the deformation mechanism of highly basal-textured magnesium alloy. The results demonstrate that the as-extruded Mg-3Zn-1Y magnesium alloy exhibits pronounced anisotropy during compression according to the loading direction. The anisotropy of the as-extruded Mg-3Zn-1Y magnesium alloy are arised from the variety of the deformation mechanisms. When the loading direction is along extrusion direction, the predominant deformation mode changes from extension twinning at a lower strain to prismatic slip at a higher strain. While compressed along extrusion radial direction (ERD), the predominant deformation mode changes from contraction twinning to a coordination of basal and second order pyramidal slip with the increasing of strain.

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Current Research and Future Prospect on Microstructures Controlling of High Performance Magnesium Alloys During Solidification
Guohua WU, Yushi CHEN, Wenjiang DING
Acta Metall Sin    2018, 54 (5): 637-646.   doi:10.11900/0412.1961.2017.00503
Accepted: 12 March 2018

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The researches on development, application and solidification microstructures of high performance magnesium (Mg) alloys have received considerable interest recently. The solidification microstructures of Mg alloys can be effectively controlled by using directional solidification technology, rapid solidification technology and the application of external field during solidification, and thus the enhanced comprehensive mechanical properties of the materials are obtained. The current researches on solidification microstructure controlling of high performance Mg alloys by using directional solidification technology, rapid solidification technology and electromagnetic stirring were reviewed. Finally, the development trend on the controlling of solidification microstructure was proposed.

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Theoretical Calculation of Schmid Factor and Its Application Under High Strain Rate Deformation in Magnesium Alloys
Yanyu LIU, Pingli MAO, Zheng LIU, Feng WANG, Zhi WANG
Acta Metall Sin    2018, 54 (6): 950-958.   doi:10.11900/0412.1961.2017.00398
Accepted: 22 January 2018

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As an important parameter, the Schmid factor has been widely applied to analyze the deformation modes in metals. In order to analyze the deformation mechanisms of magnesium alloys under high strain rate, the Schmid factors of four slip modes (basal, prismatic, pyramidal <a> and pyramidal <c+a> slips) and two twinning systems ({101?2} tension and {101?1} contraction twinnings) were systematically calculated in this work. The experimental values of Schmid factor of as-received AZ31 rolling magnesium alloy sheets were obtained by electron backscatter diffraction (EBSD) technique, and then the theoretical calculated values were compared with those values. The high strain rate compression test of AZ31 rolling magnesium sheets was conducted by using split Hopkinson pressure bar at the strain rate of 1600 s-1, and the microstructures after compression were observed by optical microscopy. The Schmid factors and microstructures are combined to discuss the predominant deformation mechanisms for different orientation samples under different loading directions. The results showed that the theoretical calculated values of Schmid factors are in good agreement with their experimental values. Therefore, the Schmid factor, owing to its simplicity and conveniene, could be used to analyze the predominant deformation mechanism and interpret the unique characteristics of "true stress-true strain" curves in magnesium alloys. Furthermore, since the Schmid factor and its variation trend associated with deformation behavior in magnesium alloys are related, the calculation result of Schmid factor can provide a theoretical analytic approach to understand anisotropic phenomena caused by strong texture in magnesium alloys.

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Comparative Study on Corrosion Behavior of Cast and Forged Mg-5Y-7Gd-1Nd-0.5Zr Alloys
Jinhui LIU, Yingwei SONG, Dayong SHAN, En-Hou HAN
Acta Metall Sin    2018, 54 (8): 1141-1149.   doi:10.11900/0412.1961.2017.00484
Accepted: 19 April 2018

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Magnesium and its alloys have become increasingly attractive in the automotive, 3C products and aerospace industries because of their advantages such as low density and high specific strength. In recent years, rare earth-Mg alloys have attracted much attention due to their high mechanical properties at room and elevated temperatures. Adjusting the microstructures by deformation treatment is a common method to improve the mechanical properties of Mg alloys. The microstructure especially the size, volume fraction and distribution of second phases in rare earth-Mg alloys will be changed during deformation treatment, which has a great effect on the corrosion resistance of Mg alloys. However, the studies on the effect of deformation treatment on the corrosion resistance of rare earth-Mg alloys are far away from sufficient. In this work, the corrosion behavior of cast and forged Mg-5Y-7Gd-1Nd-0.5Zr (EW75) alloys were studied by using SEM, XRD, mass loss measurements and electrochemical tests. The results indicate that the second phases are distributed along the grain boundaries of cast and forged EW75 alloys. Meanwhile, the second phases in forged EW75 alloy are finer and lower volume fraction than that in cast EW75 alloy. The micro-galvanic corrosion of the forged EW75 alloy is weaker in comparison with the cast EW75 alloy owing to the smaller size and lower volume fraction of second phases as well more compact surface film, resulting in the better corrosion resistance.

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Biomedical Magnesium Alloys: Composition, Microstructure and Corrosion
Rongchang ZENG, Lanyue CUI, Wei KE
Acta Metall Sin    2018, 54 (9): 1215-1235.   doi:10.11900/0412.1961.2018.00032
Accepted: 07 May 2018

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Magnesium alloys, with good biocompatibility and mechanical-compatibility, can be developed as next generation promising biomaterials. This paper summerizes the principle and the cutting-edge advances of alloying of magnesium alloys as degradable biomaterials. The effects of alloy elements on the material and biological properties of magnesium alloys are analyzed. The focus is laid on the influence of microstructure (grain size, secondary phase or intermetallic compound, long-period stacking ordered (LPSO) phase and quasi-crystal phase), heat treatment and surface oxide film on degradation and their critical progress on corrosion morphology and mechanism. Several outlooks on bio-magnesium alloys are proposed.

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