Materials Science Forum Vols. 546-549

Abstract: Microstructural evolution and mechanical properties of the AZ31 magnesium alloy during rolling and annealing process were investigated. The sheet samples were prepared after different stages of the hot rolling, cold rolling and annealing processes. The hot rolling temperature was between 300-450
C and the final thickness of the cold rolled sheets was 1.5mm. The cold rolled sheets were annealed at different annealing temperature (260-350
C) for different time (10~120min). Tensile test was performed to investigate the mechanical properties of the samples obtained from different stages. With aid of the optical microscopy, scanning electron microscopy (SEM) techniques, the microstructure of the samples were characterized and the results were related to the mechanical properties. It was found the hot-rolled sheets exhibit higher ductility comparing with the cold-rolled sheets. The microstructural investigation showed that the microstructure of the hot-rolled samples was dominated by recrystallized equiaxed grains while the microstructure of the cold-rolled samples dominated by deformation twining. By applying annealing on the cold-rolled sheets, fine recrystallization grains were obtained and ductility of the samples was improved. The effects of the grain size and twining on mechanical properties of the AZ31 sheet were further discussed based on the experimental results.

Abstract: The strip casting of AZ31 were carried out by a Ø250×150mm of vertical twin roll caster at different casting speeds (9-15m/min) and in different casting temperatures (630-660°C). The solidification microstructure of the strips was examined. The experiment results showed that the crystal grain size of the casting was smaller than that of conventional ingot, and decreases while the casting speed is raised, or the casting temperature decrease. The casting temperature strongly affected the dendrite structure that changed into sphere-like when the casting temperature was lower. The appropriate casting temperature for AZ31 magnesium alloy is 640°C, nearby its liquidus temperature.

Abstract: The superplastic bulging test of AZ31B magnesium alloy sheet of 0.6mm thick was carried out on Alliance RT/50 tensile machine at 573K and 3.3×10 −4 −1 s . It is found that either in tensile-compressive deformation or in bi-axis tensile deformation, the judgment criterion for local necking of superplastic deformation is 0 2 dε = . The superplastic forming limit diagram
FLDat 573K and 3.3×10 −4 −1 s was established for the first time.

Abstract: Developing new alloys and techniques is important for the applications of magnesium alloy products. The greatest challenge in the area is to exploit new wrought magnesium alloys[1]. In this paper, the effects of Zn addition on the microstructures and mechanical properties of the MZK60 wrought alloy which is modified from ZK60 have been investigated. The microstructures of these alloys at various states were evaluated by optical microscopy. The mechanical properties at room temperature of these alloys were studied systematically by tensile test. Experimental results indicated that increasing Zn content to 7~10%wt is able to get not only higher tensile strength and yield strength, but also higher elongation.

Abstract: The powders of Al-Mn phases were obtained from the as-cast, as-rolled and as-extruded AZ magnesium alloys (AZ31, AZ61) by using the extraction technology, which was found to be one of the effective methods for analyzing Al-Mn phases in the AZ magnesium alloys. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion X-ray detector (EDX) were used to investigate the structure and morphology of these compound powders. The results showed that manganese existed in both the way of Al8Mn5 phases and dissociated manganese particles in the AZ magnesium alloys, and rolling or extrusion process did not change the type of the Al8Mn5 phases. SEM observation showed that Al8Mn5 phases were formed mainly in dendrite and leaf shape with the size of 15-100μm in the as-cast AZ magnesium alloys, and were broken into fine spherical fragments with the size of about 4-5μm after rolling or extrusion.

Abstract: A novel way producing magnesium billets, LFEC (low frequency electromagnetic casting processing), was developed in Northeastern University in China. The high-quality magnesium billets with less macrosegregation, refined microstructure, and better surface quality were achieved because the temperature field and the flow pattern of magnesium DC casting were improved significantly after applying low frequency electromagnetic field. Extrusion is an important plastic deformation process for magnesium alloys. In this research, the magnesium billets from LFEC were extruded through a special designed die into sheets. The results of investigation on AZ31B indicated that the extrusion velocity has obvious effects on their microstructures and mechanical properties and the sheets from LFEC had finer microstructure and higher mechanical properties than that from conventional DC casting.

Abstract: The experiments of hot extrusion, hard drawing and annealing treatment were carried out in this paper in order to study the effects of these processes to the mechanical properties and structure of MB5B wrought magnesium alloy. The experimental results showed that, the eutectic phase was to melt into magnesium matrix greatly during the heating process and precipitated from matrix with point-like state in transverse section after extrusion. The tensile strength and elongation of billets after above mentioned treatment were equal to those after homogenization treatment. The as-received billets are then drawn continuously under the circumstance that the percent reduction in area(ψ) of each pass was no more than 15% and the total reduction in area was no more than 50%. As the results the tensile strength increased and elongation decreased sharply at the same time. The grains and eutectic phase of alloy after drawing were prolonged significantly. The subsequent heat treatment had little effects on the properties of the hard drawn bar.

Abstract: The fatigue crack propagation (FCP) behavior of magnesium alloy AZ61 at room temperature (RT), elevated temperature (60°C, 120°C) , and in ambient and wet air was investimated. The mechanisms of FCP were discussed in detail. It was demonstrated that The FCP rate of AZ61 magnesium alloy increased with increasing temperature and relative humidity (RH). Obvious change in the microstructure occurred during fatigue at elevated temperature, particularly at 120°C, compared to its original microstructure. Grain growth, deformation twin, grain boundary (GB) immigration and precipitates were observed in the microstructure at 120°C after fatigue. A bend occurred in the curves of FCP rate versus stress intensity factor at 120°C, which corresponded to a transition of failure mode from a mixed intergranular and transgranular fracture to a transgranular fracture. At first stage, the FCP rate increased sharply, and then went up slowly due to the growth of grain size. Secondary phase particles facilitated the fatigue crack initiation. The Hydrogen embrittlement (HE) may be primarily responsible for accelerating FCP rate in wet air.

Abstract: The effect of Nd and Y addition on the microstructure and mechanical properties of as-extruded Mg-Zn-Zr magnesium alloy has been investigated in this study. The Mg-Zn-Zr alloy had 11 μm average grain size due to dynamic recrystallization during hot extrusion; but the average grain sizes of the Mg-Zn-Nd-Y-Zr alloys were markedly reduced to 4 μm by Nd and Y additions. The lamellar α-Mg + T phase or α-Mg + W phase eutectics at grain boundaries in as-cast Mg-Zn-Nd-Y-Zr alloys were broken up and MgZn2 precipitates in the matrix are obtained during hot extrusions. The mechanical properties of as-extruded Mg-Zn-Zr alloy were improved significantly by Nd and Y additions, especially the elevated temperature strength, which was above 150 MPa in ultimate tensile strength at 250 °C. These may be ascribed to the very fine grain size, the dispersed T phase or W phase and the MgZn2 type Laves phase precipitating during hot extrusion.

Abstract: Titanium, zirconium and magnesium alloys are considered to be biocompatible, and can be used as implants such as hip ball and sockets and to make medical equipments. Biomaterials with hybrid structures in some applications utilizing the beneficial properties of different metals together are considered potential implant materials. Therefore, in this study, experimental trials were attempted to bond pure magnesium, AM60 (6 wt% Al-0.27 wt% Mn), and AZ31 (3 wt% Al-1 wt% Zn) alloys to pure zirconium and Ti6Al4V (6 wt% Al-4 wt% V) alloy to experimentally evaluate the forming bimetallic structures by diffusion bonding technique by vacuum hot pressing. SEM analysis showed the presence of a significant diffusion zone and a presence of diffusion bonding in some metallic couples. It may be suggested that novel hybrid implant materials, composed of diffusion couples of magnesium, zirconium and titanium alloys may emerge in the future.