Materials Science Forum Vols. 488-489

Abstract: An AZ81 magnesium alloy, which was processed by hot extrusion in advance, was tested in the temperature range of 473 ~ 623 K and at various initial strain rates, ranging from 1×10-1 to 1×10-4 s-1. True stress-true strain curves under various temperatures and activation energy within different temperature range were investigated for the analysis of effect of temperature on superplastic behaviors. Elongation of the as-extruded AZ81 alloy increased with temperature when tested at the same initial strain rate. Elongation values, obtained at the same initial strain rate of 1×10-3 s-1, were 94% at 473K and 446% at 623K, respectively. Deformation mechanisms varied with increasing temperature, from the grain boundary sliding (GBS) accommodated by grain boundary diffusion to the GBS accommodated by lattice diffusion, transition temperature was 573 K.

Abstract: Equal channel angular extrusion (ECAE) was applied to an extruded ZW1101 (Mg - 11wt%Zn - 0.9wt%Y) Mg alloy containing quasicrystallines. The as-extruded ZW1101 alloy had an initial grain size of about 12 µm and bands of quasicrystalline phases parallel to the extrusion direction. After the extruded alloy was subjected to ECAE processing, the grain size was refined to about 0.5 µm, and the quasicrystalline phases were further broken and dispersed in the matrix. After the ECAE processing, the micro-hardness and yield strength of the alloy were increased, however, the ultimate tensile strength and the ductility of the alloy were slightly decreased.

Abstract: 4-pass ECAE processing was applied to two kinds of magnesium alloys, AZ31 and ZK31. Subsequently, hot rolling was carried out as a secondary forming process. After ECAE processing, the samples have fine and equiaxed grains. The basal plane of AZ31 alloy is inclined at about 450 to the extrusion direction, while that of ZK31 alloy is parallel to the extrusion direction. As a result, tensile strength and 0.2% proof stress of ZK31 alloy are higher than those of AZ31 alloy. On the other hand, the basal plane orients parallel to the rolled surface in the Subsequently rolled samples and, therefore, tensile strength and 0.2% proof stress are improved significantly. Furthermore, the AZ31 alloy has higher ductility than ZK31 alloy.

Abstract: Grain refinement in a magnesium alloy AZ31 was studied in multi-directional forging (MDF) at a strain rate of 3×10-3s-1 with decreasing temperature from 623K to 423K. The MDF was carried out up to large cumulative strains with changing the loading direction during decreasing temperature from pass to pass. The structural changes were characterized by generation of many mutually crossing kink bands at low strains, followed by development of very fine grains at large strains. The results showed that MDF with decreasing temperature can accelerate uniform generation of much finer grains, resulting in the minimal grain size of 0.36µm in a cumulative strain of 4.8 at 423K. The mechanism of grain refinement was discussed.

Abstract: Microstructure evolution and mechanical properties of AZ31 Mg alloy during equal channel angular extrusion (ECAE) at the temperature range of 453-498K was investigated. The processing temperature is an important factor to affect the microstructure and mechanical properties of the Mg alloy during ECAE. ECAE processing can be practiced at 498k or upon the temperature for as-received AZ31 alloy. A new two-step ECAEed processing was successfully used with lowing the processing temperature to 453k. The ductility increased but yield stress decreased though the grain refinement after ECAE at 498k because the recrystallization took place and large angle grain boundary formed. However both the ductility and yield stress were increased after two-step ECAE, which was ascribed to grain refinement as well as incomplete dynamic recovery and recrystallization during the processing.

Abstract: Technological barriers that relate to limited capability of magnesium extrusion technology still exist, especially for the higher alloyed grades. Part of this is associated with the unfavorable crystallographic structure of the magnesium, which is Hexagonal Close Packed (HCP). This work suggests an innovative and novel technology for reducing the production cycle from cast to finished product. A unique Semi-Solid casting system was designed and constructed in order to enable casting of large scale commercial billets having thixotropic properties. These have been further hot extruded via conventional direct extrusion press. It is shown that all essential parameters have improved significantly. These include extrusion temperature reduction (up to 1500C), press pressure reduction, increase of extrusion rate (up to four times), and, enhancement of surface quality. In addition good mechanical properties and corrosion resistance were achieved.

Abstract: The microstructural evolution during compression (at 350°C and a strain rate of 0.01s-1) was examined for magnesium alloy AZ31 received in the "as-cast" condition. It was revealed that at low strains, many twins are produced and dynamically recrystallized (DRX) grains form as a necklace along pre-existing grain boundaries. At higher strains, DRX stagnates, most likely due to the accommodation of deformation in the DRX fraction of the material. It was also observed that twin boundaries act as sites for the nucleation of DRX grains. The analysis was repeated for samples pre-compressed to a strain of 0.15 at room temperature prior to the hot deformation step. The idea of these additional tests was to increase the degree of twinning and therefore the density of sites for the nucleation of DRX. It was found that statically recrystallized (SRX) grains developed at the twins during heating to the test temperature. When these samples were deformed, the peak flow stress was reduced by approximately 20% and the development of DRX was enhanced. This can be attributed to the accelerated nucleation of DRX in the refined SRX structure.

Abstract: In order to improve the poor workability and solve the problem of difficult roughing rolling of magnesium billet, the twin roll strip castor (TRSC) was used for the manufacture of magnesium thin strip directly from its molten metal. Since the cooling rates achieved by TRSC casting process are in the range of five hundred to several thousands of degrees per second and the strip is formed under the pressure, therefore, this near- rapid solidification process can leads to homogeneity of microstructures, refined grain size and increased solid solubility. The experiments were carried out on the existing equipments at State Key Laboratory of Rolling and Automation (RAL), Northeastern University. Strip samples 200~350 mm wide and 1.5~3.5 mm thick have been successfully cast in standard AZ31B alloy. The surface of the as cast magnesium strip was smooth and the edge was tidy. The microstructure analysis of the as cast magnesium strip showed that the gain size was much refined compared to that of ingot samples, and there was no segregation in the strip. The tensile strength of the strip in the cast state was about 220 MPa. The as cast magnesium strips were cold rolled directly on a two-high rolling mill, the total reduction up to 40% can be reached. X- ray diffraction result showed that the main phase in the as cast state AZ31B magnesium strip was a (Mg), the g (Mg17Al12) phase existing in the conventional wrought magnesium AZ31B sheet was vanished, therefore, the ductility of the strip increased.

Abstract: The yield and plastic deformation behaviour of wrought Mg-alloy AZ31 sheets were investigated at elevated temperatures from ambient to 300°C. It is found that the 0.2% proof stress and ultimate strength decrease linearly with increasing temperature, and the Young’s modulus goes down in the same way. Whereas the ductility, expressed by elongation, increases with temperature, in contrast to the decrease in uniform elongation. The plastic deformation anisotropy, represented by the ratio of transverse and thickness strain of the plate sample, decreases as the temperature increases, and at about 225°C, coincidentally, reaches to unity. The activation volumes for yield stress and ultimate strength are discussed with a thermal activation law-Arrhenius Equation.