Papers by Author: Wei Min Gan

Abstract: Crystallographic texture includes much information on a material’s behaviour that depends on the processed material itself, the particular thermomechanical processing it has undergone, and the quality of the process. ECAP processed pure Mg and Mg alloys were characterised by their texture and are discussed in terms of texture symmetry. For all types of deformation, including ECAP, the basal plain orientation has to follow certain general rules. Particularly for larger number of passes the ECAP texture can be explained in terms of standard rolling or extrusion textures.

Abstract: Single pass ECAP of pure Magnesium was performed and stopped after 50 % deformation; textures thereafter at the ingoing, the deformation and the outgoing channel were characterized by neutron radiation and related to the deformation mode. A gradient texture evolution was obtained, which was attributed to the inhomogeneous distribution of the effective stress and strain, and also the inevitable existence of friction.

Abstract: An ultrafine-grained (UFG) Mg-5.0wt%Zn-0.9wt%Y-0.2wt%Zr magnesium alloy with a grain size of about 0.8 µm was produced by subjecting the extruded alloy to equal channel angular pressing (ECAP) for 8 passes at 473 K. Compressive testing was performed on the ECAPed alloy in a temperature range from 423 K to 523 K and under strain rates from 1.67×10-3 to 1.67×10-1 s-1. The ultrafine grains of the ECAPed alloy were stable during compression because of the presence of the dispersion of a fine quasicrystal I-phase and of precipitates in the alloy, which restricted grain growth. The activation energy for the compression at the temperature range from 423 K to 523 K is close to the value for grain boundary diffusion in magnesium, indicating that the compressive deformation is mainly controlled by grain-boundary sliding.

Abstract: The wrought magnesium alloys AZ80 and ZK60 were extruded at 175°C ≤ T ≤ 350°C at an extrusion ratio of ER = 12. With decreasing extrusion temperatures a marked refinement in grain size was found for both alloys resulting in higher values of yield stress while UTS values were hardly affected. As opposed to AZ80, a marked yield stress differential between loading in tension and compression was observed in ZK60, this effect being explained by the differences in crystallographic texture.

Abstract: Equal channel angular pressing (ECAP) was performed on extruded Mg-Zn-Y-Zr (Mg-5.0wt%Zn-0.9wt%Y-0.2wt%Zr) alloy at 300 oC. After 8 ECAP passes, average grain size of the alloy was reduced to about 1.4 μm, and the quasicrystalline phases were broken and dispersed in the matrix. In addition, nano- quasicrystallines were precipitated from the matrix during ECAP processing. After ECAP, the elongation to failure of the extruded material was significantly improved. Only after 2 ECAP passes, the elongation to failure was 29%, and after 8 ECAP passes, it reached 35%, which was three times larger than that of the as-extruded alloy. However, both yield strength and ultimate tensile strength were decreased with the increasing ECAP passes, which was considered to be resulted from the {0002} basal plane texture modification during ECAP.

Abstract: ZWK510 (Mg-5.0wt%Zn-0.9wt%Y-0.2wt%Zr) magnesium alloy containing Mg3YZn6 quasicrystal phase was prepared by conventional permanent mold casting. Part of the cast ingot was subjected to equal channel angular pressing (ECAP) directly; another part of the cast ingot was extruded initially, then ECAP was applied to the extruded alloy. After 4-pass ECAP, the fraction of coarse grains of the as-cast alloy was decreased to about 30%, and the grain size of fine grain was decreased to about 2 μm. Both strength and ductility of the as-cast ZWK510 alloy were significantly improved with increasing ECAP passes, which was resulted from broken and dispersed I-phase, and fine grains formed due to recrystallization. The as-extruded ZWK510 had an initial grain size of about 2 μm and bands of quasicrystal phase parallel to the extrusion direction. After the extruded alloy was subjected to ECAP, the grain size of the extruded alloy was further refined, the grain size was refined to below 0.5 um after 8-pass ECAP; and the quasicrystal phase was further broken and dispersed in the matrix. After ECAP, the elongation to failure of the extruded alloy was improved. However, both yield strength and ultimate tensile strength were decreased, which is considered to be resulted from the texture modification during ECAP.

Abstract: Compressive deformation behaviors of extruded SiCw/AZ91 were investigated in Gleeble-1500 thermal simulator at temperatures from 743 K to 783 K and strain rates from 6.4×10-2 s-1 to 1.0×101s-1. Results showed that high strain rate sensitivity (~0.5) occurred during compression; deformation activation energy normalized by threshold stress was higher than the lattice self-diffusion activation energy of magnesium. Dynamic recovery (DRV) and dynamic recrystallization (DRX) took place during compression, which refined the grains. The increase of deformation energy was attributed to non-basal planes slip and climbing of dislocations and also the presence of liquid phase.

Abstract: Compressive behaviors of SiCw/AZ91composite and AZ91 alloy were investigated at temperatures from 423 K to 723 K and strain rates from 0.002 s-1 to 0.25 s-1. Microstructure evolutions after compressed at 623 K and 0.01 s-1 were observed by SEM and TEM. Results showed that compressive flow stress decreased with the increase of temperature; whiskers were broken and redistributed to the direction normal to the compression direction. At the initial stage of compression, dislocation sliding is the mainly deformation mode for the composite, while for AZ91 alloy, twining was the dominant mechanism.

Abstract: The compressive behavior of squeeze cast SiCw/AZ91 composite in the temperature range of 423-723K and in the strain rate range of 0.001-0.25 s-1 was investigated. The compressive true stress-true strain curves were measured and hot deformation microstructures were observed. The strain rate sensitivity exponent (m) of the SiCw/AZ91 composite increased with the increasing of temperature. The activation energy of deformation varied over the range of test conditions examined indicated that the deformation was controlled by more than one mechanism. The reorientation of SiC whiskers in the composite was observed during compression. During the compression, dynamic recovery and dynamic recrystallization occurred in the SiCw/AZ91 composite.