Papers by Author: Shi Ding Wu

Abstract: Equal channel angular pressing (ECAP) has been conducted on as-cast Mg-3%Li-1%Sc alloy for four-passes to study the microstructure uniformity and tensile properties. After ECAP, the microstructure become muddled, contains about 65% of deformed coarse grains with abundant low angle grain boundaries and about 35% of recrystallized small grains. Meanwhile, a strong basal texture is formed in the ECAP sample. The texture type of the recrystallized grains and the deformed grains are the same, however, the texture strength of the recrystallized grains is much lower than the deformed ones. Tensile strength is improved effectively and the elongation is maintained after ECAP. The increment of strength results from the microstructure refinement and residual dislocations produced by ECAP, while the recovery of ductility may be attributed to a shear type texture formed in the alloy during ECAP.

Abstract: The microstructural evolution and grain refinement of Cu-Al alloys with different stacking fault energies (SFEs) processed by equal-channel angular pressing (ECAP) were investigated. The grain refinement mechanism was gradually transformed from dislocation subdivision to twin fragmentation with tailoring the SFE of Cu-Al alloys. Concurrent with the transition of grain refinement mechanism, the grain size can be refined into from ultrafine region (1 m~100 nm) to the nanoscale (<100 nm) and then it is found that the minimum equilibrium grain size decreases in a roughly linear way with lowering the SFE. Moreover, in combination with the previous results, it is proposed that the formation of a uniform ultrafine microstructure can be formed more readily in the materials with high SFE due to their high recovery rate of dislocations and in the materials with low SFE due to the easy formation of a homogeneously-twinned microstructure.

Abstract: The deformation mechanisms of various kinds of single crystals and bicrystals during the process of equal channel angular pressing (ECAP) have been paid more attention world wide. This paper reviews the recent progresses in the understanding of the deformation mechanisms of single crystals and bicrystals subjected to one-pass ECAP, and discusses the effect of initial crystallographic orientation and grain boundary on the microstructural evolution of these crystals. Based on those experimental results and analysis, it is suggested that in addition to the shear deformation along the intersection plane (IP) of ECAP die, the shear along the normal of IP also plays an important role in affecting the microstructural evolution and deformation mechanisms of these single crystals and bicrystals.

Abstract: The microstructures of pure Cu processed by equal channel angular pressing (ECAP) from 4 to 24 passes were investigated. It was found that the microstructures of Cu samples with a small number of ECAP passes (4-8) were not inhomogeneous and the fraction of high-angle grain boundary (HAGB) was low (25~43%). While for the samples with many number of ECAP passes (12-24), the grains became more equiaxed-like and the GB misorientations exhibited double-peak distribution with high fraction (51~64%) of HAGB. It was dislocation cells formed in large grains of the few-pass samples, but subgrains in the many-pass samples. These characterizations suggested that ultrafine-grained (UFG) microstructures in the few-pass samples were not fully accomplished, while it was obtained after many passes (>12). It is believed that dynamic recovery during processing for many passes was attributed to the formation of UFG microstructures.

Abstract: Equal channel angular extrusion (ECAE) is an effective means of producing ultrafine-grained materials with extraordinary mechanical properties. Texture evolution and microstructure in pure copper single crystals processed by ECAE for up to five passes via route C are investigated to understand mechanisms of plastic deformation and grain refinement during ECAE. The experimental textures after the third pass ECAE process recovers that after one pass ECAE process. The main textures approaches a stable one after four passes of ECAE process via Route C while the intensity of main texture components decreases gradually. Local TEM-OIM measurements shows that grain subdivision in ECAE-processed samples occurs with the formation of many low angle grain boundaries.

Abstract: The orientation evolution in Copper single crystal during ECAE (in Route C) has been investigated experimentally for 90° die angle by X-ray diffraction and TEM-OIM. It is shown that after the previous three passes of ECAE, orientation evolutes with similar mechanism to the simple shear deformation. The grain subdivision occurs with the appearance of many low angle grain boundaries. After four times extrusions the ED of the samples remains stable, their ND rotates around the ED by about 20°. After five times extrusions the main texture component remains stable.

Abstract: The superplasticity and microstructure of magnesium alloy ZK40 processed by ECAP for 1 and 4 passes were investigated. Tensile experiments at 523 K showed that the elongation of ZK40 was improved with the increasing passes. The elongation-to-failure of ZK40 processed by ECAP for 4 passes tested at 523 K and strain rate of 1×10-4s-1 was 660%. The microstructural analyses explained these results.

Abstract: The room temperature tensile strength and ductility of Mg-3.3%Li alloy were improved simultaneously by two kinds of different equal channel angular pressing (ECAP) treatments. Microstructural analyses showed that grain refinement and texture modification are the principal reasons for the improvement of mechanical properties.

Abstract: Ultrafine-grained (UFG) pure Cu processed by equal channel angular pressing (ECAP) was subjected to cyclic deformation and subsequent ageing treatment at room temperature (RT) in order to investigate the stability of defects and grain size. Cyclic deformation for 1000 cycles at RT leads to a large decrease of internal stress. X-ray diffraction (XRD) shows that the stability of defects and grain size at RT in as-cyclic deformed sample is lower than that in as-processed sample and that a reduction of internal stress takes place prior to grain growth. TEM observations show that the microstructural evolution during ageing is characterized by normal grain growth accompanied with recovery within grain interior.