Xinkai Ma

Microhardness Distribution and Microstructural Evolution in Pure Aluminum Subjected to Severe Plastic Deformation: Elliptical Cross-Sectioned Spiral Equal-Channel Extrusion (ECSEE)

Abstract Elliptical cross-sectioned spiral equal-channel extrusion (ECSEE), one of the severe plastic deformation techniques, is of great efficiency in producing bulk ultrafine or nanostructured materials. In this paper, the simulation and experimental researches on ECSEE of high-purity aluminum were conducted to investigate the equivalent strain distribution and microhardness distribution on three orthogonal planes, as well as microstructural evolution. Simulation result shows a significant strain gradient on three planes. Microhardness tests comprise the similar results to strain distribution. According to transmission electron microscopy (TEM) results, microstructural evolution ranged from coarse structures to ultrafine structures by undergoing the shear bands, subgrains, high-angle misorientation grain boundaries and equiaxed structures. There are also some distinctions with reference to grain refinement level, grain boundary styles and dislocation distribution on different positions. The TEM investigations are in good agreement with microhardness tests.

Microstructure and Microtexture Evolution of Pure Titanium during Single Direction Torsion and Alternating Cyclic Torsion

Systematic experimental studies of microstructure and crystallographic texture of pure titanium during the Single Direction Torsion (SDT) and Alternating Cyclic Torsion (ACT) are carried out at room temperature. The microstructure evolution indicates that the grain size can be refined during SDT, while the grain morphology can be controlled during ACT. Also, lots of {10-12} and few {11-22} twins are observed and their area percentages increase with increasing torsion angles during SDT. The microtexture evolution states that the deformation texture first approaches to the B fiber (0, 90, 0 to 60 deg), and then stays away from B fiber (0, 90, 0 to 60 deg) with increasing plastic strain during SDT. The change of deformation texture is mainly attributed to the appearance of {10-12} twin. However, the deformation texture is always close to B fiber (0, 90, 0 to 60 deg) during ACT. Finally, the effects of different dislocation movements caused by SDT and ACT are discussed. Quantities of subgrains with high density dislocation are observed during SDT while the {10-12} and {11-22} twins intersect with each other, and high density dislocations distribute the twin during ACT.

Influence of Deformation Stress Triaxiality on Microstructure and Microhardness of Pure Copper Processed by Simultaneous Torsion and Tension

Simultaneous torsion and tension deformation (STTD) modes were applied on commercial pure copper to investigate the influence of stress triaxiality on microstructure evolution and hardness distribution at room temperature. STTD was divided into pure torsion (PT, a special STTD) and general STTD according to tension loading. Microstructure evolution was observed by optical microscopy, electron backscattering diffraction and transmission electron microscopy. The microhardness distribution was measured on the cross section, and the fracture morphology was observed by scanning electron microscopy. Microstructure observations show that ultrafine grains are separated by high-angle grain boundaries. Microhardness measurements exhibit hardness increased more significantly and uniformly in the specimen processed by general STTD mode than PT mode. Additionally, the fracture morphology indicates the fracture mechanism is different between STTD and PT.