Materials Science Forum Vols. 503-504

Abstract: The commercial purity zirconium (Zr702) has been deformed by Equal Channel Angular Press (ECAP) up to eight passes using route A and BC. The deformation microstructure and the bulk texture have been characterized by Electron Backscattered Diffraction (EBSD) in SEM and X-rays diffraction (XRD), respectively. The homogeneously lamellar deformation structure in route A and the equiaxed deformation structure with local heterogeneity in route BC have both been determined. In both cases the subgrain size flattened at about 0.33 ìm after 2 passes but average misorientation increased with increasing strain. After 8 passes high angle fraction is over 70% both in route A and route BC. The bimodal {0002} pole texture (high strain rolling texture) in route A and the strong unimodal {0002} pole texture with weak high strain rolling texture in route BC have been identified. The boundary texture has been analysed both in sample frame and crystallographic frame. It was found in sample frame that the boundary texture of high angle boundaries developed gradually in route A and more strongly than that in route BC, implying that the boundary characters evolved from the mixed characters (tilted and twist) to tilted boundary in route A. Based on the deformation structures and the bulk textures, the deformation mechanism of large local orientation rotation and the stability of the dislocation structures have been proposed to interpret the grain refinement and the texture development of Zr702 during ECAE processing.

Abstract: The nanostructured TiNi-based shape-memory alloys were synthesized by multi-step SPD deformations – ECAP plus cold rolling or drawing. It is found that the SPD processing changed the morphology of the martensite and temperature of martensite transformation. Also, we found that the mechanical and shape memory properties can be enhanced by forming nanostructures in these alloys. SPD processing renders higher strength, higher yield dislocation strength and in results - higher recovery stress and maximum reverse strain of shape memory.

Abstract: The analysis is made of effect and physical mechanisms of the influence of pressure on metal materials that are in the state of plastic flow, as well as on characteristics of materials undergone severe plastic deformation under pressure.

Abstract: Single roll drive equal channel angular process (SRD-ECAP) is derived from classical equal channel angular pressing (ECAP). It overcomes the drawbacks of ECAP, which is a discontinuous process and can only process short billets with invariable square or round section. SRD-ECAP, however, can process long billets continuously. In addition, it can also process sheet or strip. Another merit is that diameter or thickness of billets processed is adjustable. The structure of SRD-ECAP is quite simple and easy to be realized. In this paper, stress analysis has also been carried out. It is believed that SRD-ECAP can be a potential severe plastic deformation (SPD) process for Industrial Application.

Abstract: A coarse-grained W-25%Cu composite with a tungsten particle size between 2 and 10 μm is subjected to high pressure torsion (HPT) at room temperature to different strains. The evolution of microstructure with increasing strain is studied. At low strains (εeq ≤ 16), an inhomogeneous deformation of the material is found. This inhomogeneity is studied in detail by nanoindentation experiments. At larger strains (εeq ≥ 16), a fragmentation of tungsten particles is observed. The specimen deformed to a strain of 256 exhibits a homogeneous microstructure with a tungsten particle size between 10 and 20 nm. A further increase of strain does not cause a further significant tungsten particle refinement. A possibility of industrial application of HPT to produce nanocomposite materials is discussed.

Abstract: The purpose of this work is producing bismuth telluride based alloys with both well-orientated texture and fine grains to improve thermoelectric properties. The angular extrusion process was applied to Bi1.9Sb0.1Te2.7Se0.3 foils which were prepared by a liquid quenching method [1]. The extrusion temperatures examined ranged from 380 to 540 °C. The microstructures of the specimens were characterized with an optical microscope and OIM. Recrystallized grains sizes ranging from 3.61 to 10.01 μm were obtained at these temperatures. The textures of the specimens show the basal plane preferably aligns along to the extruded direction. A maximum power factor of 4.18 × 10-3 W / mK2 was obtained in this work. These results indicate that the angular extrusion process is effective to improve the thermoelectric properties of bismuth telluride based alloy.

Abstract: Mechanical milling (MM), which is one of SPD process, is applied to W powder and W-Remixed powder. MM processed W and W-Re powders easily form nano grain structure even though they have high melting temperature. The nano grain formation mechanism in these powders is as follows: multi axial deformation of the powders by milling, at the temperature of 333K at most, produces pan-cake grain structure at first. Extremely dense dislocations result in grain sub-division, and finally nano grain structure with high angle boundary forms. Nano grains with approximately 10 nm in diameter are obtained. The MM powders are sintered using Spark Plasma Sintering process. Sintering and high temperature deformation behaviors of the MM powders are also investigated. The MM treatment enables W powder to be able to sinter at 1273 K while the powder without MM could never sintered at the same temperature. Re addition prevents grain growth during sintering and thus increases hardness of the compacts. A large deformation of W-10mass%Re sintered compact, whose grain size is approximately 450nm, is observed at elevated temperatures.

Abstract: Microstructure and mechanical properties of pure nickel processed by high pressure torsion (HPT) and equal channel angular pressing (ECAP) have been investigated in the present paper. The uniformity of microstructure and misorientation angles in the SPD samples were studied using transmission electron microscopy and electron back scattering diffraction. Microstructural requirements for achivement of high strength and ductility in SPD metals are discussed.

Abstract: Recent studies have demonstrated that ultrafinе-grainеd (UFG) alloys processed by equal channel angular pressing (ECAP) and high pressure torsion (HPT) can exhibit enhanced supеrplacticity at relatively low temperature and/or high strain rates. At the same time severe plastic deformation (SPD) of aluminium alloys is often carried out at elevated temperatures leading to various grain size and volume fraction of precipitates. The significance of the SPD temperature for commercial 1420 and 1421 aluminium alloys has been evaluated in this paper using in-situ heating of thin foils in the column of a transmission electron microscope. Superplastic characteristics of alloys processed at various ECAP temperatures are discussed.