Processing by equal-channel angular pressing: An investigation of high temperature mechanical properties.

摘要

Equal-Channel Angular Pressing (ECAP) provides an opportunity to fabricate bulk metallic materials having ultrafine grains without any change in cross-sectional area of the sample. These ultrafine-grained materials are attractive for attaining both unique physical characteristics and excellent mechanical properties leading to high potential for industrial applications.; The microstructural evolution caused by repetitive ECAP passes was studied using Electron Back-Scattering Diffraction (EBSD) technique to pure Al after ECAP for several numbers of passes at room temperature. The results showed that repetitive ECAP passes are capable of producing materials having ultrafine and equiaxed grains surrounded by high-angle grain boundaries in a reasonably random texture. However, inhomogeneously recovered grains with concurrent grain growth were observed in the sample after ECAP for 12 passes.; Tensile creep testing was conducted on pure Al pressed for 4 passes and tested at 423--523 K under constant stress. In addition, evolution of microtexture during creep testing was investigated using the High-Pressure Preferred Orientation neutron time-flight diffractometer (HIPPO) facility. The creep deformation at higher testing temperatures was dominated by a dislocation-controlled mechanism, whereas the creep mechanism at lower testing temperature was not well understood. There was no change in microtexture during the creep testing although grain growth was observed in the samples at all testing temperatures.; Tensile properties were examined using the Zn-22% eutectoid alloy processed by ECAP for 8 passes at 473 K. It showed the highest elongation of 2230% at 1.0 x 10-2 s-1 representing high strain rate superplasticity. The sliding offset of grain boundaries was measured using Scanning Electron Microscope (SEM) on the as-pressed Zn-Al alloy pulled to an elongation of 30% to evaluate the significance of grain boundary sliding. The highest fraction of sliding offset was observed in the sample under the testing condition providing the highest ductility, thus GBS is an important mechanism to achieve superplastic elongation.; Quantitative cavity measurements and cavity volume analysis were taken to evaluate the importance of cavitation in the tensile testing of a superplastic Zn-22% Al alloy processed by ECAP. A predicted cavity growth mechanism with growth rate is proposed for the Zn-22% Al alloy.