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Grain refinement and texture development of cast Bi₉₀Sb₁₀ alloy via severe plastic deformation
The purpose of this work was to study learn about grain refinement mechanisms and texture development in cast n-type Bi₉₀Sb₁₀ alloy caused by severe plastic deformation. The practical objective is to produce a fine grained and textured microstructure in Bi₉₀Sb₁₀ alloy with enhanced thermoelectric performance and mechanical strength. In the study, twelve millimeter diameter cast bars of Bi₉₀Sb₁₀ alloy were encapsulated in square cross section aluminum 6061 alloy containers. The composite bars were equal channel angular (ECAE) extruded through a 90 degree angle die at high homologous temperature. Various extrusion conditions were studied including punch speed (0.1, 0.3 and 0.6 in/min), extrusion temperature (220, 235 and 250̊°C), number of extrusion passes (1, 2 and 4), route (A, BC and C), and exit channel area reduction ratio (half and quarter area of inlet channel). The affect of an intermediate long term heat treatment (for 100 hours at 250°C under 10⁻³ torr vacuum) was explored. Processed materials were characterized by optical microscopy, x-ray diffraction, energy dispersive spectroscopy, wavelength dispersive spectroscopy and scanning electron microscopy. Texture was analyzed using the {006} reflection plane to identify the orientation of the basal poles in processed materials. The cast grains were irregularly shaped, had a grain size of hundreds-of-microns to millimeters, and showed inhomogeneous chemical composition. Severe plastic deformation refines the cast grains through dynamic recrystallization and causes the development of a bimodal microstructure consisting of fine grains (5-30 micron) and coarse grains (50-300 micron). ECAE processing of homogenizied Bi-Sb alloy causes grain refinement and produces a more uniform microstructure. Texture results show that ECAE route C processing gives a similar or slightly stronger texture than ECAE route A processing. In both cases, the basal-plane poles become aligned with the shear direction. Reduction area exit channel extrusion is more effective for both grain refinement and texture enhancement than simple ECAE processing.
Title from author supplied metadata (automated record created 2010-03-12 12:08:51).
Doctor of Philosophy Texas A&M University 2007 http://hdl.handle.net/1969.1/ETD-TAMU-1346