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 250oC), 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 250oC 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.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-1346 |
| Date | 15 May 2009 |
| Creators | Im, Jae-taek |
| Contributors | Hartwig, K.Ted. |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | electronic, application/pdf, born digital |
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