Processing of Bi90Sb10 by Equal Channel Angle Extrusion

Cast billets of Bi₉₀Sb₁₀ have been processed by equal channel angle extrusion (ECAE) in an effort to make the alloy in a highly textured, polycrystalline form for use as a low-temperature n-type thermoelectric material. In this initial stage, we have focused on studying the evolution of the microstructure. The cast material has a random, mm-scale grain structure. As the material is deformed, there is a strong tendency toward a bimodal grain size distribution. Much of the sample readily refines to a fine grain size of ~10 microns, while the remainder maintains a coarse grain size of ~300 microns even after a large amount of plastic strain is imparted. We have found that the coarse grains are Bi-rich, while the fine grains are Sb-rich. An intermediate heat treatment that improved chemical homogeneity also improved microstructure uniformity achieved in subsequent multi-pass ECAE processing. We consider possible mechanisms by which chemical inhomogeneity leads to non-uniform grain refinement in Bi_1-xSb_x. In addition to homogeneity, texture is required to make good quality Bi_1-xSb_x alloys. Pole figure analyses of our samples show that the ECAE process can be effective in creating crystallographic texture.