Fabrication of a Thermoelectric Nanocomposite via Multiple Roll Bonding of Cast Bi-22at.%Sb Alloy

A Bi-22at.%Sb alloy was water-quenched from 973 K; its microstructure was dendritic with primary Sb-rich arms up to 50 mu m in diameter. The cast alloy was first rolled to tape, then cut into equal lengths and subjected to multiple roll bonding (MRB) processes. The evolution of the microstructure during deformation was followed by optical microscopy, scanning electron microscopy (SEM), field-emission scanning electron microscopy (FESEM), differential scanning calorimetry (DSC), and x-ray diffraction analysis. After three MRB cycles, an overall area reduction ratio of 4 x 10(3) was achieved. Due to the relatively low process temperature, a relevant fraction of dendrites was preserved. Plastic strain caused dendritic arms to elongate in the rolling direction and to flatten in the transverse direction. As a result of different starting size and orientation, the dendritic arms were transformed into Sb-rich inclusions of various shapes and dimensions at the end of the MRB process. In particular, inclusions with a transverse size down to 20 nm were observed.