Simple shear processing of bulk BI₂TE₃ alloys

Abstract

The objective of this work is to determine the appropriate extrusion conditions of cast Bi₂Te₃ alloys via equal channel angular extrusion (ECAE) to produce material that has a fine grain size (5~30[]m), uniform grain morphology and low grain boundary misorientation for improving thermoelectric and mechanical properties. The range of variables examined in this study includes: alloy type (p- and n-type) extrusion temperature (400 ~ 525⁰C), extrusion route (A, B, C and E), extrusion speed (0.3 ~ 0.01 in/min), and the number of extrusions (1, 2, 4 and 8). The microstructure is characterized by a polarized optical microscopy. Successful extrusions were accomplished at a temperature and strain rate that enabled dynamic recrystallization. Holding in the hot die after extrusion encouraged grain growth. The post processed grain size is found to be strongly dependent on extrusion temperature and extrusion speed. The microstructural uniformity is found to be dependent on the number of extrusions and extrusion route. Uniform equiaxial grain of 10 to 30mm average size are obtained in the p-type alloy after four extrusion passes following route E at a temperature of 500⁰C and an extrusion speed of 0.1 in/min. The results encourage further work and indicate ECAE is a viable method of grain refinement of Bi₂Te₃ alloys.