Microstructural Refinement and Performance Improvement of Cast n-Type Bi2Te2.79Se0.21 Ingot by Equal Channel Angular Extrusion

Abstract

How to form a uniform fine-grained structure in n-type polycrystalline Bi2Te3 based alloys to stabilize and improve the thermoelectric properties is the burning issue to be solved. In the present work, equal channel angular extrusion (ECAE) method was used to extrude the cast ingot directly at various temperatures to obtain the BiTeSe polycrystalline bulk alloys with uniform microstructure. The initial lamellae coarse grains (hundreds microns to several millimeters) were efficiently and uniformly refined to a minimum of 4.07 μm at 703 K, which fully demonstrates the feasibility of grain refinement by ECAE for cast n-type BiTeSe ingot. The average grain size of the extruded billets rapidly increased from 5.35 to 29.55 μm with the increase of extrusion temperature from 703 to 823 K. Meanwhile, the behavior of abnormal grain growth became more evident as the extrusion temperature increased because the growth rate increased more rapidly than the rate of nucleation. As the extrusion temperature increased, the Vickers hardness decreased gradually due to grain coarsening. BiTeSe alloys extruded at 703 K achieved the maximum Vickers hardness of 59.8 HV0.05, which was nearly three times higher than that of the initial cast ingot. All extruded BiTeSe billets possessed the much higher dimensionless figure of merit (ZT), compared with the initial cast ingot due to the simultaneously optimization of electrical and thermal transport performances caused by ECAE process. Finally, the specimen extruded at 823 K achieved the maximum ZT of 0.685 at 383 K.