A high-withdrawing-rate method to control the orientation of (γ+α2) lamellar structure in a β-solidifying γ-TiAl-based alloy

Abstract

β-solidifying γ-TiAl-based alloys with well-controlled lamellar orientation possess excellent mechanical properties, and low withdrawing rates are usually used to control the lamellar orientation by directional solidification. Herein, a high-withdrawing-rate method to control the lamellar orientation in a β-solidifying γ-TiAl-based alloy, TNM alloy (Ti-43.5Al–4Nb–1Mo-0.1B at.%), is proposed by Bridgeman directional solidification. Using this method, polysynthetic twinned (PST) single crystal of TNM alloy with a good lamellar controlling result is obtained. The mechanism of lamellar orientation controlling lies in the process of solidification and β/α transformation, which are governed by thermal stabilization treatment and withdrawing rate. Thermal stabilization treatment for 60 min can make the sample maintain stable interfaces of liquid/solid and β/α phase with a high temperature gradient before the withdrawing process starts, and it also leads to the incline of β dendrites; a high withdrawing rate of 100 μm/s can accomplish the grain selection of α phase during β/α transformation and make complete peritectic reaction occur. Compared with as-cast one, PST single crystal specimen has its room-temperature tensile property enhanced greatly, and presents an ultimate tensile strength/strain of 476 MPa/1.75% with a trans-lamellar fracture morphology.