A high-withdrawing-rate method to control the orientation of (?+?2) lamellar structure in a ?-solidifying ?-TiAl-based alloy

beta-solidifying gamma-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 beta-solidifying gamma-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 beta/alpha 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 beta/alpha phase with a high temperature gradient before the withdrawing process starts, and it also leads to the incline of beta dendrites; a high withdrawing rate of 100 mu m/s can accomplish the grain selection of alpha phase during beta/alpha transformation and make complete peritectic reaction occur. Compared with as-cast one, PST single crystal specimen has its roomtemperature tensile property enhanced greatly, and presents an ultimate tensile strength/strain of 476 MPa/ 1.75% with a trans-lamellar fracture morphology.

Automated summary

This study proposes a high-withdrawing-rate method to control the lamellar orientation in a beta-solidifying gamma-TiAl-based alloy, resulting in the successful preparation of polysynthetic twinned single crystal specimen with good lamellar control. The key to the control method lies in the thermal stabilization treatment and withdrawing rate.