Evading the strength-ductility trade-off at room temperature and achieving ultrahigh plasticity at 800℃ in a TiAl alloy

Improving the room temperature (RT) strength/ductility and hot-working capacity based on lamellar microstructures is of great significance for the practical application of TiAl alloys. However, the microstructure of these alloys has not been clearly identified yet. In this work, two new microstructures, here named triple-phase triple-state (T-T) and triple-phase dual-state (T-D) structures, were developed using a two-step heat treatment process in the Ti-43.5Al-4Nb-1Mo-0.1B (TNM) alloy, which also contains the pearlitic-like microstructure (PM) transformed through triggering a massive cellular response (CR). These two microstructures significantly improved the alloy strength. Furthermore, their ductility at RT and 800 ℃ was enhanced twice and 5, 6 times with respect to that of the lamellar microstructure with nano-scale interlamellar spacing, respectively. It was revealed that the formation of abundant deformation twins and their intersections in PMs during plastic deformation, cause prominent strain hardening and the dynamic Hall-Patch effect. This results in a simultaneous improvement of the RT strength and plasticity and promotes dynamic recrystallization at temperatures lower than 800 ℃; thus, the plasticity is dramatically enhanced at elevated temperatures. This structural design strategy should be extendable to other TiAl systems that can undergo a CR and provides a promising new pathway for solving the severe engineering challenges caused by the low RT plasticity and poor hot-working capacity of TiAl alloys.