Effect of heat treatments on the microstructure and mechanical properties of Ti_2AlNb intermetallic fabricated by selective laser melting

摘要

Ti_2AlNb (Ti-22Al-25Nb at.%, nominal composition) intermetallic is essential to aerospace industry from a weight reduction perspective, and brings a possibility of replacing some Ni-based superalloys. Despite evaporation of element Al (from its original composition 22.8 at.% to 18.5 at.%), selective laser melting (SLM) technology offers distinct advantages in fabricating Ti_2AlNb intermetallic with superior room-temperature properties. A full understanding of corresponding heat treatment mechanism is the key to utilize well the SLM-prepared Ti_2AlNb at high temperatures. This study aims to clarify the relationship between the micro-structure and mechanical propensities of SLM-prepared then heat-treated Ti_2AlNb, to ultimately achieve promising mechanical performances at high temperatures. For this research purpose, a series of heat treatments were conducted, and the corresponding microstructures, mechanical properties were systematic studied by advanced characterization techniques. Results show that the mechanical properties at both room and high temperatures depend on heat treatment adopted. The samples solution-treated at 950 °C and then aged at 700 °C show promising room and high temperature strengths due to the formation of acicular O, α_2 and grain boundary α_2 phases. However, their ductility was poor. Aging at 830 °C reduces the strength, but significantly improves the ductility due to the appearance of rodlike O phase and the increase of B2/β phase proportion. The alloy possesses promising strength and ductility (611 MPa, 10.0%) and high thermal stability at 650 °C. It is further demonstrated that the heat-treated Ti_2AlNb possesses promising specific strength, even higher than that of as-cast Inconel 718 superalloy. The developed alloy, therefore, has a high potential for the industries requiring high-temperature performance and weight reduction.