Tougher TiAl alloy via integration of hot isostatic pressing and heat treatment

A special processing method integrating the hot isostatic pressing and heat treatment was developed to produce TiAl alloy. As compared with the traditional two-step method, which consists of separate hot isostatic pressing and subsequent heat treatment in vacuum, the integrated approach will introduce an isostatic pressure throughout the whole process from the consolidation of powder to the heat treatment of coupon. Two types of microstructure, i.e., lamellar and duplex, have been generated by integrated and separate methods. Tensile test at room temperature indicates that the yield strength and tensile elongation of lamellar microstructure generated by integrated method are 650±30 MPa and 2.1±0.2% respectively, which are improved simultaneously as compared with 550±28 MPa and 0.6±0.1% for sample generated by separate approach. Moreover, such simultaneous enhancement of yield strength and ductility is also observed in duplex microstructure, where the yield strength and tensile elongation increase from 420±18 MPa and 2.4±0.4% in samples with separate method, to 540±25 MPa and 3±0.3% in those generated by integrated approach. Additionally, the microstructural examination also revealed the influence of microstructure to the mechanical performance. The results show that the simultaneous improvement of yield strength and tensile elongation is mainly attributed to the suppression of cracking, which is prone to happen during heat treatment without atmospheric pressure. Using the integrated method, the isostatic pressure could sustain the equilibrium pores during heat treatment, and provides an exterior force to balance the potential internal stresses due to phase transformation.