Controlling the microstructure and fracture toughness of the Ti–5Al–5Mo–5V–1Cr–1Fe alloy by multiple heat treatments

This study presents the effects of the microstructure on the corresponding fracture toughness of the Ti–5Al–5Mo–5V–1Cr–1Fe (Ti-55511) alloy by undergoing STA (α/β solution treatment plus aging) and STAA (α/β solution treatment plus first-step aging and second-step aging) processes. The results show that the microstructural evolution of Ti-55511 as well as the corresponding fracture toughness is very sensitive to the heat treatment mechanism. Here, an increase in the solution temperature can reduce the content of αp, while increasing the content of α precipitates during the STA and STAA processes. An increase in the aging temperature coarsens the α precipitate and decreases its content, which deflects the crack and consumes additional energy, thus resulting in an enhanced fracture toughness. Note that the STAA process results in the further formation of α precipitates, which will cause a larger stress concentration that promotes the growth and coalescence of voids; notably, this is inevitably accompanied by a decrease in toughness. In addition, ductile fracture dominated by dimples is mainly experienced in the crack instability and growth stage, and a higher toughness increases the size of the dimples. Overall, this study indicates that the toughness of the alloy can be controlled from 38.2 to 110 MPa⋅m0.5 by multiple heat treatments.