Effect of deformation-induced transformation on the fracture toughness of commercial titanium alloys

Materials used in this study were hot-forged bars of Ti-6Al-2Sn-4Zr-6Mo (Ti-6.2.4.6) and Ti-3Al-8V-6Cr-4Mo-4Zr (Ti-3.8.6.4.4) and hot-rolled plates of extra-low interstitial (ELI) grade Ti-6Al-4V (Ti-6.4) and Ti-15V-3Al-3Sn-3Cr (Ti-15.3). The effect of deformation-induced transformation of metastable β phase on ductility and toughness was investigated. Tensile tests, Charpy impact tests, and both static and dynamic fracture toughness tests were carried out at temperatures between 77 and 473 K on four titanium alloys containing metastable β phase. Deformation-induced α″ (orthorhombic martensite) was observed in an (α + β)-type Ti-6Al-2Sn-4Zr-6Mo alloy. The dynamic fracture toughness increased at 223 K compared to that at other temperatures. In (α + β)-type Ti-6Al-4V, the static fracture toughness at 123 K and dynamic fracture toughness at 223 K were increased by deformation-induced martensite. The strength increased as temperature decreased in this alloy. An abnormal elongation of β-type, Ti-15V-3Al-3Sn-3Cr at 123 K is attributed to the mechanical twinning of the β phase. An effect of deformation-induced transformation on the fracture toughness of Ti-3Al-8V-6Cr-4Mo-4Zr was not observed.