Effect of post-forging heat treatment on the microstructure and room temperature tensile properties in Ti-25 Al-10Nb-3V-1Mo (super α2)

Abstract

The room temperature tensile and fracture behaviour of super α2 have been assessed as a function of a range of thermomechanical treatments. It has been shown that the best properties are obtained in samples which have been forged in the β phase field using cool dies, followed by aging at 800 °C for 2 h. These property measurements have been correlated with the microstructural changes caused by the various heat treatments. It has been found that the highest strength is associated with a two-phase structure of transformed β phase containing about 17% primary α2 and that improved fracture behaviour is associated with a transformed fine Windmanstätten structure. It is concluded that the room temperature tensile ductility improved by aging at 800 °C for 2 h, which reduces the dislocation density and the stress concentration caused by the heavy forging deformation within the α2 and B2 grains. Grain boundary α2 films and the high Nb interface α2 phases at primary α2 and B2 matrix interfaces form after high temperature solution treatments. Also, these two kinds of α2 phase result in low room temperature tensile ductility. Grain boundary strengthening plays a dominant role in super α2, and transgranular fracture is the main failure feature of this alloy.