DYNAMIC FRACTURE-BEHAVIOR OF TI-6AL-4V ALLOY WITH VARIOUS STABILITIES OF BETA-PHASE

The effect of stability of the body-centered cubic (bcc) beta phase on the dynamic fracture behavior of Ti-6Al-4V alloy at room temperature and 77 K has been studied. The presence of a highly unstable beta phase in the quenched alloy leads to a decrease in both the dynamic fracture toughness and the crack propagation energy, and this decrease becomes more pronounced when test temperature is reduced to 77 K. Somewhat improved fracture characteristics were obtained by applying anneal procedure to receive a fully stable beta phase. The highest fracture toughness as well as the greatest crack propagation resistance were observed in the air-cooled grade, where the lattice parameter of the bcc phase was intermediate between those pertaining to quenched and annealed Ti-6Al-4V alloys. The effect is attributed to the vanadium content in the beta phase, which is sufficiently high to suppress deformation-induced transformation. On the other hand, the V content should be low enough to retard ductile-brittle transition, typical for the bcc metals at cryogenic temperatures. As a result, marked toughening can be achieved, so that the lowest application temperature of high-strength titanium alloys containing the bcc phase can be decreased significantly.