Toughening of titanium alloys by twinning and martensite transformation

Two kinds of titanium alloys, beta titanium alloy (Ti-10V-2Fe-3Al) and alpha titanium alloy (Ti-5Al-2.5Sn) were used to investigate the toughening mechanisms with new approaches. The results show that Ti-5Al-2.5Sn alloy possesses good combination of strength and ductility as well as satisfied low-cycle fatigue life both at 293 K and 77 K. As for Ti-10V-2Fe-3Al alloy, the microstructure with metastable beta phase shows lower strength and ductility but higher threshold stress intensity factor (DeltaK(th)) than solution treated and aged microstructure composed of alpha and beta phases. The microstructures also show that twinning occurs in deformation of Ti-5Al-2.5Sn alloy at 77 K. Twinning seems to be helpful for improving the low-cycle fatigue life to a great extent at cryogenic temperature. It's also found that owing to stress-assisted martensite transformation in metastable Ti-10V-2Fe-3Al alloy, the fatigue crack propagation path shows a very tortuous way, which decrease the effective stress intensity factor (DeltaK(eff)) at crack tip, and increase threshold stress intensity factor (DeltaK(th)). (C) 2002 Kluwer Academic Publishers.