Toughening of titanium alloys by twinning and martensite transformation

Two kinds of titanium alloys, β titanium alloy (Ti-10V-2Fe-3Al) and α 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 β phase shows lower strength and ductility but higher threshold stress intensity factor (ΔK_th) than solution treated and aged microstructure composed of α and β 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 (ΔK_eff) at crack tip, and increase threshold stress intensity factor (ΔK_th).