Twinning-induced high impact toughness of titanium alloy at cryogenic temperature

Abstract

The impact toughness of titanium alloys is one of the key factors determining their safe service in cryogenic temperature environments. In this work, the temperature-related impact properties of grade 2 pure titanium and Ti-2.5Al–3Zr–1Mo alloys are investigated. It is found that the impact properties and deformation mechanisms of the above two materials are different with decreasing temperature (20 °C, 0 °C, −50 °C, −100 °C and −196 °C). Speaking of pure titanium: the impact toughness doesn't change obviously, and crack initiation energy (Wi) gradually increases while crack propagation energy (Wp) decreases (Wp of −196 °C is 40% lower than that of 20 °C while Wi is 90% higher). The slip-dominated deformation mechanism at 20 °C gradually transforms into twinning-dominated deformation mechanism under cryogenic temperature. High-density twins formed at −100 °C and −196 °C can effectively relieve stress concentration and lead to better cryogenic impact toughness. By contrast, Wi and Wp of Ti-2.5Al–3Zr–1Mo alloy decrease with decreasing temperature. The impact toughness of Ti-2.5Al–3Zr–1Mo alloy is higher than that of pure titanium when the temperature is above −100 °C. At 20 °C, the synergistic effect of the tortuous crack path and deformation twins of Ti-2.5Al–3Zr–1Mo alloy promotes its impact toughness higher than that of pure titanium. However, the twinning density of Ti-2.5Al–3Zr–1Mo at −196 °C is much lower than that of pure titanium, which cannot release stress effectively and cause a brittle fracture. The research provides theoretical and experimental basis for the application of titanium alloys in cryogenic environment, and provides guidance for the development of titanium alloys with better cryogenic impact toughness.