Low-temperature impact toughness and deformation mechanism of CT20 titanium alloy

The impact toughness and deformation mechanism of near-α-type low-temperature titanium alloy CT20 at different temperatures (20 °C, 0 °C, −50 °C, −100 °C and − 196 °C) are studied. The results show that the impact toughness is as high as 90 J/cm² at 20 °C, and does not decrease obviously at 0 °C, −50 °C and − 100 °C, but brittle fracture occurs at −196 °C (29 J/cm²). The tortuosity of the crack path decreases with decreasing temperature. At 20 °C, the grain boundary and α colony interface with high-angle grain boundary deflect the crack propagation direction effectively, forming a tortuous crack path, while crack tends to cut through α lamellae, forming a relatively straight path at cryogenic temperature. The lamellar α undergoes kink deformation at 20 °C, showing severe plastic deformation characteristics. In addition, deformation twins and geometrically necessary dislocations (GNDs) are observed near and far from the fracture surface, indicating that the plastic zone is large. As the temperature decreases, the twin density near the crack path tends to increase. However, few twins are seen in the region far from the fracture surface, and GND densities are low at −196 °C, implying that the plastic zone is small. In a word, the synergistic effect of tortuous crack path, dislocation slip and deformation twinning enable CT20 to achieve high impact toughness at 20 °C. At −196 °C, the plastic deformation capacity of the microstructure decreased significantly, resulting in the formation of a small plastic zone and brittle fracture.