Enhanced strength-ductility-impact toughness synergy of CT20 titanium alloy by introducing a multi-level lamellar martensitic α structure

In this work, different morphologies and structures of lamellar alpha are obtained by controlling the cooling rates, i. e., water cooling (WC), air cooling (AC) and furnace cooling (FC), and their effects on tensile properties and impact toughness of a new near alpha CT20 alloy (Ti-3Al-2Zr-1Mo) are investigated. Results showed that a heterogeneous three-dimensional network lamellar martensite alpha structure was formed after WC, a mixed structure composed of fine lamellar alpha/beta and a small amount of martensite was formed after AC, and a coarse lamellar alpha structure was formed after FC. The tensile strength gradually decreased with decreasing cooling rate, and AC had the superior strength-plasticity match (yield strength was 13% higher than FC while their plasticity is equivalent). However, the impact toughness of AC (comparable to that of WC) was about 16% lower than that of FC (90 J/cm2). During tensile deformation, the synergistic effect of the good plasticity of the alpha colony, the obstruction of dislocations by the colony interface and the plastic deformation of the martensite of the AC contribute to the best strength-plasticity matching. During impact, crack initiates and propagates rapidly at the V-notch root, and the finite element simulation results showed that the plastic deformation was concentrated in a tiny region near the crack tip, resulting in the coordination deformation among the microstructures in AC cannot be fully realized. By contrast, the coordinated deformation of multi-level martensite and the generation of nano twins in WC make its impact toughness comparable to that of AC. The highest impact toughness of FC was attributed to the superior plasticity of coarse lamellae, slip transfer between coarse alpha and beta lamellae, the generation of twins and the formation of a small jagged and large wave-like composite crack propagation path.