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In this work, different morphologies and structures of lamellar α 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 α CT20 alloy (Ti-3Al-2Zr-1Mo) are investigated. Results showed that a heterogeneous three-dimensional network lamellar martensite α structure was formed after WC, a mixed structure composed of fine lamellar α/β and a small amount of martensite was formed after AC, and a coarse lamellar α 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 α 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 α and β lamellae, the generation of twins and the formation of a small jagged and large wave-like composite crack propagation path. • Cooling rate has a significant effect on lamellar α morphology and structure of CT20 alloy. • The multi-level lamellar martensitic α structure exhibits better strength-ductility-impact toughness synergy. • Coordinated deformation of multi-level martensite and generation of nano-twins result in high impact toughness. • The overall microstructure's deformation determines tensile properties, while impact toughness depends on the micro-deformation near crack tips. [ABSTRACT FROM AUTHOR]

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. • α colony interface deflects crack propagation direction at 20 °C, while crack tends to cut through α lamellae at cryogenic temperature. • The synergistic effect of tortuous crack path, dislocation slip and twinning enables CT20 to achieve the highest impact toughness at 20 °C. • The coordinated deformation of dislocation slip and twins keeps the impact toughness at a high level at 0 °C ∼ -100 °C. • The plastic deformation ability decreases significantly, resulting in the formation of a small plastic zone and brittle fracture at −196 °C. [ABSTRACT FROM AUTHOR]