Intrinsic Control Mechanism of Tensile and Impact Toughness of TC21 Titanium Alloy
TC21 alloy has high strength and fracture toughness.However,the differences in the intrinsic control mechanisms of their tensile and impact toughness are not clarified.In this work,different microstructures were prepared by regulating the solid solution temperature and cooling rate to study the tensile and impact properties.The results show that tensile performance and impact toughness exhibit different variation laws.The impact toughness of the bimodal structure with better plasticity is lower than that of the full lamellar structure with the worst plasticity,indicating that the intrinsic control mechanisms of tensile properties and impact toughness are different,which is further confirmed by the post-aging properties(no significant change in plasticity but significant decrease in impact toughness after aging).During tensile deformation,plastic deformation occurs in the whole region of the specimen before necking occurs,and the coordination deformation between αp and βt in the bimodal structure is fully developed;while the full lamellar structure has a larger colony size,its internal lamellar α orientation is uniform,and the dislocation slip length is larger,making it susceptible to plastic strain localization,and resulting in a poorer strength plasticity matching than that of the bimodal structure.Under the influence of high strain rate,the crack initiation and propagation at the notch root are rapid,and the plastic deformation is concentrated in a small range near the crack tip,resulting in the fact that coordination deformation between αp and βt in bimodal structure cannot be fully played.By contrast,the large α colony size of the full lamellar structure leads to less influence of the α colony interface on plastic deformation,and the lamellar α and β become an effective unit to control plastic deformation.The coarse lamellar α and β have good plastic deformation ability,resulting in higher crack initiation energy,which is inconsistent with the poor plasticity during tension.In addition,the large angle interface of α colony causes the deflection of crack and forms a tortuous path,resulting in higher impact toughness than that of the bimodal structure.To obtain a good match between tensile properties and impact toughness,a bimodal structure consisting of a small amount of αp and βt,which has coarse lamellar α and β internally,can be prepared by heat treatment.
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