Research On Fracture Toughness Of CT20 Titanium Alloy At Low Temperature

Titanium alloy as a majestic development of the"third metal"with its low density,high strength and good corrosion resistance and other advantages,in aerospace and marine shipping and other fields have a wide range of applications.However,the mechanical properties of most structural materials in these fields are affected by the low temperature conditions in the service environment,resulting in brittle fracture and other catastrophic accidents.Therefore,in order to ensure that the low temperature titanium alloys in service can normally service,it is necessary to investigate the low temperature mechanical properties and crack propagation behavior of the material at different temperature.This paper systematically investigated the low temperature mechanical properties of a new nearαphase low temperature titanium alloy at 20°C to-90°C.The stress evolution and dimensional changes in the crack tip plastic zone during crack propagation were characterized and analyzed by finite element simulations,the contribution of internal and external fracture toughness was calculated by combining the crack tip plastic zone and the crack propagation path,the main influencing factors of fracture toughness at different temperatures were determined,the influence of laminarα-phase on crack propagation behavior and the evolution of dislocation activity at different temperatures were analyzed,the crack propagation mechanism of CT20 titanium alloy at low temperatures was clarified and the crack propagation model at different temperatures was established.The yield strength of CT20 titanium alloy increased from 537.56 MPa to 719.64 MPa from20 to-90°C,but the reduction of area and elongation after fracture showed a linear decrease trend.The fracture toughness of CT20 alloy was decreased from 430.89 KJ/m~2 to 171.67 KJ/m~2,and the fracture mechanism was gradually changed from typical ductile fracture to quasi-cleavage fracture with the decreasing testing temperature.The reduction of fracture toughness was mainly influenced by the internal factor(the crack tip plastic zone)and external factor(the tortuosity of the crack propagation path),in which the contribution of the internal factor increased from 87.06%to 95.68%as the temperature decreased.Therefore,the high contribution of the internal factor means the plastic zone at the crack tip and lamellarαcould mainly affect the crack propagation behaviors.CT20 titanium alloy at different temperatures were along the{0001}and{101?0}crystal extension and transgranular fracture.The long and thickαplatelets significantly retarded crack propagation and deflected the direction,resulting in extra energy consumption and improving fracture toughness.The Peierls-Nabarro stress increased significantly and the density of the available dislocation at the crack tip decreased significantly with the decrease of temperature.Therefore,the size of the plastic zone at the crack tip and the ability of the lamellarαphase to resist crack growth inevitably decreased at-90°C.Meanwhile,the deformation twins could alleviate the local stress concentration and limit the movement of dislocation entanglement,which effectively slowed down the reduction of fracture toughness at low temperature.