Experimental Study On Mechanical Properties Of Nickel-titanium Alloy After Annealing Based On DIC

NiTi alloy,as a new intelligent metal material with super elasticity and shape memory effect,is widely used in civil engineering,such as shock absorber,energy absorbing device and active control equipment.After annealing,the residual stress can be effectively eliminated,grains can be refined,fracture toughness can be improved,the resistance to change of body phase and the temperature of precipitate can be changed.The enhancement of NiTi alloy basic mechanical properties and special mechanical properties by annealing treatment is of certain reference significance for the design and use of NiTi alloy components and the design of alloy annealing treatment process.Digital image correlation(DIC)method can conduct dynamic,real-time and non-contact measurement on the surface of specimen,and it has been widely used for observation of full-temperature deformation,microscopic deformation and dynamic crack at high temperature.In this paper,the grain size change,basic mechanical properties,material phase transition,ultra-elastic properties and fracture mechanical properties of Nitinol alloy at different temperatures after annealing were studied by combining digital image correlation methods,which can more intuitively describe the mechanical deformation characteristics of Nitinol alloy at different temperatures after annealing.In order to study annealing basic mechanical properties of NiTi alloy,the micro grain and the influence of the phase transition,to 400 ℃,500 ℃,600 ℃,700 ℃,800 ℃,900 ℃ annealing treatment after one hour and furnace cooling of the nickel titanium alloy plate specimen uniaxial tensile experiments get the annealing temperature on the properties of NiTi alloy conventional mechanical parameters to describe indicators(such as: nu elastic modulus E and poisson’s ratio,yield strength,tensile strength,etc.).The effect of annealing temperature on the properties of Ni Ti alloy such as martensitic phase transition temperature,austenitic phase transition temperature and thermal hysteresis was measured by DSC after annealing.The effect of annealing temperature on the grain size and morphology of Nitinol alloy was obtained by metallographic microscopic observation of annealed Nitinol alloy plates,and the effect of annealing temperature on the yield strength of alloy plates was explained by combining hall-petch theory.Through the mechanical comparison of the vertical plate rolling direction,the non-annealing specimen along the plate rolling direction and the specimen treated by annealing at 400 ℃,it is found that low temperature annealing can effectively eliminate the mechanical properties difference of different orientations caused by the plate rolling process.The conclusion of the study on the basic mechanical properties of NiTi alloy after annealing at different temperatures provides a basis for the experimental study on the superelastic properties and fracture mechanical properties of NiTi alloy.In order to study the different temperature hyperelastic properties of NiTi alloy after annealing,about three along the rolling direction(along the rolling direction,vertical direction of rolling and and 45 ° to the rolling direction)processing not annealed specimen in the laboratory temperature(17 ℃)and high temperature(150 ℃)under the environment of hyperelastic experiment,the rolling direction of the super elastic properties of the alloy.The effect of annealing temperature on the superelastic properties of Ni Ti alloy was obtained by conducting the superelastic experiment at room temperature(17 ℃)on NiTi alloy specimens after annealing at different temperatures for one hour and cooling the furnace.Combined with DIC method,the deformation information of Nitinol alloy plate during the whole loading process after annealing was obtained.In order to study the different temperature of NiTi alloy after annealing at room temperature(19 ℃)and high temperature(150 ℃)fracture mechanics performance,different temperature annealing treatment for 1 hour and furnace cooling zone open type(Ⅰ)cracks of nickel titanium alloy plate temperature in the laboratory(19 ℃)and high temperature(150 ℃)unidirectional tension to the environment.The full-field displacement field,strain field and other information of the crack tip in the loading process were calculated by using the digital image correlation method,and the corresponding stress intensity factor(SIF)value of the crack tip was calculated.By comparing the selected series loading displacement of each key point in the process of deformation field information and the crack tip stress intensity factor of instantaneous value,describe the plane stress condition with Ⅰ type fracture nickel titanium alloy under the condition of normal temperature and high temperature stress and strain accumulation and the fracture process.