Experimental Research On Mechanics Behavior Of NiTi Alloy After Annealing

NiTi alloy is a shape memory alloy with excellent properties such as shape memory effect,superelasticity and biocompatibility.It is commonly used in architecture,biomedicine,aerospace and other fields,such as used as damper,stent,antenna in space and so on.At present,the research focus of NiTi alloys is mainly on superelasticity,shape memory effect,mechanical behavior of materials after adding different elements,and the change of grains in materials.In realistic applications,the required material properties are usually obtained by heat treatment.The annealing process can reduce the hardness,eliminate the residual stress,refine the grain size,change the precipitation temperature and phase change resistance of the material.It is one of the most widely used heat treatment processes.The study of NiTi alloy with annealing treatment is focus on cooling rate,deformation restoring force and grain size,but the mechanical behavior of annealed NiTi alloy is rarely studied,so the mechanical behavior of annealed NiTi alloy is studied experimentally.Digital image correlation(DIC)is a dynamic,real-time and non-contact measurement method with the advantages of simple equipment,low light requirement and full-field measurement,which meet the experimental requirements of this study.Therefore,DIC is used to measure the displacement field and deformation of NiTi alloy,and micro-scale speckle(the necessary deformation information carrier of DIC)was fabricated.The influence law and mechanism of mechanical behavior after annealing treatment were systematically studied by a series work of annealing treatment and in-situ loading tensile experiments of before and after annealed NiTi alloy.In order to study the effect of different defects on the mechanical behavior of materials,three kinds of defects,groove,semi-circle and circle,were designed on NiTi alloy specimens and retreated with transfer speckles.Tensile tests were performed on three kinds of defect specimens using in-situ loading system.The experimental results were processed by VIC-2D(DIC method supporting software),and the deformation under different stresses were obtained.The effect of three kinds of defects on the mechanical behavior of materials was studied.In order to research the effect of different annealing temperatures on the phase transformation of NiTi alloy,annealing treatment was carried out at different temperatures.The temperature range was 300~oC~900~oC and the temperature step were 25~oC.The effect of annealing temperature on the phase transformation of NiTi alloy was tested by DSC.It was noted that the second phase precipitation occurs in the range of 325~oC~525~oC,while the intersection of heat absorption and heat release lines occurs in the range of 325~oC,475~oC,600~oC,675~oC,775oC and 875~oC.In order to study the effect of annealing temperature on the mechanical behavior of materials,in situ tensile tests were carried out on five kinds of annealing temperature treated specimens(300~oC,450~oC,600~oC,750~oC and 900~oC).It was found that the mechanical behavior of three kinds of defective specimens changed compared with that of annealed materials.For example,under 300~oC annealing,the strain in the tensile direction of the groove defect specimens decreases from increasing with stress to the region near the defect.