Study On Phase Transformation And Deformation Characteristics Of Ti-45Ni-5Cu-0.3Cr Shape Memory Alloy

In order to develop the Ti-Ni base shape memory alloys and heat treatment processes with narrow temperature hysteresis and excellent properties,Ti-45Ni-5Cu-0.3Cr(atomic fraction)shape memory alloy wire and its spring were designed and manufactured,and the effects of annealing and aging processes on the phase composition,microstructure,phase transformation behavior,mechanical properties,shape memory behaviors and stress-strain cyclic stability of the Ti-45Ni-5Cu-0.3Cr alloy were studied by means of X-ray diffractometer,optical microscope,transmission electron microscope,differential scanning calorimeter and microcomputer-controlled electronic universal testing machine.It shows that the Ti-45Ni-5Cu-0.3Cr alloys annealed at 350?700? for 30 min and aged at 300?600? for1?50 h are composed of martensite phase(M,monoclinic B19?)and parent phase(A,Cs Cl type B2)at room temperature.The Ti-45Ni-5Cu-0.3Cr alloy undergoes the stages of recovery,recrystallization and grain growth with the increase of annealing temperature,the microstructure morphology changes from fibrous to equiaxed,and the recrystallization temperature is between 550? and 600?.The microstructure morphology of the solution aged alloy is equiaxed,and no precipitated phases are found in the microstructure.The reversible phase transformations of A?M/M?A type occur in the annealed and aged Ti-45Ni-5Cu-0.3Cr alloy upon cooling/heating.The martensitic transformation temperature is stable,and changes in the range of 14?21?.The temperature hysteresis is narrow,and changes in the range of 17?21?.With the increase of annealing temperature,the tensile strength of the annealed Ti-45Ni-5Cu-0.3Cr alloy increases at first and then decrease,and the maximum value of 880 MPa was obtained in the alloy annealed at 400?500?.The elongation of the alloy increases at first and then decreases,and the maximum value of 68% was obtained in the alloy annealed at 650?.The strength and plasticity of the annealed alloy are better than those of the aged alloy.At Room temperature,both annealed and aged Ti-45Ni-5Cu-0.3Cr alloys exhibit shape memory effect.With the increase of the number of stress-strain cycles,the martensitic reorientation stress is decrease,the residual strain is increase,and the area of the stress-strain hysteresis loop is reduction in the annealed Ti-45Ni-5Cu-0.3Cr alloy;the martensitic reorientation stress increases firstly and then tends to be stable,the residual strain decreases firstly and then tends to be stable in the alloy.The stress-strain cycling stability of aged Ti-45Ni-5Cu-0.3Cr alloy is better than that of annealed alloy.With the increase of deformation temperature,the stress-strain plateau stress increases,the residual strain decreases,the alloy characteristics change from shape memory effect(SME)to superelasticity(SE),and the transition temperature is about 60? in Ti-45Ni-5Cu-0.3Cr alloy.At room temperature,the Ti-45Ni-5Cu-0.3Cr alloy springs annealed at 350?600?exhibit SME.With increasing deformation temperature,the spring characteristics change from SME to SE,the height and area of the stress-strain hysteresis loop increase,and the superelasticity improves.With the increase of stress-strain cycles,the strain recovery rate of Ti-45Ni-5Cu-0.3Cr alloy spring decreases at first and then tends to be stable.Stress-strain cyclic training can enhance the stability of shape memory behavior of alloy spring.