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 B19ˊ)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.