Phase Transitions And Performance Characterizations Of TiNiCuCr Shape Memory Alloys

The present study investigated the microstructure,martensitic transformation, mechanical properties and electrochemical behavior of Ti_49.6Ni_45.4-xCu₅Cr_x(x=0, 0.3,0.5) alloy.Optical metallography microscope,DSC and XRD were employed to study the microstructure and phase composition of the alloy.Mechanical property and the stability of superelasticity of samples under different cold-rolling and annealing conditions were investigated by universal materials testing(tensile) machine using stress-strain cycling method.Bending test method was used to measure the shape memory effect.Electrochemical behavior and the composition of the passive film on the surface of the alloy were investigated by electrochemical method and XPS respectively.Furthermore,the Ni ion release behavior was tested by immersion and ICP-OES.It was shown that solution treated TiNiCuCr alloy possessed a homogenous microstructure.XRD results confirmed Ti₂Ni and Ti₂Cu precipitation.The transformation temperature decreased with the increase of Cr content.Single B2-B19' martensitic transformation was found during cooling.The transformation temperature of annealed sample increased with the increase of annealing temperature at first,and then it decreased,and finally stabilized.The annealing treatments over a wide temperature range didn't induce R phase transformation.The results of phase transformation test showed that martensite was stabilized after one stress-strain cycling,indicated by the increase of A_s and A_f. Two transformation peaks were found during heating which were corresponding to the reverse transformation of the thermo-elastic martensite and the stabilized martensite.Further cycling changed the transformation temperatures of samples treated at different annealing temperatures,at the same time,the transformation heat decreased with the increase of cycling times.The mechanical test results showed that with the increase of Cr content the yielding strength increased and the tensile strength decreased respectively.The critical stress of stress-induced martensitic transformation increased to a maximum with the increase of annealing temperature,and then decreased. Annealing temperature had significant influence on the stability of superelasticity. Annealing at 350℃rearranged the dislocations induced by cold-rolling,resulting in strong strengthen effect.So the alloy treated under this condition had a stabile superelasticity.Annealing at 500℃resulting in stabile stress plateau,and Lüders-like deformation was found even after 30 cycling.Complete recrystalization occurred above 650℃,therefore the stabilization of superelasticity decreased.The residual strain accumulated with the increase of cycling time and Lüders-like deformation changed to homogenous deformation.The residual strain of Ti_49.6Ni_45.1Cu₅Cr_0.3 increased with the cycling times of shape memory,and the maximum two-way shape recovery strainε_tw increased at the same time.Annealing temperature was found to have an influence onε_tw. When cycling time N increased to 20,the residual strainε_p reached to 10%,andε_tw increased to 3%.Ti_49.6Ni_45.1Cu₅Cr_0.3 and Ni_51.3Ti_48.7 alloys had a similar electrochemical behavior as indicated by electrochemical test.The pH value of the electrolyte had influence on both E_corr and E_b.When the pH equaled to 2.4,5.3 and 6.2,the electrochemical impedance was 0.28 MΩ·cm²,0.31 MΩ·cm² and 0.29 MΩ·cm², respectively.Ni ion release behavior of Ti_49.6Ni_45.1Cu₅Cr_0.3 and Ni_51.3Ti_48.7 alloy showed time dependence effect that the release rate decreased with the increase of immersion time.The XPS results showed that the outmost passive film consisted of TiO₂ and some Ni oxide.After the sputtering of Ar ion,Ni:Ti ratio increased from 0.27 to 2.57,indicating the existence of a Ni-riched layer.Cu and Cr didn't affect the formation of the outmost passive film.