Microstructure Evolution And Functional Properties Of Aged TiNi Alloys Processed By Equal Channel Angular Pressing

TiNi alloy has been widely used due to its excellent shape memory properties and biocompatibility.Equal channel angular pressing（ECAP）is one of the important methods to improve the functional properties of Ti Ni alloy.For Ni-rich Ti Ni alloy,ECAP may induce the re-dissolution of Ti₃Ni₄ phase.The effect of Ti₃Ni₄ phase re-dissolution on microstructure evolution,superelasticity and mechanical properties of the alloy remains unkown.In this paper,the typical Ti_49.2Ni_50.8 alloy was selected as the target material.Firstly,different sizes of Ti₃Ni₄phase were obtained by aging treatment,and then the alloy was processed by ECAP.The microstructure evolution was studied by transmission electron microanalysis（TEM）and X-ray diffraction（XRD）analysis.The re-dissolution mechanism of Ti₃Ni₄ phase was analyzed.The martensitic transformation behavior was studied by differential scanning calorimetric（DSC）analysis,and the superelasticity and its cycle stability were studied by mechanical testing machine.After aging at 450℃,Ti₃Ni₄ phase presented in the sample.With the increase of aging time,Ti₃Ni₄ phase grows up with the increase of volume fraction.After 1 pass of pressing,the Ti₃Ni₄ phase is re-dissolved for the samples aged for 10 minutes and 30 minutes.The Ti₃Ni₄phase is partially re-dissolved for the sample aged for 60 minutes,but the size decreases.After annealing at 450℃for 10 minutes,the Ti₃Ni₄ phase presented again for the sample processed by 1 pass.The size of Ti₃Ni₄ phase increases at first and then decreases due to the difference of dislocation density in different samples.After 4 passes,Ti₃Ni₄ phase is partially re-dissolved.The re-dissolution of Ti₃Ni₄ phase may promote grain refinement.When the pass number is more than 4,the equiaxed grains are formed,and the grain size is reduced to 100～200nm.During cooling,the aged alloys show three-step phase transformation of B2→R→B19’and B2→B19’due to the inhomogeneous microstructure.After ECAP,all the samples show two-step phase transformation of B2→R→B19’during the cooling process.With the increase of passes,the martensitic transformation start temperature decreases at first and then increases,and reaches the lowest at 2 passes.After 1 pass,the critical stress to induce martensitic transformation and yield strength of the samples decreases.With the increase of passes,the critical stress to induce martensitic transformation decreases at first and then increases and decreases finally.While the yield strength,in contrast,reaches its maximum after 8 passes.After 20 cycles of deformation under constant stress of 600MPa,the slope of stress plateau increases,the critical stress decreases,and the residual strain increases and tends to be stable with the increase of cycles.The sample aged for 10 minutes and processed for 1 pass shows the best cyclic stability of superelasticity.