Microstructure And Properties Of TiNi Shape Memory Alloys Subjected To Equal Channel Angular Pressing

The microstructure, martensitic transformation behaviors, shape memory effect,mechanical properties and superelasticity behaviors of ultrafine grained TiNi shape memoryalloys (Ti_49.2Ni_50.8, at.%and Ti_50.2Ni_49.8, at.%) subjected to equal channel angular pressing（ECAP） were systematically investigated using optical microscope, scanning electronmicroscope （SEM）, X-ray diffraction （XRD）, transmission electron microscope （TEM）,differential scanning calorimetry （DSC）, bending and tensile tests. The emphasis was laid onthe influence of chemical composition and annealing treatment on microstructure, martensitictransformation, shape memory and superelasticity behaviors. The relationship betweencomposition, processing, microstructure and properties has been revealed.The results show that the microstructure of Ti_49.2Ni_50.8alloy mainly consists of parentphase with B2structure at room temperature, while Ti_50.2Ni_49.8alloy mainly consists ofmartensite phase with B19′structure at room temperature. Both microstructures of the twoalloys contain a little amount of Ti2Ni phase. TEM observation reveals that ECAP treatedTi_49.2Ni_50.8alloy has a grain size of290nm. The grain size almost keeps constant when theannealing temperature is lower than500℃, however, it increases sharply when the annealingtemperature is higher than500℃. On the other hand, the grain size for ECAP treatedTi_50.2Ni_49.8alloy is880nm. The grain size almost keeps constant when the annealingtemperature is lower than600℃, and it starts to increase when the annealing temperature ishigher than600℃.Compared with its coarse grain counterpart, ECAP treated Ti_49.2Ni_50.8alloy shows asharp decrease in martenstic transformation temperature, an obvious increase in thermalhysteresis for martensitic transformation, significantly enhanced thermal stability ofmartensitic transformation and a transformation sequence of B2→R→B19′during cooling.The yielding strength of ECAP treated Ti_49.2Ni_50.8alloy is1047MPa, which is almost twicegreater than its coarse grain counterpart. The stress value difference between yielding stressfor plastic deformation and critical stress to induce martensitic transformation is increased tothree times as compared to its coarse grain counterpart, resulting in suppression for themovement of dislocation. Thus one-way shape memory effect and superelasticity properties aswell as their stability for ECAP treated Ti_49.2Ni_50.8alloy are significantly improved. One-wayshape memory effect of ECAP treated Ti_49.2Ni_50.8alloy deteriorates gradually with increasing annealing temperature. Annealing at a temperature lower than600℃improves thesuperelastic properteis of ECAP treated Ti_49.2Ni_50.8alloy, while annealing at600℃decreasesits superelastic properteis. It is also found that two-way shape memory effect of Ti_49.2Ni_50.8alloy is obviously improved after annealing at600℃for ECAP treated Ti_49.2Ni_50.8alloy.After subjected to the same ECAP treatment with Ti_49.2Ni_50.8alloy, Ti_50.2Ni_49.8alloyshows a yielding strength of890MPa, which is increased by30%compared to its coarsegrain counterpart. There is a slightly decrease in martensitic transformation temperature andstill deteriorated martensitic transformation stability for ECAP treated Ti_50.2Ni_49.8alloycompared with its coarse grain counterpart. The reason for deteriorated martensitictransformation stability mainly lies in the larger grain size after ECAP treatment, resulting inhigher dislocation density during thermal cycling. Two stage phase transformation duringcooling is induced by thermal cycling for Ti_50.2Ni_49.8alloy subjected to ECAP treated andannealing at a temperature lower than600℃, with the first step being B2→R+B19′, and thesecond step being B2+R→B19′. Compared to its coarse grain counterpart, ECAP treatedTi_50.2Ni_49.8alloy shows an enhanced one-way shape memory effect, which almost keepsconstant if the annealing temperature is lower than600℃, and starts to decrease when theannealing temperature increases up to600℃.