Study On Plastic Deformation Mechanism Of NiTiCu Shape Memory Alloy Under Plane Strain Compression

NiTi-based shape memory alloy(SMA)is a promising candidate which can be widely employed in biomedical and aeronautical fields on account of its excellent shape memory effect,superelasticity,corrosion resistance,biocompatibility as well as mechanical properties.Plastic processing is an important step to facilitate the engineering applications of NiTi-based shape memory alloys owing to the fact that several important NiTi-based SMA products are manufactured from wire,bar,strip,sheet and tube by virtue of plastic processing and some important NiTi-based SMA devices are obtained by means of plastic processing as well.Most products made from NiTi-based SMA should be are manufactured at high temperature due to its significant work hardening ability at room temperature.Therefore,it is of great scientific significance to explore the hot plastic deformation behavior of NiTi-based SMA and reveal the hot plastic deformation mechanism of NiTi-based SMA.In the present study,hot-rolled NiTiCu SMA bar acted as the research object,hot plastic deformation mechanism and plane strain plastic deformation law of NiTiCu shape memory alloy was thoroughly studied based on a variety of experimental methods and finite element simulation.The specific research work and results are as follows.The phase transformation behavior and the microstructure of original hot-rolled NiTiCu SMA were analyzed based on Differential Scanning Calorimetry(DSC)test,Transmission Electron Microscope(TEM)experiment,Optical Microscopy(OM)experiment and X-ray Diffraction(XRD)test.It turned out that the phase transformation temperatures of the NiTiCu alloy were determined as Ms=53.8?,Mf=8.3?,As=73.1?,and Af-113.5?.Morever,the NiTiCu SMA is mainly composed of B19' martensite phase with acicular morphology and a little Ti2Ni phase at room temperature.The true stress-strain curves of NiTiCu SMA subjected to plastic deformation at high temperature were obtained on the basis of the hot compression experiments,which were conducted at the temperatures ranging from 600 to 1000? and at the strain rates ranging from 0.0005 to 0.5s-1.Finally,it can be found that the true stress-strain curves of NiTiCu SMA are sensitive to the strain rates at high temperature,where the flow stress increases with the increase in the strain rate.Arrhenius-type constitutive equation of NiTiCu SMA at high temperature was constructed according to these true stress-strain curves.In order to describe the hot workability and optimize the parameters of thermal processing of NiTiCu SMA,hot processing map was developed based on dynamic material model according to the flow stress of NiTiCu SMA subjected to plastic deformation at high temperature.The hot processing map shows that the plastic flow instability zone mainly occurs in the high strain rates region,and the instability area expands with the increase of strain value.Consequently,it would be optimal to process NiTiCu SMA at temperatures ranging from 750 to 850? and at the low strain rates according to the hot processing map and the results of microstructure observation.Based on the Arrhenius-type constitutive equation of NiTiCu SMA at high temperature,rigid-viscoplastic finite element method(FEM)was employed with the help of DEFORM software in order to simulate the plane strain compression deformation of NiTiCu SMA on the basis of various deformation temperatures(700?,800?,900 ?)and various deformation degrees(20%,40%,60%).The metal flow discipline of plane strain compression deformation of NiTiCu SMA was revealed.The simulation results show that in the process of plane strain compression deformation of NiTiCu SMA,the intermediate main deformation zone is in the state of three-direction compressive stress,and the stress state gradually changes with the intermediate main deformation zone towards two free ends.That is to say,it remains the state of compressive stress along the loading direction and constrained direction,but it turns into tensile stress gradually along the direction of the free end.In the entire deformation zone of the specimen,the strain state along the loading direction remains to be compressive strain,but it keeps elongation strain along the free end direction.