Phase Field Simulation On Thermo-mechanical Coupled Cyclic Transformation Of Super-elastic NiTi Shape Memory Alloy

Super-elastic NiTi shape memory alloy(SMA)has been utilized extensively in the automotive,aerospace,mechanical,and bio-medical fields due to its unique super-elasticity and shape memory effect,as well as good biological compatibility.In these applications,NiTi SMA devices are often subjected to cyclic loading involving a variation of loading rate.Therefore,it is very necessary to realize the cyclic deformation of super-elastic NiTi SMA with different frequencies in order to optimize the service reliability and improve the fatigue life of SMA smart devices.In recent years,extensive experimental studies have been carried out on the uniaxial tension-unloading cyclic deformation of super-elastic NiTi SMA with different frequencies.It is shown that the super-elastic NiTi SMA exhibits a super-elasticity degradation and thermo-mechanically coupled cyclic deformation.However,most of the existing studies only focus on the overall cyclic deformation of super-elastic NiTi SMA,but few on the cyclic evolution of local phase patterns.Therefore,it is necessary to carry out experimental study on uniaxial cyclic deformation of super-elastic NiTi SMA,in-situ record the evolution of local phase pattern and analyze the characteristics of cyclic deformation.Based on the phase field simulation method,crystal plasticity theory and thermodynamics framework,a phase field model is established to reveal the micro-mechanism of thermo-mechanically coupled cyclic deformation of super-elastic NiTi alloy.In this paper,the thermo-mechanically coupled cyclic deformation of super-elastic NiTi SMA is systematically studied as follows:(1)The evolutions of strain and temperature fields in the super-elastic NiTi SMA plate during uniaxial cyclic tension-unloading tests are recorded in situ by the digital image correlation(DIC)method and infrared thermal imaging one at 296 K.Based on the experimental data,the effects of loading level and loading rate on the cyclic deformation of super-elastic NiTi SMA are discussed.The cyclic deformation characteristics and the evolution law of local phase patterns in the super-elastic NiTi SMA are revealed.Then the in-situ experimental data for the cyclic deformation of NiTi alloy are enriched.(2)Based on the phase field simulation method,crystal plasticity theory and thermodynamics framework,a phase field model is established to describe the cyclic deformation of three-dimensional super-elastic NiTi SMA single crystal.The phase field simulation reveals that the micro-mechanism of super-elasticity degradation is the residual martensite accumulation and plastic deformation.Besides,the thermo-mechanically coupled deformation and rate-dependent effect of super-elastic NiTi SMA are analyzed and interpreted by considering the correspondent temperature variation.(3)Then the three-dimensional phase field model for NiTi SMA is degenerated to a two-dimensional version.Based on the finite element implementation of the proposed model,the rate-dependent thermo-mechanically cyclic deformation of super-elastic NiTi SMA polycrystalline plate under uniaxial cyclic tension-unloading is simulated.Furthermore,the mechanism of the effect of loading rate and loading level on the cyclic deformation of polycrystalline NiTi SMA plate is revealed.