Studies On Superelastic-plastic Behavior And Grain Size Effects Of Porous Shape Memory Alloys

Porous shape memory alloys(SMAs)benefit from a combination of the smart responses of dense SMAs and the characteristics of porous materials.Hence,a broad set of advantages of porous SMAs are presented,such as shape memory effect,superelasticity,low density,biocompatibility and high permeability.Holding so many excellent features mentioned above,porous SMAs are successfully adopted in several advanced applications,such as artificial bones,energy absorbers,lightweight structures,electrodes and heat exchangers.In addition to reversible phase transformation,another important mechanical behavior of porous SMAs is plastic deformation.In general,there are two types of irrecoverable deformation,known as transformation-induced plastic deformation and conventional plastic deformation at high-stress levels.Moreover,grain size effects on the mechanical response of nanostructured Ni Ti SMAs has been experimentally investigated.Incorporation of porosity alters the microstructure evolution process of SMAs,it is necessary to investigate the effects of grain size and porosity on the superelastic behavior and superelasticity degradation of porous SMAs.Development of the constitutive models has been a key issue in the application of porous SMAs.In this dissertation,the superelastic-plastic behavior,transformation-induced plastic deformation and grain size effects of porous SMAs has been studied.The main research contents are listed as follows:(1)Superelastic constitutive model.The porous SMA is treated as a two phase composite with the SMA matrix and the second phase representing voids.Based on the Gurson-Tvergaard-Needleman(GTN)model,a constitutive relationship incorporating the influences of hydrostatic stress and strain hardening is developed.Model validation was performed by reproducing the key features of porous SMAs such as superelasticity,internal loops and tensile-compressive asymmetry under uniaxial and combined loading.(2)Superelastic-plastic response at high-stress levels and cyclic superelastic deformation of porous SMAs.Firstly,the transformation function and plastic yield function considering the transformation-plasticity coupling and void shape effects are developed.The plastic back stress tensor accounting for forces generated due to incompatibility between yielded regions is introduced.Numerical results prove that the mechanical response of porous SMAs is affected by porosity and void shape,and the void shape effect is more pronounced for higher levels of porosity.The evolution of reverse transformation including critical stress is found to be degraded by plastic deformation.Secondly,the extended Mori-Tanaka homogenization method is adopted to develop a micromechanics-based model describe the transformation-induced plasticity.In the micromechanics-based model,the inelastic strain is decomposed into two parts:martensitic transformation and transformation-induced plasticity.The predictions are compared with experimental data to demonstrate the model's ability to reproduce the superelasticity degradation of porous shape memory alloys during cyclic loading.(3)Grain size effects of nanostructured Ni Ti SMAs.Based on the Gibbs free energy including the spatial gradient of the martensite volume fraction and the hardening modulus that depends on the grain size,a new transformation function determining the evolution law for transformation strain is derived.Stress concentration factor is adopted to derive the overall tangent stiffness of the composite which consists of the grain-core phase(inclusion)and the grain-boundary phase(matrix).Based on the experimentally observed tension-compression asymmetry of nanocrystalline Ni Ti SMA,a new constitutive model is developed to describe the asymmetric behavior.(4)Grain size effects of porous SMAs.On the basis of the constitutive model of nanocrystalline Ni Ti SMA,the voids of porous SMAs are regarded as one phase of the three-phase(grain-core phase,grain-boundary phase,void phase)composite material.For porous SMAs with a given grain size,it can be observed from the numerical results that the critical transformation stress decreases and the stress-strain hysteresis loop narrows with the increase of porosity.For a given porosity,porous SMAs exhibit strain hardening behavior under different grain sizes,and the critical transformation stress increases as the grain size decreases.To describe the superelasticity degradation of porous nanocrystalline SMA under cyclic loading,two mechanisms of inelastic deformation,martensitic transformation and transformation-induced plasticity are considered.Based on the secant-modulus method,the macroscopic secant bulk and shear modulus of porous nanocrystalline Ni Ti SMA during cyclic deformation are calculated.The numerical results display that the the superelasticity degradation has a strong dependence on the grain size of porous nanocrystalline Ni Ti SMA.