Stress-induced Martensitic Transformation The Experimental Observations, Theoretical Calculations And Numerical Simulation Of Microstructure Evolution

Shape memory alloy(SMA) is an important active/functional material and has founded many applications due to its shape memory effect(SME) and superelasticity (SE).With the fast development of sputtering and micro manufacturing technology, SMA is also considered as an ideal candidate for actuating and controlling component in MEMS.In order to improve the performance of the material,and to optimally design the future SMAs-based micro-scale devices or medical devices,it requires a further study on microstructures' evolution of SMAs and their relationship with the macro mechanical behavior.In this work,the characteristics of the stress-induced transformation and its microstructure evolution in Cu-based SMA were studied.Due to the specialty of the testing material,a self-decigned testing system was used in order to measure the macro-mechanical behavior and the microstructures' evolution simultaneously.An image analysis technique is proposed to quantitatively analyze the microstructures' evolution along the length direction of the whole sample.Certain mesoscopical averages were then constructed,such as interface number,phase fraction and the distribution of width of martensitie bands,in order to analyze the characteristics of the stress-induced martensitie transformation.It was found that MT had three periods:initial formation accompanied by a stress drop, mixed formation and growth while the stress changed little,merging due to growth. Moreover,the transformation could be of multi-stage:sharp stress drops at several strains and correspondingly,the initial formation and growth process occurred quasi independently in several parts(two or three) of the sample.It was also clear from the band width distribution that the martensite bands are far from uniform during MT.In order to simulate and interpret the experimental phenomena,a one-dimensional dynamic model was constructed and both the isothermal and non-isothermal cases were considered.It was indicated that each part of energies would change a lot at the initiation of the transformation,and the stress drop would happen.The multi-interfaces microstructures of certain thickness could be also found.Moreover, the effects of the parameters of the model were studied.The homotopy analysis method(HAM) was used to find accurate series solution of the nonlinear Euler-Lagrange equation which used for describing the transformation. It seemed to be quite powerful to solve the nonlinear equations even with large number of interfaces.Based on the symmetry of the solutions,it was possible to find all the solutions which satifying the solution expression of that equation. Multi-interface microstructures and stress drop were obtained in the cases as the minimizes of the total energy functional.These results agreed the results of experiments and numerical simulation.