Deformation Behavior And Microstructure Evolution Of TiNiFe Shape Memory Alloy

TiNiFe shape memory alloy which has a low Martensite transformation temperature is chosen as the experimental material in this thesis. The characteristics of hot deformation behavior of TiNiFe shape memory alloy have been studied by isothermal compression tests performed on a Gleeble-3500thermal simulation machine at deformation temperature ranging from450℃to1050℃and strain rate of0.01s-1to10s-1with maximum strain of0.8. On the basis of data obtained from isothermal compression tests, processing maps are drawn by using3-spline interpolation fitting method with the help of data processing software Origin. The characteristics of flow stress in the process of high temperature plastic deformation are studied via linear regression analysis. The constitutive model of TiNiFe alloy is established. Under hydraulic press, TiNiFe alloy is compressed on an oil press with different deformation mounts of0.2to0.4, and then kept at temperatures ranging from450to750℃for1h respectively. By means of metallographic microscope, vickers hardness tester, electron backscatter diffraction, transmission electron microscope as well as other test methods, in this alloy on the microstructure evolution under different deformation conditions of TiNiFe alloy is researched and analyzed, especially high temperature deformation behaviors of TiNiFe alloy. The main conclusions are as follows:(1) The relationships between flow stress, deformation temperature and strain rate can be concluded through the research on hot compression deformation behavior of TiNiFe shape memory alloy, that is to say, flow stress increases with a decrease in deformation temperature or increasing strain rate.(2) Based on dynamic material model, the processing maps of TiNiFe alloy with different strain are obtained, according to the hot compress test data, by using the Origin software. The optimal processing zones as well as instability areas are determined through analysis.(3) Microstructural evolution of the two ideal processing zones under different thermal deformation conditions are concluded as follows:A complete dynamic recrystallization with fine equiaxial grains occurs at1050℃-10s-1; there exists dynamic recrystallization as well as dynamic recovery under the condition of850℃-0.1s-1.(4) The high temperature plastic deformation process of TiNiFe shape memory alloy is a hot activation process. It can be found by comparing that the relationship between flow stress, strain rate and deformation temperature during high temperature plastic deformation, meets the power exponent function. After figuring out deformation activation energy respectively Q, strain rate sensitive index n, material constants alpha and A respectively, the constitutive model of thermal deformation is established. By comparison, calculated values agree well with measured ones, which means that the model calculated above meets the engineering calculation requirement.(5) Microstructure evolution of TiNiFe shape memory alloy deformed at room temperature after different annealing temperatures is studied. It concluded that gains of TiNiFe shape memory alloy deformed at a certain cold deformation amount are prone to grow abnormally when the annealing temperature is low.