Reverse Deformation Plastic Deformation Mechanism And Microstructure Simulation Of NiTiFe Shape Memory Alloy

As the novel functional materials, shape memory alloys are widely used in the engineering fields, such as medicine, aviation, aerospace, military and civilian fields because of their excellent shape memory effect and superelasticity. Nickel-titanium shape memory alloy is hard to manufacture at room temperature since it possesses the high yield strength. Isothermal precision plastic forming theory is growing maturity such that it provides a new method of forming process. With the development of finite element simulation and the theory of cellular automaton, software of finite element could be used to analyzed the influence of technological parameter of plastic deformation by the results of simulation, and then the technological parameter could be optimized. Innovation of this topic is to combine the result of finite element numerical simulation with cellular automaton to analyze thermal mechanical coupling. Finally the metal flow, stress, strain distribution cloud in isothermal plastic forming and the evolution of microstructures in the recrystallization process are achieved.The experimental materials in the research are the rolled NiTiFe shape memory alloy,where its atomic ratio is Ni:Ti:Fe=47:50:3. The original microstructure is isometric crystal through observing. The temperature of the hot compression experiments is 700?, 800?,900?,1000?, 1100? and the strain rates are 0.01s^-1, 0.1s^-1, 1s^-1,10s^-1 with Gleeble1500 thermal simulation testing machine. Finally, the true stress-strain curve is obtained.NiTiFe alloy is sensitive to the temperature and strain rates which we can get from the true stress-strain curve. The enhancing temperature and the increasing strain rate are able to lead to the obvious change of the yield strength.Based on the Arrhenius equation, the constitutive equations for hot deformation are established in order to describe the flow mechanics behavior in the process of high temperature deformation. According to the DMM theory, the processing map is established on the basis of the flow stress at the different strain. The reasonable processing temperature range is determined by means of analyzing the processing map. The reasonable processing temperature is finally determined as the one between 700? and 950?.On the basis of establishing a three-dimensional model, the backward extrusion is simulated by virtue of DEFORM finite element numerical simulation program. By taking into account the influence of three variables ?temperature-700? 800 ?, 900?,frictional coefficients-0.3, 0.7, fillet radius during pressing-R=0.5, 2.5mm ? in the simulation,it is obviously found that we can get the flow characteristics of metal during the isothermal backward extrusion, change of extrusion strength, the distribution of strength and strain. Analyzing the simulation results of different parameters, make the reasonable parameters of process.Finite element numerical simulation and cellular automaton are coupled by means of compiling the cellular automaton programs which are based on the output of finite element numerical simulation through Matlab. By simulating microstructural evolution during isothermal backward extrusion, we can get the dynamic recrystallization process, the change of grain size and dislocation density evolution during isothermal precision plastic forming.