Study On Hot Deformation Behavior And Processing Map Of As-Cast NiTi Alloy

NiTi shape memory alloy has excellent shape memory effect, mechanical and physical properties of super-elasticity, making it a promising alternative material for aerospace applications. In this paper, isothermal constant strain rate compression experiment of NiTi alloy has been carried out in the temperature range of650℃~1000℃ and strain rate range of 0.001s-1~10s-1to analyze flow behavior under different deformation conditions using Gleeble-3500 thermal simulation testing machine.Research on high temperature deformation behavior of the alloy are carried out by the processing maps technology and microstructure features of the alloy corresponding area of the processing maps are analyzed using an optical microscope, The studied results can be used as a theoretical basis for hot working optimization of NiTi alloy.According to stress-strain curve of NiTi alloy collected by friction correction, at the same strain rate, the flow stress decreases with the increase of deformation temperature significantly; At the same deformation temperature, flow stress increases obviously with the increase of strain rate, which indicate that the alloy has a positive strain rate sensitivity. The constitutive equation of NiTi alloy deformed at high temperature was constructed by using the Z parameters method including the Arrhenius term. The relative error was only 2.92% comparing the calculated stress values with the measured stress, indicating that the constitutive equation can better predict the flow behavior of Ni Ti alloy.The processing maps of NiTi alloy under different strain was established based on the Prasad criterion, and the microstructure and deformation mechanism of the different zones in the processing maps were analyzed by the microstructure observation, which verify the accuracy of the prediction results of the processing maps drawing method.According to the processing maps of Ni Ti alloy and microstructure observation, the unstable deformation zones are in the temperature ranges and the strain rate ranges of650℃~685℃ and 0.006s-1~0.1s-1, 685℃~930℃ and 0.1s-1~10s-1, 930℃~1000℃ and0.3s-1~10s-1, 650℃~860℃ and 0.001s-1~0.003s-1, 875℃~900℃ and 0.001s-1~0.002s-1,and instability in these regions mainly are flow localization and mechanical instability;The better thermomechanical parameters for hot deformation are in the temperature ranges and the strain rate ranges of 685℃~815℃ and 0.003s-1~0.1s-1, 815℃~950℃ and0.002s-1~0.1s-1, and the corresponding thermal deformation mechanism of these regions mainly are dynamic recovery.According to the results of the processing maps and microstructure observation,the optimum thermomechanical parameters for hot deformation are in the temperature ranges and the strain rate ranges of 750℃~800℃ and 0.01s-1~0.03s-1, 850℃~900℃ and0.01s-1~0.03s-1, and the corresponding thermal deformation mechanism of these regions mainly are dynamic recrystallization.