Study On Hot Deformation Behavior And Deformation Mechanism Of NiTi Shape Memory Alloys

NiTi shape memory alloy is widely used in biomedical fields,bridge building fields and aerospace fields because of its shape memory effect,super elasticity and high damping.In this study,NiTi shape memory alloy were taken from the nozzles of XX fighter jet engine.And its hot deformation behavior and deformation mechanism were systematically studied to provide theoretical support for the improvement of hot forming technology of core components of China's autonomous aviation weapons and equipment.NiTi shape memory alloy was carried out on Gleeble-3500 thermal simulation testing machine at temperatures and strain rates ranges of 750-1050?and 0.005-10s^-1.And degrees of deformation were 0.45 and 0.7.The stress-strain curves of the alloy were analyzed.And the constitutive equation of the alloy was constructed.Then the strain rate sensitivity indexes m and the hot deformation activation energy Q were calculated.The microstructures of the alloy under different deformation conditions were observed and characterized,and the hot processing maps of the alloy under Prasad flow instability conditions were established.The hot processing parameters of the alloy were predicted and optimized.The main conclusions are as follows:?1?The flow stress of NiTi shape memory alloy decreases with the increase of temperatures or the decrease of strain rates.?2?The Arrhenius hot deformation constitutive equation of the alloy was established.The constitutive model could well predict the hot flow stress of NiTi shape memory alloy under different deformation conditions.?3?Plastic flow bands and macroscopic cracks appear when the strain rate is high.The dynamic recrystallization occurs when the deformation temperature is high and the strain rate is low,and the microstructures tend to be equiaxial crystal grains that play positive roles in improving the properties of the alloy.The degree of dynamic recrystallization increases with the increase of deformation.?4?The dynamic material model?DMM?theory and Prasad instability criteria were used to draw the hot processing maps with the strain of 0.1,0.2 and 0.3 respectively.The flow instability regions of the alloy are mainly concentrated in the low temperature regions and the high strain rate regions(Area 1:750-850?,0.02-10s^-1,Area 2:850-1050?,1-10s^-1).The steady-state deformation regions of the alloy are mainly concentrated in the middle and high temperature regions and the middle and low strain rate regions(875-1050?,0.005-1s^-1).By selecting the characteristic points to observe the microstructure characteristics of the alloy,it is shown that the hot processing maps can predict the hot deformation behavior of the alloy well.