Growth Behavior Of Ni₄Ti₃ And Consequent Control Mechanism To Two-way Shape Memory Effect In The Constrained Aged Ni₅₁Ti₄₉ Alloy

With a focus on the intrinsic mechanism of optimizing two-way shape memory effect?TWSME?of the Ni₅₁Ti₄₉ alloy by constrained aging,in this thesis work,a systematic study has been carried out to investigate the growth behavior of Ni₄Ti₃ precipitates,which dominate the martensitic transformation-induced TWSME,under different constrained aging conditions.The micro-mechanism how Ni₄Ti₃ precipitates influence and control the TWSME of the Ni₅₁Ti₄₉ alloy has also been revealed.This study aims to provide an important guidance for the modulation of the functional behavior based on the martensitic transformation of Ni–Ti shape memory alloys in the aspects of materials preparation and processing optimization.In this thesis study,Ni₅₁Ti₄₉ alloy strips prepared by rapid solidification technique were investigated.A hybrid macro/micro-scaled characterization combining macroscopic two-way shape memory recovery ratio analysis and microscopic TEM observation was performed to reveal the coherency evolution and growth behavior of Ni₄Ti₃ precipitates.Based on the growth mechanism of Ni₄Ti₃ precipitates,TWSME of the Ni₅₁Ti₄₉ alloy strip is optimized by using orthogonal experiments to obtain optimal parameters of two-step constrained aging process.In addition,the influences of aging parameters,including constrained aging time,strain and stress state on TWSME of the Ni₅₁Ti₄₉ alloy are demonstrated respectively.The results show that Ni₄Ti₃ precipitates in the B2 matrix begin to lose coherency after the alloy is subjected to constrained aging at 425°C for 5 h with an average length of40.6±11.7 nm,and completely lose coherency after constrained aging at 575°C for 5 h with an average length of 439.3±168.4 nm.The length of Ni₄Ti₃ precipitates on different stress sides of constrained aged Ni₅₁Ti₄₉ alloy strips exhibits the log-normal distribution and the average length of precipitates on the compressive stress side is slightly larger than that on the tensile stress side.By increasing the aging temperature,the average length of Ni₄Ti₃precipitates increases linearly,both of the average width and the interval between Ni₄Ti₃precipitates increase exponentially,the volume fraction of Ni₄Ti₃ precipitates gradually decreases,and the aspect ratio increases first and then decreases.The orthogonal experiment results demonstrate that different constrained aging parameters in the two-step aging treatment process show the distinct degrees of the influence on the recovery ratio,varying from high to low as follows:aging temperature of first-step,aging time of first-step,aging time of second-step and aging temperature of second-step.The optimal two-step constrained aging parameters are:450°C/1 h+250°C/40 h.Furthermore,with the variations of the constrained strain,stress state and duration of constrained aging,it is found that increasing the constrained strain leads to a significant reduction in the recovery ratio of Ni₅₁Ti₄₉ alloy strips.Prolonging the aging time does not show significant effect on the recovery ratio.In the two-step aging of stress-free aging followed by constrained aging,the longer the stress-free aging time and the higher the aging temperature,the smaller the recovery ratio.Moreover,reverse constrained aging of the constrained aged Ni₅₁Ti₄₉ alloy leads to significant decrease in the recovery ratio.