Influence Of External Stress Applied During Two-step Aging On The Two-way Shape Memory Effect Of Ni₅₁Ti₄₉alloy And Its Underlying Micro-mechanism

To illustrate the underlying mechanism for tuning the two-way shape memory effect(TWSME)of Ni₅₁Ti₄₉ alloy by external stress applied during two-step aging,in this thesis work,a systemic study has been carried out to investigate the precipitation behavior of Ni₄Ti₃and its effect on martensitic transformation and consequently induced two-way shape memory effect of Ni₅₁Ti₄₉ alloy.The results of this study provide fundamental understanding in developing micro-mechanism of phase transformation and optimizing functional properties of NiTi alloys.In this thesis study,rapidly solidified Ni₅₁Ti₄₉ strips were trained by novel two-step aging to obtain two-way shape memory effect,in which external stress was applied in different stages.The mechanism of Ni₄Ti₃ precipitation under external stress applied during different stages of precipitate growth was revealed,by combing recovery ratios measurement,martensitic transformation analysis and microstructure characterization.The effect of important two-step aging parameters,including external stress,aging temperature and aging time,on martensitic transformation and TWSME of Ni₅₁Ti₄₉ alloy was also clarified.The results indicate that four variants of Ni₄Ti₃ grown equivalently in Ni₅₁Ti₄₉ alloy after stress-free aging can be varied during the subsequent constrained aging,yielding selective growth of single variant with strong orientation preference,which finally endues the alloy with TWSME.By elevating aging temperature and prolonging aging time in the first step stress-free aging to introduce larger Ni₄Ti₃precipitates,this variants variation can be weakened.Moreover,the two-step aged Ni₅₁Ti₄₉ alloy undergoing stress-free aging at 300? for 1 h followed by constrained aging at 450? at 10 h exhibits superior recovery ratios to those of conventional one-step aged alloy.In contrast,the preferential orientation of Ni₄Ti₃ precipitates originated from constrained aging at different temperature and time,which yields abnormal or normal TWSME of the Ni₅₁Ti₄₉ alloys,can retain after the second step stress-free aging at 450? for 10 h.Moreover,the degree of orientation preference of the precipitates shows no dependence on the conditions of the first step constrained aging.All the two-step aged alloys exhibited normal two-way shape memory effect with close recovery ratios.Besides,the two-step aging conditions of Ni₅₁Ti₄₉ alloy were optimized aiming at the applications above 0?.The result indicate that the martensitic transformation temperatures of the alloy are elevated significantly after stress-free aged at 400? for 1 or 10 h following constrained aging at 450? for 10 h,resulting in increased recovery ratios at 0? and 25? correspondingly.