Functional Fatigue Behavior Of Current-driven Ni-Ti Shape Memory Alloy

Aiming at one of the crucial fundamental problems,such as functional fatigue,lying on the application of Ni–Ti shape memory alloys?SMAs?in current-driven smart devices,in this thesis work a systematic study has been carried out to investigate the influence of cyclic martensitic and reverse transformations induced by current-driving on the functional fatigue behavior of Ni-Ti two-way shape memory alloys?TWSMAs?and corresponding mechanism.In this way,a better understanding on the fundamental materials issue concerning martensitic transformation behavior and shape memory effect of Ni–Ti SMAs subjected to multi-driving force will be offered.In this thesis study,Ni₅₁Ti₄₉ alloy strips with different two-way shape memory effects?TWSMEs?were prepared via rapid solidification and constrained-aging techniques.A functional fatigue testing system is designed and optimized with controllable input of current and simultaneous acquisition of output signals including temperature and displacement.The macroscopic law of the functional fatigue behavior of Ni₅₁Ti₄₉ SMA driven by current was grasped.Quantitative correlation between the characteristic parameters of martensitic transformation and SME was revealed,and the microscopic mechanism of the instability of martensitic transformation and its impact on the SME were thus demonstrated.An effective evaluation approach for the degradation of SME in the Ni₅₁Ti₄₉ SMA driven by current was established.The results indicate that the Ni₅₁Ti₄₉ SMA after constrained-aging at 450?/10 h with TWSME and the Ni₅₁Ti₄₉ SMA after constrained-aging at 350?/1 h with abnormal TWSME present obvious deformation hysteresis of 7.0? and 5.0? under cyclic current loading,in lack of swift responding.However,the vertical displacement recovery ratios of them are only0.01%and 0.11%,respectively,indicating outstanding stability of TWSME induced by R-phase transformation.Further,both alloys show relevant instable TWSME related to B19',with a decrease of TWSME recovery ratio and an increase of abnormal TWSME recovery ratio.The characteristic temperatures of R-phase transformation remain unchanged in the Ni₅₁Ti₄₉ of TWSME while the ones of B19' martensitic transformation shift toward high temperature.Meanwhile,the characteristic temperatures of R-phase transformation in the Ni₅₁Ti₄₉ with abnormal TWSME shift toward low temperature.Electron microscopic analysis indicates that no obvious microstructure variation can be observed in these two alloys after cyclic current loading.The comparative studies of functional fatigue behavior induced by different loading modes of thermal cycling,constant heating and constant current show that current,phase transformation cycles and Joule heating have significant influence on the martensitic transformation and SME by varying the Ni concentration in the matrix of the alloy with abnormal TWSME and changing the stress field in the matrix of the alloy with normal TWSME.The influence resulted from above three factors?i.e.,current,phase transformation cycles and Joule heating?shows sequentially decrease trend with a competition between the current and other two.