Abstract
Ni47Ti44Nb9 shape memory alloy(SMA)is a promising material in the aerospace field due to its wide transformation hysteresis.The application of shape memory effect depends on multistep thermomechan-ical loading,viz.,low-temperature deformation and subsequent heating to recovery.Low-temperature deformation prestrain plays a pivotal role in shape memory properties tailoring of SMA components.However,microstructure evolution and deformation mechanisms of Ni47Ti44Nb9 SMA subjected to vari-ous prestrain levels are still unclear.To this end,microstructure evolution and shape memory behaviors of Ni47Ti44Nb9 alloy subjected to multistep thermomechanical loading with prestrain levels of 8％-16％at-28 ℃(Ms+30 ℃)were investigated.The results demonstrate that the stress-strain curve of the specimen exhibits four distinct stages at a maximal prestrain of 16％.Whereas stage Ⅱ and stage Ⅲ end at prestrains of～8％and～12％,respectively.In stage Ⅱ,the stress-induced martensitic transformation is accompanied by the dislocation slip of the NiTi matrix and β-Nb inclusions.In stage Ⅲ,in addition to the higher density of dislocations and further growth of stress-induced martensite variants(SIMVs),(00 1)compound twins are introduced as a result of the(0 0 1)deformation twinning in stress-induced martensite.More{2 0-1｝ martensite twins are gradually introduced in stage Ⅳ.Correspondingly,after subsequent unloading and heating,a higher density of {1 1 4｝ austenite twins form in the specimen with a larger prestrain of 16％.With increasing prestrain from 8％to 16％,the recoverable strainεreT upon heating increases first and then decreases.The εreT obtains a maximum of 7.03％at 10％prestrain and de-creases to 6.17％at 16％prestrain.The increase of εreT can be attributed to the formation of new SIMVs,the further growth of existing SIMVs,and the recoverable(0 0 1)compound twins.While the decrease of εreT is mainly associated with the irrecoverable strain by ｛2 0-1｝ martensite twins.The effect of β-Nb inclusions on the evolution of SIMVs is also found herein that deformed β-Nb inclusions can significantly hinder the growth and recoverability of adjacent stress-induced martensite.
基金项目
National Natural Science Foundation of China(51775441)
National Science Fund for Excellent Young Scholars(51522509)
NETL
NSTL
万方数据