NiTi合金超弹性和弹热冷却效应的微观组织演变

Microstructure Evolution of Superelasticity and Elastocaloric Cooling Effect in NiTi Alloy

摘要:

通过对具有不同马氏体含量、位错密度和晶粒尺寸的3种NiTi合金进行绝热循环加载试验，研究了不同微观组织对NiTi合金变形过程中呈现出的超弹性、变形模式和弹热冷却效应的内在影响机制。结果表明，大量位错和马氏体的存在以及小的晶粒尺寸可以降低NiTi合金超弹性功能退化的程度和局部不均匀变形的可能性，但弹热冷却能力较弱，并且应变值越小超弹性越好（最小ε_residual=0.23%），弹热冷却能力越差（最大?T_cooling=0.63 K）。完全消除位错和马氏体以及增大晶粒尺寸可以获得较大的弹热冷却能力（?T_cooling=25 K），但其功能退化现象严重（由25 K降至9.6 K，下降61.6%）。通过400 ℃/15 min退火工艺调控位错和马氏体含量以及晶粒尺寸，既可以获得良好的超弹性和均匀的变形能力，又可以得到可观的弹热冷却能力（?T_cooling=7.2 K），同时可改善功能退化程度。

Abstract:

By conducting adiabatic cyclic loading tests on three types of NiTi alloys with different martensite contents, dislocation densities, and grain sizes, the intrinsic influence mechanisms of different microstructures on the superelasticity, deformation modes, and elastocaloric cooling effect during the deformation process of NiTi alloys were investigated. The results show that the presence of a high dislocation density, high martensite content, and small grain size can reduce the degree of superelastic functional degradation and the possibility of local uneven deformation in NiTi alloys. However, the elastocaloric cooling ability is weak. A smaller strain value results in superior superelasticity (minimum ε_residual=0.23%), but inferior elastocaloric cooling ability (maximum ?T_cooling=0.63 K). Completely eliminating dislocations and martensite, as well as increasing grain size, can achieve a significant elastocaloric cooling capacity (?T_cooling=25 K), but induces severe functional degradation (a drop from 25 K to 9.6 K, a decrease of 61.6%). Annealing at 400 ℃ for 15 min to tailor the dislocation density, martensite content and grain size results in good superelasticity, uniform deformation ability and a considerable elastocaloric cooling ability (?T_cooling=7.2 K), along with improved resistance to functional degradation.