Mechanisms by which grain size affects stress-induced martensitic transformation and superelastic stability in nanocrystalline NiTi alloys
晶粒尺寸效应对纳米晶镍钛合金应力诱发马氏体相变及超弹性稳定性的影响机制

摘要

镍钛合金因其优异的超弹性成为生物医学介入器械的核心材料，但循环服役条件下的性能衰减制约了其长期可靠性。本文系统研究了晶粒尺寸对Ni50.8Ti49.2合金应力诱发马氏体相变及超弹性循环稳定性的影响机制。通过大变形量冷轧结合不同温度退火，制备了涵盖纳米晶（~30 nm）至粗晶（>5μm）的梯度晶粒尺寸试样。采用DSC、EBSD、TEM及循环拉伸试验，系统表征了相变行为、微观组织及超弹性稳定性随晶粒尺寸的演化规律。结果表明，晶粒尺寸对超弹性稳定性具有显著的尺度依赖性：粗晶（>1μm）呈现典型应力诱发马氏体相变，但循环性能快速衰减；超细晶（200-500 nm）中相变与晶界协调变形共存，在~300 nm时获得最优循环稳定性，50次循环后仍保持80%以上可恢复应变；纳米晶（<100 nm）马氏体相变被显著抑制，超弹性特征消失。本文首次确定了维持良好超弹性的临界晶粒尺寸下限为80-100 nm，并提出200-400 nm为最优“窗口区间”。该研究为高性能长寿命镍钛合金器件的组织设计提供了理论依据。

Abstract

NiTi shape memory alloys have become core materials for biomedical interventional devices due to their excellent superelasticity. However, performance degradation under cyclic service conditions severely limits their long-term reliability. This paper systematically investigates the effect of grain size on stress-induced martensitic transformation and superelastic cyclic stability of Ni50.8Ti49.2 alloy. A series of specimens with gradient grain sizes ranging from nanocrystalline (~30 nm) to coarse-grained (>5 μm) were prepared through heavy cold rolling combined with annealing at different temperatures. The evolution of phase transformation behavior, microstructure, and superelastic stability with grain size was systematically characterized using DSC, EBSD, TEM, and cyclic tensile tests. The results demonstrate a significant size-dependent effect of grain size on superelastic stability. Coarse-grained specimens (>1 μm) exhibit typical stress-induced martensitic transformation but rapid degradation of cyclic performance. In ultrafine-grained specimens (200-500 nm), martensitic transformation coexists with grain boundary coordinated deformation, with optimal cyclic stability achieved at ~300 nm, maintaining over 80% recoverable strain after 50 cycles. In nanocrystalline specimens (<100 nm), martensitic transformation is significantly suppressed, and superelastic characteristics essentially disappear. This study quantitatively determines, for the first time, the critical lower limit of grain size for maintaining good superelasticity to be 80-100 nm, and proposes an optimal "window range" of 200-400 nm. This research provides theoretical guidance for microstructure design of high-performance and long-life NiTi alloy devices.