NiTiHf高温形状记忆合金热-力耦合循环变形行为的宏微观实验及本构模型研究

NiTi-based high temperature shape memory alloys (HTSMAs) presents highly application value in aerospace and nuclear industries, its deformation mechanisms are more complicated than that of binary NiTi shape memory alloys (SMAs) since they involve martensite transformation and dislocation slip/climb simultaneously. Thus, the functional failure induced by the degradation of super-elasticity during cyclic loading is a key issue that in urgent need to be solved. In this proposal, the cyclic deformation of NiTiHf high temperature shape memory alloy is investigated. At first, detailed macroscopic experiments are conducted to thoroughly understand the thermo-mechanical cyclic deformation of HTSMAs. Meanwhile, the microscopic observations are also performed to investigate the evolution characteristics of microstructures, the configuration/density of dislocation, the morphology of martensite and their interactions during cyclic loadings are all considered, which are useful to reveal the micro-mechanisms of the cyclic deformation of NiTiHf HTSMAs. Moreover, with the macroscopic and microscopic observations, a new multi-mechanism thermo-mechanical coupled constitutive model is established to describe and predict the thermo-mechanical cyclic deformation of NiTiHf HTSMAs. The experimental observations and established constitutive model provide a solid foundation for the reliability analysis of NiTi-based HTSMA devices, and then prompt the application of the NiTi-based HTSMAs and the development of phase-transformation solid mechanics.

NiTi基高温形状记忆合金在航空及核工业等高温工作环境中具有较高的应用价值，其变形机制同时涉及马氏体相变、奥氏体位错滑移及位错攀移，较二元NiTi形状记忆合金更具复杂性。该类合金在实际服役中常受循环载荷作用，超弹性劣化导致的功能性失效是亟待解决的一个关键问题。本项目针对NiTiHf高温形状记忆合金的循环变形行为，首先进行系统的宏观实验，全面了解该材料的热-力耦合循环特性；进而开展深入的微观实验研究，观察循环加载中的微观结构演化特征，对其位错组态、密度、马氏体形貌及变形过程中的交互作用进行研究，揭示循环变形的微观机理；同时在上述宏观循环特性及微观机理分析的基础上，建立基于物理机理的宏观多机制热-力耦合循环本构模型，对该材料的循环变形行为进行模拟和预测。本项目将为后续NiTi基高温形状记忆合金器件的可靠性分析提供坚实的理论基础，促进该智能材料更为广泛的工程应用，同时促进相变固体力学学科的发展。

形状记忆合金的的力学行为随温度变化会产生极大差别，其主要来源于六种非弹性变形机制的影响。即：马氏体相变、相变诱发塑性、马氏体重定向、重定向诱发塑性、孪生、位错滑移等。上述机制及其交互作用使得该材料的循环变形和疲劳失效行为极具复杂性。本项目针对宽温范围内（常温至Af+120℃）形状记忆合金的热-力耦合循环变形和疲劳失效的宏微观机理展开了较为系统的实验和理论研究，主要工作及成果如下：(1) 对形状记忆合金常温至Af+120℃下每20℃为间隔进行了五种不同应变下的循环变形实验及微观形貌分析，系统揭示了该材料相变、塑性、孪生等非弹性变形机制的交互作用及其对循环变形行为的影响。(2) 对形状记忆合金在人体温度下(37℃)进行了五种不同非比例路径的多轴疲劳实验及断口分析，提出了马氏体相变及相变诱发塑性是引起形状记忆合金疲劳损伤的最重要因素，同时揭示了该材料的相变及重定向特性对全寿命棘轮演化及疲劳失效的作用。(3) 建立了形状记忆合金基于物理机理的疲劳寿命预测模型，该模型考虑材料的多种损伤演化机制，可将传统疲劳模型预测结果的20倍以上误差减小到3倍之内，同时对多种温度下的情况均适用。(4) 在单晶尺度上考虑三类不同的马氏体相，提出了马氏体多步相变准则，建立了能够合理预测不同温度下相变路径的单晶本构模型。上述研究成果不但可以进一步推动形状记忆合金在航空航天等服役温差较大的工程领域的应用，同时对其他具有复杂变形机制的金属材料的循环变形及疲劳失效的理论研究提供了新思路。