纳米晶镍钛形状记忆合金晶粒尺寸效应及其疲劳寿命的研究

NiTi shape memory alloy (SMA) are increasingly used in many fields due to its superelasticity and shape memory effects. Long fatigue life is the key requirement of many SMA devices experiencing cyclic phase transition (PT) under periodic thermomechanical loadings. Unfortunately, the fatigue performance of existing NiTi polycrystalline SMAs with typical grain size ≥ 100 nm has so far proved to be disappointing due to material’s instability, which is caused by the generation of dislocation, the damage accumulation during cyclic PT and the thermomechanical coupling effect. To enhance the fatigue resistances of the material has been an important task for material scientists and design engineers. Grain size (GS) is one of the key microstructural factors of NiTi SMA and controls many physical and mechanical properties of the material. The current fabrication techniques have been able to reduce the GS of nanocrystalline (nc) NiTi down to 5-10 nm. Recent research further shows that grain size reduction can decrease the latent heat, hysteresis heat and temperature dependence of stress, so that thermomechanical coupling effect was suppressed. As a results, nc NiTi (GS<100nm) can significantly suppress the crack nucleation and enhance the fatigue life. In this project, the applicant aim to perform a systematic experimental study, at both macro- and micro-levels, of the GS effects on the fatigue response and the fracture mechanism of NiTi SMA. Superelastic NiTi with average GSs from 5 nm to 100nm will be fabricated and characterized. Fatigue experiments of dog-bone specimens under cyclic tension at different levels of stress, strain, temperature and frequency will be performed to obtain systematic data. We will examine the fatigue behavior of the material, and explore different physical mechanisms underlying the observed correlations between GS and fatigue life at macro-, micro- and nano-scale, and establish quantitative relationships among GS, hysteresis and fatigue resistance of the material. The project is motivated by both the scientific importance of the material internal length scale and the practical need to design high fatigue resistance SMAs by grain size engineering. The project would setup foundation to the future high stability, high fatigue resistance nanocrystalline NiTi SMA.

镍钛形状记忆合金由于其独特的超弹性及形状记忆效应，在医疗、航空航天、国防等领域有着广泛的应用。但该材料在循环加载过程中表现出强烈的热力耦合以及温度和应变场的时空震荡等行为，导致其循环稳定性和抗疲劳性较差，严重地制约其广泛的工程应用。本项目将研制新型纳米晶镍钛形状记忆合金，以达到增强其循环稳定性和疲劳寿命的目的。为此，申请者将采用宏微观实验方法，系统研究：（1）纳米晶镍钛形状记忆合金的制备工艺；（2）晶粒尺寸对材料热力耦合效应的影响；（3）晶粒尺寸对材料疲劳失效行为的影响。并通过分析和量化疲劳过程的晶粒尺寸效应，揭示材料内部尺度对相变力学宏观性能影响的规律，创建新的疲劳寿命预测准则，同时挖掘不同晶粒尺寸镍钛形状记忆合金的疲劳破坏机理。研究结果将为开发新一代高稳定性，高抗疲劳性的纳米晶镍钛形状记忆合金奠定基础。

镍钛形状记忆合金由于其独特的超弹性及形状记忆效应，在医疗、航空航天、国防等领域有着广泛的应用。但该材料在循环加载过程中表现出强烈的热力耦合以及温度和应变场的时空震荡等行为，导致其循环稳定性和抗疲劳性较差，严重地制约其广泛的工程应用。本项目通过宏微观实验方法研制了新型纳米晶镍钛形状记忆合金及其循环稳定性和疲劳寿命，系统研究了：（1）纳米晶镍钛形状记忆合金的制备工艺；（2）晶粒尺寸对材料热力耦合效应的影响；（3）晶粒尺寸对材料疲劳失效行为的影响。本项目通过冷轧和热处理方法成功制备了晶粒尺寸为10nm至235nm的纳米晶镍钛形状记忆合金，并通过表征证实了晶粒尺寸与热处理工艺之间的关系。在成功制备样品后，项目组通过原位同步实验系统的观测了不同晶粒尺寸镍钛形状记忆合金的应力应变曲线、应变场和温度场的时空演化。通过对实验结果的分析，量化了疲劳过程的晶粒尺寸效应，揭示了材料内部尺度对相变力学宏观性能影响的规律。同时，项目通过声发射等手段创建了新的疲劳寿命预测准则，并通过观察材料的微结构演化揭示了不同晶粒尺寸镍钛形状记忆合金的疲劳破坏机理。除以上结果外，本项目还通过实验验证了镍钛形状记忆合金在压缩疲劳下高达7千万周的超高周疲劳寿命，通过对破坏样品的分析，结果显示了在循环压缩状态下形状记忆合金收到了极低的应力强度因子幅度，导致了极低的裂纹扩展速率，大大增加了压缩疲劳寿命，为形状以及合金的长时间服役应用提供了切实的实验依据。