形状记忆合金强化析出相应力状态演变规律的原位同步辐射研究

It remains an important scientific problem how the stress field induced by precipitation in shape memory alloys evolves during phase transformations. Synchrotron X-ray diffraction technique, which has high spatial resolution and high penetration capability, can be used for in situ tracking the evolution of crystal structure and stress state under external fields simultaneously and promises to solve this basic scientific problem. Focusing on the evolution mechanism of the stress field induced by precipitation in NiTi-based shape memory alloys, on one hand this project aims to establish the new methodology and quantitative analysis mode of using synchrotron X-ray diffraction technique to in situ characterize the crystal structure and microstructure (crystallographic orientation distribution, stress state, etc.) of shape memory alloys during phase transformations. On the other hand, this project also aims to disclose the evolution of stress state of both matrix and precipitates during phase transformations (R phase transformation and martensitic transformation) and to elucidate the underlying micro-mechanisms that are responsible for the influence of precipitates on mechanical properties of low-temperature phases (R phase and martensite), phase transformation temperatures and variant selection and establish the corresponding physical models, in order to provide theoretical guidance for developing high-performance shape memory materials.

形状记忆合金强化析出相所致应力场在相变过程中的演变规律是一重要科学问题。同步辐射X射线衍射技术分辨率高、穿透能力强，能够原位追踪外场作用下晶体结构和应力状态的演化，有望解决这一基本科学问题。本项目拟以NiTi基形状记忆合金为研究对象，围绕强化析出相应力状态演变规律，建立同步辐射X射线衍射技术应用于形状记忆合金相变过程中晶体结构与微结构（晶体取向分布、应力状态等）原位表征的新方法与定量分析模式；揭示相变（R相相变、马氏体相变）过程中基体与析出相内部应力状态的演变规律，阐明析出相对低温相（R相、马氏体相）力学性能、相变温度、变体选择影响的内在微观机制并建立相应的物理模型，以期为开发高性能形状记忆材料提供理论指导。

形状记忆合金强化析出相所致应力场在相变过程中的演变规律是一重要科学问题。然而，由于强化析出相尺寸小、体积分数低且其大部分衍射峰与基体衍射峰相互重叠，普通衍射技术难以分辨出析出相的衍射信息以致此科学问题一直悬而未解。本项目以NiTi形状记忆合金为研究对象，利用同步辐射高能X射线衍射技术分辨率高、穿透能力强、能原位追踪外场作用下晶体结构和应力状态演化等优点，探索了同步辐射高能X射线衍射技术应用于形状记忆合金相变过程中晶体结构与微结构（晶体取向分布、应力状态等）原位表征的新方法与定量分析模式。揭示了温度诱发相变和应力诱发相变以及马氏体变体再取向过程中析出相和基体应力状态的演变规律，并阐明了析出相对力学行为的影响机制。发现无论在温度诱发相变、应力诱发相变还是马氏体变体再取向过程中，只要涉及到基体的切变过程，析出相的点阵应变就会突然增加（增加量高达0.5%，对应520MPa内应力的增加），这表明基体切变过程中发生了应力重新配分，大部分应力传递到析出相。在整个变形过程中，仅占体积分数很少的细小析出相承担了相当数量的载荷，说明析出相在析出强化形状记忆合金的变形过程中起了重要的作用。这些研究成果对深入理解共格析出相在析出强化形状记忆合金变形过程中的作用和设计高性能形状记忆合金具有重要的意义。上述研究成果已在Applied Physics Letters等SCI期刊发表论文8篇。