镍钛形状记忆合金形变/相变机理与应变速率关联性的中子衍射研究

Nickel-titanium shape memory alloys (known as nitinols) possess excellent ductility, high strength, good corrosion resistance and biocompatibility, which leads wide applications in crucial areas of military-civil technique. However, as applied to more complex scenes that the materials may suffer high speed deformation or shock loading, such as aeronautical actuators, protection structures in seism, and protective layers of satellites, the use and design of nitinols has to be prudent. Then, the study of deformation/transformation mechanisms of nitinols under high strain rate should guide those applications. Neutron diffraction techniques are powerful tools of that work. Neutron, acts as a bulk probe, could nondestructively penetrate several centimeters of material, and statistically bring out the microstructural information within the block. In this proposal, we focus on the change of transformation and deformation mechanisms in nitinols under different strain rates (mainly from 0.0001 1/s to 10000 1/s). By controlling alloy compositions and production processes, we will obtain nitinols with different crystal structures and initial textures. Then, combining with the strain-frozen technique in shock experiments, we will acquire a series of recovered samples that loaded at various strains and strain rates. By using the high-resolution neutron diffraction, neutron diffraction texture, and in-situ neutron diffraction measurements, we will analyze the effect of strain rate on dislocation density, preferred orientation, and phase volume fraction in nitinols after shock loading or low speed compression. A constitutive model of the nitinols will also be provided in terms of the additive decomposition of the rate of strains. Based on all the above results, the evolution behaviors and physical origins of the main deformation/transformation mechanisms in nitinols, such as slip, twinning, and martensitic transformation, will be systematically investigated as strain rate changes. Our research will helpfully guide the design and application of nitinol shape memory alloys in the future.

镍钛形状记忆合金兼具优异的力学性能和稳定性，已在军民两用关键技术领域得到广泛应用。而高速变形和冲击等新型复杂使役场景的出现，对镍钛合金的设计和使用提出了新的需求。因此，研究镍钛合金高应变速率下的变形机理具有重要的指导意义。中子衍射方法因其具有的无损和深穿透特性，可作为体探针统计性地获得材料变形后的微结构演化信息，进而成为理解镍钛合金高速变形机理的关键。本项目以研究不同应变率下镍钛合金的变形机制为目的，通过控制合金成分和工艺，获得不同相结构和初始织构的试样；结合冻结实验技术，得到一系列不同应变率和应变量的力学加载试样；利用高分辨中子衍射、中子衍射织构和应变测试，分析合金位错密度、晶体取向和相变体积分数随应变速率的变化。综合实验结果并辅以本构模型构建，系统研究应变率变化时镍钛合金的滑移、孪生和相变三类主要变形机制的演化规律及物理原因。本研究可为今后形状记忆合金的设计和应用提供实验和理论依据。

镍钛合金是目前使用最广泛的形状记忆材料。优异的力学性能及出色的生物相容性使镍钛合金具有广泛的应用前景。经过数十年发展，人们已经对镍钛合金独特的力学性质有了较为清晰的认识，即形状记忆效应和超弹性效应源于无扩散的马氏体相变。然而，这些认识是建立在准静态条件下的。在动态条件下，特别是面向高频、高速、冲击等应用场合的出现，镍钛合金的服役有效性值得关注。目前镍钛合金的应变率效应研究多集中在宏观性能上，对材料性能改变的微观机制、模型描述等工作较少。本项目则将常规力学测试与中子衍射技术和本构模型相结合，尝试分析镍钛合金不同应变速率下的微观形变行为，进而基于实验认识构造物理模型解释宏观现象，形成可扩展应用的研究方法。本项目以两种结构的镍钛合金为研究对象，利用霍普金森压杆获取材料的动态力学响应。通过原位的或动态软回收样的中子衍射分析，给出了材料形变的织构演化、物相结构、相变份额、位错密度、晶格应变等微观信息；通过对比变形过程的微结构差异，解释了镍钛合金力学性能受应变速率影响的原因；发展了适用于形状记忆合金体系的宏观唯象模型与微观自洽模型，两种模型均能很好地拟合实验数据，其中宏观模型的物理图像清晰，便于应用，微观模型能够量化微观力学行为，实现形状记忆合金的多尺度性能模拟。