Abstract Microstructural elements in NiTi shape memory alloys (SMAs) – precipitates, phase boundaries, inclusions, grain boundaries – can be viewed as sources of multiscale constraint that influence their deformation response. We characterized in situ, and in 3D, the deformation and the evolution of microstructure during a tension test in a superelastic NiTi specimen containing some of these sources of constraint. The method used was far-field high-energy X-ray diffraction microscopy (ff-HEDM), complemented by electron microscopy. We simulated the local stress state in the specimen using a microstructural model informed by the experimental data. Using these combined microstructure, deformation, and stress data, we report three phenomena, and relate them to specific sources of constraint. During initial elastic loading, axial lattice strain in austenite increased monotonically. On partial stress-induced phase transformation to martensite, the stress redistributed to both phases leading to a stress relaxation in austenite. The specimen contained a dense distribution of inclusions, which led to the activation of martensite habit plane variants that produce less than theoretical maximum transformation strain. Large Ni4Ti3 precipitates potentially contributed to the poor transformation response. Under load, proportional gradients in local rotation and elastic stretch developed in the martensite phase, because of the constraint at phase interfaces. This combined ff-HEDM, electron microscopy, microstructural simulation toolbox provides a versatile method to understand the effect of constraint on inelastic deformation in other alloys with hierarchical microstructure.

中文翻译：

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多尺度约束下超弹性 NiTi 形状记忆合金单晶变形力学的原位 3D 表征

摘要 NiTi 形状记忆合金 (SMA) 中的显微结构元素——析出物、相界、夹杂物、晶界——可以被视为影响其变形响应的多尺度约束的来源。我们在原位和 3D 中表征了超弹性 NiTi 试样拉伸试验期间微观结构的变形和演变，其中包含其中一些约束源。使用的方法是远场高能 X 射线衍射显微镜 (ff-HEDM)，辅以电子显微镜。我们使用由实验数据提供的微观结构模型来模拟试样中的局部应力状态。使用这些组合的微观结构、变形和应力数据，我们报告了三种现象，并将它们与特定的约束源联系起来。在初始弹性载荷期间，奥氏体中的轴向晶格应变单调增加。在部分应力诱导相转变为马氏体时，应力重新分配到两相，导致奥氏体中的应力松弛。该样品包含密集分布的夹杂物，这导致马氏体习性平面变体的激活，产生小于理论最大转变应变。大的 Ni4Ti3 沉淀物可能导致较差的转变响应。在负载下，由于相界面的约束，局部旋转和弹性拉伸的比例梯度在马氏体相中发展。这种结合 ff-HEDM、电子显微镜、微观结构模拟工具箱提供了一种通用方法来了解约束对具有分层微观结构的其他合金的非弹性变形的影响。