The stress-induced martensite transformations (SIMTs) dramatically affect the recoverable strain and mechanical response of polycrystalline NiTi. An in-depth understanding of the propagation manner and orientation preference of SIMTs is therefore crucial. In this work, we present a unique asymmetric anisotropy of SIMTs and lattice strains induced by Lüders-type deformation in polycrystalline NiTi, achieved through a combination of in-situ synchrotron X-ray diffraction and uniaxial tensile loading and unloading experiments. Our experimental findings reveal that in polycrystalline NiTi under uniaxial deformation, the asymmetry of SIMTs is attributed to the inhomogeneous strain field caused by the Lüders-type mechanism. The asymmetrical SIMT starts with the forward Lüder band and disappears along with the backward Lüder band. The austenite with the favored orientation of 〈110〉//loading direction (LD) transformed and back at a higher rate compared to other orientations during both loading and unloading.

中文翻译：

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通过基于原位同步加速器的高能 X 射线衍射了解多晶 NiTi SMA 中的不对称取向和应力状态

应力诱导马氏体转变 (SIMT) 极大地影响多晶 NiTi 的可恢复应变和机械响应。因此，深入了解 SIMT 的传播方式和方向偏好至关重要。在这项工作中，我们通过结合原位同步加速器 X 射线衍射和单轴拉伸加载和卸载实验，提出了 SIMT 的独特不对称各向异性和由多晶 NiTi 中 Lüders 型变形引起的晶格应变。我们的实验结果表明，在单轴变形下的多晶 NiTi 中，SIMT 的不对称性归因于 Lüders 型机制引起的不均匀应变场。非对称SIMT从前向Lüder带开始，并随着后向Lüder带消失。在加载和卸载期间，与其他取向相比，具有<110>//加载方向（LD）的有利取向的奥氏体以更高的速率转变和返回。