Plastic deformation significantly alters the microstructure and properties of TiNi shape memory alloys. This work investigates the formation of a unique herringbone-like sub-grain structure with extended twin boundaries in the B2 phase of a cold-rolled TiNi50.8 (at. %) and its impact on the superelasticity of the alloy. Detailed electron back-scatter diffraction (EBSD) analysis reveals abundant sub-grain areas in different orientations distinct from the matrix grain after multiple passes in cold-rolling deformation. Notably, unique band and herringbone structures emerge within the sub-grain areas, with all the boundaries identified as coincident site lattice (CSL) boundaries. Further analysis reveals spontaneous CSL boundary generation following periodic <011>B2 and <001>B2 tilt axes perpendicular to the normal direction of the deformed surface in [100]B2 oriented grains, forming a characteristic quadruple junction within the herringbone structure, distinct from the single [011]B2 tilt axis identified in the band structure. Transmission electron microscopy (TEM) identifies B2 twin relations corresponding to the special CSL Σ boundaries, while dark-field and high-angle annular dark-field (HAADF) images show B19′ martensitic nanodomains decorating the twin boundaries. This extended twin boundary structure enhances drastically cyclically stable superelasticity characterized by much more limited functional fatigue, 27 % lower modulus and 76 % larger recoverable strain as compared to the solution-treated sample after 100 training cycles. Our study provides new insights into the microstructure evolution and enhancement of the superelastic properties of TiNi SMAs by cold-rolling, offering a novel approach to optimize the functionality of SMAs by introducing special sub-grain structures with martensitic-nanodomains-nested B2 twins through simple cold-rolling process.

中文翻译：

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具有独特性能的冷轧 TiNi 形状记忆合金 B2 中的新型亚晶粒结构


塑性变形显著改变了 TiNi 形状记忆合金的微观结构和性能。这项工作研究了冷轧 TiNi50.8 （at. %） 的 B2 相中具有扩展孪晶界的独特人字形亚晶粒结构的形成及其对合金超弹性的影响。详细的电子背散射衍射 （EBSD） 分析揭示了在冷轧变形中多次经过后，不同取向的大量亚晶粒区域与基体晶粒不同。值得注意的是，在亚晶粒区域内出现了独特的带状和人字形结构，所有边界都被标识为重合位点晶格 （CSL） 边界。进一步的分析揭示了在 [100]B2 取向晶粒中垂直于变形表面法线方向的周期性 <011>B2 和 <001>B2 倾斜轴之后，自发地产生 CSL 边界，在人字形结构内形成特征性的四重连接，与带状结构中确定的单个 [011]B2 倾斜轴不同。透射电子显微镜 （TEM） 识别对应于特殊 CSL Σ 晶界的 B2 孪晶关系，而暗场和高角度环形暗场 （HAADF） 图像显示装饰孪晶晶界的 B19′ 马氏体纳米畴。这种扩展的孪生边界结构大大增强了周期稳定的超弹性，其特点是经过 100 次训练循环后，与固溶处理的样品相比，功能疲劳更加有限，模量降低了 27%，可恢复应变增加了 76%。 我们的研究为冷轧 TiNi SMA 的微观结构演变和增强超弹性性能提供了新的见解，提供了一种通过简单的冷轧工艺引入具有马氏体-纳米域嵌套 B2 孪晶的特殊亚晶结构来优化 SMA 功能的新方法。