In superconducting Nb₃Sn layers with coherence lengths of approximately 3 nm, grain boundaries act as effective pinning sites. Thus, grain refinement is an essential issue that directly affects the superconducting critical characteristics of the Nb₃Sn layer. In recent years, Hf addition to Nb₃Sn wires co-doped with Ta has attracted notable interest as a method that enables grain refinement down to several tens of nm. In-depth characterization of the Nb₃Sn grain morphology in Hf-doping is crucially important to correlate the microstructure with the flux pinning characteristics. In this article, the grain morphologies of Ti–Hf and Ta–Hf-doped Nb₃Sn layers were clarified by scanning transmission electron microscopy (STEM) and TEM-based automated crystal orientation mapping (ACOM-TEM). STEM/energy dispersive x-ray spectroscopy (EDS) revealed no significant oxide precipitates in our samples. The grain size distribution was attained by ACOM-TEM. Although Hf-doping attained a grain refinement effect in the Nb₃Sn layer in both doping cases, the degree of this effect was relatively small for Ti–Hf. Kernel average misorientation analysis by scanning electron microscopy-electron backscattered diffraction unveiled no appreciable difference between the internal strain states of the Nb-alloy parent phases in Ti–Hf and Ta–Hf. One remarkable new finding through STEM/EDS was the presence of a Cu–Hf compound phase in the Nb₃Sn layer. The Cu–Hf compound sounds analogous to the Cu–Ti compounds that form when Nb–47Ti with Cu matrix is heat treated. The STEM/EDS maps revealed a larger amount of Cu flow from the Cu–Sn side along the grain boundaries. The large Cu deposition on the grain boundaries might facilitate grain growth in Nb₃Sn. Those findings make a novel contribution to the literature as they provide a deep insight into Nb₃Sn phase formation via Hf doping.

中文翻译：

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Ti-Hf 和 Ta-Hf 掺杂 Nb3Sn 层的深度 S/TEM 观察

在相干长度约为3 nm的超导Nb _{3 Sn层中，晶界充当有效的钉扎位点。因此，晶粒细化是直接影响Nb 3} Sn层超导临界特性的重要问题。近年来，将Hf添加到与Ta共掺杂的Nb ₃ Sn线中作为一种能够将晶粒细化至数十nm的方法引起了人们的广泛关注。_{Hf 掺杂中 Nb 3} Sn 晶粒形态的深入表征对于将微观结构与通量钉扎特性关联起来至关重要。在本文中，通过扫描透射电子显微镜（STEM）和基于TEM的自动晶体取向映射（ACOM-TEM）阐明了Ti-Hf和Ta-Hf掺杂Nb _{3 Sn层的晶粒形貌。}STEM/能量色散 X 射线光谱 (EDS) 显示我们的样品中没有明显的氧化物沉淀。通过 ACOM-TEM 获得晶粒尺寸分布。_{尽管在两种掺杂情况下，Hf 掺杂都在 Nb 3} Sn 层中实现了晶粒细化效果，但对于 Ti-Hf 来说，这种效果的程度相对较小。通过扫描电子显微镜-电子背散射衍射进行的核平均取向误差分析表明，Ti-Hf 和 Ta-Hf 中 Nb 合金母相的内应变状态之间没有明显差异。_{STEM/EDS 的一项显着新发现是 Nb 3} Sn 层中存在 Cu-Hf 化合物相。Cu-Hf 化合物听起来类似于 Nb-47Ti 与 Cu 基体经过热处理时形成的 Cu-Ti 化合物。STEM/EDS 图显示大量 Cu 从 Cu-Sn 侧沿晶界流动。晶界上大量的Cu沉积可能有利于Nb ₃ Sn中的晶粒生长。这些发现对文献做出了新颖的贡献，因为它们提供了对通过 Hf 掺杂形成 Nb ₃ Sn 相的深入见解。