Atomic scale configuration of planar defects in the Nb-rich C14 Laves phase NbFe2
Atomic scale configuration of planar defects in the Nb-rich C14 Laves phase NbFe2
复制标题
DOI：
10.1016/j.actamat.2019.11.004
复制
发表时间：
2020-01-15
期刊：
影响因子：
9.4
通讯作者：
Stein, F.
中科院分区：
文献类型：
--
作者：
Slapakova, M.;Zendegani, A.;Stein, F.
LaVES相属于四面体密排金属间化合物相，其晶体结构可用离散的层状排列来描述。它们通常具有扩展的同质范围，通常的概念是，与化学计量学的偏差是由反位原子或空位来调节的。本工作表明，富Nb的NbFe2C14Laves相中多余的Nb原子也可以被引入各种类型的平面缺陷中。用像差校正的扫描电子显微镜和密度泛函理论计算表征了这些缺陷的原子构型，并建立了它们的稳定性判据。平面缺陷可分为延伸型缺陷和约束型缺陷。扩展的缺陷平行于周围C14Laves相的基面，并且是完全共格的。它们含有在邻近的Nb6Fe7Mu相中发现的特征的Zr4Al3型(0)单元。对化学键的分析表明，电荷转移的局部减少可能是这种原子排列优先的原因。然而，整体的层堆积偏离了完美相的堆积。从头计算确定了为什么这些特殊的层状缺陷可以比完美pi相的共格纳米沉淀更稳定的组态。通过锥面和基面的习惯面观察到受限缺陷。锥体缺陷只有1 nm厚，类似于完美的相。相反，受限的基底缺陷可以看作只有一个单一的O单元，似乎堆积顺序被打乱了。从头计算证实这种构型是亚稳态的。(C)2019 Acta Materialia Inc.由Elsevier Ltd.出版。保留所有权利。
Laves phases belong to the group of tetrahedrally close-packed intermetallic phases, and their crystal structure can be described by discrete layer arrangements. They often possess extended homogeneity ranges and the general notion is that deviations from stoichiometry are accommodated by anti-site atoms or vacancies. The present work shows that excess Nb atoms in a Nb-rich NbFe2 C14 Laves phase can also be incorporated in various types of planar defects. Aberration-corrected scanning transmission electron microscopy and density functional theory calculations are employed to characterize the atomic configuration of these defects and to establish stability criteria for them. The planar defects can be categorized as extended or confined ones. The extended defects lie parallel to the basal plane of the surrounding C14 Laves phase and are fully coherent. They contain the characteristic Zr4Al3-type (0) units found in the neighboring Nb6Fe7 mu phase. An analysis of the chemical bonding reveals that the local reduction of the charge transfer is a possible reason for the preference of this atomic arrangement. However, the overall layer stacking deviates from that of the perfect phase. The ab initio calculations establish why these exceptionally layered defects can be more stable configurations than coherent nano-precipitates of the perfect pi phase. The confined defects are observed with pyramidal and basal habit planes. The pyramidal defect is only similar to 1 nm thick and resembles the perfect phase. In contrast, the confined basal defect can be regarded as only one single O unit and it appears as if the stacking sequence is disrupted. This configuration is confirmed by ab initio calculations to be metastable. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.