The structure of long period stacking/order Mg-Zn-RE phases with extended non-stoichiometry ranges
The structure of long period stacking/order Mg-Zn-RE phases with extended non-stoichiometry ranges
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DOI：
10.1016/j.actamat.2011.09.030
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发表时间：
2012-01-01
期刊：
影响因子：
9.4
通讯作者：
Abe, E.
中科院分区：
文献类型：
--
作者：
Egusa, D.;Abe, E.
基于 Z 对比扫描透射电子显微镜观察和第一原理计算，我们提出了 Mg-Zn-RE 合金中形成的独特长周期堆叠/有序 (LPSO) 相的结构模型。 LPSO结构是hcp Mg结构的长周期堆叠衍生物，并且Zn/RE分布被限制在形成局部fcc堆叠（即局部ABCA堆叠）的四个密堆积原子层处。 Mg(97)Zn(1)Er(2)（14H型）和Mg(85)Zn(6)Y(9)（18R型）合金中形成的LPSO相的化学顺序非常发达，具有明显的超晶格反射，并且相关的Zn/RE分布在Z对比原子图像中清晰地出现。最初的三元有序模型是通过将所有原子放置在理想的蜂窝位置来构建的，从而导致 14H 类型的 P6(3)/mcm 和 18R 类型的 C2/m、P3(1)12 或 P3(2)12 的合理空间群。局部 Zn(6)RE(8) 团簇很好地体现了特征有序特征，这些团簇按照 L1(2) 型短程顺序嵌入 fcc 堆叠层中。初始模型的能量有利于结构松弛导致 Zn/RE 位置显着位移，这意味着强 Zn-RE 相互作用可能在相稳定性中发挥关键作用。 LPSO相似乎可以容忍Zn和RE位点上相当程度的无序性，并与Mg统计共占据，将沿着Zn/RE等原子线界定的非化学计量相区域从类似于Mg(94.0)Zn(2.0)Y(4.0)扩展到类似于Mg(83.3)Zn(8.3)Y(8.3)。 (C) 2011 Acta Materialia Inc. 由 Elsevier Ltd 出版。保留所有权利。
We propose structural models of the unique long period stacking/order (LPSO) phases formed in Mg-Zn-RE alloys, based on Z-contrast scanning transmission electron microscopy observations and first principles calculations. The LPSO structures are long period stacking derivatives of the hcp Mg structure, and the Zn/RE distributions are restricted at the four close-packed atomic layers forming local fcc stacking (i.e. a local ABCA stacking). Chemical order is well developed for the LPSO phases formed in Mg(97)Zn(1)Er(2) (14H type) and Mg(85)Zn(6)Y(9) (18R type) alloys with pronounced superlattice reflections, and the relevant Zn/RE distributions clearly emerge in the Z-contrast atomic images. Initial ternary ordered models were constructed by placing all the atoms at the ideal honeycomb sites, leading to plausible space groups of P6(3)/mcm for the 14H type and C2/m, P3(1)12 or P3(2)12 for the 18R type. The characteristic ordered features are well represented by local Zn(6)RE(8) clusters, which are embedded in the fcc stacking layers in accordance with the L1(2) type short-range order. Energy favored structural relaxations of the initial model cause significant displacement of the Zn/RE positions, implying that strong Zn-RE interactions may play a critical role in phase stability. The LPSO phases seem to tolerate a considerable degree of disorder at the Zn and RE sites with statistical co-occupations by Mg, extending the non-stoichiometric phase region bounded along the Zn/RE equiatomic line from similar to Mg(94.0)Zn(2.0)Y(4.0) to similar to Mg(83.3)Zn(8.3)Y(8.3). (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.