Crystal structure and stability of phases in Mg-Zn alloys: A comprehensive first-principles study
Crystal structure and stability of phases in Mg-Zn alloys: A comprehensive first-principles study
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DOI：
10.1016/j.actamat.2022.118443
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发表时间：
2022-10-26
期刊：
影响因子：
9.4
通讯作者：
Zhou, Bi-Cheng
中科院分区：
文献类型：
--
作者：
Cheng, Du;Wang, Kang;Zhou, Bi-Cheng
Mg- zn合金由于其沉淀淬透性、生物相容性和低成本，形成了各种商业轻质镁合金的基础。尽管取得了重大进展，但在这一重要的二元体系中，各种复杂析出物的晶体结构和稳定性仍存在争议。在这项工作中，对有关Mg-Zn体系的晶体结构和相稳定性的信息进行了批判性的回顾，并确定了三个关键的开放性问题：(1)什么是Guinier-Preston （GP）带的晶体结构？(2)观察到的β(1)′沉淀的无数相的相对稳定性是什么？(3)为什么β(2)′相与α - mg具有两种不同的取向关系（ORs） ？为了阐明这些问题，本文基于密度泛函理论、簇展开和蒙特卡罗模拟进行了全面的第一性原理计算。预测了GP区的原子结构，并分析了相干应变对其稳定性的影响。得到了由菱形MgZn2和细长六角形Mg6Zn7单元组成的β(1)′相结构。结果表明，随着菱形MgZn2单元分数的增加，β(1)′析出物趋于稳定，从而形成C14 MgZn2 Laves相的局部区域。β(2)′相与基体之间两种不同的ORs的成因可以追溯到两种形成路径，即HCP基体上相干Zn有序的弛豫和β(1)′相中C14 MgZn2区域的粗化。最后，提出了Mg-Zn合金中可行的析出顺序。(C) 2022材料学报股份有限公司Elsevier Ltd.出版。版权所有。
Mg-Zn alloys form the basis of a wide variety of commercial light-weight Mg alloys due to their precipitation hardenability, biocompatibility, and low cost. Despite significant progress, there exist controversies over the crystal structures and stabilities of various complex precipitates in this important binary system. In this work, the information about crystal structures and stabilities of phases in Mg-Zn system is critically reviewed and three key open questions are identified: (1) What are crystal structures of Guinier-Preston (GP) zones? (2) What are relative stabilities of a myriad of phases observed for beta(1)' precipitates? (3) Why does the beta(2)' phase have two distinct orientation relationships (ORs) with alpha-Mg? To shed light on these questions, comprehensive first-principles calculations based on density functional theory, cluster expansion, and Monte Carlo simulations are performed. The atomic structures of GP zones are predicted, and the effect of coherency strain on their stabilities are analyzed. The structures of beta(1)' precipitates composed of the rhombic MgZn2 and the elongated hexagonal Mg6Zn7 units are provided. It is shown that the beta(1)' precipitate can be stabilized with increased fraction of rhombic MgZn2 units, which leads to local regions of the C14 MgZn2 Laves phase. The origin of the two distinct ORs between beta(2)' phase and the matrix is traced back to two formation paths, i.e., relaxation of the coherent Zn ordering on HCP matrix and coarsening of C14 MgZn2 region in beta(1)' precipitates. Finally, a feasible precipitation sequence in Mg-Zn alloys is suggested. (C) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.