Abstract We have investigated thermodynamic behaviors of dilute Mg-Zn-Y ternary alloys to form a unique solute-enriched stacking-fault (SESF), which is an intrinsic-II type stacking-fault (I2-SF) enriched by the Zn and Y atoms and represents the structural-unit of the long-period stacking/order (LPSO) phase. SESF in the hexagonal-close-packed (hcp) Mg matrix forms a local face-centered-cubic (fcc) environment, and hence our thermodynamic analysis is based on the Gibbs energy comparison between hcp and fcc phases over the Mg-Zn-Y ternary composition ranges, using the calculation of phase diagrams (CALPHAD) method aided by the first principles calculations. We find that the Zn/Y co-segregations at the SESF provide a remarkable condition that the fcc layers become more stable than the hcp-Mg matrix. Furthermore, within the SESF, the following spinodal-like decomposition into the Mg-rich solid-solution and the Zn/Y-rich L12-type order phase causes a significant reduction of the total Gibbs energy of the system. These spontaneous thermodynamic behaviors explain why the fault layers can be remarkably stabilized in the LPSO-forming ternary Mg alloys, and also support a phenomenological origin of the Zn-Y clustering with the L12-type short-range order, which is known to occur for the LPSO phases and has been confirmed for the present SESF by electron microscopy experiments.

中文翻译：

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稀Mg-Zn-Y合金中富溶质层错的热力学起源

摘要 我们研究了稀释的 Mg-Zn-Y 三元合金的热力学行为，形成了独特的富溶质堆叠断层 (SESF)，这是一种富含 Zn 和 Y 的本征 II 型堆叠断层 (I2-SF)。原子，代表长周期堆积/有序 (LPSO) 相的结构单元。六角密堆积 (hcp) Mg 基体中的 SESF 形成局部面心立方 (fcc) 环境，因此我们的热力学分析基于 Mg-Zn-Y 上 hcp 和 fcc 相之间的吉布斯能量比较三元组成范围，使用相图计算 (CALPHAD) 方法辅助第一性原理计算。我们发现 SESF 处的 Zn/Y 共偏析提供了一个显着的条件，即 fcc 层变得比 hcp-Mg 基质更稳定。此外，在 SESF 内，随后进入富镁固溶体和富锌/钇 L12 型有序相的旋节线状分解导致系统总吉布斯能量显着降低。这些自发的热力学行为解释了为什么在形成 LPSO 的三元镁合金中断层层可以显着稳定，并且还支持具有 L12 型短程有序的 Zn-Y 簇的现象学起源，已知这种现象发生在LPSO 相，并已通过电子显微镜实验证实了目前的 SESF。