In Mg-Ca alloys the grain refining mechanism, in particular regarding the role of nucleant substrates, remains the object of debates. Although native MgO is being recognised as a nucleating substrate accounting for grain refinement of Mg alloys, the possible interactions of MgO with alloying elements that may alter the nucleation potency have not been elucidated yet. Herein, we design casting experiments of Mg-xCa alloys varied qualitatively in number density of native MgO, which are then comprehensively studied by advanced electron microscopy. The results show that grain refinement is enhanced as the particle number density of MgO increases. The native MgO particles are modified by interfacial layers due to the co-segregation of Ca and N solute atoms at the MgO/Mg interface. Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy, we reveal the nature of these Ca/N interfacial layers at the atomic scale. Irrespective of the crystallographic termination of MgO, Ca and N co-segregate at the MgO/Mg interface and occupy Mg and O sites, respectively, forming an interfacial structure of a few atomic layers. The interfacial structure is slightly expanded, less ordered and defective compared to the MgO matrix due to compositional deviations, whereby the MgO substrate is altered as a poorer template to nucleate Mg solid. Upon solidification in a TP-1 mould, the impotent MgO particles account for the grain refining mechanism, where they are suggested to participate into nucleation and grain initiation processes in an explosive manner. This work not only reveals the atomic engineering of a substrate through interfacial segregation but also demonstrates the effectiveness of a strategy whereby native MgO particles can be harnessed for grain refinement in Mg-Ca alloys.

中文翻译：

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原生 MgO 颗粒细化 Mg-Ca 合金的晶粒

在镁钙合金中，晶粒细化机制，特别是有关成核基质的作用，仍然是争论的对象。尽管原生 MgO 被认为是镁合金晶粒细化的成核基质，但 MgO 与可能改变成核效能的合金元素之间可能的相互作用尚未阐明。在此，我们设计了原生 MgO 数密度不同的 Mg-xCa 合金的铸造实验，然后通过先进的电子显微镜对其进行了全面研究。结果表明，随着MgO颗粒数密度的增加，晶粒细化程度增强。由于 MgO/Mg 界面处 Ca 和 N 溶质原子的共偏析，原生 MgO 颗粒被界面层改性。使用像差校正扫描透射电子显微镜和电子能量损失光谱，我们在原子尺度上揭示了这些 Ca/N 界面层的性质。无论 MgO 的晶体学终止如何，Ca 和 N 在 MgO/Mg 界面处共偏析，并分别占据 Mg 和 O 位点，形成几个原子层的界面结构。由于成分偏差，与 MgO 基体相比，界面结构略有扩展，有序度较低且有缺陷，因此 MgO 基体被改变为较差的模板来使 Mg 固体成核。在 TP-1 模具中凝固后，无能的 MgO 颗粒解释了晶粒细化机制，表明它们以爆炸方式参与成核和晶粒引发过程。这项工作不仅揭示了通过界面偏析对基材进行原子工程的原理，而且还证明了利用原生 MgO 颗粒细化 Mg-Ca 合金晶粒的策略的有效性。