Density functional theory study of solute cluster growth processes in Mg-Y-Zn LPSO alloys

Mg-Y-Zn長周期積層秩序合金の溶質原子クラスタ成長過程の第一原理計算

板倉 充洋  ; 山口 正剛   ; 江草 大佑*; 阿部 英司*

Itakura, Mitsuhiro; Yamaguchi, Masatake; Egusa, Daisuke*; Abe, Eiji*

シンクロ型長周期積層(LPSO)合金において溶質原子クラスタはキンク変形やキンク強化といった特異な塑性に中心的役割を果たす。強化機構を解明にはその原子構造を決定することが必須であるが、クラスタに含まれる格子間原子に関して不明な点が残っていた。本研究では第一原理計算によりクラスタが形成される過程を詳細に調べ、その成長過程で中心付近にある原子が自発的に格子間原子となり、同時に空孔が形成されることを示した。これにより実験で不明であった格子間原子の種類と割合が決定された。更に自発的な空孔形成はこれまで金属材料では知られていない現象であり、これによってLPSO構造の形成が加速されていることを初めて示した。

Solute cluster in LPSO alloys plays a key role in their idiosyncratic plastic behavior such as kink formation and kink strengthening. Identifying the atomistic details of the cluster structure is a prerequisite for any atomistic modeling of LPSO alloys aiming for their improved strength and ductility, but there have been uncertainty about interstitial atom in the cluster. While density functional theory calculations have shown that inclusion of interstitial atom is energetically favorable, it has been unclear how the extra atom is provided, how much of the cluster have interstitial atoms, and what kind of element they are. In the present work we use density functional theory calculations to investigate the growth process of the solute cluster, specifically that of Mg-Y-Zn LPSO alloy, to determine the precise atomistic structure of solute cluster. We show that a pair of an interstitial atom and a vacancy is spontaneously created when a certain number of solute atoms are absorbed into the cluster, and all the full-grown cluster should include interstitial atom. We also show that interstitial atom is either Mg or Y atom, while Zn interstitial atom is extremely rare. These knowledge greatly simplifies atomistic modeling of solute clusters in Mg-Y-Zn alloy. Owing to the vacancies emitted from the cluster, vacancy density should be over-saturated in regions where solute clusters are growing, and the increased vacancy density accelerates cluster growth.