Formation of LPSO Phases in As-Cast Mg-Al-Zn-Gd Quaternary Alloys

Mg alloys with a long-period stacking-order (LPSO) phase are categorized into two types. Those in which the LPSO phase forms in as-cast state are referred to as Type I, while those in which the LPSO phase does not appear until a heat treatment is performed are categorized as Type II. However, the reason that gives rise to these two types of alloy is still not well understood. In the present study, in an attempt to clarify this issue, three different Mg-85(Al,Zn)(6)Gd-9 quaternary alloys were prepared. The alpha-Mg, Al2Gd, Mg3Gd and LPSO phases were identified in the as-cast quaternary alloys by microstructural observations, composition analysis and crystal structure analysis using electron probe microanalysis and X-ray diffraction. The results indicated that all the quaternary alloys could be categorized as Type I, even though ternary Mg(85)Al(6)GD(9) and Mg85Zn6Gd9 alloys were Type II. The crystal structure of the LPSO phase in the as-cast alloys is considered to be18R with dilute solute elements, and the fraction of this phase increases with increasing Zn content. The presence of this phase is thought to be the cause of destabilization of the primary Al2Gd phase. It is thus concluded that the relative stability of phases in the vicinity of the LPSO phase is crucial in determining the type of Mg alloy formed.