Mg-Al-Zn-Gd 4元系合金鋳造まま材におけるLPSO相の生成挙動

抄録

Mg alloys with long period stacking ordered (LPSO) phases are categorized into two types. Mg alloys where the LPSO phase forms in as-cast state are categorized as Type I, while those where the LPSO phase does not appear in as-cast state but forms during heat treatment are categorized as Type II. However, the origin of the formation type of Mg alloys with LPSO phases still remains poorly understood. In the present study, three different Mg₈₅(Al, Zn)₆Gd₉ quaternary alloys were prepared to elucidate the dominant factors that determine the formation type of Mg alloys with LPSO phases. The α-Mg, Al₂Gd, Mg₃Gd and LPSO phases were characterized in the as-cast quaternary alloys by microstructural observation, composition analysis and crystal structure analysis by using electron probe microanalysis and X-ray diffraction. It indicates that all the quaternary alloys are categorized as Type I even though the original two ternary Mg₈₅Al₆Gd₉ and Mg₈₅Zn₆Gd₉ alloys are categorized as Type II. The crystal structure of the LPSO phases observed in the as-cast alloys is considered to be 18R with dilute solute elements. The amount of the LPSO phases in the as-cast state increases with increasing the Zn content in the alloys. It is considered to be the cause of destabilization of the primary crystallized Al₂Gd phase by the addition of Zn and hence relative stabilization of the LPSO phases. It is thus concluded that the relative phase stability in the vicinity of the LPSO phase is considered to be crucial for determination of the formation type of Mg alloys with LPSO phases.

Fig. 1 Backscattered electron images of the as-cast (a) Mg₈₅Al₄Zn₂Gd₉, (b) Mg₈₅Al₃Zn₃Gd₉ and (c) Mg₈₅Al₂Zn₄Gd₉ alloys. Each microstructure consists of the black, white, lamellar and plate-like regions.