Formation of metastable phases in rapidly solidified Mg 32(Al xZn 1−x) 49 alloys

Abstract

In the pentagonal Frank-Kasper phases, a large proportion of the atoms are present in an icosahedral coordination. A near five fold diffraction symmetry observed in some of these phases indicates structural similarity of these with quasicrystals. Ramchandrarao et al. and Zhang et al. were the first to predict that these alloys are likely to yield quasicrystals on rapid solidification. One of the most important phases falling in this category is the T-phase Mg[sub 32](Al,Zn)[sub 49]. The T-phase is an equilibrium phase and is known to form in slowly cooled specimens. This study has indicated that the T-phase can also from on rapid solidification. It has also been observed in this study that the amount of quasicrystalline phase in these ternary alloys is a strong function of the Al/Zn ratio in the alloys and contrary to the observations made in some of the earlier studies, the alloys containing higher Al contain the highest amount of quasicrystalline phase. It should be noted that though Yamane et al. have carried out studies on some alloys of this system, they have limited their studies only to X-ray Diffraction (XRD) experiments. Investigations using XRD sometimes fail to reveal the presence of phases present in smallmore » quantities. This study uses XRD as well as TEM for characterization of the phases in the different alloys. It has been possible to observe the formation of the quasicrystalline phase in many of the compositions examined in this study.« less