Abstract
X-ray diffraction study reveals that the metastable phase Ag3Al, which is quenched from a high temperature beta phase region and has the b.c.c. structure, exhibits a new type of phase transformation. When heated and/or aged below about 380 K, the quenched beta phase undergoes a displacive and irreversible transformation to the metastable 9R phase, named the 9R(H) phase in this paper, with stacking sequence (2\bar1)3 of non-ideally close packed basal planes. This phase change is simultaneously and cooperatively accompanied with that to the stable mu phase caused by ordering reactions, but it occurs without composition change. Based on the considerations about the formation mechanism of the 9R(H) phase, we can conclude that the transformation to the 9R(H) phase belongs to an extended category of the bainitic transformation. Characteristics of the martensitic transformation to the disordered 9R structure occurring at about 225 K by cooling are also discussed.
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