Metastable phase formation and decomposition in a rapidly solidified aluminium-platinum alloy

Abstract

The metastable phase formation and decomposition behaviour of a rapidly solidified Al-2 at.% Pt alloy were studied by electron microscopy and X-ray diffraction techniques. Two metastable phase were detected. One of them has a cubic structure with α = 5.67 Å and can be considered as a metastable extension of the equilibrium Al₂Pt phase. The other phase, of composition Al₆Pt, has an orthorhombic structure with lattice parameters α = 15.762 , Å. b 12.103 and c = 8.318 Å and is isostructural with the equilibrium phase Ga₆Pt. In the as-splat foils, the Al₂Pt phase forms at the interdendritic regions whereas Al₆Pt forms at the predendritically solidified regions. After prolonged aging the Al₆Pt phase grows with the dissolution of the Al₂Pt phase. The mechanisms of formation of the Al₆Pt phase during artificial aging and natural aging are different. While normal nucleation processes control the decomposition during artificial aging, sympathetic nucleation of Al₆Pt is the dominant mode of decomposition in the naturally aged foils and leads to a star-like morphology. The reasons for the formation of the above metastable phases are also discussed.