METASTABLE PHASE FORMATION AND DECOMPOSITION IN A RAPIDLY SOLIDIFIED ALUMINUM-PLATINUM ALLOY

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 a = 5.67 A ̊ 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 a = 15.762 A ̊, b = 12.103 A ̊ and c = 8.318 A ̊ 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. © 1979.