METASTABLE PHASE IN THE AG-GE RAPIDLY QUENCHED ALLOYS

Metastable phases can appear in metallic alloys if their surface respo nse function (liquidus temperature versus cooling rate) is less steep than that of the stable ones. in this case, rapid solidification proce dures can result in their formation. On the other hand, metastable all oys are expected to yield special microstructures when the equilibrium phase composition is restored by their transformation or decompositio n. These microstructures depend on the thermal history of the sample ( e.g., the heating rate and temperature). Metastable phase was obtained by melt-quenching in the Ag1-xGex (x = 0.2, 0.35). X-ray diffraction revealed in Ag-Ge samples a metastable h.c.p. The h.c.p. Ag-Ge phase i s the Hume-Rothery type. Besides, an alpha-Ag-Ge solid solution was pr esent. Amounts of non-reacted Ge were also found, with a lattice const ant close to the ideal value. Differential scanning calorimetry (DSC) runs revealed stability of the Ag-Ge metastable phase up to temperatur es of 521 K (x = 2.0) and 626 K (x = 0.35), where exothermic peaks app eared with relatively high heat release (46.6 and 48.8 J g(-1), respec tively). Work is in progress on the inhomogeneous lattice distortions in the metastable phases, which could be induced in the rapid solidifi cation process and are expected to affect their stability. (C) 1997 El sevier Science S.A.