Nonequilibrium behavior in the Al-Ge alloy system: Insights into the metastable phase diagram

Abstract

The competitive formation of metastable and stable phases during nonequilibrium processing of Al-Ge alloys and the corresponding metastable phase equilibria have been investigated. For germanium concentrations in the range 30 to 50 at. pct, it is shown that the four metastable phases can be ranked in order of decreasing stability as follows: monoclinic (P21/c), rhombohedral (R-C), orthorhombic (Pbca), and hexagonal (P6/mmm). Their formation depends not only on the transformation temperature(e.g., the liquid undercooling), but also on the presence of appropriate heterogeneous nucleation sites. For example, the orthorhombic phase has only been observed in amorphous films after rapid annealing/crystallization treatments. It is also shown that all of these phases form metastable equilibria with α-aluminum only,i.e., no metastable phase equilibria appear to exist between any metastable phase and β-germanium or between any two metastable phases. Consequently, it is not possible to draw a single metastable phase diagram that incorporates all of these phases with phase boundaries that represent their metastable equilibria; rather, separate diagrams should be drawn for each metastable phase. It is noted that these diagrams should extend only to the metastable phase field rather than all the way to pure germanium: for compositions richer in germanium, the results indicate that the metastable phase forms and then remelts upon the formation of germanium or a more stable, germanium-enriched metastable phase. Furthermore, it is proposed that this behavior is rather general in nature. Finally, it is concluded that the production of metastable phases in bulk form, in systems such as this where so many reactions occur simultaneously and competitively, might be impossible using solidification processing approaches.