Competitive devitrification and metastable phase selection in amorphous Al-Sm

Abstract Competitive phase selection during devitrification of Al-Sm glassy alloys is investigated for compositions ranging from 9.0 to 14.1 at% Sm. The crystallization response under isothermal and constant-heating-rate treatments is observed to involve metastable phases that lie well beneath (energetically above) the equilibrium landscape. Certain phases have also been shown to contain particular sub-crystalline atomic motifs that may be present and even dominant in the undercooled liquid and glass. Experimentally observed structures and associated chemical variations are investigated within the context of comprehensive solution-based thermodynamic models and the corresponding relationship between driving force and chemical partitioning. It is found that initial selection of complex large-unit-cell intermetallic phases is largely governed by the energetics of the partitionless or nearly partitionless crystallization transition. Evidence from transmission electron microscopy and small-angle X-ray scattering further suggests that cooperative precipitation phenomena may be operative and instrumental in alleviating the excess solute burden, as might be expected under conditions of limited atomic transport.