A geometric model for atomic configurations in amorphous Al alloys

A representative model for the atomic structure of amorphous Al alloys is proposed based on the fundamental structure-forming principle of high packing efficiency, topological concepts, and available partial and total radial distribution functions from diffraction studies. Selection of rare earth (RE)-centered atomic clusters as representative structural elements in this model is supported by the large coordination number (∼17±2), efficient atomic packing (100±5%), and small mean intersolute spacing (∼2 atom diameters center-to-center) associated with RE solutes in amorphous Al alloys. Using Al–Y and Al–Y–Ni alloys as a base, five idealized Y-centered clusters and two Ni-centered clusters are described with specific atomic configurations that are consistent with the observed coordination numbers and high density relative to crystalline alloys of the same composition. Significant configurational complexity, required for an amorphous structure, is offered by this structural model. A distribution in Y–Y intersolute spacing is provided by the model that is consistent with the expectation of a random distribution of Y atoms. Topological similarities with other amorphous metal alloy systems suggest that the structure described here for amorphous Al may also be relevant for many other amorphous metals with marginal glass-forming ability (critical cooling rate⩾1000 K/s), including alloys based on Mg, Fe, Ni and Co.