Crystalline or amorphous? A critical evaluation of phenomenological phase selection rules

Abstract

In this work, we reviewed the available phase selection rules comprising atomic size and topological aspects, entropies, enthalpies, melting points, valence and itinerant electron concentrations, as well as electronegativity, and validated them using a carefully prepared database of CuTiZr alloys, to assess their strengths and weaknesses. Substantially increasing interest over the last two decades in high-entropy alloys and metallic glasses motivated the search for phase selection rules that could support the designing of new alloys. These rules are most often based on phenomenological correlation between structure and topological-, thermodynamic- and/or electronic structure-related properties of alloy systems. The available phase selection criteria are, therefore, not scientifically derived, but rather statistically constructed from available experimental data. Thus, they contain potential pitfalls, hindering the rational interpretation of obtained results. With this in mind, this work discusses the importance of defining the influence of phase transformation kinetics and synthesis method on an alloy’s final structure, while highlighting the challenges of using phenomenological alloying guidelines. Furthermore, we show that the interpretation of available phase selection criteria without caution may lead to the designing and fabrication of alloys with undesired phases.

Keywords

• Microstructure
• Thermodynamics
• Multicomponent alloys
• Phase transformations
• Phase selection