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A Critical Review of High Entropy Alloys and Related Concepts (Postprint)

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Journal Article

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High entropy alloys HEAs are barely 12 years old. The field has stimulated new ideas and has inspired the exploration of the vast composition space offered by multi-principal element alloys MPEAs. Here we present a critical review of this field, with the intent of summarizing key findings, uncovering major trends and providing guidance for future efforts. Major themes in this assessment include definition of terms thermodynamic analysis of complex, concentrated alloys CCAs taxonomy of current alloy families microstructures mechanical properties potential applications and future efforts. Based on detailed analyses, the following major results emerge. Although classical thermodynamic concepts are unchanged, trends in MPEAs can be different than in simpler alloys. Common thermodynamic perceptions can be misleading and new trends are described. From a strong focus on 3d transition metal alloys, there are now seven distinct CCA families. A new theme of designing alloy families by selecting elements to achieve a specific, intended purpose is starting to emerge. A comprehensive microstructural assessment is performed using three datasets experimental data drawn from 408 different alloys and two computational datasets generated using the CALculated PHAse Diagram CALPHAD method. Each dataset emphasizes different elements and shows different microstructural trends. Trends in these three datasets are all predicted by a structure in - structure out SISO analysis developed here that uses the weighted fractions of the constituent element crystal structures in each dataset. A total of 13 distinct multi-principal element single-phase fields are found in this microstructural assessment. Relationships between composition, microstructure and properties are established for 3d transition metal MPEAs, including the roles of Al, Cr and Cu. Critical evaluation shows that commercial austenitic stainless steels and nickel alloys with 3 or more principal elements are MPEAs.

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  • Properties of Metals and Alloys

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