Origin of Properties in High Entropy Alloys

Abstract The novel class of alloys known as high entropy alloys (HEAs) present two fundamental problems; 1) prediction of their properties and reaction to alloying adjustments, 2) prediction of compositions capable of forming the random solid solution with simple crystal structure that appears to be key to their behaviour. Here DFT is applied to model the electronic structure of HEAs based on the CoCrFeNi pseudo base metal. This approach explains a number of properties such as preferred crystal structure and allows fundamental properties such as elastic moduli to be calculated accurately. The stability of HEAs is discussed and compared to that of bulk metallic glasses and a composition is produced which is capable of forming both a glassy and high entropy solid solution phase. A simple thermodynamic model is proposed to allow likely HEA solid solution forming compositions to be identified. This modelling approach using both DFT and thermodynamics is used to assess two potential high entropy alloys based on light metals. The approach shows that the electronic structure of HEAs may be used to predict their properties and therefore their behaviour is due to a free electron structure, it also suggests that the most important consideration in their stability as solid solution alloys is a lack of strong covalent interactions, ie a close to zero entropy of mixing