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The understanding of the structure and the stability of high entropy alloys is still incomplete and the mechanisms behind the composition-property relationships are unclear. One reason is that few systematic and accurate determinations of their composition-dependent structure on atomic level have been made. In this paper some results on the structure obtained by X-ray and neutron diraction of the CoCrFeNi alloy, to which Pd, Sn and Cu have been added in dierent amounts, are reported. The investigations make it obvious that none of the alloys is completely homogeneous, as has earlier been suggested, and that they do not form a perfect solid solution.

Multicomponent systems, termed High Entropy Alloys (HEAs), with predominantly single solid solution phases are a current area of focus in alloy development. Although different empirical rules have been introduced to understand phase formation and determine what the dominant phases may be in these systems, experimental investigation has revealed that in many cases their structure is not a single solid solution phase, and that the rules may not accurately distinguish the stability of the phase boundaries. Here, a combined modelling and experimental approach that looks into the electronic structure is proposed to improve accuracy of the predictions of the majority phase. To do this, the Rigid Band model is generalised for magnetic systems in prediction of the majority phase most likely to be found. Good agreement is found when the predictions are confronted with data from experiments, including a new magnetic HEA system (CoFeNiV). This also includes predicting the structural transition with varying levels of constituent elements, as a function of the valence electron concentration, n, obtained from the integrated spin-polarised density of states. This method is suitable as a new predictive technique to identify compositions for further screening, in particular for magnetic HEAs.

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