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The discovery of high entropy alloys at the turn of the millennium lead to a multitude of investigations of different types, focus and aims. With an increased knowledge of the new family of materials, it was possible to make a separation into true single phase high entropy alloys (HEA) and multi-phase compositionally complex alloys (CCA), which both fulfil the initial definition criteria. This review focuses on CCA that have been investigated and developed with a mechanical application in mind. A special importance is attributed to the mechanical testing methods, and priority is given to tensile testing at both room temperature and up to 700°C. Precise microstructural characterization techniques like transmission electron microscopy and/or atom probe tomography ensure the determination of small scale phases, which could be overlooked when using only scanning electron microscopy and/or X-ray diffraction. Comparison of the investigations that meet these criteria are summarized in several tables and figures.

Abstract:The most commonly investigated high entropy alloy, AlCoCrCuFeNi, has been chosen for optimization of its microstructural and mechanical properties by means of compositional changes and heat treatments. Among the different available optimization paths, the decrease of segregating element Cu, the increase of oxidation protective elements Al and Cr and the approach towards a γ-γ 1 microstructure like in Ni-based superalloys have been probed and compared.

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High entropy or compositionally complex alloys provide opportunities for optimization towards new high-temperature materials. Improvements in the equiatomic alloy Al17Co17Cr17Cu17Fe17Ni17 (at.%) led to the base alloy for this work with the chemical composition Al10Co25Cr8Fe15Ni36Ti6 (at.%). Characterization of the beneficial particle-strengthened microstructure by scanning electron microscopy (SEM) and observation of good mechanical properties at elevated temperatures arose the need of accomplishing further optimization steps. For this purpose, the refractory metals hafnium and molybdenum were added in small amounts (0.5 and 1.0 at.% respectively) because of their well-known positive effects on mechanical properties of Ni-based superalloys. By correlation of microstructural examinations using SEM with tensile tests in the temperature range of room temperature up to 900 °C, conclusions could be drawn for further optimization steps.

During solidification of Fe-containing Al-Si casting alloys, various different intermetallic phases are formed. As this can have an impact on mechanical properties we investigated phase formation by applying in-situ time resolved synchrotron X-ray tomography. For a commercially pure Al-10Si-0.3 wt.%Fe alloy, phase morphology and spatial arrangement during solidification was revealed with 1°C temperature resolution. We find that many of the intermetallic phases that have been identified as β-AlFeSi phases in previous studies are in fact δ phases. It was found that δ plates mainly nucleate on the eutectic Al and Si. δ phase formation terminates almost simultaneously with the final solidification of eutectic phases. Features of δ phase growth such as impingement, branching and deformation of plates are discussed. The phase separation sequence during solidification is described and explained by the existence of "cells" of residual liquid in which δ phase formation progresses rapidly due to the supersaturation of solute atoms.