The elaboration of High Entropy Alloys (HEA) was investigated by means of powder metallurgy. We studied the combination of mechanical activation and reactive sintering in the case of AlCoCrFeNi. Elemental metallic powders were first processed by planetary ball milling over a long duration (28 h). The ratio K between the rotating speed of the sun wheel and the relative rotating speed of the grinding vials was set at 0.2 and 1 corresponding to "medium" energy milling. Given the particular hardness of chromium as compared to other elements, the effect of Cr powder size was investigated and optimized. In addition to experimental characterizations of milled powders, Molecular Dynamics simulations were carried out in order to assess the formation of solid solutions. The activated powders were then consolidated by Spark Plasma Sintering at 1000 °C and 1100 °C. A nanostructured lamellar microstructure exhibiting the coexistence of the FCC and BCC phases was synthesized by this solid-state route. The sintered materials exhibited hardness of up to 670 HV. Our final results (i.e., after optimization of the milling and sintering parameters) suggest that mechanical activation combined with reactive sintering is an efficient route to elaborate dense HEA materials.

AlCoCrFeNi High Entropy Alloys were here synthesized by the combination of Planetary Ball Milling and Spark Plasma Sintering at 1100 • C. The relatively low rotating speed led to a peculiar agglomerate state referred to as "Mechanical Activated". The reactive sintering of activated agglomerates leads to a unique dual phase microstructure: the sintered sample exhibited a distinctive nanostructured lamellar microstructure consisting of two main phases (FCC and BCC). Atom Probe Tomography (APT) was used to ensure that the sintered sample was chemically homogeneous at the nanoscale in each phase. APT also revealed the presence of a Cr-rich sigma phase and oxide nanoprecipitates. X-ray Photoelectron Spectrometry (XPS) results demonstrated that most of the oxygen originated from the commercial powders. Calphad calculations revealed that the presence of oxides could alter the microstructure by modifying the global chemical composition.

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