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.