Crystal structure evolution in mehcanical alloying and spark plasma sintering of AlxCoCrCuFeNi HEAs

AlxCoCrCuFeNi high entropy alloys were synthesised through mechanical alloying and spark plasma sintering. Different alloys were produced by varying the aluminium content (x = 0.5, 1.5, 2.5 and 4). The influence of the milling duration on the evolution of microstructure, constituent phases and morphology was studied. Milling time increasing resulted in grain refinement and higher solid solution homogenisation characterised by a high internal strain. As a consequence of aluminium addition, the materials' microstructure evolved from FCC and BCC phases to FCC, BCC and ordered BCC phases. Both mechanical alloying and SPS conditions as well as aluminium content let to grain refinement and mechanical properties variations. In particular, hardness increased with increasing the aluminium content. The aluminium percentage and the consequent phases evolution is responsible for the microstructural stability at high temperatures.