Nanohardness and Microstructure of NiCoAlFeCu and NiCoAlFeCuCr Alloys Produced by Mechanical Alloying

The development of high-entropy (HE) alloys expanded the path of advanced materials research. These alloys with at least 5 principal elements in equiatomic or near equiatomic ratios have the tendency to form solid solutions (SS) phases, usually with FCC and/or BCC structures, instead of intermetallic compounds [1]. This effect is reflected with superior mechanical properties compared with conventional alloys which are composed for one or two principal elements [2]. The structures and microstructures in HE alloys, as well as their mechanical properties depend on the nature and number of the alloying elements. The mechanical properties of several high-entropy alloys have been reported. However, evaluations by nanoindentation in HE alloys have not been deeply investigated. The AlCoCrCuFeNi alloy has been the most widely investigated in terms of microstructure and mechanical properties, but at this moment there are no reports about hardness determinations of individual phases. This paper first reviews the microstructure of high entropy alloys in relation to their chemical composition, particularly to the contribution of Cr adittion, and its effect on microhardness of bulk samples and the mechanical properties of individual phases by nanoindentation tests.