Low temperature micromechanical properties of nanocrystalline CoCrFeNiMn high entropy alloy

In the temperature range 77–290 K, the microhardness of high entropy alloy CoCrFeNiMn (Сantor alloy) samples in three structural states was measured: (i) initial coarse-grained samples (ii) nanocrystalline samples obtained by high pressure torsion (HPT) at room temperature and (iii) nanocrystalline samples processed by HPT at liquid nitrogen temperature. With a decrease in temperature from 290 K to 77 K, the microhardness of all types of samples monotonically increased by about 40–60% which indicates the thermally activated nature of plastic deformation under the indenter. The highest microhardness was observed in nanocrystalline samples obtained by the torsion at room temperature. It is shown that the ratio of Vickers microhardness to the conventional yield strength remains constant in the range of 77–290 K, and that the ratio value depends on the microstructure of samples. In the coarse-grained Cantor alloy samples the normal indentation size effect was observed: the microhardness increased with a decrease of the indenter penetration depth; the obtained dependence is discussed in the framework of geometrically necessary dislocations model. In the HPT-nanocrystallized samples in the low load range, an essential microhardness drop with the load decreasing was observed, being a result of mechanical polishing of the sample surface, which destroys the microstructure created under the torsion and weakens the surface layer.