Over the last few years compositionally complex alloys consisting of five or more principal elements, also known as high entropy alloys (HEA), gained large attention within the materials science community. Refining the microstructure of a five-element HEA (CoCrFeMnNi) by applying high pressure torsion, the grain size can be reduced to less than 50 nm paired with a significant increase of strength to 1950 MPa and a hardness of ~520 HV. During further isochronal and isothermal annealing treatments an additional hardness increase was found, and by applying high resolution microscopy techniques such as TEM and ATP the occurrence of newly formed nanophases could be detected.
To gain further insights into the microstructure-property relationships, nanoindentation testing was used to compare hardness, Young’s modulus, as well as strain rate sensitivity between these different microstructures and annealing states. By applying simple strain rate controlled testing protocols it was found that the formation of these nanophases directly leads to a significant increase in the Young’s modulus from 205 GPa in the as-deformed state to ~255 GPa after 1 h at 550 °C. Afterwards the modulus decreases down to the coarse-grained (cg) and ufg-value. Moreover, a strong fluctuation of the modulus of the cg-samples was found leading to the conclusion that the investigated HEA is highly elastically anisotropic. Additionally, strain rate sensitivity values for all microstructural states and annealing treatments could be deduced by nanoindentation strain rate jump tests. As expected for an fcc nc-structure, strain rate sensitivity was increased in the ultrafine grained states. However, also the single crystalline and cg-states show some significant influences on the hardness upon changing the strain rate. This can be directly related to the high lattice distortions proposed to be present in HEAs, causing a high lattice friction. Overall, it was shown that nanoindentation is an elegant and very versatile tool to shed more light onto the microstructure-property relationships of complex materials and was proven to be beneficial as a high throughput testing technique.