Nanoindentation study on the creep characteristics of high-entropy alloy films: fcc versus bcc structures

Using the magnetron sputtering technique, two typical high-entropy alloy (HEA) films namely CoCrFeNiCu (Al-0) with a face-centered cubic (fcc) structure and CoCrFeNiCuAl2.5 (Al-2.5) with a body-centered cubic (bcc) structure were prepared by alloy targets. The as-deposited HEA films have a columnar-growth mode and nanocrystalline grains. The creep behaviors of both HEA films were systematically investigated by nanoindentation with a Berkovich indenter. The bcc Al-2.5 exhibited a stronger creep resistance than the fcc Al-0. In addition, with the increase of holding load and/or loading rate, the creep deformation was significantly enhanced in the fcc Al-0. Interestingly, it was almost history-independent in the bcc Al-2.5. The creep characteristics of HEA films could be related to the distinct lattice structures, which apparently affect the kinetics of plastic deformation. The strain rate sensitivity (SRS) and activation volume of the dislocation nucleation were carefully estimated for both HEA films. In view of the large differences of activation volumes between Al-0 and Al-2.5, we present discussions to explain the observed creep characteristics in HEA films. (C) 2015 Published by Elsevier Ltd