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

Abstract

Using the magnetron sputtering technique, two typical high-entropy alloy (HEA) films namely CoCrFeNiCu (Al-0) with a face-centered cubic (fcc) structure and CoCrFeNiCuAl 2.5 (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.

High-Temperature Nano-Indentation Creep Behavior of Multi-Principal Element Alloys under Static and Dynamic Loads

“…CoCrNi showed the lowest creep displacement and highest activation energy supporting its superior creep resistance. Hardness or strength has a pronounced effect on creep resistance as previously reported [16,18]. CoCrNi showed the highest hardness compared to the other two systems and thus better creep resistance.…”

“…At smaller depth, dislocations are closer to the free surface and therefore have higher mobility and diffusion leading to lower n. also be related to the mobility and diffusion of dislocations and how far they are from the free surface [41]. At smaller depth, dislocations are closer to the free surface and therefore have higher mobility and diffusion leading to lower n. The high values of stress exponent obtained from nano-indentation of the three alloys may be attributed to the complex stress state underneath the indenter [16,21,38]. The stress exponent is a complex function of microstructure, mobile dislocation density and/or the activation area underneath the indenter [42].…”

“…From Figure 4, it was concluded that the stress exponent determined from dynamic tests was slightly lower than that in static test. This may be because of oscillatory load which resulted in better propagation of dislocations and reduction of n. The high values of stress exponent obtained from nano-indentation of the three alloys may be attributed to the complex stress state underneath the indenter [16,21,38]. The stress exponent is a complex function of microstructure, mobile dislocation density and/or the activation area underneath the indenter [42].…”

“…Nano-indentation technique has been widely used to characterize the small-scale deformation behavior of materials [9][10][11][12][13][14][15]. However, there are limited reports on the creep behavior of MPEAs using nano-indentation, and majority of the studies are at room temperature with a maximum load of 100 mN [16][17][18][19][20][21][22]. There are no reports on the deformation behavior of MPEAs as a function of temperature comparing static and dynamic loads.…”

Tracer diffusion in single crystalline CoCrFeNi and CoCrFeMnNi high entropy alloys

“…In recent years, numerous HEAs have been investigated focusing on high temperature mechanical properties [7,8], creep strength [9,10,11,12], oxidation resistance [13,14,15] and coating applications [16], following an originally introduced paradigm of four 'core effect', i.e. a high entropy, severe lattice distortion, 'cocktail' effect and sluggish diffusion [2].…”