Microstructural changes in CoCrFeMnNi under mild tribological load

Abstract

The lack of a principle element in high-entropy alloys (HEA) leads to unique and unexpected material properties. Tribological loading of metallic materials often results in deformed subsurface layers. As the microstructure feedbacks with friction forces, the microstructural evolution is highly dynamic and complex. The concept of HEAs promises high solid solution strengthening, which might decrease these microstructural changes. Here, we experimentally investigated the deformation behavior of CoCrFeMnNi in a dry, reciprocating tribological contact under a mild normal load. After only a single stroke, a surprisingly thick subsurface deformation layer was observed. This layer is characterized by nanocrystalline grains, twins and bands of localized dislocation motion. Twinning was found to be decisive for the overall thickness of this layer, and twin formation within the stress field of the moving sphere is analyzed. The localization of dislocation activity, caused by planar slip, results in a grain rotation. Fragmentation of twins and dislocation rearrangement lead to a nanocrystalline layer underneath the worn surface. In addition, oxide-rich layers were found after several sliding cycles. These oxides intermix with the nanocrystalline layer due to material transfer to the counter body and re-deposition to the wear track. Having revealed these fundamental mechanisms, the evolution of such deformation layers in CoCrFeMnNi under a tribological load might lead to other HEAs with compositions and properties specifically tailored to tribological applications in the future.

A critical review on mechanically alloyed high entropy alloys: processing challenges and properties

“…So, the result indicated that the transition occurred from adhesive and abrasive wear to delamination as the duration of wear is continued and examined the variation of CoF with load, sliding speed, and time [175]. Dollmann et al [176] investigated the wear behaviour of CoCrFeMnNi HEAs in a separate study. A soft fcc HEA has a low hardness value for the undeformed surface, while the hardness value increases significantly to 500 HV for the deformed surface.…”

Friction of Metals: A Review of Microstructural Evolution and Nanoscale Phenomena in Shearing Contacts

“…Other sophisticated routes exist for imparting higher strength to metals, and providing thermal stability to the underlying microstructural features, like grain boundaries and dislocations. Some relatively recent examples include the formation of thick boundaries (sometimes called complexions) [253][254][255], solution strengthening [46,81] in high entropy alloys [256][257][258][259][260][261][262][263] (where combinations of multiple elements increases the energy required for dislocation motion), and metastable amorphous bulk metallic glasses [264][265][266][267].…”

Dislocation-mediated and twinning-induced plasticity of CoCrFeMnNi in varying tribological loading scenarios

“…In turn, the resulting deformation layer leads to a change in the friction coefficient. So far, various microstructural characteristics have been observed in coarse grained, face-centred cubic (fcc) metallic materials under tribological load, such as a dislocation trace line [4,5], subgrain formation [2,6], deformation twinning [7,8], band-like patterns [9][10][11] and nanocrystalline grains [7,12].…”

“…The two normal loads they applied resulted in different grain sizes in the subsurface area [12]. In our earlier work, we analysed the microstructural evolution with increasing cycle number with a focus on the onset of microstructural changes at high friction coefficients [7]. In contrast, the influence of the friction and normal force is investigated after a single-trace experiment in this manuscript.…”

“…It is known from uniaxial tests that the initial grain orientation is decisive for the occurrence of deformation twinning [23,24]. In contrast to high SFE materials [25][26][27], there is only little data available on medium SFE materials [7,13,28]. For example, nanoscratch tests in austenitic steel have demonstrated the influence of the initial crystal direction parallel to the normal direction on the formation of slip traces next to the wear track, pile-up and the wear track depth [28].…”