Evidence of Inverse Hall-Petch Behavior and Low Friction and Wear in High Entropy Alloys

Abstract

We present evidence of inverse Hall-petch behavior for a single-phase high entropy alloy (cocrfeMnni) in ultra-high vacuum and show that it is associated with low friction coefficients (~0.3). Grain size measurements by STEM validate a recently proposed dynamic amorphization model that accurately predicts grain size-dependent shear strength in the inverse Hall-Petch regime. Wear rates in the initially soft (coarse grained) material were shown to be remarkably low (~10-6 mm 3 /n-m), the lowest for any HEA tested in an inert environment where oxidation and the formation of mixed metal-oxide films is mitigated. the combined high wear resistance and low friction are linked to the formation of an ultra-nanocrystalline near-surface layer. The dynamic amorphization model was also used to predict an average high angle grain boundary energy (0.87 J/m 2). this value was used to explain cavitationinduced nanoporosity found in the highly deformed surface layer, a phenomenon that has been linked to superplasticity. Since their discovery in 2004 1,2 , high entropy alloys (HEAs) have been extensively investigated and shown to exhibit remarkable thermomechanical properties 3,4. Studies of grain-size dependent mechanical behavior are limited 5,6 , however, especially in the regime of ultra-nanocrystalline grain size where softening (i.e. inverse Hall-Petch behavior) occurs. Recent work has also shown that these alloys are ideally suited for processing using laser-based additive manufacturing techniques 7-9 , as they exhibit high phase stability and derive much of their strength from thermal history-insensitive mechanisms like solution strengthening 10. While recent publications have presented experimental evidence of low friction or high wear resistance (but typically not both) with HEAs 11-25 , they were performed in environments where oxidation and the formation of mixed metal-oxide surface films was prevalent. These phenomena can greatly impact strength and deformation/wear resistance. We show that in an ultra-high vacuum (UHV) environment, i.e., in the absence of chemically reactive species, an additively manufactured initially coarse grained (soft), single-phase HEA exhibited a tendency toward low friction coefficients (i.e., low shear strength) and some of the lowest wear rates currently reported for these materials. The low friction and wear are attributed to a propensity for extreme grain refinement under sliding contact. Specifically, we present evidence of that this high strain rate shear leads to dynamic amorphization and inverse Hall-Petch behavior in the surface layer. In the present context of sliding contacts, dynamic amorphization refers to the continuous shear-induced generation of a structurally amorphous layer that accommodates deformation 26. This is in competition with stress-and temperature-driven grain growth and recrystallization, all of which reestablish crystallinity and order in the shear layer. The shear layer must be amorphized again in subsequent contact passes, thus we refer to this process as dynam...

Re-Melting Behaviour and Wear Resistance of Vanadium Carbide Precipitating Cr27.5Co14Fe22Mo22Ni11.65V2.85 High Entropy Alloy

“…Nevertheless, on the basis of various experiments and procedures, such as CALPHAD [ 1 , 2 , 3 ], it has been possible to identify and investigate promising systems for different applications. The fields of application range from high-temperature or cryogenic materials to corrosion-resistant and wear-resistant materials [ 4 , 5 , 6 , 7 ]. In addition to the single-phase high entropy alloys, other alloys have been developed which form precipitates in order to further influence the properties of these alloys [ 8 ].…”

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].…”

Recent Advances in Additive Manufacturing of High Entropy Alloys and Their Nuclear and Wear-Resistant Applications

“…The wear rates were measured to be 3.64 × 10 −6 and 1.42 × 10 −6 mm 3 •N −1 •m −1 for Nb content of x = 0 and x = 2, respectively. Jones et al [358] reported that CoCrFeNiMn showed a remarkably low wear rate of 10 −6 mm 3 •N −1 •m −1 with the hardness of 1.6 GPa. Zhu et al [359] added V and Nb in equiatomic ratio into the CoCrFeNiMn (FCC phase) coating and studied the wear resistance.…”