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Experimental results reveal that the apparent activation-energy for grain-growth in an fcc-based AlxCoCrFeNi high entropy alloy (HEA) system increases from 179 to 486 kJ/mol when the Al content increases from x = 0.1 to 0.3. These unexpectedly high apparent activation-energy values can be potentially attributed to solute clustering within the fcc solid-solution phase that develops with increasing Al content in this HEA. Detailed microstructural analysis using atom-probe tomography and density functional theory (DFT) calculations strongly indicate the presence of such nanoscale clusters. This phenomenon can change grain-growth from a classical solute-drag regime to a much more sluggish cluster-drag based mechanism in these HEAs.

High entropy alloys represent a new paradigm of structural alloy design consisting of (near) equal proportions of constituent elements resulting in a number of attractive properties. In particular, eutectic high entropy alloys offer a remarkable combination of high strength and good ductility from the synergistic contribution of each phase in the eutectic, thereby circumventing the strength-ductility trade-off in conventional structural materials. In the present study, wear and corrosion behavior were evaluated for the AlCoCrFeNi2.1 eutectic high entropy alloy consisting of BCC (B2), and FCC (L12) lamellae. A transition from adhesive to oxidative wear was observed in reciprocating wear analysis. The L12 phase with lower hardness preferentially deformed during the wear test. The ratio of hardness to modulus was almost two times higher for the B2 phase as compared to L12. The overall corrosion resistance of the eutectic high entropy alloy was comparable to 304 stainless steel in 3.5 wt% NaCl solution. However, detailed microscopy revealed preferential dissolution of the B2 phase. Phase-specific scanning kelvin probe analysis showed relatively higher electropositivity for the B2 phase as compared with L12, supporting the selective corrosion and higher coefficient of friction of B2.

Laser surface engineering was successfully used to engineer a nano carbide composite coating on a low carbon steel substrate. Two powder precursors of B4C and B4C+ 4 wt% Fe were utilized with different laser powers and processing speeds. A smooth crack free coating layer was achieved with B4C+ 4 wt% Fe powder precursor. The phase evolution and microstructure characterization of the coating layer were conducted using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Microscopy observations revealed a fine dendritic microstructure consisting of α-Fe phase dendrites with nanosized B4C precipitates, while the interdendritic regions contained mainly Fe2B and Fe3C phases. The retention of B4C in the coating layer was observed only at high laser power with high scanning speed. This is due to the lower stability of B4C with respect to Fe2B and Fe3C phases as confirmed with density functional theory based thermodynamic calculations. The hardness increased nearly seven fold for the nano-composite coating compared to the steel substrate due to evolution of high hardness ceramic phases, in addition to the refinement of the microstructure. This laser coating has a high potential in advance engineering applications where a thick coating layer with high hardness is required. Keywords: Laser surface processing, Composite coatings, Boron carbide, Thermodynamic modeling