Nano- and micro-mechanical properties and corrosion performance of a HVOF sprayed AlCoCrFeNi high-entropy alloy coating

In this work, a gas atomized feedstock was used to fabricate an AlCoCrFeNi HEA coating using the high-velocity oxygen fuel (HVOF) process. The coating's resistance to room temperature surface degradation was evaluated using dry sliding wear and seawater corrosion testing. The coating retained the feedstock phase structure with negligible in-flight oxidation and was composed of a majority BCC phase with a minor B2 phase, resulting in a high micro- and nano-hardness of ~7 GPa. These observed phase compositions were consistent with thermodynamically calculated phase predictions using a CALPHAD model. Microstructure-mechanical property mapping revealed uniform microstructural characteristics. However, the multiscale wear resistance of the coating was critically affected by the presence of the hard BCC/B2 phase composition, which led to severe brittleness. Combinatorial assessment of the worn surface, wear debris and counter body indicated that wear was dictated by a combination of abrasive, surface fatigue, tribo-oxidation and adhesive wear. In addition, the coating exhibited superior general corrosion resistance compared with conventional SS316L, but the selective dissolution of the B2 phase preceded poor localized corrosion resistance, ultimately leading to pitting corrosion

Review of thermo-physical properties, wetting and heat transfer characteristics of nanofluids and their applicability in industrial quench heat treatment

The success of quenching process during industrial heat treatment mainly depends on the heat transfer characteristics of the quenching medium. In the case of quenching, the scope for redesigning the system or operational parameters for enhancing the heat transfer is very much limited and the emphasis should be on designing quench media with enhanced heat transfer characteristics. Recent studies on nanofluids have shown that these fluids offer improved wetting and heat transfer characteristics. Further water-based nanofluids are environment friendly as compared to mineral oil quench media. These potential advantages have led to the development of nanofluid-based quench media for heat treatment practices. In this article, thermo-physical properties, wetting and boiling heat transfer characteristics of nanofluids are reviewed and discussed. The unique thermal and heat transfer characteristics of nanofluids would be extremely useful for exploiting them as quench media for industrial heat treatment

Specimen- and grain-size dependence of compression deformation behavior in nanocrystalline copper

The compression deformation behavior of electrodeposited nanocrystalline copper pillars with average grain sizes (d) of 360, 100, and 34 nm has been investigated as a function of specimen size (D). The yield stress for nanocrystalline pillars with d = 360 and 100 nm does not depend on specimen size, exhibiting essentially the bulk yield stress until the specimen size is reduced down to the critical values ((D/d)∗ = 35 and 85), below which the yield stress decreases with the decrease in specimen size. In contrast, the yield stress for nanocrystalline pillars with d = 34 nm does not depend much on specimen size, exhibiting the bulk yield stress value for all specimen sizes investigated. The dominant deformation mechanism changes from dislocation glide for pillars with d = 360 and 100 nm to grain boundary diffusional creep for pillars with d = 34 nm. Grain-size induced softening occurs for pillars with d = 34 nm being consistent with the occurrence of change in deformation mechanisms, whereas the bulk yield stress for pillars with d = 360 and 100 nm increases with the decrease in grain size according to the classical Hall–Petch relationship. The critical (D/d)∗ values determined for nanocrystalline Cu pillars with d = 360 and 100 nm increases with the decrease in grain size so as to conform to the same power law scaling obtained for coarse-grained Cu polycrystals. This is the first indication that the specimen size-induced softening extends from micrometer to nanometer scales as far as the dominant deformation mechanism is dislocation glide. The considerably large critical (D/d)∗ values determined for nanocrystalline Cu pillars with d = 360 and 100 nm are discussed in terms of strain continuity among neighboring grains and the generation of geometrically necessary dislocations to maintain strain continuity at the grain boundaries