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The temperature-dependent hardness of additively-manufactured near-eutectic Ni–Nb was investigated. This alloy was found to have solidified into a two-phase nanoscale microstructure with peak hardness of H ≅ 14–17 GPa at temperatures up to 400 °C, above which irreversible softening was observed despite retention of significant strength compared to traditionally-synthesized Ni-based superalloys. Experiments and molecular-dynamics simulations show that deformation for single-phase nanocrystalline volumes was confined to intragranular slip-band formation in δ-Ni3Nb and to intergranular grain-boundary sliding in μ-Ni6Nb7. However, microscopy in the nanostructured two-phase regions after severe plastic deformation indicated that phase boundaries acted as nucleation sites for dislocations, promoting twinning-induced plasticity (TWIP) in the μ-Ni6Nb7 grains. This work highlights (1) that additive manufacturing techniques enable formation of unique microstructures that exhibit superior mechanical properties, and (2) that multi-phase intermetallic compounds provide a route to mitigate brittle fracture though the promotion of twinning-induced plasticity. High strength and the absence of interface decohesion (cracking) suggests that multi-phase intermetallic systems may be a viable route for design of new printable superalloys. These results suggest that additive manufacturing methods and rapid solidification via non-equilibrium pathways may enable a pathway for achieving high combined strength and ductility. [ABSTRACT FROM AUTHOR]

We report an investigation of the friction mechanisms of MoS2 thin films under changing environments and contact conditions using a variety of computational and experimental techniques. Molecular dynamics simulations were used to study the effects of water and molecular oxygen on friction and bonding of MoS2 lamellae during initial sliding. Characterization via photoelectron emission microscopy (PEEM) and Kelvin probe force microscopy (KPFM) were used to determine work function changes in shear modified material within the top few nanometers of MoS2 wear scars. The work function was shown to change with contact conditions and environment, and shown by density functional theory (DFT) calculations and literature reports to be correlated with lamellae size and thickness of the basally oriented surface layer. Results from nanoscale simulations and macroscale experiments suggest that the evolution of the friction behavior of MoS2 is linked primarily to the formation or inhibition of a basally oriented, molecularly thin surface film with long-range order. [ABSTRACT FROM AUTHOR]