This article investigated the microstructure of Ti6Al4V that was fabricated via selective laser melting; specifically, the mechanism of martensitic transformation and relationship among parent β phase, martensite (α’) and newly generated β phase that formed in the present experiments were elucidated. The primary X-ray diffraction (XRD), transmission electron microscopy (TEM) and tensile test were combined to discuss the relationship between α’, β phase and mechanical properties. The average width of each coarse β columnar grain is 80–160 μm, which is in agreement with the width of a laser scanning track. The result revealed a further relationship between β columnar grain and laser scanning track. Additionally, the high dislocation density, stacking faults and the typical (10true1¯1) twinning were identified in the as-built sample. The twinning was filled with many dislocation lines that exhibited apparent slip systems of climbing and cross-slip. Moreover, the α + β phase with fine dislocation lines and residual twinning were observed in the stress relieving sample. Furthermore, both as-built and stress-relieved samples had a better homogeneous density and finer grains in the center area than in the edge area, displaying good mechanical properties by Feature-Scan. The α’ phase resulted in the improvement of tensile strength and hardness and decrease of plasticity, while the newly generated β phase resulted in a decrease of strength and enhancement of plasticity. The poor plasticity was ascribed to the different print mode, remained support structures and large thermal stresses.

Here, we demonstrate that nonprecious CoFe-based oxide nanoarrays exhibit excellent electrocatalytic activity and superior stability for electrochemical oxygen evolution reaction (OER) at large current densities (>500 mA cm–2). Carbon fiber paper (CFP) with three-dimensional macroporous structure, excellent corrosion resistance, and high electrical properties is used as the support material to prevent surface passivation during the long-term process of OER. Through a facile method of hydrothermal synthesis and thermal treatment, vertically aligned arrays of spinel Co x Fe3–x O4 nanostructures are homogeneously grown on CFP. The morphology and the Fe-doping content of the CoFe oxide nanoarrays can be controlled by the Fe3+ concentration in the precursor solution. The arrays of CoFe oxide nanosheets (NSs) grown on CFP (Co2.3Fe0.7O4-NSs/CFP) deliver lower Tafel slope (34.3 mV dec–1) than CoFe oxide nanowire (NW) arrays grown on CFP (Co2.7Fe0.3O4-NWs/CFP) in alkaline solution, owing to higher Fe-doping content and larger effective specific surface area. The Co2.3Fe0.7O4-NSs/CFP electrode exhibits excellent stability for OER at large current densities in alkaline solution. Moreover, the morphology and structure of CoFeO nanoarrays are well preserved after long-term testing, indicating the high stability for OER.

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Owing to unstable electrical energy generation from intermittent solar and wind energy sources, the development of efficient and durable nonnoble electrocatalysts is extremely desirable, in particular, those that can be used over a wide range of current densities for the hydrogen evolution reaction (HER), especially >1 A cm −2 , which is extremely desirable for energy transformation in power systems. Here, a nanosponge-like NiMo solid solution based on a nanoscale Kirkendall effect is fabricated via a one-step high-temperature sintering method. A polycrystalline layer of NiMo oxyhydroxide is formed on the surface of the asprepared electrode after aging, thus achieving a low overpotential and Tafel slope of 37 mV at 10 mA cm −2 and 39.2 mV dec −1 , respectively. The electrode displays superior electrochemical stability over a wide range of current densities without compromising its electrocatalytic activity; in particular, it is durable for over 300 h at 2 A cm −2 . This is attributed to the combination of the 3D-interconnected nanosponge structure, synergistic effect between the Ni and Mo species, lattice distortion of the solid solution, and its high conductivity. This work provides a possible commercialization for the fabrication of nonprecious electrocatalysts with high performance in large-scale energy conversion.

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