To investigate the effect of Nb on the dehydrogenation properties of Mg–Nb composite films, Mg/Nb eight-layer film and Mg-10 at% Nb alloy film with the similar Mg-to-Nb atomic ratio were prepared by magnetron sputtering. Results show that both Mg/Nb eight-layer film and Mg-10 at% Nb alloy film exhibit excellent de/hydrogenation properties. Mg-10 at% Nb alloy film starts to release hydrogen at 108 °C, and its desorption peak temperature is lower to 146 °C, which is much better than that of pure MgH2 under the same condition. Scanning electron microscopy (SEM) results demonstrate that the dispersive Nb nanoparticles in Mg/Nb eight-layer film may serve as nucleation sites for Mg ↔ MgH2 reactions, which can provide channels for H diffusion. For Mg-10 at% Nb alloy film, the uniform distributions of Nb can accelerate the hydrogen diffusion and effectively improve the dehydrogenation kinetics for MgH2. This study provides an enlightening way for designing and preparing Mg-based composite films with excellent dehydrogenation properties.

•Microstructure changes of austenitic steel PH15-7Mo were due to alloying elements, service condition and carbide M23C6.•Lath-shape martensitic laths keep pseudo {112} twinning relationship.•β-NiAl particles hold a typical cubic-to-cubic orientation relationship with martensite.•M23C6 carbide kept a cubic–cubic orientation relationship (OR) with austenite and an unusual G-T OR with martensite.•Multiple orientation relationship between M23C6 and austenite is correlative with their structural similarity.The microstructure and crystallographic features of a semi-austenitic precipitation hardening steel PH 15-7Mo during solution treatment, roddrawing and aging were investigated by means of optical microscope, X-ray diffraction analyzer and transmission electron microscope. It was found that the microstructure of the steel was consist of dominant austenite, small amount of martensite and 10–15 vol.% δ-ferrite after solution treatment at 1050 °C followed by cooling in water at room temperature. The austenite transformed into lath martensite during tensile roddrawing about 60% deforming companied with some coherent fine β-NiAl particles precipitated within martensite. With higher aging temperature and longer holding time, tiny carbide M23C6 particles precipitated from martensite, which kept the cubic–cubic orientation relationship (OR) with austenite and G-T OR with martensite which is different with all the reported orientations. The OR between tiny carbide M23C6 particles G-T OR with martensite was discussed in terms of crystallography of phase transformations.