YF 3 modification NiTi-Si coatings were prepared on Ti-6Al-4 V substrate by laser cladding. On this foundation, heat and cryogenic treatments were conducted respectively to explore the effect of after-treatment procedures. The consequence of post-treatment methods on the phase constituent, microstructure, mechanical property, and phase transition characteristics of post-treatment coatings was studied. The phase constituent of the three coatings was roughly the same, composed of NiTi and NiTi 2 . The results show that heat treatment led to the coarsening of NiTi phase grain size and increased Si content in the NiTi 2 phase. In addition, due to the rapid cooling rate provided by cryogenic treatment, the spinodal decomposition of the Si element occurred in the NiTi 2 phase. Furthermore, the post-treatments also affected the phase transformation characteristics of the coatings. As the temperature of martensitic transformation is sensitive to the content of Ni element in the NiTi phase, the M s and M f points of the heat treatment coating were slightly higher. In contrast to the heat treatment, the M s and M f points of the cryogenic treatment coating decreased. The microhardness of the three coatings is as follows: cryogenic treatment > original coatings > heat treatment. The cryogenic treatment coating has the highest microhardness due to grain refinement and spinodal decomposition. In addition, the cryogenic treatment coating showed the best wear morphology.
In the present work, NaBH 4 based hydrogen storage materials, 3NaBH 4 -(x)YF 3 -(1-x)GdF 3 composites, were prepared via mechanical ball milling with different values of x (2/3, 1/2, 1/3). The de-/rehydrogenation thermodynamic and kinetic behaviors of 3NaBH 4 -(x)YF 3 -(1-x)GdF 3 composites were systematically investigated. These composites showed a single endothermal peak of hydrogen desorption even though two metal fluorides were added simultaneously into NaBH 4 . All the 3NaBH 4 -(x)YF 3 -(1-x)GdF 3 composites showed reversible hydrogen sorption ability and the best hydrogen absorption kinetics was observed in the 3NaBH 4 -0.5YF 3 -0.5GdF 3 composite, with about 2 wt% hydrogen absorbed at 370 °C under 3.2 MPa H 2 pressure in 1 h. Its hydrogen absorption kinetic behaviors were correlated closely to a First-order reaction model based on experimental results. According to the pressure-composition-temperature (PCT) tests, the reversible hydrogen storage capacity increases, and the hydrogen desorption enthalpy decreases along with more GdF 3 addition. In particular, the desorption enthalpy with regard to the apparent Pauling's electronegativity (χ p ) of added metal cations can be described as Δ H d = −2748.21χ p +3852.99 kJ/mol H 2 , where χ p =(x)∙χ p (Y 3+ )+(1-x)∙χ p (Gd 3+ ). This research helps us to clarify the effect of co-addition of two rare earth metal fluorides on reversible hydrogen sorption in NaBH 4 .
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