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Abstract Mg 80– x Ni 20 Y x (x = 0–7) alloys were successfully prepared by using the vacuum induction melting method. The structural characterizations of the alloys were performed by using X-ray diffraction, scanning electron microscope, transmission electron microscope and X-ray photoelectron spectroscopy measurements. The effects of yttrium content on the microstructure and hydrogen storage properties of the as-cast alloys were investigated. The alloys are composed of a primary phase of Mg 2 Ni, lamella eutectic composites of Mg + Mg 2 Ni, and some amount of YNi 3. The YNi 3 has completely transformed into in situ formed nanoscale YH 2 and YH 3 , with the formation of Mg 2 NiH 4. Pulverization of the alloy particles was observed during the hydrogen absorption and desorption cycles. However, the phase composition remains unchanged even after 20 hydrogenation cycles. Yttrium addition significantly improved the hydrogen-absorption kinetic performance of the alloy. In addition, pressure-composition-temperature measurements indicated that the entropy and enthalpy changes for the formation and decomposition of MgH 2 and Mg 2 NiH 4 gradually decreased with the increase of yttrium content. Highlights • YNi 3 , Mg 2 Ni and lamella eutectic Mg + Mg 2 Ni phases were observed in the alloys. • YNi 3 fully decomposed, with the formation of nanometer YH 2 and Mg 2 NiH 4. • Increasing Y content significantly improves the hydrogen absorption kinetics. • The decomposition peak temperature was reduced from 586 K to 523 K. • Δ H and Δ S of the Mg/MgH 2 and Mg 2 Ni/Mg 2 NiH 4 transformations decreased. [ABSTRACT FROM AUTHOR]