•Alloying Mg with Cu cannot occur in MgCu diffusion couples under 453 K.•Mg2Cu can grow under 453 K in Mg/Cu super-laminates composites (SLCs).•Activation energies for layer growth process of Mg2Cu in Mg/Cu SLCs were evaluated.The formation mechanism of microstructures in Mg/Cu super-laminates composites (SLCs) fabricated by accumulative roll-bonding (ARB) during initial activation was investigated. It is revealed that Mg2Cu can grow with sufficient growth rate at low temperatures (<453 K) in Mg/Cu SLCs. The growth of Mg2Cu layers for short growth length is diffusion-controlled. The activation energies for layer growth process of Mg2Cu layers in Mg/Cu SLCs are 107 ± 8 kJmol−1 with acid cleaning prior to ARB and 103 ± 12 kJmol−1 without acid cleaning prior to ARB, respectively, which are about 2/3 of those in MgCu diffusion couples. It is considered that the high density of lattice defects in Mg/Cu SLCs fabricated by ARB contributes the much lower activation energies for layer growth process of Mg2Cu layers in Mg/Cu SLCs than in MgCu diffusion couples.

•Mg/Cu SLCs could be hydrogenated by three kinds of processes.•The micro/nano-structures of Mg/Cu SLCs after initial hydrogenation can be classified in three.•These are an open and a sheathed network of MgCu2 coexisting with MgH2, and a layer of MgCu2 existing between MgH2 and Cu layers.•A sheath of MgCu2 retards hydrogenation of Mg2Cu.Formation mechanism of micro/nano-structures through competitive reactions in Mg/Cu super-laminate composites (SLCs) during initial hydrogenation was investigated experimentally. Three types of MgCu2 structures, an open network, a sheathed network and a layer, were observed after initial hydrogenation of Mg/Cu SLCs under the conditions of 573 K, 86.4 ks in H2 atmosphere of 3.3 MPa. Hydrogenation of Mg (1) and alloying Mg with Cu to form Mg2Cu (2) followed by hydrogenation of Mg2Cu (3) should be competitive. It is proposed that an open network of MgCu2 can be formed when (1) occurs later than (3), a sheathed network when (1) occurs later than (2) and earlier than (3), and a layer when (1) occurs earlier than (2). However, the reaction of MgH2 to Cu under H2 atmosphere is not known in general. Therefore, in order to prove the existence and the generality of this reaction, pellets of MgH2 and Cu powder were heated under the condition of 673 K, 86.4 ks in H2 atmosphere of 8.0 MPa. The formation of a layer of MgCu2 was confirmed by SEM observations, EDS analyses and XRD measurements.

•The effect of initial structures of Mg/Cu super-laminates on hydrogen absorption/desorption properties was investigated.•Three types of specimens were prepared in cold rolling. Each of them had a fine, medium, coarse microstructure, respectively.•Two types of MgCu2 forms, 3D-network and layered forms, were observed after hydrogenation.•Alloying with Mg and Cu, and hydrogenation of Mg are competitive processes.•The dominant process depends on the initial structures of Mg/Cu super-laminates.The effect of initial structures of Mg/Cu super-laminates on hydrogen absorption/desorption properties was investigated experimentally. Mg/Cu super-laminates were prepared by a repetitive fold and roll method using a conventional two-high roll mill. Three types of specimens were prepared by changing rolling reduction in cold rolling. Each of them had a fine, medium, coarse microstructure, respectively. Hydrogen absorption/desorption properties were measured with HP-DSC and a Sieverts’ type instrument. Microstructures were observed with SEM. Two types of MgCu2 forms, 3D-network and layered forms, were observed after hydrogenation under the conditions of 573 K, 86.4 ks and 3.3 MPa of H2, respectively. This implies that Mg/Cu super-laminates can be hydrogenated by two kinds of processes. The dominant process depends on the initial structures of Mg/Cu super-laminates. Repetitive hydrogenation/dehydrogenation tests using HP-DSC indicate that, even after 20 cycles, the initial microstructures of Mg/Cu super-laminates can affect their hydrogen absorption/desorption properties.