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 Mg-Cu 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 Mg-Cu diffusion couples. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.