Characterization of accumulative fold-forged magnesium-nickel multilayered composite structures

Abstract The newly developed severe plastic deformation (SPD) process of accumulative fold forging (AFF) was implemented for the production of multilayered composite structures by incorporating magnesium powder between nickel foil layers. The process was carried out up to 20 folding steps to produce a composite structure with more than a million number layers. By controlling the content of introduced magnesium powder and cyclic repeating of fold-forging steps, a reverse composite of magnesium matrix reinforced by fragments of nickel foil layers was produced. Microstructural characterization according was conducted using channeling contrast imaging by field emission-scanning electron microscopy (FE-SEM) and focused ion beam (FIB)/transmission electron microscopy (TEM) analyses, which revealed the formation of ultra-fine grains (UFG) with the average sizes of ~900 and 400 nm in the magnesium matrix and nickel foil layers, respectively. Moreover, a well-bonded structure was characterized at the interface of nickel folded layers and magnesium matrix was noted as a result of severe plastic shear straining during the AFF process as well as the deformation-assisted elemental inter-diffusion across the interface. Mechanical behavior of the produced multilayers composite structure was interrogated by indentation micro-hardness testing while displayed a fluctuation of hardness profile across the layers depending on the location of the micro-indenter. Considerable hardness improvements up to maximum values of ~300 and 60 HV are noted for the nickel layers and magnesium matrix, respectively, i.e., increases of around 100% compared to the primary annealed state of these metals as a result of the microstructural refinements.