Development of surface composite based on Mg–Al–Ni system on AZ31 magnesium alloy and evaluation of formation mechanism

Highlights: • This research showed that by applying friction stir processing (FSP) in-situ surface composite based on Mg–Al/Ni alloying systems is produced on AZ31 plate under different FSP passes. • Thermodynamic and kinetic study of interfacial solid state reactions confirmed phase formation during different FSP passes. • Moreover, the effective Gibbs free energy of formation graphs for the compounds were plotted at 711 K. • Based on kinetic standpoint, schematic diagram of Mg–Ni intermetallic compounds during difference FSP passes is plotted. • Due to presence of intermetallic phases the mean hardness of the stir zone reached about 106 Hv, which is about two times higher than the base metal. - Abstract: The in-situ synthesis of Mg–Al–Ni composite on the surface of AZ31 plate by friction stir processing (FSP), has been investigated in this article. The unprocessed AZ31 plate consisted of grains of 25 μm size. By increasing the number of FSP passes from one to five, the grain size of the AZ31 plate decreased to 7.5 and 3 μm, respectively. A uniform distribution of the reinforcements was also obtained by increasing the number of FSP passes. Based on the results of X-ray diffraction (XRD) and Energy dispersive spectrometry (EDS) analyses, Mg₂Ni and Al₃Ni₂ intermetallic compounds are in-situ formed by a single-pass FSP of the composite specimens. By increasing the number of FSP passes, the amounts of Mg₂Ni and Al₃Ni₂ compounds are dramatically reduced and AlNi and MgNi₂ intermetallic compounds take their place. Thermodynamic and kinetic of interfacial solid state reactions were studied to determine the reactive mechanisms and phase evolutions during different passes of FSP. The maximum amount of hardness (∼106 Hv), was obtained for the composite sample after five passes of FSP