Microstructural evolution during mechanical alloying of Mg and Ni

This study investigates the microstructural evolution during mechanical alloying of Mg and Ni, and during subsequent heat treatments of SPEX milled powders. The milling was performed under an inert argon atmosphere, employing two different ball-to-powder ratios (BPR): 10:1 and 20:1. It was observed that a large number of defects were produced during the first hours of milling. These defects led to the amorphization of part of the system, and promoted an extensive refinement of the microstructure as well. Later on, the formation of the Mg₂Ni compound due to the crystallization of the amorphous phase was observed. The microstructural details showed us that a possible complete sequence of phase transformations during milling of pure Mg and Ni could be: Mg_c + Ni_c → Mg_nc + Ni_c → amorphous + Ni_nc + Mg_nc → amorphous + Ni _nc + Mg₂Ni_nc → Mg₂Ni _nc, where c and nc stand for crystalline and nanocrystalline, respectively. On the other hand, subsequent heat treatments of the milled powder showed that the amorphous phase could also be transformed into Mg ₂Ni by heating. Finally, the knowledge of the evolution of the microstructure during milling and annealing were combined to propose an optimal processing route to efficiently obtain the Mg₂Ni compound.