Microstructure and microhardness evolution of a Mg83Ni6Zn5Y6 alloy upon annealing

The microstructures and the corresponding microhardness evolution of a Mg83Ni6Zn5Y6 (at.%) alloy subjected to a series of heat treatments were investigated. In addition to the 14H long period stacking ordered (LPSO) phase, a diamond-cubic phase with the possible space group of Fd3¯m and lattice parameter of a = 0.73 nm were identified in the as-cast alloy. The combination of LPSO thin lamellae and the nanometer-sized cubic phase leads to a relatively high Vickers microhardness of about 116.2 ± 13.7. When annealed at 473 K and 573 K, the above diamond-cubic phase partially transforms to its structural variant, Fm3¯m, together with a slight change of lattice parameter (a = 0.72 nm). Precipitation of hexagonal Mg2Ni lath also happens at these moderate annealing temperatures. The remarkable increase of microhardness at this stage is believed to result from the strengthening of precipitates and the resultant coherent interface between the precipitates and their surrounding medium. When annealed at temperatures ranging from 673 K to 773 K, the Mg2Ni precipitates were dissolved, grain coarsening in the alloy occurred, Fd3¯m phase completely transformed to its Fm3¯m counterpart, and consequently microhardness of the alloy was decreased significantly.

► We have carried out a detailed study on microstructure and microhardness evolution in a Mg83Ni6Zn5Y6 alloy during annealing.
► The as-cast Mg83Ni6Zn5Y6 (at.%) alloy is composed of Mg, 14H-LPSO phase, and a diamond-cubic quaternary metastable phase (S′) with possible space group of Fd3¯m and lattice parameter of a = 0.73 nm. Multiple microtwins and highly dense stacking faults are the salient features in the S′ phase.
► Mg2Ni laths and a nanometer-sized S phase (Fm3¯m, a = 0.72 nm) co-precipitated on the consumption of the S′ phase upon annealing at 473 K and 573 K. After annealing at higher temperatures, the Mg2Ni and S′ phase transformed to the S phase, and the 14H-LPSO phase was replaced by the 18R-LPSO phase.
► The corresponding change of microhardness of Mg–Ni–Zn–Y alloy is strongly related to the microstructure evolution and phase transformation upon annealing.