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 Fd (3) over barm and lattice parameter of a = 0.73nm 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 473K and 573 K, the above diamond-cubic phase partially transforms to its structural variant, Fm (3) over barm, 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 673K to 773 K, the Mg2Ni precipitates were dissolved, grain coarsening in the alloy occurred, Fd (3) over barm phase completely transformed to its Fm (3) over barm counterpart, and consequently microhardness of the alloy was decreased significantly. (C) 2011 Elsevier B. V. All rights reserved.