Microstructure evolution of Mg-6Gd-2Y alloy during solid solution and aging process

The microstructure evolution of Mg-6 wt% Gd-2 wt% Y alloy (denoted as GW62) during solid solution and aging process were investigated by optical microscope, scanning electron microscope, energy dispersive spectroscopy, X-ray diffraction and high resolution transmission electron microscopy. The microstructure of as-cast GW62 alloy is comprised of isometric alpha(Mg) crystals, equilibrium phase Mg-5(Gd,Y), and non-equilibrium phase Mg-2(Gd,Y). With the increase of solution treatment time, the sizes of the semicontinuous Mg-5(Gd,Y) phases gradually decrease until dissolved. Rare-earth-rich phases (Mg(Gd,Y)(2)) with fcc structures a=0.56 nm were newly generated in the alloy at grain boundaries and inside grains after solution treatment, which could hinder the growth of isometric alpha(Mg) crystals in the solution process. The maximum hardness of the GW62 alloy is significantly different for different aging temperatures. The age-hardening effect is most apparent at 175 degrees C. The microstructural evolution of the alloy after aging at 175 degrees C was as follows: the beta '' phase precipitated from alpha(Mg) during the under-aging stage (4-32 h); next, the beta' phase precipitated from cc(Mg) up until peak aging was reached (32-100 h); as aging time progressed, the hardening reached its maximum at 100 h. At this stage, the precipitated phases were the beta' and beta '' phases; finally, during over-aging (> 100 h), the size of beta' phase continually increased while transforming into the beta(1) and beta phases. (C) 2015 Elsevier B.V. All rights reserved.