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 α(Mg) crystals, equilibrium phase Mg5(Gd,Y), and non-equilibrium phase Mg2(Gd,Y). With the increase of solution treatment time, the sizes of the semicontinuous Mg5(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 α(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 °C. The microstructural evolution of the alloy after aging at 175 °C was as follows: the β″ phase precipitated from α(Mg) during the under-aging stage (4–32 h); next, the β′ phase precipitated from α(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 β′ and β″ phases; finally, during over-aging (>100 h), the size of β′ phase continually increased while transforming into the β1 and β phases.