Phase transformations due to isochronal annealing of Mg - rare earth - Sc-Mn squeeze cast alloys

Abstract

Abstract Magnesium alloys containing rare earths (Gd, Y, Ce, ≈ 3–10 wt.%), Sc (< 1 wt.%) and Mn (≈ 1 wt.%) were squeeze cast. Electrical resistivity and hardness responses to isochronal annealing were measured in the range 20–580°C and phase changes responsible were identified by transmission electron microscopy. In alloys containing Gd and Y simultaneous precipitation of three phases takes place, namely the metastable c-base-centred orthorhombic phase (Mg–Y, Mg–Gd) in the form of prismatic plates, the thermally stable hexagonal Mn2Sc phase in the form of fine basal discs and a metastable hexagonal coherent phase, containing Mn and Y or Gd, in the form of very thin basal plates (missing in the Mg alloy with 10 wt.% Gd). In the Ce-containing alloy the Mg12Ce tetragonal phase, present after casting in various forms, transforms into a rod-like form. Simultaneously, fine Mn2Sc discs precipitate on basal planes and the grain boundary eutectic transforms to the equilibrium phases.