Physical metallurgy of metastable bcc lanthanide-magnesium alloys for R = La, Gd, and Dy

Abstract

Bcc La-Mg, Gd-Mg, and Dy-Mg alloys have been prepared by an ice water/acetone quench from liquid melts. Single-phase alloys could be retained in a window around the eutectoid composition: 13 to 22 at. pct Mg, 23.6 to 29 at. pct Mg, and 27 to 29 at. pct Mg for La, Gd, and Dy alloys, respectively. At the center of the windows, X-ray diffraction peaks are extremely sharp as in equilibrium bcc structures; however, as alloy composition is moved away from the eutectoid, line broadening is observed. Reversion of the bcc phase to the equilibrium micro-structure for R-Mg alloys (R = La, Gd, or Dy) has been characterized by differential thermal analysis (DTA) or differential scanning calorimetry (DSC) and isothermal annealing. La-Mg alloys revert directly to c~La (dhcp) + LaMg at about 350 °C when heated at 10 °C/min. In contrast, the Gd and Dy alloys revert by a two-step process: first, a transition to an intermediate distorted hcp phase between 300 °C and 400 °C and, second, the relaxation of this phase to αR (hcp) + RMg at about 490 °C when heated at 10 °/min. Isothermal annealing and high temperature X-ray diffraction confirm the nature of these reactions.