Thermodynamic re-assessment of the Mg–Gd–Y ternary system coupling with the driving forces for phase precipitations during aging process

Abstract

Based on the available experimental phase equilibrium relations and aging precipitation sequences, the Mg–Gd–Y ternary system has been thermodynamically re-assessed by means of CALPHAD technique. To simulate the experimentally reported aging precipitation sequence, α(Mg)SS(supersaturated)→ GP zones (D019-type, metastable)→ β'-Mg7Gd (c-bco, metastable)→ β1-Mg3Gd (fcc, metastable)→ β-Mg5Gd (fcc, stable) near the Mg–Gd side, and α(Mg)SS(supersaturated)→ β'-Mg7Y (c-bco, metastable)→ β-Mg24Y5 (bcc, stable) near the Mg–Y side, the effective nucleation driving forces obtained by deducting the nucleation resistances from the thermodynamic driving forces are calculated and analyzed. Two metastable components, GP zones (D019-type) and β' (c-bco) ordered from α(Mg)SS, do not exist in the stable equilibrium phase diagram but appear in the annealing process of typical alloys. The Redlich–Kister equations are adopted to describe three solution phases, Liquid, HCP_A3 and BCC_A2. The intermediate compounds Mg2Y, Mg24Y5, Mg2Gd, Mg3Gd and Mg5Gd are expressed by the formulas of (Mg,Y)2/3(Gd,Mg,Y)1/3, Mg24/29(Gd,Mg,Y)4/29Y1/29, (Gd,Mg)2/3(Gd,Mg,Y)1/3, (Gd,Mg)3/4(Gd,Mg,Y)1/4 and Mg5/6(Gd,Mg,Y)1/6, respectively. In particular, the two-sublattice models (Gd,Mg,Y)1/2(Gd,Mg,Y)1/2, (Gd,Mg,Y)3/4(Gd,Mg,Y)1/4 and (Gd,Mg,Y)7/8(Gd,Mg,Y)1/8 have been respectively used to describe the stable Mg(Gd,Y) (BCC_B2) alloy compound as well as the metastable GP zones (D019-type) and β' (c-bco) phase, in order to cope with the order-disorder transitions. A set of self-consistent thermodynamic parameters has been obtained to ensure the thermodynamic calculations well consistent with the reported experimental data, containing not only the stable equilibrium phase diagram but also the aging precipitation sequence.