Experimental determination of the phase equilibria of the Mg-Nd-Zn system at 320 °C

The phase equilibria of the Mg-Nd-Zn system at 320 °C were investigated with an Mg-Nd-Zn diffusion couple and 34 equilibrated alloys, by means of X-ray diffraction technique and electron probe microanalyses. Eight ternary phases, denoted as τ-τ7 and T-NdMg12, respectively, were found at 320 °C. It was revealed that the T-NdMg12 phase is based on the metastable NdMg12 compound and stabilized by the substitution of Zn for Mg. The τ1 phase with a C-centered orthorhombic crystal structure was determined to have a composition range of 7.4-7.7 at.% Nd and 25.8-40.1 at.% Zn. The τ2 phase with a hexagonal crystal structure was measured to have a composition range of 6.3-7.8 at.% Nd and 61.0-64.0 at.% Zn. The crystal structures of the five ternary phases τ3-τ7 are still unknown. Four ternary phases, viz. T-NdMg12 and τ5 to τ7, can be regarded as the newly found ternary phases in the present work. According to the present work, the τ3, τ4 and τ5 phases have the Zn composition ranges of 61.6-64.6 at.% Zn, 54.4-67.9 at.% Zn and 59.5-64.5 at.% Zn, and have the approximate Nd contents of 10.4, 13.5 and 17.2 at.% Nd, respectively. The τ6 phase has a approximate chemical composition of Nd15.7Mg8.4Zn75.9 while the τ7 phase has a composition range of 58.0-60.2 at.% Zn at about 21.3 at.% Nd.