Experimental investigation and first-principle calculations coupled with thermodynamic modeling of the Mn–Nd phase diagram

Abstract

The complete Mn–Nd phase diagram was established experimentally by means of key samples and diffusion couple techniques. The phase transformation temperatures, crystal structures and phase equilibria were studied using differential scanning calorimetry (DSC), X-ray diffraction (XRD), electron probe microanalysis (EPMA), and scanning electron microscope (SEM) techniques. Three compounds in the Mn-rich side and two terminal solid solutions in the Nd-rich side were observed. The compounds Mn 2 Nd, Mn 23 Nd 6 , and Mn 17 Nd 2 form peritectically at 850, 940, and 1025 °C, respectively. The eutectoidal decompositions of the compounds, Mn 2 Nd, and Mn 23 Nd 6 , were confirmed in the temperature ranges of 650–550 and 550–400 °C, respectively, using EPMA. The maximum solubility of Mn in DHCP-Nd was found to be 2.3 at% Mn at the 685 °C eutectic temperature. The solvus line of DHCP-Nd was determined using EPMA. The solubility of Mn in BCC-Nd was extrapolated from DSC data to be 5.0 at% Mn at 728 °C. The existence of a Mn 17 Nd 2 phase of the Th 2 Ni 17 type structure was confirmed using EPMA and XRD. The system was modelled using CALPHAD methodology. The quasi-chemical model (QCM) was used to describe the liquid phase, the terminal solution phases were modeled as substitutional solutions using the random mixing model, and the intermetallic compounds were treated as stoichiometric phases. The enthalpies of formation of the system compounds were calculated using the electronic density functional method. The resulting enthalpy of mixing was in good agreement with the literature. • The Mn–Nd phase diagram has been constructed experimentally. • First-principle calculations coupled with thermodynamic modeling have been performed. • A self-consistent set of parameters have been obtained using CALPHAD approach. • Modified quasi-chemical model has been used to model the liquid phase.