On the constitution and thermodynamic modeling of the phase diagrams Nb-Mn and Ta-Mn

Abstract

The constitution of the two phase diagrams Nb-Mn and Ta-Mn has been determined from light optical and transmission and scanning electron microscopy (LOM, TEM and SEM) with energy dispersive (EDX) as well as wavelength dispersive (WDX) X-ray spectroscopy, X-ray powder (XPD) and single crystal diffraction (XSCD), differential thermal analysis (DTA) and/or differential scanning calorimetry (DSC). The Laves phases NbMn2 and TaMn2 are the only binary compounds in these systems. High-temperature differential thermal analyses revealed congruent melting for NbMn2 with Tm(NbMn2) = 1515 ± 15 °C, whereas TaMn2 melts incongruently with Tm(TaMn2) = 1797 ± 40 °C close to a depleted peritectic reaction. Both Laves phases engage in eutectic reactions ℓ ↔ (Mn) + Nb(Ta)Mn2 (Teut = 1220 ± 10 °C at 4.9 at% Nb and Teut = 1234 ± 10 °C at 0.7 at% Ta, respectively). NbMn2 also forms a eutectic with (Nb): ℓ ↔ (Nb) + NbMn2 at Teut = 1493 ± 15 °C and 53.2 at% Nb. Mn shows remarkably large maximum solid solubilities of 19.4 at% Mn in (Nb) as well as of 21.3 at% Mn in (Ta). Detailed atom site distribution has been established for the Laves phases by means of temperature dependent X-ray single crystal data (both C14 - MgZn2-type). Combined data from XPD, EDX/WDX and SEM microstructure indicate that for both Laves phases extended homogeneity regions exist: Nb1+xMn2−x (62.5–73.0 at% Mn at 950°C: −0.19≤x≤0.125) and Ta1+xMn2−x (59.5–68.5 at% Mn: −0.055≤x≤0.215). Density functional theory (DFT) calculations favor Nb(Ta)/Mn antisite occupation rather than defects. The phases, “NbMn” and “TaMn”, adopted earlier in the literature as binary system inherent compounds, were shown (TEM, WDX electron microprobe data and X-ray Rietveld refinements) to be oxygen stabilized phases of the Ti4Ni2O type (so-called eta(η)-phases) with modified Nb(Ta)/Mn site substitution to comply with the formula Nb(Ta)3−xMn3+xO1−y (defect η-W3Fe3C-type). From magnetic susceptibility and magnetization measurements, both oxide stabilized eta phases η-Nb3Mn3O1−y and η-Ta3Mn3O1−y were found to order ferromagnetically below Tc ~ 77 K, but the Laves phases NbMn2, TaMn2 reveal weakly temperature dependent paramagnetism. No trace of the rhombohedral μ-phase (W6Fe7-type) has been encountered in our investigation of the two binary phase diagrams. Thermodynamic and transport properties (specific heat, electrical resistivity and magnetic susceptibility/magnetization) classify the Laves phases with metallic behavior whilst mechanical properties (elastic moduli from DFT and nanoindentation as well as hardness and thermal expansion) group both Laves phases among rather hard and brittle intermetallics. Based on (i) the experimentally derived constitution of the Nb-Mn and Ta-Mn systems, and (ii) on new own DFT data of the energy of formation of the Laves phases, a CALPHAD (CALculation of PHAse Diagrams) calculation of both systems was made providing a complete set of optimized thermodynamic data. Furthermore, the DFT calculations provided information on the instability of the η-Ta3Mn3 structure and the atom-site specific stabilization effect of oxygen.