Phase transformations and phase equilibria in the La-Ni and La-Ni-Fe systems. Part 2: Isothermal sections at 750, 600 and 500 °C

Abstract The ternary phase diagram La-Ni-Fe was studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Three isothermal sections, at 750, 600 and 500 °C were constructed covering the whole concentration range. In addition, vertical sections at 15 and 35 at.% La were evaluated based on the abovementioned methods and DTA analysis. Among binary compounds, LaNi5, La5Ni19 and La2Ni7 have the widest homogeneity regions. The LaNi5 phase dissolves 19.9, 19.6 and 19.4 at.% Fe at 750, 600 and 500 °C, respectively. The solubility of Fe in La5Ni19 is 14.5, 12.1 and 8.9 at.% at 750, 600 and 500 °C, respectively. For the La2Ni7 phase, the Fe-solubility was found to be 13.9, 11.3 and 10.1 at.% at 750, 600 and 500 °C, respectively. The homogeneity regions of the remaining phases are much smaller. The character of phase equilibria at 750 °C in the Ni-rich region is similar to those at solidus temperature. The character of the phase equilibria at 600 and 500 °C, however, differs from those at higher temperatures. In particular, the equilibrium (γFe,Ni) + La5Ni19 which is present in the La-Ni-Fe system at solidus temperature and 750 °C, is absent at 600 and 500 °C. Instead, the alternative equilibrium LaNi5 + La2Ni7 is present at 600 °C. Furthermore, the equilibrium La2Ni7 + La2Ni3 which is present at solidus temperature, is replaced by the alternative equilibrium (αFe) + La7Ni16 at lower temperatures.