Phase Equilibria And Thermodynamic Properties Of Fe-ni-zn And Mg-ge Study

Steel sheets are the substrate for hot-dip galvanized coatings while Ni is one additive element in the hot-dip galvanized alloys. Research on the phase equilibria of Fe-Ni-Zn ternary system is of foundamental importance to provide scientific guidance with the optimization of galvanizing process. The phase equilibria of the Fe-Ni-Zn ternary system at 550℃were investigated by using the diffusion couple technique and alloy method. The whole isothermal section of Fe-Ni-Zn system at 550℃was determined with 15 key alloys and 4 diffusion couples by means of the X-ray diffraction and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM /EDX). The existence of a ternary phase, the T phase, was confirmed. The homogeneity ranges of some intermetallic phases in the system were preliminarily determined, and their lattice parameters were calculated.Taking into account the presently determined isothermal section and the information from the literature, the thermodynamic optimization of the Fe-Ni-Zn system was performed. A set of self-consistent thermodynamic parameters for the Gibbs energies of individual phases in the Fe-Ni-Zn system was then obtained. Comprehensive comparisons between the calculated and measured phase diagrams showed that most of the experimental information is satisfactorily accounted for by the present thermodynamic modeling.The Al-Mg-Ge alloy is widely used in electronic equipment, spacecraft, and automotive industries because of its good thermal conductivity, high temperature strength and age hardening rate, etc. In addition, germanium can effectively refine the grain size of (α-Mg) phase for magnesium alloys and the ultimate tensile strengths of the alloys increase with the increment of the volume fractions of eutectic structure. Therefor, research on Mg-Ge system is of important practical significance.The Mg-Ge system was reassessed by means of the CALPHAD approach, supplemented with first-principles calculation. All of the experimental phase diagram and thermodynamic data available in the literature are critically reviewed and assessed. The substitutional solution model and associate solution model were seperately employed to describe the Gibbs energy of the liquid phase for comparison. The enthalpy of formation for Mg2Ge was calculated by first-principles calculation with a desire to clarify the discrepancies in the literature data reported for the enthalpies of formation. Two sets of self-consistent thermodynamic parameters for the Mg-Ge system were finally obtained, as the substitutional solution model and the associate solution model were alternatively employed for the liquid phase. Some improvements have been made in the present work, compared with the previous assessments.