Experimental Determination And Thermodynamic Modeling Of The Ga-Zr And Co-Cr-Nb Systems

Abstract:A thermodynamic database for multi-component materials is a crucial part of material design. A reliable and self-consistent database must be built on exact experimental determination and appropriate thermodynamic modeling of binary and ternary systems. In the present work, phase diagram information and thermodynamic information for the Ga-Zr and Co-Cr-Nb systems were experimentally investigated. And these two systems were optimized by CALPHAD method in the basis of experimental date from the present work and literatures. The major achievements of the present work are as follows:(1) Phase equilibria of the Ga-Zr system were investigated by experiment and thermodynamic modeling. In the experimental part, eleven alloys were prepared by melting the pure elements and annealed. Both the as-cast and annealed samples were analyzed by XRD, OM, and SEM. The annealed alloys were investigated by DTA and EPMA. In order to assist the thermodynamic modeling, the enthalpies of formation at0K for the GaZr2, Ga3Zr5, Ga2Zr3, Ga4Zr5, αGaZr, Ga3Zr2, Ga5Zr3, Ga2Zr and Ga3Zr phases were computed via first-principles calculations. The enthalpy of formation at298K for the aGaZr was measured by high temperature reaction calorimetry. Based on the experimental phase diagram data from the present work and the literatures as well as the present first-principles calculations, the Ga-Zr system was critically assessed by means of CALPHAD approach.(2) The phase equilibria at1100℃, vertical sections at Co-30at.%, Co-71at.%, Nb-9at.%and Nb-13at.%, and an liquidus projection of the Co-Cr-Nb system were investigated experimentally. The as-cast and equilibrated alloys were studied using XRD, SEM, and EMPA. The phase transformation temperatures were examined by DTA. The cubic C15, the hexagonal C14and C36polytypes of Laves phases occur as stable phases in the central part of the isothermal section at1100℃. With increasing Cr content, the polytype of Laves phase changes from C15to C14and back to C15in the NbCo2-NbCr2section at1100℃. The phase equilibria involving the Laves phases and the extensions of all phase fields were determined. The liquidus projection of the Co-Cr-Nb was established in the basis of as-cast microstructure analysis and DTA measurements(3) Based on the experimental data and the thermodynamic parameters of the three binary systems, the Co-Cr-Nb system were evaluated by CALPHAD method. The calculated phase diagrams agree well with the available experimental data.