Experimental and Calculated Ag + Au + Ge Phase Diagram

The investigation of the equilibrium phase diagram of the Ag + Au + Ge system has been carried out by the following ways: (a) the location of equilibrium surfaces was determined on the whole composition range by high temperature isoperibolic calorimetry and differential thermal analysis; (b) the equilibrium temperatures of the ternary system were calculated from the equilibrium temperatures and the thermodynamic functions referring to the three limiting binary alloys Ag + Au, Ag + Ge, Au + Ge. A satisfactory agreement was found between the calculated liquidus and the one obtained by calorimetry and thermal analysis. In the course of a systematic thermodynamic investigation of ternary alloys based on gold, silver, and a IV b metal, the three systems Ag + Au + Si, Ag + Au + Ge, and Ag + Au + Sn were examined; the molar enthalpies of formation of the liquid mixtures were obtained on the one hand and the equilibrium phase diagrams on the other.^1,2,3 This paper focuses on the latter topics for the ternary alloys Ag + Au + Ge; a comparison is carried out between the equilibrium temperatures measured by differential thermal analysis at the laboratory S.E.T.T. in Marseille and those calculated at the Royal Institute of Technology in Stockholm. This calculation is based on the thermodynamic data published for the limiting binary systems and also on the ternary enthalpies measured by calorimetry at very high temperature.     

Electrochemically-induced reactions at Ni/ZrO2 interfaces

A symmetrical solid state galvanic cell, Ni/ZrO₂ + 9.5 mol% Y₂O₃/Ni, was used as a model system to study and modify chemical reactions at Ni—ZrO₂ interfaces. The cell was produced by diffusion bonding Ni on either side of a single crystal of yttria-doped cubic zirconia. Different oxygen activities were established at the interfaces by applying an electrical potential across the galvanic cell. When the electrochemical potential was greater than a critical value, of the order of V, the intermetallic compound Ni₅Zr formed at the interface with low oxygen activity and NiO formed at the interface with high oxygen activity. Under these experimental conditions, the ionic transference, number of the electrolyte was ∼0.03. In order to avoid internal, electrical short circuiting of the cell, a voltage had to be applied during cooling. In a different experiment, after applying the electrical current, the opn circuit voltage of the cell was measured. During this period, the cell was short circuited internally, which caused the oxidation of the Ni₅Zr layer to Ni and monoclinic ZrO₂ and the reduction of the NiO layer to Ni. The microstructure, chemistry and morphology of the phases, grown under different conditions, were investigated using scanning and transmission electron microscopy. In addition, the measured and calculated thickness of the reaction products were compared.     

Phase transformations at steel/IN626 clad interfaces

The microstructures of 4130 and 2.25Cr-1Mo steels clad to nickel base IN625 by welding and HIPing were examined by Analytical Electron Microscopy (AEM) and Secondary Ion Mass Spectroscopy (SIMS) to determine the interfacial microstructural characteristics which could affect their mechanical properties and corrosion resistance. The interface microstructures of the clads produced by the two methods were considerably different. The clad produced by welding was characterized by a low density of carbide precipitates confined to a very narrow region (∼1 μm) at the interface of ferrite and austenite. In addition, a thin region of untempered martensite was present at the interface which could affect its resistance to hydrogen embrittlement as well as other mechanical properties. The interface of the HIP clad composite contained several regions of distinct microstructural characteristics with widely varying densities of carbide precipitates. Relative to the clad produced by welding, extensive precipitation was observed both in the steel and in the IN625 at the interface, separated by a region free from precipitation. The extent of precipitation at the interface regions appears to be controlled essentially by the extent of carbon transport across the interface. The article describes the detailed analysis of the interface characteristics, and models are proposed to explain the microstructural evolution at the interface of the HIP and weld clad composites.