Experimental investigation of the Ag-Ge-Zn system

Ag-based alloys have industrial importance also in relation to their use as high temperature solders in jewellery [1]. Silver solders usually are based on the Ag-Cu binary system, nevertheless Zn can be considered a candidate for replacing copper; it reduces the melting point of solder increases the fluidity of the alloys and deoxidizes the melt. Furthermore, the very good tarnish-resistance of Ag-Ge and Ag-Ge-Zn alloys [2] make them good candidates for bonding sterling silver (Ag92.5Cu7.5, wt %) which are commonly used in jewellery due to their very special lustre and very good mechanical properties. Moreover, the addition of Ge to sterling silver alloys can greatly reduce the oxidation during melting and casting. Concerning Ag-Ge-Zn alloys, neither literature data on their phase diagram nor on thermodynamic properties are available. The experimental investigation of the Ag-Ge-Zn phase diagram was performed by using combined microstructural, XRD and Differential Scanning Calorimeter (DSC) analyses. The samples were subjected to thermal analysis by a heat-flux DSC apparatus with heating and cooling rate of 0.5 or 0.3 °C/min. The microstructure of the samples, both after annealing and after DSC analysis, was studied by optical and scanning electron microscopy. Considering the slow heating and cooling rate, the isothermal section at room temperature was established. No ternary compounds were observed. On the basis of the experimental investigations the invariant reactions were identified. [1] Welding, Brazing and Soldering, ASM Handbook, Vol.6, eds. J.R. Davis, K. Ferjutz, N.D. Wheaton, M.S. Woods, ASM International, 1993. [2] M.R. Pinasco, G. Pellati, A. Saccone, G. Borzone, E. Ricci, R. Novakovic, A. Passerone, "Leghe a base di argento e procedimenti per la loro realizzazione, particolarmente per la fabbricazione di gioielli". Patent (2006). N. GE2006A000118