MECHANICAL-PROPERTIES OF INTERNALLY OXIDIZED SILVER ALLOYS

An increase in strength and a decrease in strain have been observed in Ag-0.4at.%Mg and Ag-0.2at.%Cu alloys during internal oxidation at 420-degrees-C under 1 atm oxygen. This phenomenon, more pronounced in Ag-Mg alloy, is explained by a difference in coalescence degree: small precipitates are found in Ag-Mg (diameter about 1.5 nm) and voluminous precipitates in Ag-Cu (diameter about 10(3) nm). A correlation between mechanical properties and gravimetric studies shows that the most important modifications of these properties occur when a maximal oxygen quantity is stored in irregular and non-compact clusters. This amount is higher than the quantity required for stoichiometric oxide, MgO. Moreover, these clusters evolve towards a more compact structure by a release of oxygen and a small evolution of mechanical properties. In Ag-Mg alloys the grain boundary segregation of magnesium induces a small grain size (about 15 mum); when oxygen penetrates this matrix, the size increases to about 40 mum, allowing an increase in plastic deformation. During internal oxidation of Ag-Mg alloys a high concentration of dislocations in observed, contributing to the strengthening of the material.