Hardness and microstructure of internally oxidized silver alloys

Alloys of Ag, Ag/1.12 at.% Mg, and Ag/0.25 at.% Mg-0.25 at.% Ni were internally oxidized at 450 to 825 C and their hardnesses and microstructure were observed. Microhardness profiles showed that hardness was high near the surface and decreased with depth into the sample. Microstructure contained regions of small and large grains, where region sizes were dependent on treatment temperature. Transitions between small- and large-grained regions were abrupt. Treatments at higher temperatures failed to alter grain structure, indicating that Mg-O clusters had indeed pinned the grain boundaries. Clustering was shown to affect the hardness of oxidized alloys, not only by inhibiting grain growth, but also by inhibiting dislocation motion

High-temperature internal oxidation of Ag/1.2at.% Mg and Ag/0.25at.% Mg-0.25at.% Ni.

High-temperature oxygen diffusion and internal oxidation in Ag, Ag/1.2 at.% Mg (Ag-Mg), and Ag/0.25 at.% Mg-0.25 at.% Ni (Ag-Mg-Ni) have been studied, mostly in air and 8% O{sub 2}, at 450-835 C. The focus of the studies was on thermogravimetric analysis, microhardness tests, and optical and electron microscopy observations of grain growth and its inhibition by oxidation. The internal oxidation of both alloys exhibited nearly identical activation energies (0.81 eV for Ag-Mg and 0.83 eV for Ag-Mg-Ni) and rate constants. The maximum O content of both alloys was superstoichiometric (e.g., O/Mg > 1.0) and the maximum O/Mg ratios were higher at lower temperatures than at higher temperatures (e.g., 1.25 at 500 C and 1.05 at 800 C). Diffusion of O in pure Ag was {approx}60 times faster at 825 C and {approx}400 times faster at 500 C than internal oxidation of either of the Ag alloys. Grain growth of both alloys and of the Ag was quantified between 450-800 C and related to internal oxidation