Phase-controlled internal oxidation and solute redistribution in Sb- and Cr-modified Ag-Sn-In-Ni alloys

The internal oxidation of multicomponent Ag-based alloys is controlled not only by oxygen diffusion through the Ag matrix but also by the constitution and reactivity of solute-rich secondary phases. However, the phase-specific roles of minor Sb and Cr additions remain insufficiently clarified. In this work, Ag-6.5Sn-2.5In-0.5Ni-0.5Sb and Ag-6.5Sn-2.5In-0.5Ni-0.5Cr alloys were investigated by high-pressure oxidation, quasi-in-situ oxidation, X-ray diffraction, and electron probe microanalysis. After oxidation at 750 °C in pure O2 under pO2 ≈ 0.9 MPa, full-thickness cross-sectional observations revealed inward oxidation from both surfaces toward the specimen center, leaving a residual Ag-rich unoxidized core. The Cr-containing alloy showed a larger inward oxidation depth than the Sb-containing alloy, suggesting a higher degree of internal oxidation. Quasi-in-situ observations further indicated distinct phase-controlled oxidation features. In the Sb-containing alloy, In and Sn dissolved in the Ag matrix appeared to oxidize preferentially, followed by delayed oxidation of Ni-Sb-Sn-rich secondary phases and accompanied by oxygen ingress and outward redistribution of Sn, Sb, and Ni. In the Cr-containing alloy, Cr-rich regions showed preferential early-stage oxidation, whereas Ni-Sn-rich and mixed Ni-Cr-Sn regions were associated with complex Ni-, Sn-, Cr-, and O-containing oxide configurations at later stages. These results suggest that Sb favors a relatively sequential oxidation pathway, while Cr promotes reactive Cr-bearing regions and multiphase-coupled oxidation. This phase-dependent mechanism provides guidance for tailoring oxide morphology and optimizing high-pressure internal oxidation processing in Ag-SnO2-based electrical contact materials. The findings also highlight the importance of controlling secondary-phase type, distribution, and reactivity during alloy design.