Effect of Chromium and Molybdenum Increment on the Crystal Structure, Nanoindentation, and Corrosion Properties of Cobalt-Based Alloys

This study investigates the influence of chromium and molybdenum content (Cr: 20–35 wt%; Mo: 5 and 10 wt%) in a cobalt–chromium–molybdenum alloy system on the phases present in the microstructure, the nanomechanical, and electrochemical behavior of the alloys. A linear increase in the hardness is found in relation to the increasing chromium content for both 5 and 10 wt% Mo contents, whereas reduced modulus demonstrates an inflection at 30 wt% Cr, which is influenced by the molybdenum content. The open-circuit potential, polarization resistivity, and linear sweep voltammetry disclose that passivity of Co–Cr–Mo alloys improves in line with increasing Cr until 30 wt%, with any further increase causing a depreciation in the corrosion properties. X-Ray diffraction and optical metallographic analysis confirm that as the chromium content transitions through the 25–30 wt% range, the sigma (σ) phase begins to develop within the combined matrix of hexagonal close pack martensite (ε) and face-centered cubic austenite (γ) phase.