Effect of chromium and molybdenum increment on the crystal structure, nanoindentation and corrosion properties of cobalt based alloys

This study investigated 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 was found in relation to the increasing chromium content for both 5 and 10 wt.% Mo contents, whereas reduced modulus demonstrated an inflection at 30 wt.% Cr, which was influenced by the molybdenum content. The open circuit potential, polarization resistivity and linear sweep voltammetry disclosed that passivity of Co-Cr-Mo alloys improved 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 confirmed that as the chromium content transitioned through the 25-30 wt.% range, the sigma (σ) phase began to develop within the combined matrix of hexagonal close pack martensite (ε) and face centered cubic austenite (γ) phase