Systematic Study on the Microstructures of Biomedical Co–20Cr–15W–10Ni Alloys with Carbon Contents Ranging from 0 to 0.2 mass pct

Abstract

Herein, the effect of carbon content on the microstructures of Co–20Cr–15W–10Ni (mass pct, CCWN) alloys was systematically studied. For this, CCWN alloys with carbon contents of 0, 0.05, 0.10, and 0.20 mass pct, i.e., 0C, 0.05C, 0.10C, and 0.20C alloys, respectively, were prepared using an induction melting furnace. The as-cast alloys were solution treated at 1523 K for 7.2 ks, followed by cold swaging and heat treatment at 1173 K–1473 K for 0.15–7.2 ks. Consequently, η-phase (M6C-M12C type, M: metallic element) precipitates were detected in the as-cast 0.10C and 0.20C alloys, whereas no precipitates were observed in the 0C and 0.05C alloys. These precipitates were dissolved via a solution treatment. After cold swaging, the ε-phase formed through a strain-induced martensitic transformation in the 0C and 0.05C alloys. Following heat treatment, a single γ-phase matrix was observed in all the alloys, and μ-phase (Co7W6-type) and η-phase precipitates were observed in the 0C and carbon-added alloys, respectively. The precipitation temperature range of the η-phase increased with increasing carbon content. The stability of the precipitates qualitatively conformed with that of the calculated phase diagram. This is the first paper that reports the microstructural changes in CCWN alloys with varying carbon contents.