Improved Mechanical Properties of EBM Biomedical Co–28Cr–6Mo–0.11N Alloy by the Dispersed Precipitation Control of ε-HCP Phase Based on ε ↔ γ Phase Transformation

Abstract

Mechanical properties of electron-beam-melted biomedical Co–Cr–Mo–N alloys can be improved by the grain refinement from reverse transform treatment, which transforms a low-temperature strong ε-phase into a high-temperature ductile γ-phase. Although mechanical properties of alloys consisting of a single ε- or γ-phase have been previously reported on, those comprising mixed ε- and γ-phases have not yet been investigated. Herein, the heat treatment conditions of the Co–28Cr–6Mo–0.11N alloy were determined to control the phase fraction while obtaining fine grains in the mixed phases with superior mechanical properties. The phase transformation behavior was analyzed. Superior mechanical properties were observed in the mixed phases containing 70 pct γ-phase and 30 pct ε-phase. The tensile and yield strengths were higher, and the elongation was approximately the same, compared to that of the single γ-phase. Moreover, the 30 pct ε-phase mixed-phase material obtained during the γ → ε heat treatment had a smaller overall average grain size and showed superior mechanical properties than that obtained during the ε → γ heat treatment. This study is expected to facilitate the application of biomedical Co–Cr–Mo–N alloys with fine grains and superior mechanical properties obtained via heat treatment.