Deformation induced FCC to HCP transformation in a Co–27Cr–5Mo–0.05C alloy

In this work, the role played by plastic straining on the FCC(γ) → HCP(ɛ) strain induced transformation (SIT) ɛ-martensite in a Co–Cr–Mo–0.05C alloy was investigated. It was found that alloy plastic deformation at room temperature promotes the development of ɛ-martensite giving rise to the formation of intragranular striations. Moreover, aging at 800 °C of pre-strained specimens initially promotes SIT ɛ-martensite. This is followed by a time plateau (1 h) delay after which isothermal ɛ-martensite develops. Apparently, prior plastic straining leads to increasing incubation times for the onset of the isothermal ɛ-martensite transformation. In addition, tensile specimens containing various amounts of isothermal ɛ-martensite were tested all the way to fracture and their respective stress–strain properties were experimentally determined. Comparisons between experimental and simulated flow curves using the rule of mixtures indicated some deviations in the simulations at volume fractions above 0.25 ɛ-martensite. In contrast, predictions of yield and tensile strength were in good agreement with the experimental outcome. In particular, linear trends were exhibited by both, the yield and tensile strength as a function of the volume fraction of ɛ-martensite. Finally, the alloy elongation remained nearly constant with the volume fraction of ɛ-martensite in the range of the 0.15 < HCP < 0.55. Higher or lower values lead to a rapid decrease in the exhibited elongation.

Research highlights▶ The strain induced transformation (SIT) in a low carbon (0.05%) Co–Cr–Mo alloy was investigated. ▶ It was found that the presence of SIT ɛ-martensite causes a time delay for the isothermal FCC to HCP transformation when the alloy is aged at 800 °C. ▶ The mechanical properties of the low carbon Co–Cr–Mo alloy containing various amounts of ɛ-martensite were determined. ▶ It was found that the alloy exhibited significant work hardening, yet the plastic flow properties did not follow the rule of mixtures.