Abstract
In the current research, we studied the role of the solution treatment and aging on the microstructure of a Co–28Cr–5Mo–0.3C alloy. We used metallographic observations, scanning electron microscopy (SEM), and hardness measurements for the evaluations. We also made a comparison between the phase equilibrium calculated with Thermo-Calc, using TCFE8 and TCNI8 thermodynamic databases and experimental findings. The experimental results showed that the transformation of the metastable FCC phase to the HCP phase during aging was extremely sensitive to the solution treatment prior to aging. The effect of the increase in the solution temperature and time was detectable through promotion of the martensitic transformation during quenching in which HCP1 (straight bands) and HCP2 (lamellar-type constitution) phases had developed. In contrast, a low solution temperature and time caused most of the primary carbides to remain in an undissolved condition in the matrix; therefore, during aging, no sign of the FCC to HCP1 (straight bands) phase transformation could be observed. However, we observed the formation of the HCP2 phase (lamellar-type constitution) at the grain boundaries. In addition, the X-ray diffraction pattern indicated that the sample solution treated at lower temperatures and shorter times had a stronger martensitic transformation during aging compared to the sample solution treated at higher temperatures and longer times. Hardness measurements confirmed the results. Thermodynamical calculations showed that an agreement existed between the experiments and calculations. We also discuss the results from the TCFE8 and TCNI8 databases.
Highlights
The ASTM F75 cast alloy, with a Co–28Cr–5Mo–0.3C nominal wt. % composition, is one of the most widely used cobalt-base alloys for orthopedic implants, such as knee, hip, and shoulder implants [1,2,3].The mechanical properties of this alloy and its microstructure are strongly related
The HCP phase can be induced from the FCC phase through plastic deformation [6], quenching from T (T > TC ), or isothermal aging below
The specific microstructural features related to the FCC- > HCP martensitic transformation seem to be strongly influenced by the alloy composition and heat treatment conditions
Summary
The ASTM F75 cast alloy, with a Co–28Cr–5Mo–0.3C nominal wt. % composition, is one of the most widely used cobalt-base alloys for orthopedic implants, such as knee, hip, and shoulder implants [1,2,3]. The HCP phase can be induced from the FCC phase through plastic deformation (strain-induced transformation) [6], quenching from T (T > TC ) (athermal martensite), or isothermal aging below. The specific microstructural features related to the FCC- > HCP martensitic transformation seem to be strongly influenced by the alloy composition and heat treatment conditions. The role of the solution temperature and aging on the microstructure of the Co-28Cr-5Mo-0.3C alloy, as well as the metastable FCC->HCP phase transformation during aging, was investigated. Kurosu et al [11] investigated an alloy with a similar composition to the alloy used in this work, without carbon addition They calculated the FCC and HCP phases, as well as the Sigma phase, owing to the lack of carbon in the composition. We provide thermodynamical calculations using both the TCFE8 and TCNI8 databases, which are compared to the experimental results
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