Evaluation of the Aging Effect on the Microstructure of Co-28Cr-6Mo-0.3C Alloy: Experimental Characterization and Computational Thermodynamics

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.

Influence of Precipitation Hardening on the Mechanical Properties of Co-Cr-Mo and Co-Cr-W-Mo Dental Alloys

“…This is also the case for pure cobalt, which shows an allotropic transformation to a γ-phase crystal structure at 400 • C [3]. Chromium, molybdenum [4] and tungsten [3] increase the transformation temperature, as both increase the stability of the ε-phase. Anyway, with isothermal aging in the temperature region between 650 and 950 • C, the transformation from γ-phase to ε-phase can be completed [5].…”

“…In Figure 5c, it can be seen that during aging a strong crystallographic orientation-dependent precipitation of smaller carbides in straight lines and along the crystal grain boundaries occurred. The literature [4] states that the γ-phase with the fcc crystal lattice transforms to ε-martensite (hcp1) during isothermal annealing at 921 • C. ε-Martensite appears in the form of long straight lines on which carbides precipitate during aging, as shown in Figure 5c.…”

“…After 12 h of ageing, we could also observe fine needle-like precipitates formed inside the crystal grains of the γ-phase. The literature [4] states that in addition to ε-martensite (hcp1), the hcp2 phase can also form during aging. This nucleates at the grain boundaries of the γ-phase.…”

“…The microstructure also consists of secondary phases, mainly blocky carbides of the M 23 C 6 type, which mostly precipitate at the grain boundaries and in the interdendritic regions. In general, M 23 C 6 carbides have been identified as the most important secondary phase in Co-Cr alloys [4]. The mechanical properties can be improved by heat treatment through precipitation hardening, i.e., dissolving the large carbide network and forming fine precipitates during aging.…”

“…Beside the chemical composition, the mechanical properties of Co-Cr-based alloys can be significantly improved by heat treatment [4,11]. The most typical heat treatment is precipitation hardening, which takes place in three steps: solution annealing, quenching and aging.…”