Microstructural evolution during isothermal aging and strain-induced transformation followed by isothermal aging in Co-Cr-Mo-C alloy: A comparative study

The present study was undertaken to investigate the effects of isothermal aging (at 850 °C for 4, 8, 16 and 24 h) and strain-induced transformation (engineering strains of 10% and 20%) followed by isothermal aging (at 850 °C for 4, 8 and 16 h) on the microstructural evolution of a Co-28Cr-5Mo-0.3C alloy. The obtained results showed that isothermal aging at 850 °C resulted in the formation of lamellar-type carbides at the grain boundaries. Moreover, X-ray diffraction analysis indicated that isothermal aging of solution treated specimens at 850 °C for 24 h did not lead to complete fcc phase transformation to hcp one. In contrast with the isothermally aged specimens, applying plastic deformation to the solutionized samples accelerated the completion and saturation of fcc(metastable) → hcp transformation after 8 h aging at 850 °C. In addition, the X-ray diffraction results indicated that implementing isothermal aging of the strain-induced specimens at the higher aging time (16 h) caused the formation of (1 1 1)fcc and (2 0 0)fcc diffraction peaks again. Also, the strain-induced specimens followed by isothermal aging showed higher amount of microhardness as compared with the other specimens aged solely.