Temperature effects on the lattice constants and crystal structure of a Co-27Cr-5Mo low-carbon alloy

Cobalt base (Co-Cr-Mo-C) alloys are known to exhibit two crystal structures, namely, face-centered-cubic (fcc) and hexagonal-close-packed (hcp). Accordingly, in this work, fcc and hcp lattice constants were measured at room and elevated temperatures in a Co-27Cr-5Mo-0.05C alloy using "in-situ" X-ray diffraction techniques. It was found that the lattice parameters corresponding to both the fcc and hcp phases increase linearly with temperature as a result of thermal expansion. Linear ([alpha]^sub l^) and volumetric ([alpha]^sub v^) coefficients of thermal expansion were computed for the main crystal structures found in this alloy. In particular, it was found that the fcc phase deviates from ideal isotropy as [alpha]^sub v^ was approximated by [alpha]^sub v^ = 2.7 [alpha]^sub l^. Also, the equilibrium fcc-hcp temperature was established to be approximately 970 °C. In particular, the thermodynamic stability of the fcc phase and its effect on the exhibited lattice parameters were disclosed. Moreover, measurements of alloy density with temperature indicated a linear decrease as a result of thermal expansion. Finally, the (c/a) ratio found for the hcp phase exhibited a nonlinear trend with temperature. This, in turn, was accompanied by a contraction along the hcp c-axis. [PUBLICATION ABSTRACT]