The allotropic phase transformation from the face-centered cubic (fcc) to the hexagonal close-packed (hcp) crystal structure is one of the many fascinating properties of the element Co. For the pure element the transformation temperature lies around 420 °C. By alloying, this transformation can be stabilized to higher temperatures due to a changed phase equilibrium and additionally an altered stacking fault energy leading to different deformation mechanisms. Suitable alloying elements to establish a twinned fcc-hcp two-phase microstructure are W and Mo, since both have the greatest impact on the stacking fault energy.
As the effect of Mo-alloying on Co has been investigated intensively in literature, this is not the case for W. Hence, this work covers the influence of W on the thermodynamics and kinetics regarding the hcp to fcc phase transformation in a CoCrW medium entropy alloy using an in-situ x-ray diffraction heating setup. By alloying 8 at.% of Cr and W, the phase transformation temperature can be increased by nearly 300 °C. Different pre-deformations up to 35 % not only show a further stabilization of hcp, but also a partial transformation back into the ductile fcc phase. Therefore, this pre-deformed two-phase microstructure is an interesting system regarding its mechanical thermophysical and mechanical properties.