Strain induced decomposition and precipitation of carbides in a molybdenum-hafnium-carbon alloy

In the present study of a sintered and thermo-mechanically processed Mo-Hf-C (MHC) alloy the strain induced decomposition of Mo-carbides and subsequent precipitation of Hf-carbides was revealed. The microstructural characterization was performed with atom probe tomography (APT), transmission electron microscopy (TEM), X-ray diffraction (XRD) and optical light microscopy (OLM). During annealing of deformed conditions it can be shown by TEM analysis that Hf-carbides were preferentially formed on dislocations and occasionally on (sub)grain boundaries. XRD and OLM investigations revealed that the required C for the Hf-carbides was delivered by the decomposition of Mo-carbides which are originally located at former grain boundaries in the as-sintered condition. It has been shown that for this decomposition mechanical strain prior to annealing was required. In the early stages of precipitation APT analyses revealed a significant segregation of C and Hf atoms to dislocations and boundaries. Finally, the authors deduced that due to the dislocation-precipitation interaction the misfit strain was effectively reduced and therefore, the Hf-carbides are located on dislocations and (sub)grain boundaries.