On the precipitation mechanism in the molybdenum based alloy MHC (Mo-Hf-C)

摘要

The precipitation mechanism of small hafnium carbides in the sintered and thermo-mechanically processed molybdenum based alloy MHC (Mo-0.65Hf-0.65C (at.-%)) is reported. Light and scanning electron microscopy revealed hafnium oxides, large hafnium carbides and molybdenum carbide layers at the grain boundaries in the as-sintered material. Additionally, atom probe tomography showed a residual dissolved content of 0.12 at.-%Hf, but no carbon in solid solution. After thermo-mechanical processing of the as-sintered material in a deformation dilatometer, transmission electron microscopy revealed small hafnium carbides with diameters of 10-100 nm. These carbides were preferentially located at dislocations and dislocation networks. Without deformation prior to aging, no formation of small hafnium carbides was observed. X-ray diffraction confirmed that decomposition of molybdenum carbides had occurred, which delivered the carbon for the formation of strain induced precipitates.