On the chemistry of the carbides in a molybdenum base Mo-Hf-C alloy produced by powder metallurgy

Abstract

The main focus of the present work is on the investigation of carbides present after sintering in the molybdenum base alloy MHC (Mo-Hf-C) which is produced via a powder route. The alloy has a nominal content of 0.65 at.% hafnium and 0.65 at.% carbon and can be hardened by precipitation of nm-sized hafnium carbides. The assessment of the molybdenum-rich corner of the ternary (quaternary) Mo-Hf-C(-O) phase diagram is required to exploit the full precipitation potential of the alloy. Thus, it is important to gain information about the accurate chemical composition of all individual phases. Therefore, the chemistry and structure of hafnium and molybdenum carbides were analyzed by means of X-ray diffraction, energy dispersive spectroscopy and atom probe tomography. The analysis of hexagonal molybdenum carbides revealed a significant amount of hafnium (∼1 at.%) in addition to molybdenum and carbon. The face centered cubic hafnium carbide phase on the other hand contains high amounts of oxygen (∼4.4 at.%) and molybdenum (∼2 at.%) in addition to small amounts of zirconium and nitrogen. In order to understand the high oxygen content in the hafnium carbide, energies of formation and lattice parameters for hafnium carbides with different contents of oxygen and molybdenum were calculated theoretically using an ab initio approach. Subsequently, the theoretically calculated lattice parameter of hafnium carbide was compared to the experimentally determined one of the carbide present in the MHC alloy. The experimental and modeling results are in good agreement. Finally, the formation mechanism of hafnium carbide during the sintering process is discussed.