On the role of carbon in molybdenum manufactured by Laser Powder Bed Fusion

Carbon is known to improve the ductility and grain boundary strength of molybdenum. Scientific consensus to explain this phenomenon is that carbon either replaces oxygen impurities at grain boundaries or suppresses the diffusion of oxygen towards the grain boundaries. The aim of this study is to clarify the mechanism on how the carbides interact with the oxygen impurities and the molybdenum matrix, thus to provide a better understanding for designing molybdenum alloy compositions. Molybdenum alloyed with 4500 μg/g carbon was manufactured in the additive manufacturing process of Laser Powder Bed Fusion (LPBF). The cellular grain structure of molybdenum cells with surrounding molybdenum carbide caused by constitutional undercooling during the solidification process was investigated by Transmission Electron Microscopy (TEM) for morphology, composition and crystal structure. The carbide was identified to be orthorhombic ζ-Mo₂C, which has a solubility for oxygen. ζ-Mo₂C shows a semi-coherent interface with α-Mo. No cracks could be found at the α-Mo / α-Mo or the α-Mo / ζ-Mo₂C interfaces. Oxygen could not be detected at grain boundaries, but only within ζ-Mo₂C, which traps the oxygen, thus avoiding oxygen segregation to the grain boundaries and improving the grain boundary strength.