Microstructure characterization of nanocrystalline Ni3C synthesized by high-energy ball milling

Preparation of a metal carbide by a conventional route requires a huge instrumentation regarding melting the metal and graphite under vacuum at a very high temperature. However, mechanical alloying process is a very useful method for the fabrication of high melting point materials like metal carbides, which additionally produce nanocrystalline structure with improved properties. This article reports the microstructure characterization of nanocrystalline Ni3C synthesized at room temperature by mechanical milling of Ni and graphite powders under inert atmosphere. The stoichiometric Ni3C phase is formed after 5 h of milling. Microstructure of the prepared materials is characterized in terms of lattice imperfections, primarily by analyzing the X-ray powder diffraction data, employing both Warren–Averbach's method of line profile analysis and Rietveld's structure refinement method. The phase transformation kinetics studied by the Rietveld method reveals that the graphite powder becomes amorphous at the early stage of milling and then diffuses into Ni matrix and a Ni–C solid solution is formed within 1 h of milling. The high-resolution electron microscopy reveals that the Ni3C phase is initiated as a thin shell on Ni–C core within 2 h of milling and becomes stoichiometric in composition within 5 h of milling. Electron microscopy observations agree well with the X-ray measurement of particle size (∼9 nm) of Ni3C phase in 5 h milled sample and Ni–Ni3C core shell structure in 3 h milled sample.