Formation of Nickel Carbide in the Course of Deformation Treatment of Ni-C Mixtures

Nonequilibrium Ni(C) solid solutions supersaturated with carbon to 10.2 at % were synthesized by mechanochemical method. An analysis of diffraction patterns showed that the formation of Ni(C) solid solutions is accompanied by an increase in the probability of appearance of deformation stacking faults. When the carbon content in the initial Ni-C mixtures is above 20 at %, the fcc Ni(C) solid solution resulting from the mechanical synthesis transforms into the metastable Ni(3)C nickel carbide with a hexagonal structure. The thermal stability of nonequilibrium Ni(C) solid solutions was determined. The solid solutions formed from the mixtures with carbon contents from 7 to 15 at % undergo partial decomposition accompanied by the carbide precipitation upon heating to 300 degrees C. The decomposition of the metastable Ni(3)C carbide starts at a temperature T(s) similar to 464.8 degrees C; the thermal effect is -Delta H = 10-13 kJ/mol. The effective radius of carbon atoms in the Ni(C) solid solutions was determined; it is equal to Rc(eff) = 0.061 nm.