Experiments and calculations on refining mechanism of NbC on primary M7C3 carbide in hypereutectic Fe-Cr-C alloy.

Two hypereutectic Fe-Cr-C hardfacing alloys were manufactured, whose Nb contents are 0 and 1.2 wt% respectively. Their phase precipitation curves were calculated by Thermal-calc software and their phase structures were detected by X-ray diffractometer, which indicate that NbC precipitates previously to the nucleation of primary M 7 C 3 carbide. The alloys were observed by metallographic microscope, which finds that the primary M 7 C 3 carbides in the alloy with 1.2 wt% Nb are obviously refined. The chemical compositions of carbides were further measured by energy diffraction spectrum, which finds that NbC particles are distributed in the primary M 7 C 3 carbides. The melting process of M 7 C 3 carbide was simulated by molecular dynamics method and the melting point of M 7 C 3 carbide is identified to be 1625±5 K. The melting behaviors of M 7 C 3 embryos with different radii were simulated, and the solid-liquid interfacial energy of M 7 C 3 carbide was calculated to be 3.312 J/m 2 based on the classical nucleation theory. Two kinds of M 7 C 3 (0001)/NbC(111) interface models with different atomic stacking modes were built. By means of first-principles calculations, M 7 C 3 (0001)/NbC(111) interface I is combined by polar covalent/ionic bonds and metallic bonds, while M 7 C 3 (0001)/NbC(111) interface II is combined by ionic bonds and metallic bonds. The works of adhesion are 1.23 J/m 2 and 2.24 J/m 2 respectively. There exists a region where the interfacial energy of interface II is lower than the solid-liquid interfacial energy of M 7 C 3 carbide. The above experimental and calculation results demonstrate that NbC particle is the heterogeneous nucleus of primary M 7 C 3 carbide and thereby refines it. [ABSTRACT FROM AUTHOR]