Refining effect of TiC on primary M7C3 in hypereutectic FeCrC harden-surface welding coating: Experimental research and first-principles calculation

Hypereutectic FeCrC coatings with and without Ti additive were developed by harden-surface welding. The equilibrium phase diagrams of the coatings were calculated by Thermal-calc software. The phase structures were determined by X-ray diffractometer (XRD). The microstructures were observed by optical microscopy (OM). The micro-morphologies of M7C3 and TiC were observed by field emission scanning electron microscopy (FESEM). The compositions of M7C3 and TiC were determined by energy dispersive spectrum (EDS). The crystal structure of primary M7C3 was determined by transmission electron microscopy (TEM). The phase diagrams indicate that TiC precipitates firstly in the coating with Ti additive. The subsequent phase transformation is the same as that in the coating without Ti additive, namely primary M7C3 precipitates and then eutectic reaction occurs. The XRD patterns are corresponding to the phase diagrams, which show that there exists TiC in the coating with Ti additive. By comparing their metallographic images, the primary M7C3 are obviously refined in the coating with Ti additive. From the micro-morphologies of M7C3 and TiC, it is found that TiC exists inside primary M7C3 and they are tightly combined. Therefore, TiC may be the heterogeneous nucleus of primary M7C3. On the basis of experimental research, the refining effect of TiC on primary M7C3 was investigated by first-principles calculation. The calculation results shows the work of adhesion of Fe3Cr4C3(0001)/TiC(111) interface is 3.48 J/m2 and the interfacial energy ranges from 0.921 J/m2 to 2.782 J/m2. The Fe3Cr4C3(0001)/TiC(111) interface is theoretically stable. The Fe3Cr4C3(0001)/TiC(111) interface is well jointed by TiC polar covalent/ionic bond and CrTi metallic bond. Therefore, the preferentially precipitated TiC in FeCrCTi alloy is the heterogeneous nucleus of primary M7C3 and thereby refines the primary M7C3.