Insights into refinement mechanism of primary M7C3 carbide by La2O3 via experiments and first-principles calculations

This study is dedicated to summarize refinement mechanism of the primary M7C3 carbide by rare earth oxide La2O3. The La2O3 was added into hypereutectic Fe–27Cr–4C alloy, and its microstructure was observed and analyzed by optical microscope (OM), X-ray diffractometer (XRD) and scanning electron microscope (SEM). The lattice mismatch degrees of La2O3//M7C3 interfaces were calculated. The interface properties of La2O3//M7C3 were calculated by the first principles. The effectiveness of La2O3 as the heterogeneous nucleus of the primary M7C3 carbide in the alloy was analyzed. The results show that the primary M7C3 carbide can be refined by La2O3 in this alloy. The lattice mismatch degree between La2O3(111) plane and M7C3(0001) plane is 2.36%, which indicates that La2O3 as the heterogeneous nucleus of M7C3 is the most effective. La2O3(111) plane and M7C3(0001) plane were chosen to construct the interface models. Two surface models, such as La-termination and O1-termination on La2O3(111) plane were constructed, which converge to 26 and 21 layers, and their surface energies are 5.256 J/m2 and 2.029 J/m2, respectively. The surface model of M7C3(0001) plane converges to 17 layers, and its surface energy is 3.199 J/m2. Among La-M7C3 and O1-M7C3 interfaces, the adhesion work (16.162 J/m2) of O1-M7C3 is larger than that (1.731 J/m2) of La-M7C3. However, the interface energy (−10.910 J/m2) of O1-M7C3 is smaller than that (6.725 J/m2) of La-M7C3, which indicates that O1-M7C3 interface has the best interface bonding property and the lowest interface nucleation resistance. Therefore, La2O3 can serve as the heterogeneous nucleus of M7C3 and tends to form an O1-M7C3 heterogeneous nucleation interface.