LaAlO3 as the heterogeneous nucleus of ferrite: Experimental investigation and theoretical calculation

Abstract

The hardfacing (harden-surface-welding) metals without and with La2O3 were prepared. The microstructures of the hardfacing metals were observed by optical microscopy. The phase structures were determined by X-ray diffraction. The morphologies and compositions of the inclusions were observed by field emission scanning electron microscope equipped with energy dispersive X-ray spectrometry. The free energies of the La-inclusions which may exist in the hardfacing molten pool were calculated according to thermodynamic theory. Subsequently, the surface properties of LaAlO3(100), ferrite(100) and ferrite(110) surfaces, the interfacial properties of LaAlO3(100)/Ferrite(100) and LaAlO3(100)/Ferrite(110) interfaces, and the effectiveness of LaAlO3 as the heterogeneous nucleus of Ferrite were studied using a first-principles density functional plane-wave ultrasoft pseudopotential method. The experiment results indicate that, in the hardfacing metal, the ferrite grains can be refined and the numbers of the harmful inclusions can be reduced effectively by adding La2O3. Before the hardfacing molten pool solidification, La2O3 transforms into the LaAlO3 inclusion preferentially. The calculated results indicate that, in LaAlO3, most outer electrons of the Al atoms transfer to the O atoms, which means that the ionic bonds are formed between Al atoms and O atoms. Besides, because of the sp2 orbital hybridization, strong covalent bonds are formed between La atoms and O atoms with certain directionality. For LaAlO3(100) surface, there are two terminated structures, which are LaO terminated structure and AlO2 terminated one. When the La chemical potential is small, surface stability of AlO2 terminated structure is higher than that of LaO terminated one. While when the La chemical potential is large, LaO terminated structure is more stable than AlO2 terminated one. For the LaO terminated structure, the interfacial ideal adhesion work of the LaAlO3(100)/Ferrite(100) interface is larger than that of the LaAlO3(100)/Ferrite(110) interface while the interfacial energy of the LaAlO3(100)/Ferrite(100) interface is smaller than that of the LaAlO3(100)/Ferrite(110) interface, which indicates that the interfacial binding strength and interfacial stability of LaAlO3(100)/Ferrite(100) interface are both higher than those of the LaAlO3(100)/Ferrite(110) one. For the two LaAlO3/Ferrite interfaces, when the La chemical potential reaches −2.447 eV and −1.009 eV, their interfacial energies are both smaller than the liquid-solid interfacial energy of Ferrite(l)/Ferrite(s), which means that LaAlO3 can be the effective heterogeneous nucleus of Ferrite on current conditions.