Xiaolei Xing

Refinement of nano-Y2O3 on microstructure of hypereutectic Fe-Cr-C hardfacing coatings

Abstract The Fe-Cr-C flux-cored wires with 0 wt.%, 0.63 wt.%, 2.54 wt.% and 5.08 wt.% additions of nano-Y 2 O 3 were developed in this work. And the different hypereutectic Fe-Cr-C hardfacing coatings were prepared. The phase structures of the coatings were determined by X-ray diffraction. The microstructures were observed by optical microscopy. The morphologies of the hypereutectic Fe-Cr-C hardfacing coatings were observed by a field emission scanning electron microscope equipped with an X-ray energy dispersive spectrometer. The effectiveness of Y 2 O 3 as heterogeneous nuclei of primary M 7 C 3 was calculated with the misfit theory. The experiment results showed that, the microstructures of the hypereutectic Fe-Cr-C hardfacing coatings consisted of M 7 C 3 , γ-Fe and α-Fe phases. With the increase of nano-Y 2 O 3 additives, primary M 7 C 3 in hypereutectic Fe-Cr-C coatings could be refined gradually. The average size of the primary M 7 C 3 was the minimum, which was 22 μm, when nano-Y 2 O 3 additive was 2.54 wt.%. The calculated results showed that, the two-dimensional lattice misfit between the face (001) of Y 2 O 3 and face (100) of orthorhombic M 7 C 3 was 4.911%, which indicated that Y 2 O 3 as heterogeneous nuclei of M 7 C 3 was middle effective to refine the primary M 7 C 3 .

Growth characteristics of primary M7C3 carbide in hypereutectic Fe-Cr-C alloy

The microstructure of the hypereutectic Fe-Cr-C alloy is observed by optical microscopy (OM). The initial growth morphology, the crystallographic structure, the semi-molten morphology and the stacking faults of the primary M7C3 carbide are observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The in-suit growth process of the primary M7C3 carbide was observed by confocal laser microscope (CLM). It is found that the primary M7C3 carbide in hypereutectic Fe-Cr-C alloy is irregular polygonal shape with several hollows in the center and gaps on the edge. Some primary M7C3 carbides are formed by layers of shell or/and consist of multiple parts. In the initial growth period, the primary M7C3 carbide forms protrusion parallel to {} crystal planes. The extending and revolving protrusion forms the carbide shell. The electron backscattered diffraction (EBSD) maps show that the primary M7C3 carbide consists of multiple parts. The semi-molten M7C3 carbide contains unmelted shell and several small-scale carbides inside, which further proves that the primary M7C3 carbide is not an overall block. It is believed that the coalescence of the primary M7C3 carbides is ascribed to the growing condition of the protrusion and the gap filling process.

Solid Solute Regularity of La Atom in α-Fe Supercell by First-principles

The structure stability, elastic property and electronic structure of α-Fe supercell with La atom were investigated by first-principles, in which, generalized gradient approximation (GGA) with the Perdew Burke Ernzerhof (PBE) was used as exchange-correlation functional. α-Fe supercells with La atom include α-Fe supercell with La atom in octahedral interstitial solid solute (La-OISS), that with La atom in tetrahedral interstitial solid solute (La-TISS) and that with La atom in substitutional solid solute (La-SSS). The results show that the La-SSS α-Fe supercell is more stable than La-OISS one. The resistance to volume change, reversible deformation and stiffness of La-OISS α-Fe supercell are stronger than those of La-SSS one. Moreover, the degrees of anisotropy and ionization in La-SSS α-Fe supercell are both stronger than those in La-OISS one. The bonding strength between La atom and Fe atom in La-SSS α-Fe supercell is larger than that in La-OISS one.