Characterization of iron-based surface multilayer-structured tungsten carbide composite layers by EBSD and FIB/TEM

Abstract

To strengthen the surface, tungsten carbide composite layers on an iron-based alloy were produced by a diffusion-controlled in-situ reaction in the solid state treated from 1000 °C to 1100 °C. Microstructure of the carbide layer was characterized by using electron backscatter diffraction (EBSD), focused ion beam (FIB) and transmission electron microscopy (TEM) techniques. The results reveal that the composite carbide layers exhibit a multiple-layer structure and orderly consists of the thin W2C layer and Fe3W3C layer, and a broad WC-Fe layer from top to bottom. Even then, the WC-Fe cemented layer dominates among them. The interface of the carbide layers/substrate is validated with an intimate metallurgical bond. Due to inadequate reaction, a thin tungsten plate covers the surface of the W2C layer. A highly distorted W2C lattice structure and an atomic transition layer with the thickness of about 6.79 nm were observed at the vicinity of the W/W2C interface by high-resolution TEM. Additionally, the relationship of crystal orientation obtained between the WC and α-Fe is WC2¯1¯2//α-Fe112¯. EBSD characterization shows that the morphology of WC grains mainly presents the equiaxial and columnar. The orientation of WC grains indexed tends to 011¯0 and 1¯21¯0 crystalline directions as the elevated temperature. Furthermore, grains of solid solution Fe3W3C with a cubic structure have a columnar morphology, and the growth direction of columnar grain is perpendicular to the W2C layer. The oriented growth of carbide grains can be attributed to the diffusion-controlled growth mechanism.