Morphologies and growth mechanism of (TiNb)C in TiNb-based surface reinforced layer

Abstract

The TiNb-based surface-reinforced layer, which has a layered structure with various (TiNb)C particles, fabricated by in-situ solid-state diffusion carbonization reaction, (TiNb)C particles were investigated in three-dimensional space. The shape evolution and growth mechanism during fabrication were also discussed. To clarify the relationship between growth mechanism and the morphologies of the particles, the methods of low-temperature brittle fracture and stress corrosion cracking were adopted to investigate the morphological characteristics of particles in each layer. In the surface-reinforced layer, there are three kinds of particles with different sizes and diversified morphologies (quasi nano-sized near-spheroidal, micro-sized fusiform-like, and submicro-sized near-cubic), and the fusiform-like particles in the middle layer have lamellar structure. The results of EBSD indicate that the orientation relationship of each layer exhibits a random-directed-random relationship from outside to inside. The variation of size, morphologies, and orientation relationship is caused by the directed diffusion of carbon atoms and changes in the nucleation-growth mechanism. The analyses reveal that the growth mechanism of the (TiNb)C particles changes from spherical to lamellar and then back to spherical. The solid-state phase transformation condition induces the formation of the special middle part in the surface-reinforced layer, which has a lamellar structure and special < 110 > orientation relationship.