Diffusion Paths of Silicide Coatings on Nb-Si-Based Alloys During Pack Cementation Process

Silicide coatings and B-modified silicide coatings were deposited on Nb-Si-based alloys by means of pack cementation method at 1573 K. The phase sequence of the coating/substrate system was investigated in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron microprobe analysis (EPMA), respectively. The substrate is composed of (Nb, X)5Si3, Cr2Nb, and (Nb, X)ss phases (X is Ti, Cr, Hf, and Al). In addition, the silicide coating exhibits a multilayer microstructure, consisting of three layers, i.e., (Nb, X)Si2 → (Nb, X)Si2 + (Ti, Nb)5Si4 → (Ti, Nb)5Si4, whereas the B-modified silicide coating consists of a sequence of four layers, (Nb, X)B2 + (Nb, X)Si2 → (Nb, X)Si2 → (Nb, X)Si2 + (Ti, Nb)5Si4 → (Ti, Nb)5Si4. The diffusion paths for the two coating/substrate systems were thereafter elucidated by inspecting the Nb-Ti-Si-Cr and Nb-Si-Ti-B/Nb-Si-Ti-Cr multinary isotherms. Of technological interest, the diffusion path in the silicide coating was extended to predict and study the diffusion path in the Ge-modified silicide coating/substrate, which well describes the Nb-Ti-Cr-(Si, Ge) tetrahedral isotherm and phase behavior. This offers an indirect validation for the diffusion path in the silicide coating/substrate system.