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.