Microstructural study of the NbC to G-phase transformation in HP-Nb alloys

• The microstructure of a centrifugally cast HP alloy was studied in its as-received state and after ageing at 900 °C. A multi-scale approach combining X Ray Diffraction (XRD), advanced electron microscopy modes (scanning and transmission electron microscopies (SEM, TEM), together with focused ion beam/SEM nanotomography (FIB-nt)) has been carried out to characterize the evolution of niobium carbides during ageing. After thermal treatment, the carbides exhibit a complex microstructure, consisting of a core of untransformed NbC, an intermediate layer of G-phase (Ni16Nb6Si7) with embedded nanometric titanium carbide precipitates, and an outer shell of alternating chromium carbides Cr23C6 and G-phase. A simple diffusion model was used to explain the faster external growth of G-phase compared to the internal NbC dissolution, and to determine a diffusion coefficient of niobium in the G-phase at 900 °C. 

Nyckelord

Advanced material characterization

Ageing

G-phase

HP-Nb alloy

Microstructure

NbC

Carbides

Chromium compounds

Diffusion

Ion beams

Nickel compounds

Niobium compounds

Scanning electron microscopy

Silicon compounds

Titanium carbide

Transmission electron microscopy

Advanced materials

Carbide precipitate

Complex microstructures

Multi-scale approaches

Nb alloys

Scanning and transmission electron microscopy

Niobium alloys

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