Silicide coating on V4Cr4Ti alloy and their oxidation behaviour at moderate temperatures (450-650°C)

摘要

Considered as one potential candidate for components core of some generation IV nuclear systems, vanadium base alloys could be used as fuel cladding in Gas Fast Reactor (GFR) and also Sodium Fast Reactor (SFR). Vanadium is a semi refractory metal (T_f ≈ 1910°C), having suitable mechanical properties under irradiation at high temperature [ ], high mechanical formability as compared to the main other candidates such a SiC/SiC or ODS steels. However, vanadium is a trap to oxygen at elevated temperature and its oxidation leads to formation of non-protective oxide layer and oxygen in depth diffusion even under low partial pressure of O_2 [2 ]. In this study, a protective coating dedicated to vanadium alloys was developed using the halide activated pack cementation (HAPC) process. The thermodynamic assessment allowed the determination of deposit conditions in terms of masteralloy, activator, filler and temperature. This method was used to deposit Si on the surface of a V4Cr4Ti alloy and to form a multi-layered silicide coating, mainly consisting in VSi_2 in the outer layer (fig.1). Thermogravimetric oxidation tests were performed in isothermal (at 450, 500, 550 and 650°C) and cyclic conditions (at 650°C), to evaluate the efficiency of the coating in air. It turns out that the vanadium disilicide coating exhibits a high oxidation resistance in both conditions in this range of temperature. In parallel, the compound was synthesized using the powder metallurgy route to assess the oxidation mechanism of VSi_2. The oxidation behaviour of the bulk material was estimated using the aforementioned conditions used for the coating oxidation study. Finally, it was observed that both the sintered compound and the coating made of VSi2 were not susceptible to the catastrophic oxidation affecting refractory metal disilicides, particularly MoSi_2 and NbSi_2, in the operating temperature range. In fact, this coating can be suitable to protect vanadium alloys.