Ta content-dependent phase evolution and corrosion resistance of ZrTiHfTa_x high-entropy alloys in 3-nitro-1,2,4-triazol-5-one (NTO) medium

High-entropy alloys (HEAs) have emerged as promising candidates for energy structural materials (ESMs) due to their superior mechanical properties and compositional flexibility. However, their corrosion resistance in contact with energetic materials, particularly NTO, a widely used insensitive high explosive, remains insufficiently understood. In this study, a series of ZrTiHfTa_x HEAs with varying Ta contents were fabricated via vacuum arc melting to explore the effect of Ta content on microstructural evolution and corrosion resistance in aqueous NTO solution. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses revealed a composition-induced phase transformation from a dual-phase HCP + BCC structure at low Ta content to a single-phase BCC structure at higher Ta concentrations. Electrochemical measurements demonstrated that increasing Ta content markedly enhanced corrosion resistance; the corrosion current density of the Ta_1.00 HEA was 44.57% of that of the Ta_0.25 HEA. Moreover, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) indicated that higher Ta content facilitated the formation of a denser and more compact passive film with reduced defect density. Density functional theory (DFT) calculations further revealed that the passive layer provides dual protection effects by physically hindering H⁺/NTO⁻ ingress and chemically suppressing nitro group dissociation and substrate oxidation. This work offers fundamental insights into the corrosion protection mechanisms of HEAs in NTO-containing environments and provides valuable guidance for the rational design of corrosion-resistant HEAs.