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• The multiple heterostructure is realized in TiZrNbTa refractory high-entropy alloy via composition and annealing design. • The strength-ductility trade-off over a wide strain rate range of TiZrNbTa refractory high-entropy alloy is overcome by the multiple heterostructures. • The quasi-static tensile yield strength, ultimate tensile strength and elongation are 18 %, 20 % and 50 % higher than those of the base alloy. • A combination of high yield strength (∼1450 MPa) and large ultimate strain (∼25 %) is achieved at high strain rate (103 s−1). To improve the mechanical properties of refractory high-entropy alloy (RHEA), whose strain hardening capacity is relatively weak, over a wide strain rate range, the multi-heterostructure strategy is investigated in this work. Based on the composition and annealing design, the multi-heterostructure consisting of two mutually coherent body-centred cubic solid solution matrices and penetrating complex phases is achieved in TiZrNbTaNi 0.05 RHEA (MH-RHEA). Under quasi-static loading, this multi-heterostructure takes both good precipitation strengthening and hetero-deformation-induced hardening effects, diversifies the deformation modes, finally promotes the strain hardening capacity and deformation homogeneity. The tensile yield strength, ultimate tensile strength and elongation of MH-RHEA are 1195 ± 12 MPa, 1235 ± 9 MPa and 22.4 ± 1.0 %, respectively, which are 18 %, 20 % and 50 % higher than those of the base alloy. Under dynamic loading (∼103 s−1), the thermal effect can be neutralized and the deformation localization is avoided. Consequently, a high yield strength of ∼1450 MPa and a large ultimate strain of ∼25 % without fracture at a high strain rate of 2390 s−1 are realized in MH-RHEA. This heterostructure strategy offers a new paradigm to enhance the strength and ductility simultaneously of RHEAs over a wide strain rate range. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract To improve the mechanical properties of high-entropy alloys (HEAs) and expand the application range of metastable engineering, the NbZrTiTa alloy was researched. The results show that this alloy exhibits uniform element distribution and a metastable single-phase body-centered cubic (BCC) structure. During loading, element diffusion occurs, and then the TiZr-rich and TaNb-rich regions form. The increased Ti and Zr content reduces the stability of the BCC structure and leads to in-situ structure transformation in the TiZr-rich region. Element diffusion and structure transformation improve ductility by absorbing the loading work and releasing internal stresses. Furthermore, interface strengthening caused by the formation of the dual-phase region and the coherent nano-precipitation due to the compositional fluctuations together enhance the strength. The co-contribution of various metastable-induced strengthening and toughening mechanisms distinguishes the strength and ductility of the single-phase NbZrTiTa HEA from those of all the reported refractory systems. More importantly, the successful utilization of the novel metastable engineering induced by element diffusion in single-phase HEA provides a useful guide to design HEAs and other structural materials. Graphical abstract Unlabelled Image Highlights • A novel metastable engineering is utilized in single-phase high-entropy alloy to overcome the strength-ductility trade-off. • Transformation-induced plasticity effect is stimulated by element diffusion and the in-situ formation of dual-phase region. • Structure transformation and coherent nano-precipitation distinguish the mechanical property of NbZrTiTa high-entropy alloy. [ABSTRACT FROM AUTHOR]

Iridium (Ir) does not form a protective oxide barrier in high-temperature oxidizing environments, leading to a limited lifetime at high temperatures. To improve the oxidation resistance, pure Ir was modified by pack cementation to produce a double-layer iridium-aluminum (Ir-Al) coating. The oxidation behavior investigation on the Ir-Al coatings at temperatures from 1400 °C to 2000 °C demonstrated that the oxidation resistance of the Ir were significantly enhanced after the pack cementation treatment. Further detailed studies showed that the morphology and mass loss of the oxidized Ir-Al coating are considerably affected by the melting and oxidation of the IrAl 2.7(3) . [ABSTRACT FROM AUTHOR]

[Display omitted] • A combination of high strength and ductility in TiZrNbTa HEAs was achieved by doping nitrogen. • Interstitial strengthening contributes to the enhanced strength. The improved ductility originates from the promoted planar dislocation slip inside grains. • Annealing further increases the strength and ductility of the nitrogen-doped TiZrNbTa HEA. • The improved strength and ductility in the annealed HEAs attributes to the nitrogen solid solution strengthening, in situ grain refinement and promoted dislocation cross slip. Refractory high-entropy alloys (HEAs) usually exhibit high strength but limited ductility. Here, we report high strength (∼1334 MPa) and large ductility (∼18.8%) in nitrogen-doped TiZrNbTa HEAs. The microstructural evolution and the corresponding mechanical properties of the as-cast/annealed nitrogen-doped HEAs were investigated. All as-cast (TiZrNbTa) 100- x N x (x = 0, 0.3, 0.6) HEAs exhibit a single-phase body-centred cubic structure. Interstitial strengthening contributes to the enhanced strength. The improved ductility of the 0.6 at.% nitrogen-doped HEA originates from the promoted planar dislocation slip inside grains. Spinodal decomposition and grain boundary (Zr, N)-enriched precipitates emerge when the nitrogen content is 0.9 at.%, which results in significant embrittlement. Isothermal annealing of the 0.6 at.% nitrogen-doped HEA at 800 °C to 1200 °C was conducted to further improve the mechanical performance. The complicated precipitates and grain boundary impurity segregation in the alloys annealed at 800 °C and 1200 °C lead to deteriorated strength and ductility. However, in the alloy annealed at 1000 °C, a slight increase in both strength and ductility appears due to solid solution strengthening, in situ grain refinement and promoted dislocation cross slip, which is induced by the coherent boundaries. This work provides a promising conduit to approach high strength and ductility in HEAs. [ABSTRACT FROM AUTHOR]

The presence and effect of lattice distortion have become the most concerned issue of high-entropy alloys (HEAs) recently. To address this issue, TiZrNbTa (TZNT) HEAs were presented and an in-depth analysis concerning their microstructure, elemental diffusion and oxidation behavior was conducted. The results showed that during the high-temperature oxidation process at 800–1400 °C, the metastable homogeneous microstructure of TZNT HEAs segregated, then TiZr-rich and TaNb-rich regions with different lattice distortion degrees formed. Although O has stronger affinities with Ta and Nb, the oxygen atoms are more likely to diffuse in TiZr-rich region due to the dislocation pipe effect caused by severe lattice distortion. The inhomogeneous diffusion of oxygen atoms leads to inhomogeneous distribution of internal stress. The stress concentration and the structure transformation from BCC to brittle HCP jointly facilitate the cracks to form easier and grow in TiZr-rich region, finally resulting in a catastrophic pesting of TZNT HEAs at 800 °C. By contrast, the heterogeneity of oxygen atoms diffusion rate decreases as temperature increases. Therefore, the TZNT HEAs exhibited a parabolic oxidation behavior between 1000 and 1400 °C. The lattice distortion, whose degree is below the threshold for the generation of dislocation pipe effect, slows down the oxygen atoms diffusion rate. The findings of this work deepen the understanding on HEAs and can be utilized in tailoring the diffusion and oxidation behavior of high-performance HEAs via controlling lattice distortion. Image 1 • Effect of lattice distortion on diffusion behavior of high-entropy alloy is revealed. • Severe lattice distortion leads to a fast diffusion of O atoms via dislocation pipes. • Gentle lattice distortion hinders the diffusion of O compared with complete lattice. • Inhomogeneous diffusion in two distorted regions results in the pesting oxidation. [ABSTRACT FROM AUTHOR]