Fig. 1 Increasing trend of alloy chemistry complexity over time [14,19] (IMs: intermetallic or metal compound, HEAs: high-entropy alloys, RHEAs: refractory high-entropy alloys)

Fig. 2 Alloys world based on the configurational entropy

Fig. 3 System expansion of RHEAs

Fig. 4 Effect of alloying elements on RHEAs was reviewed from four aspects: preparation method, microstructure, synthesis, and mechanical properties, and toughening mechanism

Fig. 5 Common preparation methods of refractory high-entropy alloy

Fig. 6 Temperature dependence of the yield stress of NbMoTaW and VNbMoTaW RHEAs and two superalloys, Inconel 718 and Haynes 230 [31]

Fig. 7 Temperature dependence of a yield strength and b specific yield strength of HfMoTaTiZr, HfMoNbTaTiZr, HfNbTaTiZr (in as-solidified condition done by this work for comparison), HfNbTaTiZr (in the hot-isostatic pressed and vacuum annealed condition) HEAs, and two superalloys, Inconel 718 and CMSX-4 (the yield strengths and specific yield strengths of Inconel 718 and CMSX-4 are tensile yield strengths). Nominal composition of Inconel 718 is Ni: 52.5%, Cr: 19.0%, Mo: 3.0%, Nb: 5.1%, Al: 0.5%, Ti: 0.9%, Fe: 18.5%, Mn: 0.2%, Si: 0.2%, and B: 0.04%. That of CMSX-4 is Cr: 6.5%, Co: 10.0%, Mo: 0.6%, W: 6.5%, Ta: 6.5%, Re: 3.0%, Al: 5.5% and Ni: As Balance (in wt%) [86]

Fig. 8 Variation in friction coefficient a and wear rate b of NbTaWMo and NbTaWMoSi0.25 RHEAs at different temperatures [93]

Fig. 9 SEM micrographs of worn surfaces of NbTaWMoSi0.25 RHEA at different temperatures: a 25 ℃, b 200 ℃, c 400 ℃, d 600℃, e 800 ℃ [93]

Fig. 10 Comparison of Epit and icorr for the Hf0.5Nb0.5Ta0.5Ti1.5Zr RHEA, previously reported HEAs, and some conventional passive alloys in the 3.5 wt% NaCl solution [99]

Fig. 11 Isothermal oxidation curves for the HEAs at 1300 ℃, show the effects of Ti, V, and Si addition [104]

Fig. 12 Mechanical properties of HfMoxNbTaTiZr alloys: a the engineering stress-strain curves, b the increase of yield strength as functions of solute concentration in the HfMoxNbTaTiZr and other HEAs [87,129,132]

Fig. 13 Room temperature tensile stress-strain curves for the as-cast TiZrHfNb (denoted as a base alloy), (TiZrHfNb)98O2 (denoted as O-2), and (TiZrHfNb)98N2 (denoted as N-2) HEAs. σy is the yield strength (squares), σUTS is the ultimate strength (diamonds) and ε is the elongation (circles). The inset shows the corresponding strain-hardening response (dσ/dε). A higher work-hardening rate is observed for the O-2 HEA variant (TiZrHfNb)98O2 compared to the base HEA TiZrHfNb and the N-2 HEA (TiZrHfNb)98N2 [142]

Fig. 14 Initial yield stress, prediction by Labusch model vs experiment, for a range of BCC RHEAs [146]

Fig. 15 Scanning transmission electron microscopy (STEM) high-angle annular dark-field image and fast Fourier transforms (inside the red squares) recorded from a survey sample extracted from the inside-grain region; The microstructure of AlMo0.5NbTa0.5TiZr refractory high-entropy alloy appears to consist of two phases, the coherent precipitation of the disordered plate-like BCC particles from ordered B2 matrix [161,166]

Fig. 16 Microstructure of Al0.5NbTa0.8Ti1.5V0.2Zr in the cast, hot iso-statically pressed (HIPed) and homogenized (1200 °C/24 h/slow cool) condition. APT re-construction of Al- (red) and Ta- (blue) rich regions (left) and compositional changes (proximity histogram) across a BCC-B2 interface (right) [163]

Fig. 17 SEM backscatter electron images of a polished cross section of the hot isostatically pressed (HIPd) NbCrMo0.5Ta0.5TiZr alloy. The three phases (BCC1, BCC2, and FCC) with different morphologies and contrasts, as well as the transition layer (TL), are indicated [63]

Fig. 18 Compressive stress-strain curves of a Mo0.5NbHf0.5ZrTi master alloy, C0.1 and C0.3 alloy [173] b the as-cast Mo0.5NbHf0.5ZrTiSix alloy [55]

Fig. 19 XRD patterns and EBSD images of the as-cast Tax HEAs. The Ta concentration significantly influences the phase constitution of this alloy system, rendering either a single (bcc) or dual-phase (bcc?+?hcp) structure. The decreasing of Ta content destabilizes the bcc matrix and promotes the formation of hcp [52]

Fig. 20 In situ neutron diffraction data reveal structural evolution as a function of applied stress in alloy Ta0.4. a Evolution of diffraction patterns with the applied stress. b Evolution of relative intensities of the (110)bcc and (101)hcp peak as a function of applied stress. c Evolution of lattice strain on (110)bcc,(200)bcc, and (101)hcp during loading-unloading [52]

Fig. 21 Mechanical behavior of the as-cast Tax HEAs at room temperature. a Representative tensile true stress-strain curves. b Corresponding strain-hardening rate curves [52]

Fig. 22 Engineering stress-strain curve and compressive yield strength - grain size plot of WNbMoTaV HEA under compression at room temperature [42]

Fig. 23 a True and engineering tensile stress-strain curves of HfNbTaTiZr alloy with different grain sizes, in which dashed lines connecting necking point with fracture point are treated to be linear, b yield stress as a function of grain size in the HfNbTaTiZr alloy[225]

Fig. 24 EBSD results for HfNbTaTiZr sheet following a 65% thickness reduction: a Image-quality map b inverse-pole-figure map of grains for the transverse direction. Longitudinal cross section; the rolling direction is horizontal [227]