Overcoming the strength-ductility trade-off in refractory medium-entropy alloys via controlled B2 ordering

Figures & data

Figure 1. Characterisation of microstructure and phase composition of the Al_x(NbTiZr)_100-x alloys: (a-d) – inverse pole figures (IPF), showing the fully recrystallised microstructure of the NbTiZr (a), Al_2.5(NbTiZr)_97.5 (b), Al₅(NbTiZr)₉₅ (c), and Al_7.5(NbTiZr)_92.5 (d) alloys; (e-h) – SADPs, acquired in [001]_bcc zone axes of the NbTiZr (e), Al_2.5(NbTiZr)_97.5 (f), Al₅(NbTiZr)₉₅ (g), and Al_7.5(NbTiZr)_92.5 (h) alloys. Pink arrows in Figures 1(g, h) highlighted the 100_B2 superlattice spots in the Al₅(NbTiZr)₉₅ (g) and Al_7.5(NbTiZr)_92.5 (h) alloys; (i-l) – intensity line profiles along the g₂₀₀ vectors (the directions are denoted with dashed blue arrows in SADPs Figures 1(e-h)) for the NbTiZr (i), Al_2.5(NbTiZr)_97.5 (j), Al₅(NbTiZr)₉₅ (g), and Al_7.5(NbTiZr)_92.5 (h) alloys. Pink arrows in Figures 1(k, l) denoted the peaks, corresponded to the 100 local maxima in the Al₅(NbTiZr)₉₅ (k) and Al_7.5(NbTiZr)_92.5 (l) alloys.

Figure 2. Detailed characterisation of the fine structure of the NbTiZr and Al_7.5(NbTiZr)_92.5 alloys: (a, b) – typical dark-field TEM images taken from the aperture position marked with yellow circles (corresponded to the half-length of the g₂₀₀ vector in SADP of each alloy), showing the absence of any contrast in the NbTiZr (a) alloy and profuse B2 domains in the Al_7.5(NbTiZr)_92.5 (b) alloy; (c, d) – typical magnified dark-field images before (b) and after (c) the threshold segmentation procedure; (e) – histogram, illustrating the size distribution of the B2 SRO domains with the estimated values of average size, $\overline{d}$, standard deviation, σ, and fraction, f.

Figure 3. Characterisation of room-temperature tensile mechanical properties of the Al_x(NbTiZr)_100-x alloys: (a) – engineering stress-strain curves; (b) – true stress-strain curves; (c) – DIC images of tensile specimens, taken at the necking formation and demonstrating the strain distribution along the cross-section; (d) – evolution of strain hardening rate, θ, with the strain.

Table 1. Yield strength, YS, ultimate tensile strength, UTS, uniform elongation, UE, elongation to fracture, EF, and apparent activation volume, V*, obtained during room-temperature tensile and stress relaxation tests of the Al_x(NbTiZr)_100-x alloys.

Alloy	YS, MPa	UTS, MPa	UE, %	EF, %	V*, b^3
NbTiZr	680 ± 5	715 ± 10	18.8 ± 0.6	34.1 ± 0.8	∼ 70–60
Al2.5(NbTiZr)97.5	785 ± 5	810 ± 5	22.0 ± 0.8	39.6 ± 0.7	∼ 67–59
Al5(NbTiZr)95	845 ± 40	860 ± 30	24.5 ± 0.4	42.2 ± 0.5	∼ 49–61
Al7.5(NbTiZr)92.5	930 ± 20	935 ± 20	25.7 ± 2.3	38.7 ± 2.0	∼ 50–68

Note: For V*, a range of values, evolving with the strain, are given.

Figure 4. Effect of Al on the strength: (a) a plot, showing the Al-induced increment of YS, ΔYS, in the studied Al_x(NbTiZr)_100-x alloys and some other Al-containing RHEAs [11,30,66,67]; (b) a plot, illustrating a comparison between the experimentally observed values of ΔYS in the Al_x(NbTiZr)_100-x alloys and the ones calculated by simplified Suzuki model for SSS. A good correlation for the Al_2.5(NbTiZr)_97.5 and Al₅(NbTiZr)₉₅ alloys should be noted. The appeared difference between the experimental and calculated ΔYS for the Al_7.5(NbTiZr)_92.5 was assumed due to the SRO strengthening (see the text).

Figure 5. Characterisation of fine microstructure of the Al_x(NbTiZr)_100-x alloys after the tensile tests: (a-d) – TEM bright-field images of the dislocation structures, observed in the near-fractured zones of tensile specimens of the NbTiZr (a), Al_2.5(NbTiZr)_97.5 (b), Al₅(NbTiZr)₉₅ (c), and Al_7.5(NbTiZr)_92.5 (d) alloys. White dashed lines in Figure 5(d) highlighted the signs of the DBs formation in the Al_2.5(NbTiZr)_97.5 alloy; (e, f) – SADPs taken from cells (denoted as ‘1’ in Figure 5(e)) and DBs (denoted as ‘2’ in Figure 5(f)) in [001]_bcc zone axes. Pink and yellow arrows in Figures 5(e, f) denoted the peaks, corresponded to the 100_B2 superlattice spots in cells and DBs, respectively; (g, h) – intensity line profiles along the g₂₀₀ vectors (the directions are denoted with dashed blue arrows in SADPs Figures 5(e, f)) for the Al_7.5(NbTiZr)_92.5 alloy. Pink and yellow arrows in Figures 5(g, h) denoted the peaks, corresponded to the 100 local maxima in cells and DBs, respectively.

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Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.