800^oC-stable D0₂₂ superlattice in a NiCrFe-based medium entropy alloy

Abstract

The metastable-stable transition and rapid coarsening of metastable D0₂₂ phase remains its Achilles hell, restricting the application of D0₂₂-strengthened alloys at service temperatures higher than 650°C. Hence, it is crucial to improve thermal stability of D0₂₂ phase. In this work, we report an 800°C-stable D0₂₂-Ni₃(W_0.24Ti_0.38Ta_0.19Nb_0.19) superlattice in the Ni₅₈Cr₂₃Fe₁₀W₅Ti₂Ta₁Nb₁ MEA. Experimental results and theoretical analysis showed that the exceptional thermal stability is due to Nb-sublattice occupying elements, low diffusivity of W, and extremely small lattice misfit between the D0₂₂ phase and FCC matrix. These findings shed lights on developing high-performance D0₂₂-hardened alloys capable of operating at temperatures above 650°C.

GRAPHICAL ABSTRACT

KEYWORDS:

• D0₂₂ superlattice
• coarsening
• phase stability
• precipitation hardening

High entropy alloys (HEAs) and medium entropy alloys (MEAs) with face centered cubic (FCC) structure have shown the highest fracture toughness [1–3] of metallic materials, stimulating a broad exploration of their strengthening and toughening mechanisms. Comparing the strengthening mechanisms of reported alloys [4–10], it is found that precipitation strengthening is a powerful strategy. Of particular interest, D0₂₂ (${\gamma }^{\mathrm{\prime \prime }}$) phase is one of the most efficient precipitates to strengthen FCC alloys without sacrificing ductility significantly to date [9, 11, 12]. D0₂₂ phase with a volume fraction of ∼7% contributes strength of about 700 MPa because of its combined ordering strengthening and coherency strengthening [9]. However, D0₂₂ phase undergoes rapid coarsening and transforms into $\delta$ (D0_a) or $ϵ$ (D0₁₉) phase upon continued aging at around 650°C [13–15], resulting in a substantial degradation in mechanical properties. Hereinto, one of the critical challenges for D0₂₂-hardened alloys is to improve the thermal stability of D0₂₂ phase.

In Ni-based superalloys, microalloying has been found useful in enhancing the stability of Ni₃Nb-type D0₂₂ phase. For example, changing the local (Al + Ti)/Nb ratio suppresses the coarsening of D0₂₂-Ni₃Nb precipitates in IN718 [16–21]. The first-principles calculation indicates that Nb-sublattice occupiers like Ti, Ta and W thermodynamically stabilize the Ni₃Nb-type D0₂₂ phase [12, 22, 23]. Despite these insightful understandings, to the best of authors’ knowledge, the D0₂₂ phase in IN718 still transforms into a D0a phase quickly at 700°C. Fortunately, Gao et al. [24] recently reported a new Ni₃(Cr_0.2W_0.4Ti_0.4)-D0₂₂ phase that is thermo-stable at 700°C, giving us opportunities to develop D0₂₂ phase that is stable at an even higher temperature. However, the coarsening behavior and phase-stability mechanism of the Ni₃(Cr_0.2W_0.4Ti_0.4)-D0₂₂ phase did not get adequate scientific attention. It also remains unknown if the microalloying could further improve the thermostability of this new D0₂₂ phase. Meantime, the extensive works of other intermetallic particles in high entropy alloys provide a key insight for our research about the phase stability of D0₂₂ [25, 26].

Therefore, we in this study developed a Ni₅₈Cr₂₃Fe₁₀W₅Ti₂Ta₁Nb₁ MEA based on the NiCrFe-based system [10] and investigated its precipitation behaviors and mechanical properties using systematic experimental and theoretical methods. Our results showed that a Ni₃(W_0.24Ti_0.38Ta_0.19Nb_0.19)-D0₂₂ phase precipitates in this MEA and this new superlattice is thermally stable at 800°C. The origins of this exceptional thermostability have been revealed using experimental and theoretical analysis.

Elements of Ni, Cr, Fe, W, Ti, Ta and Nb with high purity (>99.9 wt.%) were used as raw materials. The Ni₅₈Cr₂₃Fe₁₀W₅Ti₂Ta₁Nb₁ MEA were prepared using arc melting in an argon atmosphere and casted into a copper mold with the dimension of 50 × 10 × 5 mm³. The as-cast samples were first solution-treated at 1200°C for 48 h and then cold-rolled to approximately 1.5 mm along the longitudinal direction, followed by recrystallization at 1200°C for ∼2 min. Subsequently, isothermal aging treatments were applied to the MEA at 800°C, 750°C and 700°C at various times. All the heat treatment procedures described here were finished by water quenching. The SEM samples were polished down to 0.04 μm colloidal silica or electron-polished finish, and then analyzed by scanning electron microscope equipped with BSE and EDS detector (SEM, Tescan MIRA3). Rigaku d/max-2550 obtained the X-ray diffraction (XRD) patterns of the samples with a monochromator. For the transmission electron microscope (TEM, FEI Themis Z) analysis, sheet samples with a thickness of 0.5 mm were first cut from the annealed samples by electrical discharged machining (EDM) and then mechanically ground to a thickness of ∼70 μm using SiC papers. Afterward, the samples were punched into 3-mm-diameter discs and thinned by a twin-jet polishing technique. ImageJ software was used for image analysis and Pandat software with the newest MEA database was used to predict the precipitation behavior. For mechanical tests, the dog-bone specimens with a gauge dimension of 12.5 × 1.2 × 3 mm³ were prepared for the tensile test with a strain rate of 1 × 10⁻³ s⁻¹ in the universal testing machine (TSMT) at room temperature.

Figure 1 shows the microstructures of the Ni₅₈Cr₂₃Fe₁₀W₅Ti₂Ta₁Nb₁ MEA aged at 800°C for 12 h. Figure 1(a₁) presents typical fine recrystallized equiaxed grains, and there are $\mu$ phases enriched in W inside the grain and at the grain boundaries. The $\mu$ phase itself shows very small volume fraction and little influence on the strength of our previous results [10]. Figure 1(a₂) depicts the nano-sized lens-shaped precipitates within the grain. TEM analysis was devoted to confirming the structures of these nano-sized precipitates. Figure 1(b) exhibits a typical dark-field TEM image of the disk-shaped precipitates with different crystallographic orientations. The disk-shaped precipitates are ∼55 nm in length and ∼20 nm in width. Figure 1(c) demonstrated the FCC crystal structure of the matrix and the D0₂₂ structure of the disk-shaped precipitates using selective area diffraction patterns. The orientation relationship between the D0₂₂ phase and matrix follows [100]_γ″//[001]_m, [010]_γ″//[001]_m, and[001]_γ″//[001]_m. Furthermore, compositional analysis by TEM-EDS indicates that the Ni, W, Ti, Ta and Nb elements have a strong tendency to partitioning into the D0₂₂ phase as shown in Figure 1(d). The quantitative chemical compositions of the D0₂₂ phase and FCC matrix obtained from the TEM-EDS-point result are shown in Table 1, and the present high-entropy D0₂₂ phase follows the Ni₃(W_0.24Ti_0.38Ta_0.19Nb_0.19) stoichiometry. High-resolution TEM observation (Figure 1(e)) reveals the fully coherent interface between ordered D0₂₂ phases and disordered FCC matrix by fast fourier transform (FFT) and inverse fast fourier transform (IFFT) analysis, circled by dotted boxes of f₁ (green), f₂ (blue) and g (orange), respectively. For the ordered A₃B-type D0₂₂ superlattices, the red dot represents the A sublattice sites, and similarly, the black dot represents the B sublattice sites in Figure 1(g). Besides, the lattice misfit of the FCC matrix and D0₂₂ phase is calculated to be ∼0.56% along the a-axis.

Figure 1. Microstructural features of the Ni₅₈Cr₂₃Fe₁₀W₅Ti₂Ta₁Nb₁ MEA aged at 800°C for 12 h: (a) SEM images showing the typical microstructures, (b) Dark-field image showing the D0₂₂ phase, (c) low-index diffraction patterns taken along [001]_m, [011]_m and [$1$12]_m zone axes of the matrix, (d) High-magnification microstructure of the MEA with the corresponding TEM-EDS element mappings of Ni, W, Ti, Ta and Nb elements, and (e) High resolution TEM image showing D0₂₂ precipitate coherent with FCC matrix by FFT (marked as f₁ and f₂) and IFFT (marked as g).

Table 1. Chemical compositions of D0₂₂ phase and matrix obtained from TEM-EDS (at.%).

Phases	Ni	Cr	Fe	W	Ti	Ta	Nb
D022	74.2 ± 0.3	3.5 ± 0.0	1.7 ± 0.1	4.8 ± 0.3	8.1 ± 0.8	3.9 ± 0.2	3.9 ± 0.4
Matrix	57.4 ± 1.4	25.4 ± 1.5	10.9 ± 0.9	3.8 ± 0.1	1.7 ± 0.7	0.3 ± 0.3	0.6 ± 0.2

We further reveal the thermal stability of this newly developed Ni₃(W_0.24Ti_0.38Ta_0.19Nb_0.19)-D0₂₂ phase by annealing at 700–800°C for 12–144 h based on Fig. S1 and S2 (see details in ‘Supplementary materials’). The typical SEM micrographs of nanoparticles were carried out to track the coarsening behavior of the D0₂₂ phase under different aging conditions. As shown in Figure 2, there are two mutually vertical ellipsoidal variants and a spherical variant, which is consistent with that in Figure 1(b). Furthermore, as long as the annealing time is prolonged or the aging temperature is increased, the size of D0₂₂ precipitates increased to varying degrees while kept the disk shape. We quantitatively evaluate the average major axis size and volume fraction of nanoparticles in the MEA with the variation of annealing time at different temperatures in Fig. S3. Based on the Lifshitz-Slyozov-Wagner (LSW) theory, the relationship between the average major axis size of D0₂₂ nanoparticles and aging time can be written as follows: $d^3\left(t\right)-d^3\left(t_0\right)=k\left(t-t_0\right)$, where $d\left(t\right)$ is the average major axis size of D0₂₂ nanoparticles at the aging time of t, k is the coarsening rate constant. The corresponding normalized particle size distributions (PSD) histograms analyzed by commercial ImageJ software are also presented accordingly, as shown in Figure 2. For a clear comparison, the distribution functions predicted by the MLSW model are plotted here (blank curves) [27]. It is evident that the size distribution of the D0₂₂ nanoparticles in the MEA is in reasonable agreement with the prediction by the LSW theory.

Figure 2. SEM images and the particle size distribution of the MEA aged at different temperatures revealing the ripening of D0₂₂ precipitates, (a) 800°C, (b) 750°C, and (c) 700°C.

It is generally recognized that the Ni₃Nb phases with D0₂₂ superlattices gradually transform into $\delta$ (D0_a) phase with needle-shape or $ϵ$ (D0₁₉) phase with lath-shape [13, 14, 28, 29]. Therefore, to assess the extent to the thermal stability of the D0₂₂ phase in the MEA during the longer-term thermal exposure, and the MEA was investigated by annealing at 800°C for 240–720 h. Stunningly, the current D0₂₂ phase still showed disc-shaped morphologies without an obvious change during long-term exposure at 800°C in the Ni₅₈Cr₂₃Fe₁₀W₅Ti₂Ta₁Nb₁ MEA in Figure 3(a), and XRD test also supports that no new phase was found, as shown in Fig. S4, demonstrating the good thermal stability of Ni₃(W_0.24Ti_0.38Ta_0.19Nb_0.19)-D0₂₂ phase at 800°C. The coarsening rate of our investigated MEA and Inconel 718, modified Inconel 718 superalloy [30–32] are compared in Figure 3(b), the Ni₅₈Cr₂₃Fe₁₀W₅Ti₂Ta₁Nb₁ MEA shows a lower coarsening rate based on the LSW theory.

Figure 3. (a) SEM images of the MEA during long-term exposure at 800°C revealing the ripening of D0₂₂ precipitates, (b) Comparison of coarsening rate for present work and previous work (Alloy718 [30], IN718 [32] and M718 [31])

The observed exceptional phase stability of Ni₃(W_0.24Ti_0.38Ta_0.19Nb_0.19)-D0₂₂ phase could be understood from the following aspects. First, in the current study, the alloying elements of Ti, Ta and W are more inclined to occupy the Nb sublattice (Table 1), which can thermodynamically stabilize the ${\gamma }^{\mathrm{\prime \prime }}$-Ni₃Nb with D0₂₂ superlattices according to first–principles calculations [12]. Second, the important alloying parameters that determine the crystal structure of precipitates, i.e. valence electron concentrations (VEC) [9] and the atomic-radius ratio (${\text{R}}_{\overline{\mathrm{A}}}/{\text{R}}_{\overline{\mathrm{B}}}$) [33], of Ni₃(W_0.24Ti_0.38Ta_0.19Nb_0.19)-D0₂₂ also indicate its enhanced phase stability. Based on reported data in Fig. S5, we found that the D0₂₂ phase tends to be stable when the two alloying parameters range in 8.588 < VEC < 8.75, 0.9307<${\text{R}}_{\overline{\mathrm{A}}}/{\text{R}}_{\overline{\mathrm{B}}}$<0.9707. The two values of the Ni₃(W_0.24Ti_0.38Ta_0.19Nb_0.19) phase in this study were 8.715 and 0.9437, respectively, tending to stabilize its D0₂₂ structure. It seems plausible that, by varying gradually the radius ratio ${\text{R}}_{\overline{\mathrm{A}}}/{\text{R}}_{\overline{\mathrm{B}}}$ and VEC strategy, the stacking types of ordered A₃B-type precipitation may undergo a transformation, improving high thermal stability of ordered A₃B-type precipitation. However, designing new alloys (without much trial and error) to get precipitates with high thermal stability is still an open question and needs elaborate study. Last but not least, we calculated the phase diagram of the in Ni₅₈Cr₂₃Fe₁₀W₅Ti₂Ta₁Nb₁ MEA based on the Pandat software with the latest MEA database, as shown in Fig. S1, D0₂₂ phases can stably exist at 800°C against the transformation to the D0_a or D0₁₉ phase. This strongly supports that the designed D0₂₂ phase exhibits excellent thermostability at 800°C.

The slower coarsening kinetics in Ni₅₈Cr₂₃Fe₁₀W₅Ti₂Ta₁Nb₁ MEA are mainly related to two considerations. On the one hand, Ni₃(W_0.24Ti_0.38Ta_0.19Nb_0.19)-D0₂₂ phases are probably controlled by the diffusivity of W, which is the slowest diffusers in Ni [34, 35], but Ni₃Nb-D0₂₂ phases in Inconel 718 and modified Inconel 718 superalloy are governed by the diffusivity of Nb. This low diffusivity of W significantly reduces the migration rate of the phase interface. And one the other hand, the D0₂₂ phase has a relatively large lattice misfit of ∼3% with the FCC matrix along the c-axis, while a smaller misfit of < 1% along the a-axis in Inconel 718 superalloy [36]. Nevertheless, in Ni₅₈Cr₂₃Fe₁₀W₅Ti₂Ta₁Nb₁ MEA, the drastically reduced lattice misfit (about 2.6% along the c-axis while 0.56% along the a-axis) should lower the elastic misfit energy [37–39], leading to a substantial reduction in the driving force for coarsening.

The tensile properties of the MEA are exhibited in Figure 4(a). The yield strength and ultimate tensile strength of the MEA aged at 800°C for 12 h were measured to be ∼1 GPa and ∼1.4 GPa. In addition, the fracture elongation is approximately 33%. In addition, Fig. S6 shows the tensile properties of the MEA aged at 700°C and 750°C and Fig. S7 exhibits the fracture graphs for the failed samples at room temperature tensile test. To highlight the superior combination of mechanical properties and thermal stability of the Ni₅₈Cr₂₃Fe₁₀W₅Ti₂Ta₁Nb₁ MEA in the current work, we compare it with other reported D0₂₂-strengthened HEAs in Figure 4(b). We take the temperature that the D0₂₂ phase is thermostable as the horizontal axis and the yield strength as the vertical axis. Notably, the current D0₂₂ phase shows great superiority by being not only stable at a higher temperature but also stronger than those reported HEAs and superalloys. We also evaluated the thermal response of the Ni₅₈Cr₂₃Fe₁₀W₅Ti₂Ta₁Nb₁ MEA aged at 800°C for 12 h by measuring the variation in yield strength at elevated temperatures in Fig. S8, and a yield strength about 800 MPa is maintained at temperatures from 600°C to 700°C. Fig. S9 shows the fracture graphs for the failed samples at high temperature tensile test.

Figure 4. (a) Room-temperature tensile stress–strain curves of the MEA aged at 800°C for different times, (b) The comparative results of yield strength and annealing temperature with the reported D0₂₂-strengthened HEAs and superalloys, Ni_2.1CoCrFe_0.5Nb_0.2 [40], CoCrFeNiNb_0.25 [9], Ni_2.1CoCrFe_0.5Nb_0.2 [41], Cr₂₂Co₂₂Ni₄₈Nb₄V₄ [42], Ni₂CrCoNb_0.15 [43], CoCrNi_1.5Nb_0.2 [44], Ni₆₆Cr₂₆W₆Ti₂ [45], Ni_57.6Cr_19.2Fe_19.2Nb₄ [46], Inconel 625[47], Inconel 718[48].

In summary, we developed a new D0₂₂-strengthened Ni₅₈Cr₂₃Fe₁₀W₅Ti₂Ta₁Nb₁ MEA and investigated its thermostability using SEM, TEM, and CALPHAD. Exceptionally, a new D0₂₂ phase with a stoichiometry of Ni₃(W_0.24Ti_0.38Ta_0.19Nb_0.19) precipitates from the FCC matrix and stably exists after aging for up to 720 h at 800°C. The coarsening behavior of this newly developed D0₂₂ phase was controlled by Lifshitz-Slyozov-Wagner (LSW) theory. We attributed this excellent coarsening resistance to the low diffusivity of W and the extremely small lattice misfit between the D0₂₂ phase and FCC matrix. Furthermore, the Ni₃(W_0.24Ti_0.38Ta_0.19Nb_0.19)-D0₂₂ phase shows superior strengthening effect even after long-term thermal exposure at 800°C. These findings provide the possibility to design high performance D0₂₂-strengthened alloys with excellent thermal stability.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors are grateful for the financial support by the National Natural Science Foundation of China [grant number 52001266], the Guangdong Basic and Applied Basic Research Foundation [grant number 2023A1515012703], the Shanghai “Phosphor” Science Foundation, China [grant number 23YF1450900], Fundamental Research Funds for the Central Universities [grant number G2022KY05109], Young Elite Scientists Sponsorship Program by CAST [grant number 2023QNRC001], and the Practice and Innovation Funds for Graduate Students of Northwestern Polytechnical University [grant number PF2023073].