An ultrastrong niobium alloy enabled by refractory carbide and eutectic structure

ABSTRACT Nb alloys with high strength and relatively low density are sought for high-temperature applications. However, due to strain softening, conventional Nb alloys often exhibit insufficient high-temperature strengths. Here we report a strategy to design a novel lightweight and thermally-stable Nb alloy by combining the notion of eutectics and refractory carbide. The resulting Nb2MoWC0.96 eutectic alloy exhibits a high-temperature specific compressive strength of 77.2 MPa/·cm3·g1 at 1673 K with a retained engineering plasticity of 37.5%, both are notably higher than the existing Nb and Nb-containing alloys. This work demonstrates that both the strength and plasticity of Nb alloys can be further optimized with refractory carbide and eutectics. GRAPHICAL ABSTRACT IMPACT STATEMENT Combining the notion of eutectics with refractory carbide makes high strength, large plasticity Nb alloy with good microstructural stability.

Low-density high-temperature alloys are sought for applications in the aerospace and weapons industries. For example, advanced aero-engines and gas turbines require an operating temperature higher than 1273 K. Pure Nb has a high melting point of 2741 K and a low density of 8.57 g/cm 3 , making it an ideal element to be used at high temperatures. Also, owing to a good combination of high strength and high ductility, Nb alloys are often used as thermal protection and structural materials in aerospace, high-temperature reactors, and the aviation industry [1][2][3]. Often, based on their strength values, low-, medium-, and high-strength Nb alloys are classified. Conventionally, alloying elements such as W, Mo, Ta, and V are added to improve the hightemperature strength of Nb alloys based on strong solid solution strengthening [4][5][6]. However, their strength values are insufficient to meet a higher requirement as next-generation materials for aerospace engines [7,8]. For  Nb alloys, the F-48 (Nb-15W-5Mo-1Zr-0.1C) and C-3009 (Nb-22.4Hf-5.9W) alloys have compressive yield strength of 210 and 388 MPa at 1473 K, respectively [7,9]. These strength values are notably lower than those of Nbcontaining alloys (i.e. 868 MPa for C 0.25 Hf 0.25 NbTaW 0.5 , and 673 MPa for HfMo 0.5 NbTiV 0.5 Si 0.7 ) [10,11].
To further improve the strength of single-phase Nb alloys, the introduction of second phases such as Nb 2 C and Nb 3 Al is required [3,12]. However, the introduction of excessive second phases could lead to a significant loss in plasticity, mainly because of the easy crack formation during deformation, due to the weak interfacial bonding between the main phase and the second phase(s). For example, Nb-Si alloys (i.e. Nb x Mo y W-10Ti-18Si, x = 10 and 20, y = 0,5,10, and 15 in mol.%) consisting of an Nb-rich solid solution phase and a silicide phase exhibited a yield strength of about 400-800 MPa in compression mode at 1670 K, owing to a high-volume fraction ( > 40%) of the silicide phase [4,13]. Despite the high strength, the Nb-Si alloys suffer from crack initiation from interfacial decohesion [14,15].
Recently, a new class of refractory high-entropy alloys composed of multi-principal elements has exhibited a high strength at high temperatures owing to a strong solid solution strengthening [16]. For example, equiatomic NbMoTaW and VNbMoTaW alloys were reported to have a high strength of above 420 MPa in compression at 1673 K [16], which are notably higher as compared with their dilute alloys [17]. Further, a higher strength can be achieved through the introduction of a second phase [10,13,19]. For example, the Re 0.5 MoNbW(TaC) 0.5 alloy consisting of a solid solution phase and a carbide phase exhibited an ultrahigh yield strength of 901 MPa in compression at 1473 K [20], owing to the combination of a stable eutectic structure with a carbide phase of multiprincipal-element-mixing. However, the density of this alloy (15.01 g/cm 3 ) is significantly higher than those of Nb alloys having a density of 7.40-13.71 g/cm 3 [21].
To further improve the high-temperature strength while reducing the density of the carbide-reinforced eutectic high-entropy alloys, we redesign the alloy by tuning the chemical compositions so that the density is comparatively low as compared to the lightweight Nb alloys.
A compositionally optimized Nb 2 MoWC 0.96 alloy with a molar ratio of 2: 1: 1: 0.96 was fabricated by arc melting high purity (better than 99.99%) Nb, Mo, W, and C powders under an argon atmosphere. Each alloy ingots were remelted five times to ensure chemical homogeneity. The samples had a diameter of about 20 mm and a thickness of about 8 mm. To evaluate thermal stability, the as-cast samples were sealed in Quartz tubes filled with high-purity argon and then annealed at 1573 K for 10 and 30 h in an electric furnace, respectively. The samples were then subsequently furnace-cooled to room temperature. Compression tests were conducted on cylindrical specimens with a dimension of 4 mm × 6 mm at 1073, 1273, 1473, and 1673 K under an argon atmosphere using a Gleeble-3500 thermal simulator at a strain rate of 1×10 −3 S −1 . The engineering strain was calibrated using an extensometer.
The crystal structure was measured using an X-ray diffractometer (XRD) with a Cu target at a scanning rate of 1 degree/min. The microstructure was characterized using a scanning electron microscope (SEM, Quanta FEG250) equipped with a back-scattered electron (BSE) detector, and an electron backscatter diffraction (EBSD) detector. The EBSD data were analyzed using AZteccrystal software. Fine microstructure and chemical compositions were measured using a transmission electron microscope (TEM, Tecnai G2 F20) equipped with a high-angle annular dark-field (HAADF) detector and energy dispersive X-ray spectroscopy (EDS) detector. Figure 1(a,b) show an XRD pattern and SEM-BSE images of the Nb 2 MoWC 0.96 alloy, which reveal a lamellar eutectic microstructure consisting of alternating BCC and FCC-type NbC phases. The lattice constants of the BCC and NbC phases were identified to be a = 0.3224 nm and a = 0.4451 nm, respectively. Chemical analysis by TEM-EDS reveals that the BCC phase is partitioned by multiple elements with a composition of W 33.1 Nb 32.5 Mo 30.2 C 4.2 (at. %) [ Figure 1(c)]. Based on the chemical composition, the configurational entropy value ( S) is calculated to be 1.23 R provided that elements are randomly distributed, this value falls within the range of medium entropy alloy (0.69 R < S < 1.61 R). By contrast, the NbC is a dilute carbide phase with a composition of (Nb 53.4 Mo 5.5 W 5.5 ) 64.4 C 35. 6 [ Figure 1(c) and Table 1]. Microstructure analysis by high-resolution scanning TEM (STEM) shows that the BCC phase contains a low density of dislocations, while the NbC phase shows several stacking faults [ Figure 1 The phase stability of the alloy was evaluated by theoretical calculation of phases and supported by phase analysis using XRD. Figure 2(a) shows a pseudo-binary phase diagram of (Nb 2 MoW)-C alloys calculated using the Fastsage software. The phase diagram exhibits a eutectic reaction with a reaction product of BCC and FCC (NbC) phases. The two phases remain stable during the solidification process without precipitation of new phases, supporting high thermal stability of the predicted BCC and FCC (NbC) phases. Figure 2(b) displays XRD patterns of the Nb 2 MoWC 0.96 alloy annealed at 1573 K for 10 and 30 h, respectively. A second phase was not detectable in the XRD patterns, and a noticeable peak shift was also not observed, confirming again the high thermal stability of phases. This excellent phase stability is further supported by the fact that noticeable microstructure changes such as interfacial migration accompanied by grain coarsening were not observed in the alloy [ Figure 2 Table 2. Figure 3(c, d) displays the summarized temperature-dependent yield strength and ultimate strength normalized by their densities (Table 3), these values are then compared with those of existing Nb and Nb-containing alloys tested under compression [13,[19][20][21][22][23][24]. Notably, both specific yield strength and ultimate strength of the Nb 2 MoWC 0.96 alloy are significantly higher than those of reported Nb and Nb-containing alloys particularly at high-temperature regime (i.e. > 1273 K) because of the excellent resistance to high temperature softening. SEM-EBSD experiments were conducted to investigate the post-deformation microstructure of the Nb 2 MoWC 0.96 alloy to clarify the strengthening mechanism. The values GNDs of Nb and NbC in the ascast and high compression temperatures are summarized in Table 4. SEM-EBSD maps reveal a neglectable low dislocation density in the as-cast BCC and NbC phases (Figure 4). After compression at 1073 K, a high  density of dislocations is observed in both the BCC phase and NbC carbide phase. The dislocation density further increases with compression temperatures, as displayed in maps containing geometrically necessary dislocations (GNDs). Further increasing the compression temperature to 1673 K, however, a decrease in GNDs is observed, indicating that a recovery and recrystallization in the BCC and NbC phase took place during compression. As is revealed by the microstructures in Figure 4, the critical temperature for recrystallization is around 1673 K. Below this critical temperature, recovery should have taken place. However, the increased dislocation density upon increasing temperatures suggests that the speed for dislocation annihilation via recovery does not match with the accumulation of dislocations by a larger plastic strain with increasing deformation temperatures. Similar to the microstructure observed by TEM, the orientation relationship detected by SEM-EBSD reveals that the as-cast alloy near accommodates the  K-S orientation relationship (Figure 4 and 5), while locally the lamellar structure adopts a neater K-S relationship. Upon compression at high temperatures, the local orientation relationship nearly retains locally, despite the K-S orientation seems to be destroyed from the pole figures of the whole area, due to the increased defect density and the occurrence of recrystallization. Post-deformation microstructure was further observed by TEM to unveil the fine structure of dislocations. Figure 6  reduction in SF density. As displayed in Figure 6(a-c), a notable transition of deformation mode from SFs to mixed SFs and dislocations in the FCC carbide phase are observed. Upon compression at 1673 K, recrystallization along with reduced SF and dislocation density takes place [ Figure 6(d)]. Despite recrystallization, the BCC/NbC eutectic interface remains stable without a significant change in its orientation relationship [ Figure 6(f)]. It is worth noting that a transition of deformation mode from SFs to mixed SFs and dislocations are observed [ Figure 6(a-c)]. This transition could be associated with activation of slip systems at increasing temperatures, which promotes dislocation nucleation at phase boundaries. At 1083 K, only partial dislocations are generated at the phase interface and propagate into grain interior. With deformation temperature increased to 1273 K, the generation of both mixed partial and full dislocations is observed.
This work reports the design of a high-strength lightweight Nb alloy by combining the eutectics with refractory carbide. The resulting Nb 2 MoWC 0.96 eutectic alloy exhibited a high strength at high temperatures owing to the excellent microstructure stability. This is in sharp contrast to the reported Nb and Nb-containing alloys in which grain coarsening and grain boundary sliding are often observed at high temperatures [14,[25][26][27].
To clarify the high thermal stability of the lamellar eutectic structure, a defect migration speed V F is calculated as per the defect migration theory [28]: where D is the diffusion coefficient, γ is the surface tension between the two phases, the BCC solid solution phase is regarded as solvent and the NbC as solute, C α and C β are solute concentrations of the two phases (C α > C β ) (α and β represent the BCC and NbC phases, respectively), λ is the average lamellae spacing of the eutectic structure in the as-cast alloy and is 550 nm on average (Figure 3), λ BCC is the average lamellar thickness of the as-cast BCC phase (about 250 nm). V BCC is the partial molar volume of the BCC phase [29]. It is noted that that the interface migration is far more complicated than that simply described by the diffusion of solutes. However, this calculation with the simple model provides a qualitative estimation. The calculated ln (2λ /λ α ) and (C α -C β ) are positive, thereby the value of V F is positive. The positive sign indicates that the eutectic structure is unstable, which is opposite to our observation that microstructure is highly stable at high temperatures. This in turn suggests that the value of V F would be extremely small and the diffusion coefficient D is tiny. Conversely, the high thermal stability of the eutectic structure is considered to arise from slow diffusion in the Nb 2 MoWC 0.96 alloy. Thermodynamically, the configurational entropy of the BCC phase is calculated to be 1.23 R (where R is the gas constant) based on its chemical composition (Table 1), this value is notably larger than those of dilute alloys with an entropy value of less than 0.69 R [30]. In general, a slow diffusion could arise from the high configuration entropy effect. It has been pointed out that in the Al 0.5 CoCrCuFeNi HEAs, the sluggish diffusion kinetics ensures that the equilibrium phase at a high temperature can be frozen to room temperature even by conventional annealing treatments [31]. They carried out extensive thermodynamic calculations of the Gibbs free energy difference ( G) between the solid solution and intermetallic phases by the Miedema model and revealed a delayed diffusion kinetics resulting from the high-entropy effect.
Upon compression, the BCC/NbC phase interface remained stable at high a temperature up to 1673 K ( Figure 4). To characterize the dissimilar interface quantitively, a mismatching degree factor between the BCC and NbC phases in the as-cast and 1474 K-compressed alloy is calculated with measured interplanar distances (d) and mismatch angles. For the as-cast alloy, the mismatch angle between (11-1) NbC and (101) BCC is estimated to be ∼ 1.4 degrees, and the d 11−1 is 0.269 nm for the NbC phase while the d 101 is 0.253 nm for the BCC phase. Therefore, the mismatch parameter (δ 1 ) between The calculated mismatch parameter (δ) falls between 0.05 and 0.25, supporting the semi-coherent nature of the interface. The result also demonstrates that the semicoherent interface remains almost unchanged after hightemperature compression, further confirming the high thermal stability of the alloy. Conversely, at a deformation temperature below 1673 K, the thermal stability of the alloy is dominated by interfacial stability. However, dislocation activity becomes a major factor in controlling thermal stability when temperature increases beyond 1673 K [33], strong evidence is shown that an obvious recrystallization at 1673 K is observed, which took place at the expense of reduced dislocation density as shown in Table 4.
To conclude, a lightweight Nb eutectic alloy with a dual-phase structure was designed and fabricated. The resulting Nb 2 MoWC 0.96 exhibited a high strength at high temperatures along with excellent microstructural stability. The main findings are listed below: (1) The as-cast Nb 2 MoWC 0.96 alloy was composed of alternating BCC and FCC-type NbC phases. The composition and microstructure of the Nb 2 MoWC 0.96 composite remained stable after annealing at 1573 K for 30 h, revealing excellent microstructural stability. (2) The Nb 2 MoWC 0.96 alloy has an excellent combination of high strength and plasticity while achieving a comparable low density with conventional Nb and Nb-containing alloys. (3) The excellent microstructure stability below 1673 K is supposed to originate from a low diffusion coefficient of the BCC solid solution phase owing to a high configuration entropy, as well as from a small mismatch of coherent phase interface. At temperatures beyond 1673 K, dislocation activity plays a major role in controlling thermal stability. (4) Tensile ductility could be expected by proper thermomechanical treatment of the cast alloy because of the high plasticity of the fabricated Nb alloy.