The Effect of Nb Addition on the Microstructural Evolution and Mechanical Properties of 50W–Ni–Fe Alloy

Abstract

Optimizing the design of low-tungsten-content alloys represents an effective approach to address the insufficient strength and toughness of conventional tungsten alloys. This study focuses on the design and fabrication of low-tungsten-content alloys, specifically investigating the effects of Nb addition on the low-temperature sintering microstructure and mechanical properties of 50W–Ni–Fe alloy. The results demonstrate that Nb significantly lowers the liquid phase formation temperature, shifting the densification mechanism from solid phase sintering to liquid phase sintering. Nb primarily dissolves in the γ-(Ni,Fe) matrix phase and forms nanoscale γ″-Ni₃Nb precipitates. These γ″-Ni₃Nb precipitates maintain coherent interfaces with the γ-(Ni,Fe) matrix phase, exhibiting excellent interfacial bonding, which markedly enhances the hardness and modulus of the matrix phase. Through the strengthening effects of solid solution strengthening and precipitation strengthening, the tensile strength of the alloy increases to 1259 MPa while maintaining a total elongation of 23.1%. The fracture mode of the 50W-Ni-Fe-Nb alloy transitions to a mixed mechanism involving cleavage fracture of W and ductile rupture of the matrix phase.

1. Introduction

Tungsten alloys are typical two-phase composite materials, consisting of W particles and a matrix phase. The matrix phase is composed of elements such as Ni, Cu and Fe [1]. Due to their high density, high strength, high hardness, and excellent radiation absorption capability, tungsten alloys have important applications in military industry, aerospace, and other fields. With the expansion of tungsten alloy applications, there is an increasing demand for enhanced comprehensive mechanical properties, making the development of tungsten alloys with balanced strength and ductility a key research focus.

The balance of mechanical properties in tungsten alloys primarily relies on the design of the two-phase structure: the strength originates from hard and brittle W particles, while the ductility derives from the soft matrix phase. Optimizing composition and processing to achieve synergistic improvement in both strength and ductility remains a major challenge in this field.

W-Ni-Fe is a traditional high-tungsten-content alloy (>85 wt.% W). Researchers have improved the comprehensive mechanical properties of tungsten alloys by introducing minor alloy elements [2,3,4,5,6,7], optimizing the sintering process [8], deformation strengthening [9,10], adding second phase particles [11,12], and applying post-sintering treatments [13,14,15,16]. However, the intrinsic brittleness of W and the limited volume fraction of the matrix phase restrict further enhancement of ductility. In contrast, low-W-content tungsten alloys significantly improved ductility due to the increased proportion of the matrix phase. During sintering densification, restricted long-range diffusion of W atoms across the matrix phase effectively inhibits W grain coarsening, resulting in fine-grain strengthening [17]. Consequently, the development of high-performance low-W-content tungsten alloys has attracted widespread attention.

In low-W-content tungsten alloys, the dominant matrix phase (primarily Ni-based) offers opportunities for performance tuning through alloying. Studies on Ni-based alloys have demonstrated that Nb, as a potent strengthening element, enhances strength via solid solution strengthening and precipitation strengthening mechanisms [18,19,20]. Introducing Nb into low-W-content tungsten alloys may therefore enable synergistic strengthening effects to resolve the strength–ductility trade-off.

This study focuses on a 50W–Ni–Fe system with Nb additions, systematically investigating the effects of Nb on densification behavior, microstructure evolution, and mechanical properties, while elucidating the strengthening mechanisms and failure behavior of 50WNiFeNb alloys. This study aims to provide insights for designing tungsten alloys with high strength and high ductility.

2. Materials and Methods

In this experiment, the samples were prepared using high-purity reduced tungsten powder, carbonyl nickel powder, carbonyl iron powder, and reduced niobium powder, with detailed parameters listed in

Table 1. Characterization of raw powders.

Elemental Powder	Purity [%]	Mean Particle Size [μm]	Particle Shape	Fabrication Method
W	99.98	5	Irregular	Oxide reduction
Ni	99.6	3	Spherical	Carbonyl process
Fe	99	5	Spherical	Carbonyl process
Nb	99	5	Irregular	Oxide reduction

. Liu et al. [21] systematically investigated the influence of Nb content on the mechanical properties of Inconel 718 nickel alloys and identified an optimal amount of Nb addition. Based on their study, a 50W–35Ni–11Fe–4Nb alloy (hereafter referred to as WNiFeNb alloy) was designed in this study. For comparison, a Nb-free 50W–35Ni–15Fe alloy (hereafter referred to as WNiFe alloy) was also prepared to evaluate the effects of Nb addition on the microstructure and properties of the 50 W alloy.

The raw powders were loaded into mixing containers under an argon atmosphere to prevent oxidation during blending. Homogenization was performed using a three-dimensional mixer (model SBH-1L) for 8 h. The pre-mixed powders were then encapsulated in rubber molds and compacted via cold isostatic pressing (CIP) at 250 MPa to form green compacts.

A conventional single-step sintering process was employed: The compacts were subjected to pressureless sintering under a vacuum atmosphere using a vacuum furnace (VVP-60, Shanghai Haoyue, Shanghai, China). The sintering process involved heating to 900 °C at 10 °C/min followed by a 30 min holding period, subsequent heating to 1200 °C, 1250 °C, 1300 °C, and 1350 °C at 5 °C/min with a 2 h holding period at each stage, and finally furnace cooling to room temperature. The vacuum level was maintained below 10⁻³ Pa throughout the sintering process. The dimensions of the sintered samples and the sintering process profile are shown in the Figure 1.

The density of the tungsten alloys was measured using the Archimedes drainage method. The theoretical density of the 50 W alloy was calculated based on the theoretical densities and mass fractions of its constituent elements:

$$\rho ={\displaystyle \frac{100}{\sum \frac{W_i}{{\rho }_i}}}$$

(1)

where ρ represents the theoretical density of the tungsten alloy, and W_i and ρ_i are the mass fraction and theoretical density of element i, respectively. The relative density is obtained by taking the ratio of the experimentally measured density to the theoretical density. Tensile strength tests of the samples were carried out using a dual-column tabletop testing system (Instron^® 3369, Norwood, MA, USA). Three tensile specimens (dimensions: 26 L × 3 W × 3 H mm; gauge length: 8 mm, as shown in the Figure 2) were taken from each alloy to ensure the reliability of the tensile test data.

A field-emission scanning electron microscope (SEM, TESCAN MIRA4 LMH, Brno, Czech Republic) was used to characterize the microstructure and fracture features of the alloys. The phase composition of the alloys was characterized using X-Ray diffraction (XRD) analysis performed on a D8 ADVANCE instrument equipped with Cu Kα radiation. An electron probe micro-analyzer (EPMA, JXA-8230, Akishima, Japan) was used to characterize and analyze the chemical composition and elemental concentration variations in the matrix phase of the tungsten alloys. A focused ion beam-scanning electron microscope (FIB-SEM, TESCAN AMBER) was used to prepare transmission electron microscopy (TEM) samples, which were then observed using a spherical aberration-corrected transmission electron microscope (TEM, FEI TitanTM G2 60-300, Hillsboro, OR, USA). ImageJ software (version: 1.54h) was used to measure the grain size distribution of the alloys.

3. Results

3.1. Microstructure

Figure 1 presents the XRD patterns of WNiFe and WNiFeNb alloys sintered at different temperatures. As shown in Figure 3a,b, both WNiFe and WNiFeNb alloys exhibit nearly identical diffraction patterns at all sintering temperatures. The characteristic peaks correspond to the W phase (W(110), W(200), W(211), W(220)) and the γ-(Ni,Fe) phase (γ(111), γ(200), γ(220)), confirming the two-phase structure of the alloys. Notably, no diffraction peaks associated with elemental Nb are observed in the WNiFeNb alloy at any sintering temperature, indicating that Nb was successfully dissolved into the matrix.

A direct comparison of the 1350 °C-sintered WNiFe and WNiFeNb alloys (Figure 3c) demonstrates that Nb addition does not alter the fundamental crystalline structure of the two phases in the tungsten alloys. The enlarged view of the dashed regions in Figure 3d reveals no significant changes in the W phase diffraction peaks. However, the γ-(Ni,Fe) phase peaks in the WNiFeNb alloy show a slight leftward shift, suggesting an expansion of the γ-phase lattice constant. This phenomenon arises from the substitutional solid solution of Nb (atomic radius: 147 pm) in the γ-(Ni,Fe) matrix, where Nb atoms replace smaller Ni atoms (atomic radius: 124 pm) [22].

Figure 4 shows the SEM images of WNiFe alloys sintered at different temperatures. All microstructures exhibit white W particles embedded in a gray γ-(Ni,Fe) matrix phase. Notably, no significant microstructural differences are observed at all sintering temperatures. The alloys consistently display characteristic features of solid phase sintering: non-uniform distribution of W particles and the γ-(Ni,Fe) matrix phase, irregularly shaped W particles with no apparent growth, and high W-W connectivity

Figure 5 presents SEM images of the WNiFeNb alloy microstructure at different sintering temperatures. In addition to white W particles and a gray γ-(Ni,Fe) matrix phase, a small number of dark grey particles are distributed within the matrix phase. As shown in Figure 5a, pores are observed in the alloy sintered at 1200 °C, with a large number of fine W particles agglomerated together. With increasing sintering temperature (Figure 5b,c), porosity decreases and partial coarsening of W particles occurs. When the temperature further rises to 1350 °C (Figure 5d), significant W particle growth is observed, accompanied by smoother particle surfaces and more homogeneous distribution of both W particles and matrix phase. These microstructural features are characteristic of typical liquid phase sintering mechanisms.

To investigate the elemental distribution and identify the dark grey particles in the WNiFeNb alloy, electron probe micro-analysis (EPMA) was conducted on the alloy sintered at 1350 °C, as shown in Figure 6. Significant elemental diffusion is observed at the interface between W particles and the γ-(Ni,Fe) matrix phase. Nb is uniformly distributed within the γ-(Ni,Fe) matrix phase, while the dark grey particles are composed of Nb and O. A quantitative analysis was conducted on the tungsten particles (Position 1), matrix phase (Position 2) and dark grey particles (Position 3), with the results summarized in

Table 2. Elemental composition at the indicated position in Figure 6.

Position	W	Ni	Fe	Nb	O
1	93.84 ± 1.28	4.09 ± 0.91	1.30 ± 0.46	0.77 ± 0.14	-
2	7.15 ± 0.10	66.85 ± 0.20	22.13 ± 0.21	3.88 ± 0.18	-
3	0.38 ± 0.04	3.76 ± 1.28	1.92 ± 0.12	31.62 ± 0.36	62.32 ± 1.68

. EPMA quantitative analysis reveals that the atomic ratio of Nb to O in the dark grey particles is approximately 1:2. By referencing the Nb–O binary phase diagram, these particles are confirmed to be the NbO₂ phase.

Figure 7 shows TEM images of the WNiFeNb alloy sintered at 1350 °C. As shown in Figure 7a, no significant defects are observed at the interface between W particles and the γ-(Ni,Fe) matrix phase, indicating excellent interfacial bonding unaffected by Nb addition. Figure 7b,c confirm that the W particles exhibit a body-centered cubic (BCC) structure, while the γ-(Ni,Fe) matrix phase adopts a face-centered cubic (FCC) structure.

The selected-area dark-field (DF) image (Figure 7d) reveals abundant nanoscale precipitates (20–50 nm) within the matrix phase. Indexing of the selected-area electron diffraction (SAED) pattern (Figure 7c) identifies these precipitates as γ″-Ni₃Nb. The orientation relationship (111)_γ-(Ni,Fe)//(112)_γ″-Ni3Nb confirms a coherent interface between the γ″-Ni₃Nb precipitates and the γ-(Ni,Fe) matrix.

3.2. Mechanical Property

Figure 8 shows the relative densities of WNiFe and WNiFeNb alloys sintered at different temperatures. As the sintering temperature increases, both alloys exhibit a progressive increase in relative density. At 1200 °C and 1250 °C, the relative density difference between the two alloys is minimal. However, at 1300 °C, the WNiFe alloy achieves a relative density of 96.0%, while the WNiFeNb alloy reaches 98.1%, demonstrating a significant densification improvement.

When sintered at 1350 °C, the relative density of the WNiFe alloy remains nearly unchanged at 96.1%, showing no further enhancement compared to the 1300 °C condition. In contrast, the WNiFeNb alloy achieves a relative density of 98.7%, approaching full densification. These results clearly indicate that Nb addition markedly promotes the sintering densification of the WNiFeNb alloy.

Figure 9 presents the tensile strength and total elongation of WNiFe and WNiFeNb alloys sintered at different temperatures. As shown in Figure 9a, the tensile strength of the WNiFe alloy initially increases with rising sintering temperature, reaching a peak of 781 ± 11 MPa at 1300 °C, followed by a slight decrease to 769 ± 3 MPa at 1350 °C. Meanwhile, the total elongation of the WNiFe alloy continuously improves with temperature, reaching a maximum value of 31.0 ± 0.8% at 1350 °C.

In contrast, the WNiFeNb alloy (Figure 9b) exhibits a consistent increase in tensile strength with sintering temperature, achieving a maximum value of 1259 ± 13 MPa at 1350 °C. This represents a remarkable 478 MPa enhancement compared to the Nb-free WNiFe alloy. The total elongation of the WNiFeNb alloy first increases to a peak of 23.6 ± 0.8% at 1300 °C, then slightly declines to 23.1 ± 0.7% at 1350 °C, showing an approximate 7% reduction relative to the WNiFe alloy.

3.3. Fracture Morphology

Figure 10 presents SEM images of the tensile fracture morphology of WNiFe alloys sintered at different temperatures. As observed in Figure 10a,b, when the sintering temperature does not exceed 1250 °C, the fracture morphologies exhibit numerous W particles retaining their original powder morphology, accompanied by extensive interconnected pores. With increasing sintering temperature, sintering necks between W particles grow larger, and porosity progressively decreases.

At sintering temperatures of 1300 °C and above (Figure 10c,d), the fracture morphologies no longer show significant aggregation of original W powder particles. The dominant fracture mode transitions to W-W intergranular fracture, with no W cleavage fracture features observe. Furthermore, numerous fine dimples formed by the ductile fracture of the matrix phase are observed, which is consistent with the increase in the total elongation of the alloy.

Figure 11 displays SEM images of the tensile fracture morphologies of WNiFeNb alloys sintered at different temperatures. At 1200 °C (Figure 11a), the fracture morphology exhibits extensive intergranular separation of original particles and abundant pores. When the sintering temperature increases to 1250 °C (Figure 11b), porosity remains significant, though partial coarsening of W particles is observed, accompanied by reduced aggregation of original powder particles. Additionally, pronounced W particle debonding and minor ductile tearing ridges are visible in the matrix phase.

At 1300 °C (Figure 11c), dispersed fine tearing ridges gradually coalesce into a network-like structure, while partial cleavage fracture of W particles emerges. As shown in Figure 11d, when sintered at 1350 °C, the fracture morphology demonstrates substantial W particle coarsening, with cleavage fracture becoming the dominant failure mode for W particles. Notably, interface debonding between W particles and the matrix phase nearly disappears, correlating with the significant strength enhancement of the WNiFeNb alloy.

4. Discussion

4.1. Effect of Nb Addition on Microstructural Characteristics

According to the Ni-Fe binary phase diagram, the minimum liquidus formation temperature of the matrix phase with a Ni:Fe ratio of 7:3 is 1426 °C [17]. Consequently, within the sintering temperature range of 1200–1350 °C, the densification mechanism of the WNiFe alloy remains governed by solid phase sintering. Diffusion at contact points drives sintering neck formation and densification. The low sintering temperatures restrict diffusion rates, and the absence of a liquid phase prevents particle rearrangement, resulting in the microstructural characteristics of the WNiFe alloy: limited W particle growth, non-uniform distribution, and high W-W connectivity. At 1350 °C, the alloy achieves a relative density of only 96.1%.

The Ni-Nb binary phase diagram [23] reveals a eutectic liquid formation at 1180 °C. At 1200 °C, a minimal liquid phase forms in the WNiFeNb alloy, with microstructural features similar to those of the WNiFe alloy. As temperature increases, the liquid phase content increases, and capillary forces drive the liquid phase to infiltrate W-W interfaces, filling pores and facilitating W particle rearrangement for a more uniform distribution. Meanwhile, W dissolution and precipitation within the liquid phase contribute to localized grain growth. At 1350 °C, while significant W particle growth occurs, the increased matrix phase fraction (compared to traditional high-W alloys) restricts long-range W diffusion during dissolution/precipitation, effectively suppressing W grain growth. The resultant W particles exhibit an average grain size of 7.40 ± 0.22 μm, markedly smaller than those in conventional high-W alloys (~30 μm).

Additionally, Ni and Nb demonstrate the lowest mixing enthalpy in the matrix phase (

Table 3. Atomic radius and enthalpy of mixtures of Ni, Fe, and Nb atoms [22].

Element		Ni	Fe	Nb
Atomic radius [pm]		124	126	147
Enthalpy of mixing [KJ/mol]	Ni	-	−2	−30
	Fe	-	-	−16
	Nb	-	-	-

). According to the minimum mixing enthalpy selection theory, Ni–Nb intermetallic compounds preferentially form in the alloy. Based on the Ni-Nb binary phase diagram, a eutectic reaction occurs at 1285 °C: L ↔ Ni₃Nb + Ni. Ni₃Nb exists in two structures: stable D0a–Ni₃Nb and metastable D0₂₂-Ni₃Nb [18,22]. While the alloy’s cooling process during sintering is close to equilibrium solidification, only the D0₂₂–Ni₃Nb phase is observed in the sintered alloy. This occurs because Fe dissolution in the matrix phase increases its lattice constant, reducing interfacial misfit. Additionally, Fe adjusts the alloy’s electron concentration ratio to stabilize the D0₂₂ phase, thereby promoting its precipitation [24].

4.2. Effect of Nb Addition on Mechanical Properties

The addition of Nb significantly enhances the tensile properties of the 50W–Ni–Fe alloy, particularly increasing the ultimate tensile strength by approximately 478 MPa, while maintaining a favorable combination of strength and ductility compared to similar alloy systems and conventional high-W-content tungsten alloys (Figure 12). The strengthening mechanisms primarily involve two factors: solid solution strengthening from Nb dissolution in the matrix phase, and precipitation strengthening by coherent γ″-Ni₃Nb precipitates formed within the matrix phase.

In the 50WNiFeNb alloy, the significant atomic size mismatch between Nb atoms and Ni/Fe atoms induces substantial lattice distortion when W and Nb atoms dissolve into the γ-(Ni,Fe) matrix phase via solid solution. This dissolution alters the local atomic arrangement, creating stress fields that interact with dislocations and hinder their glide. Consequently, the hardness and modulus of the γ-(Ni,Fe) matrix phase are enhanced. The solid solution strengthening effect can be quantitatively evaluated using the following formula [31]:

$$\Delta {\sigma }_{SS}=3^{{\displaystyle \frac{3}{2}}}\cdot {\displaystyle \frac{G\cdot {\epsilon }^{\frac{3}{2}}\cdot C^{\frac{1}{2}}}{700}}$$

(2)

where 0 is the shear modulus of Ni (G = 79 GPa), ε is the interaction coefficient (ε = 0.33), and C is the solid solubility of W and Nb in the γ-(Ni,Fe) matrix phase, measured via EPMA as C_W = 7.06 at.% and C_Nb = 4.05 at.%. The calculated solid solution strengthening contribution is Δσ_SS = 37.06 MPa.

The γ″-Ni₃Nb phase with a D0₂₂ structure is an ordered tetragonal phase that maintains a coherent interface with the γ-(Ni,Fe) matrix phase. The closely matched lattice parameters result in low interfacial misfit. Additionally, the small size of γ″-Ni₃Nb precipitates (20–50 nm) reduces elastic interactions with dislocations while effectively inhibiting crack initiation at the γ″-Ni₃Nb/γ-(Ni,Fe) interface. Furthermore, the γ″-Ni₃Nb phase exhibits high antiphase boundary (APB) energy, leading to a more pronounced precipitation strengthening [32]. The precipitation strengthening contribution can be calculated using the following formula [33]:

$$\Delta {\sigma }_{OR}={\displaystyle \frac{0.13Gb}{\lambda }}\ln {\displaystyle \frac{r}{b}}$$

(3)

$$\lambda =2r\left[{\left({\displaystyle \frac{1}{2f}}\right)}^{{\displaystyle \frac{1}{3}}}-1\right]$$

(4)

where b is the Burgers vector of the matrix phase (b = 0.25 nm), f is the volume fraction of precipitates, λ is the average distance between Ni₃Nb phase, and r is the precipitate size. The calculated precipitation strengthening contribution is Δσ_OR = 462.72 MPa, demonstrating that precipitation strengthening dominates the performance enhancement induced by Nb addition.

Under the synergistic effects of Nb solid solution strengthening and nano-scale Ni₃Nb precipitation, the matrix phase exhibits significantly improved hardness and modulus (as shown in Figure 13 and

Table 4. Nanoindentation measurement of hardness and elastic modulus of the matrix phase.

Sample	Hardness of Matrix Phase [GPa]	Modulus of Matrix Phase [GPa]
WNiFe	5.11 ± 0.08	210.47 ± 5.51
WNiFeNb	8.24 ± 0.25	268.10 ± 1.70

), providing robust support for the enhanced alloy strength.

Coherent nano-precipitates not only generate significant precipitation strengthening effects but also induce distinctive multistage work-hardening behavior, effectively enhancing ductility by delaying local plastic instability [34]. However, the reduced ductility of the WNiFeNb alloy compared to the WNiFe alloy may be attributed to the presence of numerous NbO₂ particles. As observed in the fracture morphology (Figure 11) and crack path profiles (Figure 14) of the WNiFeNb alloy, the hard and brittle NbO₂ particles exhibit poor interfacial bonding with both W particles and the γ-(Ni,Fe) matrix phase. These interfaces serve as preferential sites for crack initiation and propagation, thereby degrading the alloy’s plastic deformation capability.

4.3. Alloy Failure Behaviour

As typical two-phase alloys, tungsten alloys exhibit a strong correlation between their mechanical properties and fracture modes. The relationship between fracture strength and fracture modes can be described by the following formula:

$${\sigma }_b=f_W{\sigma }_W+f_{W-W}{\sigma }_{W-W}+f_{W-M}{\sigma }_{W-M}+f_M{\sigma }_M$$

(5)

where f_W, f_W−W, f_W−M, and f_M are the proportions of W cleavage fracture, W-W intergranular fracture, W/matrix phase interfacial debonding, and matrix phase ductile rupture in the alloy’s fracture surface, respectively. σ_W, σ_W−W, σ_W−M, and σ_M are the cleavage strength of W particles, W-W interfacial strength, W/matrix phase interfacial strength, and matrix phase strength, respectively. Among these fracture modes, σ_W−W and σ_M are relatively low; thus, the alloy’s strength is primarily governed by σ_W−M and f_W.

Figure 14 shows SEM images of the crack path profiles for 1350 °C-sintered WNiFe and WNiFeNb alloys after tensile testing. In the WNiFe alloy, due to the solid phase sintering mechanism, the non-uniform distribution of W particles and weak W/matrix phase interfacial bonding result in a main crack propagation path dominated by W-W intergranular fracture and W/matrix phase interfacial debonding, leading to inferior strength.

In contrast, Nb addition in the WNiFeNb alloy enhances sintering densification, resulting in a more uniform W particle distribution, reduced W–W connectivity, and strengthened W/matrix phase interfaces. Consequently, the main crack propagation path is dominated by W cleavage fracture with no significant W/matrix phase debonding observed. Furthermore, the formation of nano-scale γ″-Ni₃Nb precipitates within the matrix phase due to Nb–Ni interactions significantly enhances the matrix phase’s hardness and modulus, contributing to a remarkable increase in alloy strength.

5. Conclusions

This study systematically investigated the effects of Nb addition (4 wt.%) on the microstructure evolution and mechanical properties of 50W–Ni–Fe alloys sintered at different temperatures (1200 °C, 1250 °C, 1300 °C, 1350 °C), with comparisons to Nb-free 50 W alloys. The main conclusions are as follows:

(1)

Nb addition significantly reduces the liquid phase formation temperature, promotes sintering densification, and improves microstructural homogeneity. The 50W–35Ni–11Fe–4Nb alloy achieves an optimal relative density of 98.7% at 1350 °C.

(2)

Nb and Ni form a large number of γ″-Ni₃Nb precipitates with sizes ranging from 20 nm to 50 nm. These precipitates maintain a coherent relationship with the γ-(Ni,Fe) matrix, exhibiting strong interfacial bonding, which significantly enhances the hardness and modulus of the matrix phase.

(3)

The WNiFeNb alloy achieves balanced strength and ductility primarily through precipitation strengthening, supplemented by solid solution strengthening. The alloy achieves an ultimate tensile strength of 1259 ± 13 MPa (61% improvement over the Nb-free alloy) and a total elongation of 23.1 ± 0.7%. The fracture mode transitions to a hybrid mechanism dominated by W particle cleavage fracture and matrix phase ductile rupture.