Synergistic Enhancement of Mechanical Properties and Electrical Conductivity of Immiscible Bimetal: A Case Study on W–Cu

Qixiang Duan; Chao Hou; Tielong Han; Yurong Li; Haibin Wang; Xiaoyan Song; Zuoren Nie

doi:10.1016/j.eng.2024.07.024

Engineering ›› 2025, Vol. 46 ›› Issue (3) :238 -249. DOI: 10.1016/j.eng.2024.07.024

Research Material Science and Engineering—Article

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Synergistic Enhancement of Mechanical Properties and Electrical Conductivity of Immiscible Bimetal: A Case Study on W–Cu

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Abstract

Immiscible bimetal systems, of which tungsten–copper (W–Cu) is a typical representative, have crucial applications in fields requiring both mechanical and physical properties. Nevertheless, it is a major challenge to determine how to give full play to the advantages of the two phases of the bimetal and achieve outstanding comprehensive properties. In this study, an ultrafine-grained W–Cu bimetal with spatially connected Cu and specific W islands was fabricated through a designed powder-mixing process and subsequent rapid low-temperature sintering. The prepared bimetal concurrently has a high yield strength, large plastic strain, and high electrical conductivity. The stress distribution and strain response of individual phases in different types of W–Cu bimetals under loading were quantified by means of a simulation. The high yield strength of the reported bimetal results from the microstructure refinement and high contiguity of the grains in the W islands, which enhance the contribution of W to the total plastic deformation of the bimetal. The high electrical conductivity is attributed to the increased mean free path of the Cu and the reduced proportion of phase boundaries due to the specific phase combination of W islands and Cu. This work provides new insight into modulating phase configuration in immiscible metallic composites to achieve high-level multi-objective properties.

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Immiscible bimetal / Phase configuration / Mechanical property / Electrical conductivity / Strain response

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Qixiang Duan, Chao Hou, Tielong Han, Yurong Li, Haibin Wang, Xiaoyan Song, Zuoren Nie. Synergistic Enhancement of Mechanical Properties and Electrical Conductivity of Immiscible Bimetal: A Case Study on W–Cu. Engineering, 2025, 46(3): 238-249 DOI:10.1016/j.eng.2024.07.024

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1. Introduction

Immiscible bimetal materials generally possess unique combined mechanical and physical properties from their metal components [1], [2]. Tungsten–copper (W–Cu) bimetal composites, as typical representatives of immiscible bimetallic systems, have the advantages of the high hardness and low thermal expansion coefficient of W and the good electrical and thermal conductivities of Cu, and have been widely applied in the fields of electronic packaging materials, heat sinks, high-voltage electric contact materials, and welding electrodes [3], [4], [5]. Achieving excellent integrated properties, such as useful mechanical properties and high electrical conductivity, is desirable and highly demanded in the development of advanced W–Cu bimetal composites.

An effective method to modulate the mechanical and physical properties of an immiscible metallic composite is to introduce a third component [6], [7], [8]. For example, due to the solution strengthening effect, the addition of zinc (Zn) was reported to induce increases in the maximum bending strength and hardness of a W–Cu bimetal to 960 MPa and 3.6 GPa, respectively [9]. As an alternative, hard ceramic nanoparticles can improve the strength of a composite by means of dispersion reinforcement. Moreover, rigid particles tend to reduce the contiguity of bimetal phases and inhibit grain growth during the sintering process of bimetal powders. However, the agglomeration of nanoparticles usually decreases the strength and toughness of the composite and deteriorates the physical properties. For example, dissolving alloying elements in a metal matrix and adding foreign particles were found to decrease the electrical conductivity of bimetal composites [10], [11]. Therefore, the design and preparation of immiscible bimetal composites that are free of a third component while possessing improved mechanical and physical properties are of great importance and have attracted considerable research attention.

There are two featured sizes in immiscible bimetal composites: the phase size and the internal grain size. Refining the sizes of the two metal phases is a common strategy to improve the strength of the composite [12]. However, this method of refining the microstructure has a negative side effect. When the sizes of the metal phases are reduced to the nanoscale, the increased volume fraction of the interfaces results in a significant deterioration in the electrical conductivity [13], [14]. Furthermore, due to the limited capacity of nanoscale phases to accommodate dislocations, the plasticity of the composite is greatly reduced. Therefore, to obtain excellent comprehensive properties in terms of strength, plasticity, and electrical conductivity, the sizes of the bimetal phases should be controlled within an appropriate range. On the other hand, refining the grain size inside the metal phases can effectively increase the hardness and strength of the composite [15], [16]. In addition, by introducing grain boundaries rather than phase boundaries, the influence of interfaces on the physical properties of the composite, such as the electrical conductivity, can be lessened.

The configuration of the immiscible bimetal phases also plays an important role in the properties of the composite. A continuous network is advantageous for the load-bearing capacity and conductivity of the bimetal composite [17]. The network configuration in a bimetal composite is generally fabricated using the infiltration method. However, when using this method, it is very difficult to simultaneously realize both a high relative density and a refined network. Recently, the co-sputtering of immiscible metals was utilized to create a bicontinuous metallic nanocomposite by tuning the deposition rate and substrate temperature [18]. Unfortunately, this method is only applicable to the preparation of thin-film materials and is difficult to use for mass production. Therefore, it is challenging to obtain immiscible bimetallic bulk materials with both a fine network of metal phases and a small internal grain size.

Due to the immiscible nature of the metal components in a bimetal, the binding strength of the phase boundaries is low. These weak phase boundaries are prone to sliding or debonding when bearing a load, which reduces the effective stress transfer between different phases [19]. A local network with high connectivity is conducive to preventing crack propagation along the path of least resistance (e.g., only within the relatively weaker phase and at the phase boundaries), thereby improving the fracture strength and toughness compared with bimetals reinforced by dispersed particles. As this analysis shows, it is essential to appropriately control the microstructure feature size and spatial configuration of the metal components to achieve excellent integration of the mechanical and physical properties of an immiscible bimetal composite.

In the present work, taking the W–Cu system as an example, a new type of ultrafine-grained bimetal composite with spatially connected Cu and W islands was fabricated. This bimetal composite exhibits outstanding integrated mechanical properties and electric conductivity. Compared with the mechanical properties of other W–Cu composites with various grain sizes, a special strengthening effect was observed, in addition to the grain refinement strengthening of the prepared W–Cu bimetal. Using a finite-element simulation based on the real microstructures of the experimental materials, the stress distribution, strain response of each phase, and fracture behavior of bimetals with different microstructural characteristics were quantitatively studied, and the mechanisms for the outstanding comprehensive properties of the prepared W–Cu bimetal were determined.

2. Experimental procedure

Ultrafine-grained W islands were constructed using ultrafine W particles as building blocks. To ensure the spatial connectivity of the W grains in the W islands and that of the W islands with Cu in the bimetal composite, W and Cu powders with different particle sizes were utilized. The much smaller particle size of the W powder compared with that of the Cu powder facilitated the formation of a combination of W islands and a continuous Cu phase. First, the ultrafine W powder (with a particle size of 100–200 nm, 99.9%; Beijing Hawk Technology Co., Ltd., China) was milled without a process control agent. A ball-to-powder mass ratio of 20:1, a rotation speed of 500 r∙min⁻¹, and a milling time of 20 h were set for the milling parameters. Due to the high surface area of the ultrafine powder, the particles were prone to forming agglomerates composed of ultrafine W particles. These agglomerates differed from the conventional ultrafine-grained W particles because the particles within the agglomerates came into contact with each other through particle surfaces instead of grain boundaries.

The W agglomerates were then mixed with the Cu powder, which had a micron-scale particle size (1 µm, 99.9%; Shanghai Aladdin Biochemical Technology Co., Ltd., China). The mass ratio of the W agglomerates to the Cu powder was 7:3. Because of the specific loosely compacted structure, the agglomerates possessed strong shape adaptability and easily attached to the surfaces of the Cu particles, forming a network composed of loosely connected ultrafine W particles (W islands) in contact with Cu particles. The ball-milling process was performed for 6 h using a rotation speed of 300 r∙min⁻¹ and a ball-to-powder mass ratio of 10:1. These milling parameters were carefully tuned to obtain the desired combination of W and Cu within a short duration, avoiding an adverse effect on the Cu conductivity. Tungsten carbide–cobalt (WC–Co) cemented carbide vessels and balls were used as the grinding medium. The entire ball milling process was contained in high-purity argon to protect the powders from oxidation. Finally, the W–Cu powder mixture was densified via fast hot-press sintering to obtain a bimetal bulk containing robust ultrafine-grained W islands. To inhibit grain growth during the sintering process, the sintering temperature and pressure were respectively optimized at 950 °C and 100 MPa. The sintering duration was 6 min, and the heating rate was 100 °C∙min⁻¹.

For comparison, a coarse-grained W–Cu (CG W–Cu) bimetal bulk was prepared with the same sintering parameters but different raw powders and powder processing. W powder with a mean particle size of 12 µm and Cu powder with a mean particle size of 1 µm (both with a purity of 99.9%; Shanghai Aladdin Biochemical Technology Co., Ltd.) were used as raw materials to prepare the CG W–Cu. The powders were milled with a ball-to-powder mass ratio of 5:1 and a rotation speed of 300 r∙min⁻¹ for 3 h.

The phase constitution was detected by means of X-ray diffraction (XRD; D/MAX-3C, Rikagu, Japan), using Cu Kα radiation and a scanning speed of 4 (° )∙min⁻¹. Scanning electron microscopy (SEM; Nova 200 NanoSEM, FEI Company, USA) and transmission electron microscopy (TEM; 200 keV; JEM-2100F, JEOL Ltd., Japan) were applied to characterize the microstructure. The grain size analysis was conducted using electron backscattering diffraction (EBSD). The samples utilized for EBSD characterization underwent mechanical and ion polishing to ensure optimal surface quality and eliminate any surface stress. To obtain a reliable statistical result, regions containing more than 300 grains of Cu and over 1200 grains of W were selected. Relative densities were determined by means of the Archimedes method. All the prepared samples had a relative density higher than 97%. The hardness was measured by a Vickers hardness tester with a load of 30 kg and a holding duration of 5 s. The electrical conductivity was tested with an eddy current conductivity meter (Sigma 2008, Xiamen Tianyan Instruments Co., Ltd, China). As a mechanical test, uniaxial compression was conducted: The samples were machined into cylinders with dimensions of ϕ3 mm × 6 mm and compressed with a strain rate of 5 × 10⁻⁴ s⁻¹. For data reliability, at least three samples were measured for each mechanical property using the same testing conditions. The finite-element analysis was performed using the ABAQUS software.

3. Results and discussion

3.1. Composition and microstructure of composites

Fig. 1(a) shows the XRD patterns of the prepared ultrafine-grained W–Cu composite powder and the sintered bimetal bulk. Only W and Cu phases are present, confirming that no additional impurities were introduced in the preparation of the powder and sintered bulk. Figs. 1(b) and (c) respectively show the grain orientation distributions of the W and Cu phases in the ultrafine-grained W–Cu bulk, indicating that there is no specific orientation of the W and Cu phases in the bimetal. Figs. 1(d) and (e) depict the inverse pole figures of the W and Cu phases. The maximum texture intensities of the W and Cu phases are 1.08 and 2.52, respectively, confirming that there is no specific texture in the two phases.

The phase distribution and grain size statistics for the W and Cu phases of the ultrafine-grained W–Cu bulk are shown in Fig. 2. In Fig. 2(a), the regions with darker contrast are the Cu phase and those with brighter contrast are the W phase, presenting the formation of the specific configuration of spatially connected Cu and W islands (denoted herein as the UFG-CW-Cu bimetal). This configuration was homogeneously distributed across the full field of the microstructure of the bimetal. A grain size analysis was conducted based on the EBSD results. As shown in Figs. 2(b) and (c), the average grain sizes of the W and Cu phases are 210 and 200 nm, respectively. When compared with the size distribution of the raw W particles (Fig. S1 in Appendix A), this finding suggests that the ultrafine grain size in the W islands was retained from the prepared ultrafine structured W particles. The refined particle size of the W powder increased the agglomeration during the powder milling process. The hot-pressing sintering technique and the parameters used in the present experiment have the advantages of a low temperature and short duration for the densification of the powder mixture. Thus, the growth of the W grains in the agglomerate can be effectively inhibited during the sintering process. Figs. 2(d) and (e) provide the statistics of the grain-boundary mismatch angles in the W and Cu phases, showing that high-angle grain boundaries (HAGBs) are dominant in the W phase, accounting for 92% of the total W grain boundaries. This finding confirms the assumption that the ultrafine-grained W islands were obtained by sintering the prepared agglomerates of W particles. In the Cu phase, 45% of the grain-boundary mismatch angles are distributed at 60°, which represents a twin structure [20]. Aside from the high proportion of twins, the proportions of the low-angle grain boundaries (LAGBs) and HAGBs in the Cu phase are 19% and 81%, respectively.

The microstructure of the UFG-CW-Cu bimetal was further characterized using TEM. Fig. 3(a) illustrates the featured structure of the W and Cu phase configuration, showing clearly that the W islands are composed of ultrafine W grains. Fig. 3(b) provides a high-resolution TEM (HRTEM) analysis of the W and Cu crystals at the W/Cu phase boundary. The W and Cu grains exhibited a Kurdjumov–Sachs orientation relationship of

{(111)}_{Cu} ‖ {(110)}_{W}

and

{[\bar{1} 11]}_{W} ‖ {[01 \bar{1}]}_{Cu}

, which is common for the interfaces between body-centered cubic (BCC) and face-centered cubic (FCC) metals [21]. The interplanar spacings of the Cu(200) and W(101) crystal planes were measured as 0.181 and 0.224 nm, respectively. These values are consistent with the standard crystal plane spacings of W (Powder Diffraction File (PDF) card 04-0806) and Cu (PDF card 04-0836), indicating that there is no obvious lattice distortion in the W and Cu phases of the UFG-CW-Cu bimetal.

3.2. Comprehensive performance

3.2.1. Hardness and electrical conductivity

The hardness and electrical conductivity of the UFG-CW-Cu bimetal were found to be (430.0 ± 3.6) HV₃₀ (HV₃₀ stands for Vickers hardness test with 30 kgf load, 1 kgf = 9.80665 N) and 42% IACS ± 3.2% IACS (IACS is the is an acronym for International Annealed Copper Standard, and the conductivity of the annealed copper (5.8001 × 10⁷ S∙m⁻¹) is defined to be 100% IACS at 20 °C), respectively. With the same Cu content, the hardness of the CG W–Cu bimetal is only (210.0 ± 2.7) HV₃₀, and its electrical conductivity is 53% IACS ± 2.8% IACS (its microstructure and grain size distribution are provided in Fig. S2 in Appendix A). Thus, the hardness of the UFG-CW-Cu bimetal is more than twice that of the CG W–Cu bimetal. Moreover, the hardness of the UFG-CW-Cu bimetal is higher than those of most reported W–30wt%Cu composites [7], [11], [13], [22], [23], [24]. This improvement in the hardness of the UFG-CW-Cu bimetal is mainly attributed to the refinement of the W grains. In this work, the prepared ultrafine W particles and the rapid sintering processing facilitated the fabrication of an ultrafine-grained W–Cu bimetal. The average W grain size in the prepared UFG-CW-Cu bimetal is smaller than those in the W–Cu composites reported in the literature, which include W–Cu composites prepared by means of infiltration (W grain size: 5–7 µm [25]), hot-pressing sintering (W grain size: 1–2 µm [26]), liquid-state sintering after high-energy ball milling (W grain size: 1 µm [13]), and low-temperature liquid-state sintering with Al₂O₃ doping (W grain size: 472 nm [27]).

For a fully dense W–Cu composite with uniformly dispersed W phases in a Cu matrix, Weiner’s rule can be used to evaluate the electrical conductivity according to Eq. (1) [28]:

(1)

\frac{δ_{C} - δ_{C u}}{δ_{C} - {2 δ}_{C u}} = f_{W} \frac{δ_{W} - δ_{C u}}{δ_{W} + 2 δ_{C u}}

where δ_C, δ_W, and δ_Cu represent the electrical conductivity of the W–Cu composite, W phase, and Cu phase, respectively; and f_W is the volume fraction of the W phase in the composite. If the electrical conductivity values of the W and Cu phases are set as 31% IACS and 100% IACS (i.e., the same as those of the pure substances), the theoretical electrical conductivity of the fully dense W–30wt%Cu composite should be 60% IACS. However, the actual electrical conductivity of the UFG-CW-Cu bimetal is about 70% of the theoretical value. The difference between the actual and theoretical electrical conductivity can be attributed to the significantly increased interfacial resistance in the UFG-CW-Cu bimetal, especially from the heterogeneous W/Cu interfaces. The influence of the interface resistivity Δρ_i can be evaluated as follows [29]:

(2)

Δ ρ_{i} = \frac{2}{3} ρ_{Me-i} (\frac{S}{V})

where ρ_Me-i is the specific interfacial resistivity, such as the resistivity of the W/Cu interface and the grain boundary; S is the interface area, V is the volume, and S/V is the interface area per unit volume. As shown in Fig. 2(a), the formed W islands are homogeneously distributed in the UFG-CW-Cu bimetal, resulting in a great increase in the degree of W–W connectivity and a substantial decrease in the proportion of W/Cu phase boundaries in the bimetal.

In order to quantitatively describe the degree of connectivity of the W grains, the contiguity of the W grains (C_W–W) was calculated based on Eq. (3) [30]:

(3)

C_{W - W} = \frac{2 L_{W - W}}{{2 L}_{W - W} + L_{W - C u}}

where L_W–W is the length of the W grain boundaries, and L_W–Cu is that of the W/Cu phase boundaries. The contiguity of the W grains in the UFG-CW-Cu bimetal was calculated to be C_W–W = 0.48, which is twice as high as that of an ultrafine-grained W–Cu composite (C_W–W = 0.24) prepared via electroless plating combined with rapid sintering [23] and nearly five time as high as that of a coarse-grained W–Cu composite (C_W–W = 0.10) prepared via the cold isostatic pressing of a Cu-plated W skeleton combined with infiltration [30]. The low contiguity of the W grains in the abovementioned ultrafine-grained and coarse-grained W–Cu composites is due to the state of the phase configuration between W and Cu. Fig. 4 shows schematic diagrams of the influence of W contiguity on the interfaces in the W–Cu bimetal, with the W/Cu phase boundaries marked in yellow. When two isolated W grains become adjacent, the two W/Cu phase boundaries merge into a single W grain boundary. Therefore, for W–Cu bimetals with the same Cu content and W grain size, the higher the contiguity of the W grains, the lower the number of W/Cu phase boundaries (comparing Fig. 4(b) with 4(a)). With such a configuration, the connectivity and mean free path of the Cu grains are increased, and the barrier to electron transmission from the interfaces is greatly reduced, as indicated by the bold arrows in Fig. 4(b). In this way, excellent electrical conductivity can be obtained in a bimetal with an ultrafine grain size.

Fig. 5 compares the hardness and electrical conductivity of the UFG-CW-Cu bimetal with those of W–Cu composites reported in the literature. The referenced composites include W–Cu bimetals, such as nanostructured W–Cu [15], [31], coarse-grained W–Cu prepared via electroless plating [32], hot extruded W–Cu [33], and fine-grained W–Cu [34]. W–Cu-based composites with alloying elements (e.g., Zr [35], Ag [36], Ce [37], and La [38]) and introduced foreign phases (e.g., graphene (Gr) [22], graphene nanoplates (GNPs) [39], carbon nanotubes (CNTs) [40], WC [41], CeO₂ [42], TiB₂ [25], AlN [26], and Al₂O₃ [27]) are also involved. As the figure indicates, the UFG-CW-Cu bimetal has outstanding comprehensive properties in terms of hardness and electrical conductivity, in comparison with W–Cu bimetals with any Cu content.

3.2.2. Compressive properties

Mechanical compression tests were conducted for the UFG-CW-Cu and CG W–Cu bimetals at room temperature; the stress–strain curves are shown in Fig. 6(a). The yield strength of the CG W–Cu bimetal is 600 MPa, while the UFG-CW-Cu bimetal exhibits a significantly increased yield strength of 1460 MPa—nearly 2.5 times that of the CG W–Cu bimetal. Compared with other ultrafine-grained W–Cu-based composites reported in the literature, such as W–Cu composites doped with Al₂O₃ [27], prepared by electroless plating [23], and reinforced by W fiber [43], the UFG-CW-Cu bimetal prepared in the present work not only has a high compressive yield strength but also exhibits a strain of 10%, revealing good comprehensive mechanical properties. Fig. 6(b) compares the compression performance of the UFG-CW-Cu bimetal with those of the composites reported in the literature. Several key factors affect the mechanical properties of W–Cu composites. Firstly, the Cu content has a significant effect on the mechanical properties of a W–Cu composite, as increasing the Cu content decreases the yield strength and increases the strain. Secondly, the W grain size in the composite plays an important role in the integrated properties of strength and plasticity. For W–Cu bimetals, liquid-state sintering usually results in a coarsening of the grain structure and hence a decrease in the yield strength [44]. Rapid sintering techniques, such as spark plasma sintering, effectively inhibit the growth of W grains, increasing the yield strength; however, the plastic strain is generally lower than those of samples prepared via liquid-state sintering [23]. Thirdly, the strength of the composites can be significantly improved by the addition of CNTs [40], WC [24], [26], Gr [45], niobium (Nb) [46], and so forth. Among the composites made in this way, the W–Cu–Cr–ZrC composite has the highest yield strength by far, as the co-addition of components stabilizes the W nanostructure; however, its plastic strain is low [16]. As shown in Fig. 6(b), the comprehensive properties of the UFG-CW-Cu bimetal in terms of yield strength and plastic strain are obviously superior to those of its W–Cu composite counterparts reported in the literature. It is worth noting that the UFG-CW-Cu bimetal has a higher compressive yield strength and plastic strain than the W–Cu composites prepared by sintering Cu-coated W powder with the same particle size.

In order to quantitatively demonstrate the strengthening mechanism, comparisons were performed of the compressive yield strength of the UFG-CW-Cu bimetal and that of W–Cu composites with different W grain sizes. It was found that, from the ultrafine-grained [16], [23] to coarse-grained W–Cu composites [24], [47], [48], [49], the relationship between the yield strength and the W grain size basically aligns with the Hall–Petch formula, as guided by the dotted line in Fig. 6(c). The predicted strength, according to the Hall–Petch relationship, indicates that the strengthening mechanism of the UFG-CW-Cu bimetal is mainly grain refinement. The inset in Fig. 6(c) shows the yield strength of W–Cu bimetals with an ultrafine-grained structure. In the present work, the UFG-CW-Cu bimetal was determined to have a mean W grain size of 210 nm using EBSD. In Ref. [23], the mean W grain size in the bimetal was evaluated by SEM as 200 nm, while the mean W grain size obtained by local TEM observation was approximately 500 nm in Ref. [46]. Although different methods were employed to evaluate the grain size in these works, it is evident that these materials exhibit a similar grain size of W, at less than 500 nm. Notably, the compressive yield strength of the UFG-CW-Cu bimetal is 260 MPa higher than that of a composite with a similar W grain size but isolated distribution of the W grains [23]. The extra strength of the UFG-CW-Cu bimetal implies that there is a special strengthening mechanism in addition to grain refinement.

3.3. Fracture analysis

The compressive fracture mechanisms of the UFG-CW-Cu and CG W–Cu bimetals were studied. Fig. 7(a) shows an obvious deformation zone near the compressive fracture surface of the UFG-CW-Cu bimetal. Fig. 7(b) provides an enlarged view of the deformation zone adjacent to the fracture surface, showing that the W islands are strongly deformed in this zone of the UFG-CW-Cu bimetal. This is different from the slight plastic deformation generally observed in ultrafine-grained W–Cu composites with isolated W distribution, where no significant plastic deformation was observed in the W phase [23]. In the CG W–Cu bimetal of this study, there is no obvious plastic deformation in the W phase near the compressive fracture zone, as shown in Fig. 7(c), implying that the W phase makes little contribution to the plastic strain of the CG W–Cu bimetal.

Fig. 7(d) shows the typical morphology of the compression fracture surface of the UFG-CW-Cu bimetal. In addition to the ductile fracture of the Cu phase in the form of tearing, the bonding of ultrafine W grains in the W islands can be observed, confirming the high yield strength and large plastic strain of the UFG-CW-Cu bimetal. The local microstructure at the tip of the crack is shown in Fig. 7(e). It can be seen that the crack stopped propagating in the W island consisting of ultrafine grains, and a transgranular fracture of the W grains did not occur. The inverse fast Fourier-transform image corresponding to the region marked by the square in Fig. 7(e) is provided in Fig. 7(f). Inside the grain of the W island in front of the crack tip, severe lattice distortion has occurred. Moreover, there are dislocations in the W grain near the phase boundary between the W island and the Cu, indicating the occurrence of plastic deformation in the W phase of the UFG-CW-Cu bimetal. This finding is consistent with the microstructure characteristics shown in Figs. 7(a) and (b).

3.4. Stress and strain distributions

3.4.1. Model setup

In order to accurately describe the stress distribution and strain response of the bimetal composites under compression, finite-element models mapping the real microstructures of different types of bimetals were set up for simulations (Fig. 8). First, typical microstructures of the UFG-CW-Cu and CG W–Cu bimetal samples were taken from experiments, as shown in Figs. 8(a-i) and (a-ii). The microstructure images were then binarized (Figs. 8(b-i) and (b-ii)), and the binarized images were vectorized to generate the geometric boundaries of the W and Cu phases (Figs. 8(c-i) and (c-ii)). Finally, the corresponding regions of the W and Cu phases were generated by identifying the W/Cu phase boundaries. A quadrilateral element with eight nodes was used for the grid division. To ensure the comparability of the simulation results, the same parameters were adopted for the grid division of different models of microstructures, as shown in Figs. 8(d-i) and (d-ii). In order to investigate the influence of the phase configuration on the mechanical properties of the bimetal, a configuration of isolated W phase distributing in a Cu phase was made for comparison, generated by separating the W phases in the W islands of the UFG-CW-Cu bimetal. The established W–Cu bimetal with separated W phases was denoted as the UFG-SW-Cu bimetal. The binarized microstructure, the generation of geometric boundaries, and the grid division of the UFG-SW-Cu bimetal are shown in Figs. 8(b-iii), (c-iii), and (d-iii). Considering the ultrafine grain size of the W and Cu phases in the UFG-CW-Cu bimetal, the constants of the basic mechanical properties of the ultrafine-grained W and Cu [50], [51] were utilized, as shown in Table 1. A plane strain mode was employed. The left edge of the model was fixed, while force and displacement were applied to the right edge. The upper and lower edges were left free and unconstrained. A free boundary condition was implemented for the upper and lower edges in the current model; this allowed for a partial relaxation of the stress on these surfaces, resulting in a reduced stress level compared with that in the center region. It is important to emphasize that this treatment was kept consistent for all three models, ensuring that there were no alterations in the trends of the stress distribution and strain response of the three modeled materials.

3.4.2. Local stress analysis

Fig. 7 shows the microstructural characteristics of the UFG-CW-Cu bimetal sample after compression fracture. The representative feature is the plastic deformation of the W and Cu phases near the fracture surface. To explore the effects of the size and contiguity of the W phase on the deformation and fracture behavior of the composite, the stress distributions in three types of composites with different microstructural characteristics were analyzed, as shown in Fig. 9, Fig. 10, Fig. 11. In the simulations, at a strain of 1%, part of the ultrafine W phases in the W islands of the UFG-CW-Cu bimetal reached the compressive strength of W (2069 MPa), as shown in Fig. 9(a-i). This suggests that the W phase in the UFG-CW-Cu bimetal was subjected to a greater stress at the initial loading stage. In contrast, at the same stage, the stress in the Cu phase was lower than 500 MPa. Moreover, the maximum shear stress was distributed along a direction that was 45° from the loading direction. The stress concentration in the Cu phase existed at the W/Cu phase boundaries, as shown in Fig. 9(b-i). This stress concentration is attributed to the strain mismatch of the two phases at the interface.

When the strain was increased to 5%, the stress in the W and Cu phases of the UFG-CW-Cu bimetal increased simultaneously, as shown in Figs. 9(a-ii) and (b-ii). At this stage, the maximum shear stress in the Cu phase reached compressive strength of Cu (675 MPa). As the strain was increased to 10%, the stress in the W and Cu phases further increased (Figs. 9(a-iii) and (b-iii)), revealing concurrent load bearing in both phases. There are two maximum shear stress directions that are 45° from the loading direction in Fig. 9. The characteristics of stress distribution in these two directions are different due to the difference in the W contiguity. It is more difficult for long-range extension of shear stress within the Cu phase to occur in regions with higher W contiguity. From the mechanical tests in the experiments, the maximum deformation capacity of the UFG-CW-Cu bimetal was reached at a strain of 10%. Consistent with the experimental results, the above simulation results indicate that the W and Cu phases underwent plastic deformation concurrently under loading, confirming the mechanism for the microstructure characteristics near the fracture region in the UFG-CW-Cu bimetal sample (Figs. 7(a) and (b)). In addition, the simulation results showed that the stress concentration tended to occur at the W/Cu phase boundaries. Due to the poor wettability and weak bonding strength between the W and Cu phases, the stress concentration may have caused the formation of microcracks and propagation along the phase boundaries, leading to the fracture failure of the composite.

Compared with the UFG-CW-Cu bimetal, at a strain of 1%, the stress level in the W phase was lower in the CG W–Cu bimetal, while that in the Cu phase was higher, as shown in Figs. 10(a-i) and (b-i). This finding indicates that the Cu phase in the CG W–Cu bimetal rapidly reached its yield strength at the early stage of loading. At a strain of 5%, the stress in the Cu phase increased significantly. In contrast, a stress increase was only found in a few regions of the W phase, as shown in Figs. 10(a-ii) and (b-ii). When the strain was increased to 10%, the stress in both the Cu and W phases increased (Figs. 10(a-iii) and (b-iii)). The configuration of the W and Cu phases affects the stress distribution in the bimetal during loading. In the CG W–Cu bimetal, due to the coarse and less-connected W phase, the Cu phase existing between the W phases reached its yield strength earlier than the W phases and thus had a larger plastic deformation than the Cu phase in the UFG-CW-Cu bimetal. These results explain the experimental observation that the W phase had an inconspicuous deformation even near the fracture region in the CG W–Cu bimetal (Fig. 7(c)).

In addition to the W grain size leading to a different contiguity of the W phase, composites with a similar W grain size can exhibit different contiguities of the W phase, resulting in a different stress distribution in the composite. Compared with the UFG-CW-Cu bimetal, the W–Cu bimetal prepared by sintering Cu-coated W particles had a uniform distribution of W in the Cu matrix, exhibiting low contiguity of the W phase. To clarify the effect of the contiguity of the W phase (with a similar grain size) on the stress distribution in the bimetal, the model of the UFG-SW-Cu bimetal was used to calculate the stress distribution in the bimetal with isolated ultrafine W grains (Fig. 11). Compared with the UFG-CW-Cu bimetal, only a few W phases in the UFG-SW-Cu bimetal had a higher stress level. However, the Cu phases located between the isolated W phases had long-range stress extension even at an early stage of loading (with a strain of 1%), as shown in Figs. 11(a-i) and (b-i). At a strain of 5%, although the stress in the W phase increased, its value was smaller than that in the W phase of the UFG-CW-Cu bimetal, as shown by a comparison between Figs. 9(a-ii) and 11(a-ii). This finding indicates that a decrease in the W contiguity reduced the load-bearing capacity of the W phase in the bimetal, even with a refined feature size. Due to the elimination of the connectivity of the W phases (in the modeling, all the W phases were separated into isolated regions) and the lack of continuous obstruction against stress transfer in the Cu phase, there were more regions with high-level stress in the Cu phases between the isolated W phases (Fig. 11(b-ii)). When the strain was increased from 5% to 10%, regions with higher stresses increased in both the W and Cu phases (Figs. 11(a-iii) and (b-iii)). The above simulation results are consistent with the experimental observation of the deformation microstructure near the fracture region in the ultrafine-grained W–Cu bimetal with isolated W phases, where the deformation of the W grains was small [23]. Therefore, the contiguity of the W phase has a significant effect on the stress distribution in each phase of the W–Cu bimetal. Under the same load, the higher the contiguity of the W phase, the more beneficial it is in hindering an increase in stress in the Cu phase and the long-range extension of the stress. As a result, a high-level stress state mainly exists in the W phase, endowing the bimetal with a high yield strength.

3.4.3. Strain response

The strain responses of the UFG-CW-Cu, UFG-SW-Cu, and CG W–Cu bimetals at different loading stages were further studied. Fig. 12(a) shows the changes in the average equivalent plastic strain of the W and Cu phases in the three types of bimetal composites under loading. It can be seen that the equivalent plastic strain of the W phase in the UFG-CW-Cu bimetal is significantly higher than those of the W phases in the other two types of bimetals (Fig. 12(b)). In particular, the increment rate of the equivalent plastic strain of the W phase in the UFG-CW-Cu bimetal is the highest, especially under higher loading strains. On the other hand, the equivalent plastic strain of the Cu phase is the greatest in the CG W–Cu bimetal among the three bimetals. The difference in the strain response is attributed to the fact that the ultrafine-grained W islands in the UFG-CW-Cu bimetal bear most of the load among the three bimetals.

In order to evaluate more directly the respective contributions of the W and Cu phases to the plasticity of the bimetal, the difference in the equivalent plastic strain between the two phases, which was defined as the strain partitioning, was calculated for the three bimetals; the results are shown in Fig. 12(c). The strain partitioning of the W and Cu phases increases in all of the UFG-CW-Cu, UFG-SW-Cu, and CG W–Cu bimetals with the applied strain, suggesting that the plastic deformation behavior of the W and Cu phases deviates with the loading. The increasing rate of strain partitioning with the applied strain declines in the following order: CG W–Cu, UFG-SW-Cu, and UFG-CW-Cu. With a high applied strain of 10%, the UFG-SW-Cu and CG W–Cu bimetals have a 24.8% and 45.6% higher strain partitioning than the UFG-CW-Cu bimetal, respectively. The plastic strain of the W in the CG W–Cu and UFG-SW-Cu bimetals only accounts for 1% and 7% of the total strain, respectively, while the plastic strain of the W in the UFG-CW-Cu bimetal accounts for up to 20% of the total strain. The calculated stress bearing (denoted as the nominal stress) of the bimetal as a whole is shown in Fig. 12(d), which indicates that—under the same loading conditions—the UFG-CW-Cu bimetal can support much higher stress than the UFG-SW-Cu and CG W–Cu bimetals.

To explore the correlation between the microstructure and the strain of the bimetal, the local strain responses of the W and Cu phases in some representative regions in the UFG-CW-Cu bimetal were analyzed. The selected locations and the corresponding equivalent plastic strains are shown in Figs. 12(e) and (f). These regions include the Cu phase close to the phase boundary between the Cu and the W island (location 1), the Cu grain interior (location 2), the W grain interior in the W island (location 3), the W phase close to the phase boundary between the W island and Cu (location 4), and the interior of an isolated W grain (location 5). The calculated local strain responses in Fig. 12(f) indicate that the increasing tendency of the local plastic strain is similar for locations 1–4 with an increase in the applied strain. This suggests that the strain responses at these locations are coordinative and harmonious during the deformation process. In particular, the coordinated strain response at the phase boundary between the Cu and the W island (curves “1” and “4” in Fig. 12(f)) prevents the occurrence of a strain concentration at the interfaces. In comparison, the local strain in the interior of an isolated W grain is obviously smaller than that in other regions and has a slow increasing rate as the loading increases (curve “5” in Fig. 12(f)), implying that the isolated W grain makes a much lower contribution to the plastic strain of the bimetal than the grain in the W island does. This finding can explain the greater plasticity of the UFG-CW-Cu bimetal in comparison with the UFG-SW-Cu bimetal. As the W contiguity increases, the mean free path of the Cu phase increases, and the proportion of W/Cu phase boundaries decreases (Fig. 4). At the same time, the relative contribution of the Cu to the total plastic strain of the bimetal declines (Fig. 12(c)). Thus, in the UFG-CW-Cu bimetal, the coordinative plastic deformation of the Cu and the W islands can extend without premature failure of the Cu phases at the W/Cu phase boundaries. Therefore, its specific configuration, with islands of the hard W phase and their connection with the ductile Cu phase, facilitates the UFG-CW-Cu bimetal in promoting concurrent deformation of the W and Cu phases and thereby increasing the contribution of the W to the total plasticity of the bimetal. The plastic accommodation between the W islands and the Cu phase in the UFG-CW-Cu bimetal results in a good combination of yield strength and plastic strain in the bimetal.

The results of this study provide inspiration for developing high-performance immiscible bimetals. In general, the mechanical properties of an alloy can be effectively improved through the refinement and homogenization of its microstructures. However, in an immiscible bimetal composite, the phase boundaries are intrinsically weak and prone to have stress and strain concentrations, due to the great differences in the fundamental properties of the two metals. Thus, it is desirable to reduce the proportion of phase boundaries in bimetals with a refined microstructure. In the present study, while maintaining the ultrafine grain size of the microstructure, dispersion of the W phase was avoided and the contiguity of the W grains was increased, effectively reducing the proportion of phase boundaries in the refined bimetal. On the other hand, refinement of the microstructure has a negative effect on the electrical conductivity. Therefore, to balance the effects of grain size refinement on the mechanical properties and electrical conductivity, the configuration of the immiscible metals is especially important. In the UFG-CW-Cu bimetal, the Cu phase is not separated by W islands and is mainly connected continuously in the space. This grants the bimetal a high electrical conductivity, based on the Cu matrix. Thus, the specific phase configuration aids the UFG-CW-Cu bimetal in synergistically achieving high mechanical properties and high electrical conductivity. The abovementioned strategy holds promise for enhancing the comprehensive properties of immiscible bimetal composites.

4. Conclusions

Taking a W–Cu system as an example, a new type of immiscible bimetal with spatially connected Cu and ultrafine-grained W islands was prepared. The configuration of the phases and the microstructural characteristics of different types of bimetals were analyzed, and the key factors resulting in excellent mechanical properties and electrical conductivity were demonstrated through experimental studies and finite-element simulations. The main conclusions are summarized below.

(1) The formation of special ultrafine-grained W islands is attributed to the designed milling process, in which the W particles were refined and agglomerated prior to being connected with Cu particles in the second step of mixing and subsequent rapid low-temperature sintering. The configuration of the W islands and the spatially connected Cu is homogeneous in the full field of the microstructure of the UFG-CW-Cu bimetal, and the ultrafine-grain structure of the W and Cu phases is stable.

(2) The compressive yield strength of the prepared UFG-CW-Cu bimetal is as high as 1460 MPa, which is about 2.5 times that of conventional coarse-grained W–Cu bimetals. In addition, the UFG-CW-Cu bimetal has a large strain of 10% and a high electrical conductivity of 42% IACS. The comprehensive properties of the UFG-CW-Cu bimetal reach the highest level among its W–Cu counterparts reported in the literature.

(3) The high yield strength of the UFG-CW-Cu bimetal is a result of the bimetal’s microstructure refinement and the high contiguity of grains in the W islands. The configuration of the W islands and Cu promotes the concurrent and coordinated deformation of the W and Cu phases, enhancing the contribution of W to the total plastic strain of the bimetal. The increased mean free path of the Cu and the reduced proportion of phase boundaries due to the presence of W islands instead of dispersed W grains grant the bimetal a high electrical conductivity.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (92163107, 52171061, and 52371128) and the National Key Research and Development Program of China (2022YFB3708800 and 2021YFB3501502).

Compliance with ethics guidelines

Qixiang Duan, Chao Hou, Tielong Han, Yurong Li, Haibin Wang, Xiaoyan Song, and Zuoren Nie declare that they have no conflict of interest or financial conflicts to disclose.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.eng.2024.07.024.

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