Ultrafine-grained W–Cu–SiC composites prepared by mechanical alloying and infiltration of copper into pre-sintered skeleton

1 Introduction

Tungsten–copper (W–Cu) composites, where W is chosen for its outstanding high-temperature resistance to deformation and erosion, and Cu is for its excellent thermal and electrical conductivities, are a typical pseudo alloy due to the immiscibility and non-reaction between W and Cu, giving a combined and synergetic effect of the two phases in the composite system [1–4]. They are extensively used in various research and application fields such as ultrahigh-voltage electric contact materials, heat sink materials and even candidate materials for International Thermonuclear Experimental Reactor project [5–7]. Industrially, W–Cu composites are mainly produced by two methods: liquid phase sintering and infiltration of Cu into pre-sintered W skeleton [3,8].

To improve the service performances of W–Cu composites, in recent years, extensive attempts have been made by the following two feasible strategies: making finer grains according to Hall–Petch relationship [9] and adding strengthening reinforcements. For the first strategy, sub-micron and nanoscale powders have been attempted to be fabricated by mechanical alloying [8,10], sol–gel [11] and freeze-casting [12] methods. However, nano-sized powders with a high surface energy and an arch bridge effect usually determine formation of blind holes in the final billet [8,10], which requires microstructure tailoring, possibly through processing control, adding additional phases or composition optimisation. For the last mentioned strategy, reinforcements such as TiN, WC, ZrC/TiC and graphene [13–18] were adopted to reinforce the W–Cu composites. Among the ceramic reinforcements specifically, SiC particles with high elastic modulus, small thermal expansion coefficient, high-temperature corrosion resistance and low price [19,20] have been widely utilised in various composites such as Al-, Mg-, high-entropy alloys and so on [21–23]. Very recently, it has also been verified that SiC/Cu functional-graded material is beneficial to fusion reactor application for its low induced radioactivity after irradiation [24] and its effectiveness in adjusting the coefficients of thermal expansion mismatch between W and Cu [25]. Therefore, in this work, addition of SiC particles in small proportions of 0.5–3 wt-% was chosen to tailor the microstructure and properties of W–Cu composites by combined processes of mechanical milling and copper infiltration under hydrogen atmosphere. The resultant characteristics including microstructure, density, electrical conductivity and compressive behaviour of the W–Cu-x%SiC (x = 0.5, 1, 2, 3) composites were studied systematically.

2 Experimental materials and methods

The specifications of initial powders used in this work were listed in Table 1. Mixed powders with 25 wt-% copper (Figure 1(a)) and 75 wt-% tungsten (Figure 1(b)) were used as the initial powders, which were then mechanical alloyed with small additions of 0.5, 1, 2 and 3 wt-% silicon carbide powders (Figure 1(c)), at a rotating speed of 400 r min⁻¹, and a ball to powder weight ratio of 5: 1 for 8 h. Hardened steel balls were utilised, composed of balls in diameter of φ6, φ8 and φ10 mm, for which the respective weight ratio was 1:2:1.5. Figure 1(a–c) shows the raw commercial powders of Cu (Shanghai Jinzheng Nickel–cobalt New Materials Co. Ltd.), W (Xiamen Golden Egret Special Alloy Co. Ltd.) and SiC (Shanghai Aladdin Biochemical Tech. Co. Ltd.) used in this work. After mechanical alloying as shown in Figure 1(d), corresponding EDS elemental mapping results clearly revealed the homogeneous distribution of constituting elements. The mechanically alloyed powders were then pressed under an XTM-40 forming hydraulic press with the pressure of 340 MPa for a dwell time of 40 s to obtain a cylindrical green compact with the relative density value of ∼65%. The diameter of the compacting die was Φ51 mm. The green body was pressureless sintered at 1350°C for 2 h under hydrogen (H₂, purity ≥ 99.9 wt-%) atmosphere to remove possible oxides, followed by pressureless infiltration of a copper block (purity ≥ 99.9 wt-%) into the sintered skeleton under H₂, resulting in a relative density value of ∼90%. After that, the samples were cooled by furnace cooling and machined to remove the excessive copper used for infiltration.

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Figure 1

Secondary electron SEM images of raw powders of Cu (a), W (b) and SiC (c), as well as mixed powders with 3% SiC after high energy ball-milling (d) and EDS mapping results (e-i).

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Table 1

Powder specifications used in this work.

Powder	Particle size	Purity (%)	Company
Cu	<50 μm	≥99.9	Shanghai Jinzheng Nickel-cobalt New Materials Co. Ltd.
W	<500 nm	≥99.98	Xiamen Golden Egret Special Alloy Co. Ltd.
SiC	<150 nm	≥99.9	Shanghai Aladdin Biochemical Tech. Co. Ltd.

For the resulting infiltrated samples, XRD analyses have been conducted using MAXima XRD-7000 with Cu-Ka-radiation at the scanning rate of 3° min⁻¹. The density of the samples was measured according to Archimedes’ principle as stated in ASTM B963-2011 using FA2004 analytical electronic balance, and the electrical conductivity was measured by Model 7501 eddy current conductivity meter. Room temperature uniaxial compression tests according to ASTM E9-89a were carried out on WE-100 hydraulic universal material testing machine under a constant strain rate of 5 × 10⁻⁴ s⁻¹, using cylindrical specimens with a diameter of 3 mm and a height of 4.5 mm. For each material, three parallel tests were performed. Scanning electron microscopy (SEM) images of the polished samples were characterised using JSM-6700F SEM. Processing and analysis of related images were conducted using Image-Pro-Plus 6 software. Localised crystallographic misorientation was characterised by electron backscattering diffraction (EBSD) on Merlin-Compact SEM with a step size of 60 nm at the acceleration voltage of 20 kV.

According to the results shown in Figure 2(a), it can be seen that bcc structured W and fcc structured Cu were confirmed in all four composites, without diffraction peaks of SiC, which is considered to be due to the insensitivity of XRD to the low content of SiC, also having small atomic scattering factors [26]. After Rietveld refinement of the patterns, the infiltrated composites of W–Cu-x%SiC (x = 0.5, 1, 2, 3) were calculated to be W75.5Cu24.5, W75.0Cu25.0, W78.3Cu21.7 and W76.7Cu23.3, respectively, which are in good agreement with the starting nominal composition of W75Cu25. In Figure 2(b), the calculated profile of the WCu-3%SiC composite was also given as an instance.

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Figure 2

XRD patterns of the infiltrated WCu-x%SiC (x = 0.5, 1, 2, 3) composites (a) and the Rietveld derived pattern for the infiltrated WCu-3%SiC as an instance (b).

The microstructure of the infiltrated W–Cu-x%SiC (x = 0.5, 1, 2, 3) composites are shown in Figure 3. It can be seen that obvious particle necks appeared after sintering, and it is hard to differentiate W and SiC from the phase contrast except for Cu phase with the dark colour from the secondary electron images. By counting the number of particle/Cu phase interfaces N_PL and particle/particle interfaces N_PP passing through the bisector, the contiguity of the strengthening particles can be calculated according to the formula of C_P–P= 2N_SS/(2N_SS+N_SL) [27,28]. In the formula, C_P–P stands for the calculated value of particle–particle contiguity, N_SS for the number of intersects between straight lines and solid–solid interfaces and N_SL for the number of intersects between straight lines and solid–liquid interfaces. The calculation process can be seen with more details as shown in Figure S1 in the supporting information. The results showed that the WCu-1%SiC composite exhibited the highest particle–particle contiguity value (C_P–P) of 0.404, implying its superior mechanical properties due to the well-necked skeleton [27–30]. In addition, owning to the high surface energy and arch bridge effect of the mechanically alloyed powders and also the retarded rearrangement of tungsten particles due to the poorly wetted SiC, unnegligible blind holes formed increasingly as shown in Figure 3(c, d) with increasing amount of SiC, which could result in deteriorated mechanical properties due to the inclined crack initiation around these voids.

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Figure 3

SEM images of the infiltrated WCu-x%SiC (x = 0.5, 1, 2, 3) composites, with inserted particle size distribution and calculated particle–particle contiguity values C_P–P.

Furthermore, based on the particle size distribution results, fitted by Gaussian Function as shown in the inserts of Figure 3, it is revealed that with the increasing SiC content, the particles exhibited a growth from 1.225 to 1.873 μm gradually, which clearly confirmed the additional effect of SiC for sintering growth of W particles, presented in Figure 4 (square points), which can be well fitted by the formula y₂ = 1.067 + 0.412x − 0.0494ײ (the red line in Figure 4), where y denotes the W strengthening particle size and x stands for the content of SiC. For more thorough information on particle size changing, if the sample without any SiC under the same processing parameters [31] was also considered, a first decreasing and then increasing fitting curve (the black line in Figure 4) can be obtained as y₁ = 1.430 + 0.0695x − 0.0753ײ, which means that the small content of SiC under 1% was beneficial for particle refinement even after sintering and infiltration, while redundant addition would facilitate the particle growth of ultrafine-grained tungsten powders.

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Figure 4

Summarised strengthening particle size with the change of the added SiC content and two fitting formulae of the particle growth curves.

In Figure 5(a), the determined values of electrical conductivity and density values of the prepared WCu-x%SiC (x = 0.5, 1, 2, 3) composites are plotted vs. the SiC wt-% content. Consistent with the above SEM observation, the composites with SiC over 2% contained more obvious pores, which definitely resulted in a low density value. Basically, the density decreased with the increase of SiC due to its small density value of 3.2 g cm⁻³ and its inhibiting effect on the W particle rearrangement for sintering densification. With the same variation tendency, the electrical conductivity also decreased with the increase of SiC content, which can be ascribed to more electron scattering from the introduced voids and SiC particles. The stress–strain curves (Figure 5(b)) of the prepared WCu-x%SiC (x = 0.5, 1, 2, 3) composites revealed that the strength values of respective 1367, 1416, 1050 and 1114 MPa exhibit good agreement with the variation trend of C_P–P values of 0.360, 0.404, 0.307 and 0.301. Besides these, suffering from unnegligible pores introduced as revealed in Figure 3(c, d), the composites of WCu-2%SiC and WCu-3%SiC begin to fail at relatively low stress values. Therefore, the optimum content of SiC for W–Cu composites can be chosen to be 1%, leading to excellent mechanical properties (σ: 1416 MPa, ε: 44%) with a satisfactory electrical conductivity value of 15.4 MS m⁻¹.

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Figure 5

The values of electrical conductivity and density (a), as well as the compressive stress–strain curves (b) of the infiltrated WCu-x%SiC (x = 0.5, 1, 2, 3) composites.

For more detailed microstructural information of the WCu–SiC composites, EBSD experiment providing statistical grain information was carried out. The band contrast image, Euler angle distribution and phase map with marked grain boundaries of WCu-1%SiC are presented in Figure 6(a–c). It is revealed that W and SiC particles were homogeneously embedded in the Cu matrix. The calculated average grain size of W, Cu and SiC are 219, 201 and 155 nm, respectively. After pole figure analysis of the three phases, it is detected that only Cu exhibited a strong {001}<110> textured structure, as shown in Figure 6(d). Besides, Schmid factor (SF) distribution was also analysed on the whole detected section as shown in Figure 6(e, f). It can be seen that rather disrupted and scattered SF values are distributed on the section. More specifically, the SiC particles with lower SF values (far-red and near-blue colored regions in Figure 6(e)) would hinder the dislocation motion, contributing to the above strain-hardening effect during compression.

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Figure 6

EBSD band contrast image (a), Euler angle distribution (b), phase map with marked grain boundaries (c) and Pole figure of Cu (d), as well as the SF distribution (e) on the detected section of the infiltrated WCu-1%SiC composite alloy.

In this work, results of investigations are presented that were performed for production of WCu-x wt-%SiC (x = 0.5, 1, 2, 3) composites by mechanical alloying of the components W and SiC, compacting and sintering followed by infiltration with copper under hydrogen. The main conclusions are summarised as follows:

Phase constituents of the infiltrated composites, consisting of bcc-W, fcc-Cu and hcp-SiC have been confirmed by XRD and EBSD. A first decreasing and then increasing fitting formula was obtained as y₁ = 1.430 + 0.0695x − 0.0753x², indicating that the small content of SiC under 1% is beneficial for particle refinement of ultrafine-grained tungsten. Redundant addition of SiC facilitates their growth and the formation of micro-voids in the resultant composites.