Structural changes during synthesizing of nanostructured W–20 wt% Cu composite powder by mechanical alloying

https://doi.org/10.1016/j.msea.2006.09.005Get rights and content

Abstract

Nanostructured W–20 wt% Cu composite powder was synthesized by mechanical alloying (MA) in an Attritor ball mill. The morphological changes and structural evolution of the composite powder during MA was studied by employing scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX), laser particle size analyzer (LPS), inductively coupled plasma (ICP) spectrometry, atomic absorption spectrophotometery (AAS), and the bulk powder density measurement. The results were compared with those obtained from attrition milling of monolithic W and Cu powders processed at the same condition. Whereas the milling mechanism of the monolithic powders follow the ductile (for Cu) and semi-brittle (for W) systems, the W/Cu powder mixture exhibits different behavior. At the early stage of milling, the copper particles are fragmented and incorporated into the W matrix, resulting in the formation of W/Cu composite with laminar structure. With increasing milling time and due to continuous fracturing, the laminar structure is refined and a homogenous distribution of fine Cu particles (0.3–0.6 μm) in the W matrix is formed. The analysis of XRD patterns indicated that the composite powder composes of nanostructured grains with the size of 49 nm for Cu and 23 nm for W. A faster grain refinement in the composite powder compared to the monolithic particles was noticed. The XRD peak intensity also revealed that partial mutual solubility of the constituent elements (≈4–7 at% for Cu in W and ≈2–3 at% for W in Cu) was induced by prolonged mechanical milling.

Introduction

Tungsten/copper composites are commonly used for electrical and thermal objectives like heat sinks and electrical conductors [1], [2]. Good thermal and electrical conductivity combined with low thermal expansion coefficient (CTE) have extended the application of these composites as an advanced engineering material [3]. It is noteworthy that with tailoring the amount of copper in the composite composition, a range of CTE values with controlled conductivity can be obtained. The tailored W/Cu composites are widely used for thermal management applications such as microelectronic packaging [4].

W/Cu composites are generally produced by infiltration, liquid phase sintering and hot pressing [5], [6]. However, lack of appreciable solubility of tungsten in liquid copper (10−5 at% at 1200 °C [1]) limits the densification during sintering. Additionally, it is important to establish a homogenous and well-mixed powder mixture to improve sinterability and to restrict dilation and distortion. In order to improve the homogeneity and uniformity of W/Cu components, conventional ball milling [7], oxide co-reduction [8], mechano-chemical methods [4], and mechanical alloying [9], [10], [11], [12] have been examined. The latter process is particularly attractive since it possesses the advantages of the synthesis of nanostructured materials with high purity at large quantities. Additionally, the components can be fabricated at lower temperatures without the need to addition of sintering activators [13]. Finally, better properties compared to conventional processed materials are obtainable.

The synthesis of W/Cu composites by MA has been attempted by many researchers. The prime aim of the most studies has focused on fabrication of fine and homogenous mixture of the initial components. Meanwhile, particular interest has been devoted on the sintering behavior of milled W/Cu powders [13]. Also, manufacturing of complex-shaped parts by powder injection molding has recently attracted much attention [1], [2]. Moreover, metastable phase transformation induced by high-energy ball milling in W/Cu system has been the subject of interest. It should be mentioned that a large positive heat of formation (35.5 kJ mol−1 [11]) in this binary system makes it difficult, if not impossible, to extend the mutual solubility, and in more severe case, induce amorphisation. Whereas Gaffet et al. [11] reported the amorphisation of the alloy by a lengthy period of milling times, Zhang and Massalski [14] stated that the amorphisation might be assisted by other elements such as oxygen and nitrogen. The claims of amorphisation of alloys with large heat of formation are, as yet, not convincing [10]. The grain refinement process of W/Cu composite powders by mechanical milling has recently been addressed by Alam [15]. Although several researchers have performed work on W/Cu nanocomposites developed by mechanical alloying, for example [9], [10], [11], [12], [13], [14], [15], [16], the morphological changes and structural evolution of the powder mixture upon milling and the progress in homogenizing has not been addressed exclusively. The evaluation of microstructural refinement and degree of homogenization occurring during MA provides information about the milling process and the stages involved in the formation of a uniform nanostructured composite. This paper presents experimental results on the effect of MA on the morphological and structural changes in W–20 wt% Cu system. The milling mechanism is addressed and the variations in the particle size and morphology, grain size and microstrain, the lattice parameter, and mutual solid solubility are reported. The progress in the microstructural homogenization during MA of Cu (soft metal)/W (hard metal) system is shown.

Section snippets

Experimental procedure

The characteristics of the materials used in the present study are reported in Table 1. The elemental Cu powder was provided from ECKA Granulates GmbH (Kaiserstraße, Germany). The powder was produced by electrolysis process and has particles with dendritic shape (Fig. 1a) with the mean diameter of 32.8 μm. The particle size characteristics of the powder were determined by a laser particle size analyzer (Mastersizer 2000, Malvern Instruments). Reduced W powder was supplied by Hubei Minmetals

Morphological analysis

Fig. 2 shows the morphology of copper powder at different milling times. The milling stages include: plastic deformation of the dendrite arms to form flake-like particles (Fig. 2a); formation of large flattened particles due to severe plastic deformation of copper (Fig. 2b); micro-welding and fracture of the flakes to form equiaxed particles (Fig. 2c). This behavior is in consistent with the typical mechanical milling mechanism of soft (ductile) materials [12].

The morphology of the monolithic W

Conclusion

Nanostructured W–20 wt% Cu was synthesized by attrition milling. It was shown that at the early stage of milling, the initial Cu particles was deformed and filled the free voids between polygonal W grains, which led to higher bulk powder density. After 8 h MA, the Cu particles were captured between the deformed W grains, and a composite laminar structure was attained. With continuing milling, repeated fracture of the laminate structure resulted in the formation of equiaxed and fine particles. The

Acknowledgement

The grant of the Office of Vice Chancellor for Research and Technology, Sharif University of Technology, is appreciated.

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