Composite Ta–Cu powders prepared by high energy milling

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Abstract

A mixture of Ta and 20 wt%Cu elemental powders were milled in a high energy mill under two conditions of milling intensity. Composite particles in both conditions were formed. The original Ta particles were firstly fragmented and then pierced in the deformed Cu particles. Cu hardens due to cold work and breaks. The final powder is finer than the initial powders. The final composite particles are homogeneous and consist of amorphous, separated Cu and Ta phases. The more intense milling condition produced composite particles with shorter milling time and amorphization of both phases after 50 h of milling. The less intense milling condition amorphized only the Cu phase. The composite particles can produce denser structures than mixed powders, if heated above the Cu melting point.

Introduction

High energy milling and mechanical alloying are terms utilized to name the synthesis of alloys and compounds by means of intensive milling that causes mixing at an intimate level and deformation of the materials’ microstructure [1], [2]. Here, high energy milling (HEM) is used.

The Ta–Cu system can be compared to the W–Cu system. Ta and W are high melting point transition elements, which do not dissolve Cu nor are dissolved by it [3], [4]. It is expected that liquid Cu wets poorly Ta. These characteristics make Ta–Cu a hard-to-sinter system. Depending on the Ta content, the Ta–Cu alloy can have different applications. If the Ta content exceeds 70 wt%, the alloy can be an alternative to W–Cu, WC–Cu, Mo–Cu as heat sink for semiconductor devices [5]. If the Ta content is lower than 10 at.%, Cu can be dispersion strengthened by fine Ta precipitates, as it occurs with Nb [6], [7].

During HEM the following processes may happen: severe deformation that could lead to amorphization of the phases, formation of solid solutions (also with extended solubility), formation of composite particles, excellent dispersion of phases [1], [2]. For the W–Cu system, literature [8], [9] reports formation of composite particles, good dispersion, but does not mention amorphization of any phase. In the Nb–Cu system, the formation of a saturated solid solution of nanocrystalline Cu with up to 10 at.% Nb has been reported [6]. The absence of XRD and electron diffraction peaks, and SEM images that did not show any evidence of Nb particles supported this affirmation. According to the authors, Nb dissolves and diffuses in the Cu lattice along the dislocation lines. As the Ta–Cu system, Nb–Cu presents mutual insolubility.

Complete amorphization of both Ta and Cu by HEM of the elemental powders is reported [4] for milling times longer than 50 h in mixtures whose Cu content is in the 30–50 at.% range. Cu richer mixtures do not completely amorphize. On the other hand, Lee et al. [10] reported that in a Ta–30 at.%Cu mixture amorphization started after 30 h milling and had not finished after 100 h.

Systems such as W–Cu, Nb–Cu and Ta–Cu have positive enthalpy of mixture. Therefore formation of a solid solution of these elements is not expected. Nevertheless the introduction of strain and interfaces in the crystalline material can elevate its free energy to a value over that of a metastable solid solution (crystalline or amorphous) [4], [10]. HEM is able to produce both effects.

This work focuses on the HEM milling of a Ta–20 wt%Cu mixture of elemental powders. The evolution of the structure of the particles and of the crystal lattice of the phases is followed by SEM and XRD.

Section snippets

Experimental procedure

Ta and Cu elemental powders were used in this work. The Ta powder had a mean particle size of 235 μm, and Cu 28 μm. Fig. 1a and b shows SEM micrographs of the as-supplied powders.

The Ta and Cu powders in a proportion of 20 wt%Cu were placed into a hardmetal lined vial together with hardmetal balls and cyclohexane for milling in a high energy planetary Fritsch Pulverisette 7 mill for 100 h, but samples were collected at shorter milling times (2, 10, 20, 50 and 75 h) so that the evolution of the

Results and discussion

Fig. 2 shows a SEM micrograph (BSE mode) of the mixed powder. The particles stand side by side without the formation of agglomerates. Due to the significant size and density difference, segregation is expected if such a powder is milled.

Fig. 3a–f shows SEM (BSE mode) of the powders milled 2, 20, 50, 75 and 100 h under condition 1. First, the flake-like Ta particles are fragmented (Fig. 3a). The Ta debris is smaller than the original Cu particles. The Cu particles are deformed but still Ta and Cu

Conclusions

HEM for 100 h of Ta and Cu with the composition Ta–20 wt%Cu produces composite particles in which both phases are amorphous. Cu amorphizes earlier than Ta. Depending on the milling intensity Cu can amorphizes after 20 h and Ta after 50 h. Ta is harder than Cu. The Ta particles are pierced in the deformed Cu particles and fracture, but they are also deformed. No signal of solid solution is found. Both phases can coexist separately in amorphous state. The composite particles do not resemble the

Acknowledgements

The authors are thankful to FAPERN and CNPq for financial support (Projects 35.0118/2005-1(NV)) and to the laboratories of NEPGN/UFRN through the Projects CTPETRO-INFRA and FINEP/LEM.

References (10)

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