Synthesis and characterization of nano-structured Cu–Zn γ-brass alloy

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Abstract

In the present investigation, γ-Cu5Zn8 intermetallic compound having an electron to atom ratio of about 21:13 and being structurally one of the most complex Hume–Rothery phases was selected for mechanical milling/alloying. The detailed characterization was carried out by X-ray diffraction and transmission electron microscopy to study the microstructural evolution and stability during the synthesis and processing. Attempts have been made to explore the possibility of the formation of nano-structured and amorphous phases by mechanical milling and to provide a thermodynamic explanation based on an existing semi-empirical model. It was found that γ-phase is quite stable and there was a decrease in crystallite size up to ∼20 nm with an increase in milling/alloying duration up to 40 h. However, amorphization could not be achieved even after 40 h of milling. Detailed Miedema calculation showed that amorphization of the present compound would be possible if the crystallite size can be made below a certain critical value.

Introduction

It is known that non-equilibrium processing techniques can be exploited to generate novel materials with suitable crystal structures and microstructures for various technological applications [1], [2]. There are intense activities to understand the origin and role of complex intermetallic phases for developing advanced materials while experimenting with the various non-equilibrium processing techniques. Among the non-equilibrium techniques developed during the past few decades to synthesize novel materials include rapid solidification processing, mechanical alloying/milling, plasma processing, vapor deposition, ion or electron or neutron irradiation, severe plastic deformation, etc. [3], [4], [5]. Mechanical alloying which is a solid-state powder processing technique involving repeated cold welding, mechanically activated interdiffusion, fragmentation and dynamic recrystallization of powder particles in a high energy ball mill is an ideal processing route to develop nanocrystalline materials at ambient temperature [6], [7]. In the recent years it is shown that mechanical alloying/milling can convert an intermetallic compound or elemental powder blend into nanocrystalline or amorphous aggregate [8], [9], [10], [11]. One of the advantages of this conversion to nanocrystalline or amorphous state is the enhancement of ductility and toughness [12], [13], [14], [15]. Hence it has a possibility for use as a coating material on structural components. Among the possible mechanisms of solid-state amorphization by mechanical alloying/milling, it is suggested that accumulation of deformation generated defects including significant increase in internal surface area (grain boundaries) could raise the Gibbs energy of the system above that of the corresponding amorphous phase and convert a crystalline phase into amorphous/glassy material [11], [15], [16].

In the present investigation, we have selected γ-Cu5Zn8 intermetallic phase, being the most complex Hume–Rothery phase for studying its stability during mechanical milling/alloying. These alloys after appropriate structural/microstructural modification may be suitable for usage as coating materials. The structure of γ-brass alloy was first described by Bradley and Thewlis [17] in terms of a cubic unit cell consisting of 27 cubic units in a 3 × 3 × 3 array of a body centred cubic (bcc) lattice such that the unit cell of γ-brass contains 52 sites with two atoms—one at the centre and another at the vertices of this block being removed. The same structure was also described by Bradley and Jones [18], [19] in terms of a cluster of four concentric shells centered on the vacant sites (Fig. 1). The first shell is a regular tetrahedron of four atoms. The second one comprises four more atoms and forms a larger tetrahedron surrounding the faces of the first one. The third shell is an octahedron having six sites—one over each edge of the smaller tetrahedron. The fourth one is cuboctahedron having twelve sites. Thus, Cu–Zn γ-brass is a bcc structure with a cluster of 4 + 4 + 6 + 12 = 26 atoms occupying the body centre and vertex position of the cubic unit cell [20]. In addition to the bcc γ-brass alloy, simple cubic (sc) and face centered cubic (fcc) γ-brass structure have been reported in Al–Cu and Al–Cu–Cr alloys, respectively [21]. Earlier work has been reported on mechanical alloying as well as milling of various phases of the Cu–Zn system [22], [23], [24], [25], [26]. Although the crystal structure of γ-brass is fairly well understood, the scope of converting this complex intermetallic compound into nanocrystalline or amorphous state by non-equilibrium processing techniques has not yet been explored. A thermodynamic modeling for prediction of the glass forming range in the present system has not been reported.

The present work is aimed at synthesizing a single-phase γ-Cu5Zn8 alloy for studying the stability and the possibility of synthesizing nanocrystalline or amorphous aggregate by mechanical alloying/milling. An attempt has been made to rationalize our results on the basis of free energy calculation using the semi-empirical Miedema model.

Section snippets

Experimental

About 60 gm of γ-Cu5Zn8 alloy was prepared by induction melting in a graphite crucible using commercially pure (99.9%) constituent elements under protective argon atmosphere to prevent oxidation. An additional 23 wt.% Zn was added to compensate for evaporation loss of Zn during melting. This amount was estimated after several trials of melting experiments. The composition of the ingot obtained was analyzed in an atomic absorption spectrophotometer. Bulk density of as-cast γ-brass ingot was

Results and discussion

The chemical composition of the γ-brass alloy was found to have 34.4% of Cu and 65.56% Zn (by wt.%). The chemical composition obtained confirms that the composition of the alloy lies within the single-phase γ-brass region having composition of Zn in the range of 57.5–70.6 wt.% [27]. The ingot of the as-cast alloy has a diameter of 17 mm and height of 42 mm. Fig. 2 shows the optical microstructure of the γ-brass alloy after homogenization that reveals a polygonal, mainly rectangular type and single

Conclusions

High energy ball milling of Cu5Zn8 γ-brass and mechanical alloying of Cu–Zn elemental powder lead to the formation of nanocrystalline Cu5Zn8 γ-brass phase under the present experimental conditions. It was found that γ-brass phase is quite stable and there was a decrease in crystallite size up to ∼20 nm with an increase in milling/alloying duration up to 40 h with higher BPR (30:1). Amorphization could not be achieved even after 40 h of milling. Thermodynamic calculation using Miedema model

Acknowledgements

Authors would like to thank Prof. S. Ranganathan, Prof. G.V.S. Sastry, Dr. R.K. Mandal and Dr. Rajesh Prasad for many stimulating discussions and useful suggestions. The financial support of the Department of Science and Technology (DST Project: SR/S3/ME/051/2005-SERC-Engg), New Delhi, India is gratefully acknowledged for carrying out this work. Authors also thankfully acknowledge the suggestion/critical comments during the revision of this paper.

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