Elsevier

Acta Materialia

Volume 61, Issue 9, May 2013, Pages 3482-3489
Acta Materialia

High-pressure torsion for enhanced atomic diffusion and promoting solid-state reactions in the aluminum–copper system

https://doi.org/10.1016/j.actamat.2013.02.042Get rights and content

Abstract

This study reports that solid-state reactions occur by the application of high-pressure torsion (HPT) to the Al–Cu system even at low homologous temperature. A bulk form of disc consisting of two separate half-discs of pure Al and pure Cu are processed by HPT at ambient temperature under a pressure of 6 GPa. X-ray diffraction analysis and high-resolution transmission electron microscopy confirm the formation of different intermetallic phases such as Al2Cu, AlCu and Al4Cu9, as well as the dissolution and supersaturation of Al and Cu in each matrix. It is shown that the diffusion coefficient is enhanced by 1012–1022 times during the HPT processing in comparison with the lattice diffusion and becomes comparable to the surface diffusion. The enhanced diffusion is attributed to the presence of a high density of lattice defects such as vacancies, dislocations and grain boundaries produced by HPT processing.

Introduction

There are several processing methods to achieve solid-state reactions in metallic systems, such as diffusion bonding, mechanical milling of elemental powders, rolling and folding of laminated thin layers, and drawing or extrusion of banded fibrous wires [1]. Solid-state reactions can be promoted by increasing temperature and/or by imposing intense plastic strain, and both approaches thus enable the reactions to occur faster in bulk forms [1]. In particular, severe plastic deformation (SPD) is currently attracting much attention because it can achieve not only significant grain refinement of bulk metallic materials [2] but also consolidation and bonding of metallic powders [3], composites [4] and machining chips [5] at ambient temperatures without a conventional sintering process.

Typical SPD processes include equal-channel angular pressing (ECAP), accumulative-roll bonding (ARB) and high-pressure torsion (HPT) [6]. Solid-state alloying has been achieved using such processes in different metallic systems: Al–Cu [7], Al–Zn–Mg–Cu [8] and Cu–Cr–Ag [9] by ECAP; Cu–Ag [10] and Cu–Zr [10] by ARB; and Al–Mg [11], Al–W [12], [13], Cu–Ag [14], Cu–Ni [15], Cu–Cr [16], Cu–Co [17], Cu–Fe [17], Cu–W [18], W–Ti [13] W–Ni [13], and Ni–Al–Cr [19] by HPT.

Among the various SPD processes, HPT is unique [20]. It provides grain refinement of hard-to-deform materials such as W [21] and intermetallics [22], and induces phase transformation because of high pressure [23] or high strain [24]. Furthermore, solid-state reactions can be achieved with HPT: amorphization in Cu–Zr [25], Cu–Ag [25], Cu–Zr–Ti [26] and Ni–Ti [27], intermetallic formation in Al–Mg [11], Al–Ni [28] and Al–Ti [29], carbide formation in Cu–Nb–C [30], and hydride formation in Hf–H [31].

In this study, HPT is applied to a bulk form of discs in the Al–Cu system at ambient temperature, and it is demonstrated that solid-state reactions readily occur during the HPT operation because of the unusually enhanced lattice diffusion.

Section snippets

Experimental procedures

Rods 10 mm in diameter were prepared from high-purity Al (99.99%) and Cu (99.96%). They were annealed for 1 h at 773 K for Al and at 873 K for Cu. Each rod was cut into two halves along the longitudinal axis using a wire-cutting electrical discharge machine. The half-rods were sliced to a thickness of 0.8 mm. One half disc of Al and one half disc of Cu were placed together in a circular shallow hole of the lower HPT anvil, as shown in Fig. 1a. The lower anvil was then raised to contact with the

Results

Fig. 2 shows back-scattered electron (BSE) SEM images of the cross-sectional views for the samples after 1 turn (Fig. 2a) and 100 turns (Fig. 2b–d). The brighter contrast corresponds to Cu-enriched regions because all images were taken by BSE. For the sample after 1 turn, as shown in Fig. 2a, an Al layer is present between the two Cu layers and the boundaries between Al and Cu are clear. For the sample after 100 turns, as in Fig. 2b, a fine layered structure is visible throughout the disc with

Discussion

A question arises from the current investigation as to how the solid-state reaction and the formation of intermetallic phases are achieved by the HPT processing. It is reasonable that the atomic reaction may be enhanced by the following three factors during the HPT operation: (1) an increase in temperature due to plastic deformation and shear friction; (2) a reduction in atomic diffusion distance by microstructural refinement as discussed in an earlier report [28]; and (3) an increase in the

Summary and conclusions

HPT was applied to a bulk form of disc consisting of two separate half-discs of pure Al and pure Cu. The following conclusions were obtained.

  • 1.

    Alternating Al–Cu layered structures with well-defined Al/Cu interfaces are formed with a stacking sequence along the disc normal at an early stage of straining. With increasing the imposed strain (the number of turns), distorted lamellar Al–Cu structures are extended over the disc.

  • 2.

    Processing by HPT promotes the solid-state reaction of Al and Cu so that

Acknowledgments

This work was supported in part by the Light Metals Educational Foundation of Japan, in part by a Grant-in-Aid for Scientific Research from the MEXT, Japan, in Innovative Areas “Bulk Nanostructured Metals” and in part by Kyushu University Interdisciplinary Programs in Education and Projects in Research Development (P&P).

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