Microstructural evolution in high purity aluminum processed by ECAP

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Abstract

High purity (99.99%) aluminum was processed by equal-channel angular pressing (ECAP) through 1–12 passes and examined using orientation imaging microscopy. The results reveal two distinct processing regimes: from 1 to 4 passes the microstructure evolves from elongated subgrains to an essentially equiaxed array of ultrafine grains and from 4 to 12 passes there is no measurable change in the average grain size and grain aspect ratio. The boundary misorientation angle and the fraction of high-angle boundaries increase rapidly up to 4 passes and at a slower rate from 4 to 12 passes. Anomalously large grains were visible in the central region of the billet pressed through 12 passes due to dynamic recovery and grain growth. The results suggest optimum processing is achieved by pressing through 4–8 passes.

Introduction

Equal-channel angular pressing (ECAP) is an attractive processing method for achieving exceptional grain refinement in bulk metals [1]. However, experiments have shown that the grain sizes achieved after ECAP are critically dependent upon the material. For example, the processing of high purity (99.99%) aluminum leads to a stable equilibrium grain size of ∼1.3 μm but much smaller equilibrium grain sizes are achieved in Al–Mg solid solution alloys including a grain size of only ∼0.27 μm in the Al–3% Mg alloy [2].

An earlier report documented the grain refinement occurring in high purity (99.99%) aluminum when processing by ECAP [3] and subsequently this material was subjected to a more detailed analysis where samples were pressed up to 4 passes and microstructural observations were recorded on three orthogonal planes of sectioning [4]. This latter report provided important information on the process of grain refinement in ECAP but nevertheless it was conducted without the benefit of orientation imaging microscopy so that the nature of the boundaries was inferred indirectly from the spreading of the diffraction spots in the selected area diffraction patterns.

More recent studies have suggested that grain refinement in pure aluminum is dependent also on the precise purity level of the material. Experiments showed that a commercial (99%) purity aluminum was successfully refined to a grain size of ∼1.5 μm by ECAP whereas a higher purity (99.998%) aluminum produced a grain size of ∼20 μm due, it was suggested, to the occurrence of discontinuous static recrystallization during the pressing operation [5].

Two difficulties arise in any attempts to compare the more recent data with the earlier results describing grain refinement in ECAP [4]. First, the early experiments were conducted only to a total of 4 separate passes through the ECAP die so that information is missing at the higher strains. Second, no quantitative measurements were recorded to document the distributions of the grain boundary misorientations. The present investigation was initiated specifically to address this deficiency. Tests were conducted using an aluminum of 99.99% purity which is identical to the purity used in the earlier study [4], the billets were pressed through a total of up to 12 passes in ECAP and these samples were then examined to provide quantitative microstructural information using orientation imaging microscopy incorporating a computer-aided facility for electron-backscattered diffraction (EBSD) analysis.

Section snippets

Experimental material and procedures

A block of high purity (99.99%) aluminum, with a diameter of ∼30 mm and length of ∼150 mm, was swaged into a rod with a diameter of 10 mm at room temperature. Billets with lengths of ∼60 mm were cut for processing by ECAP and each billet was annealed in air at 773 K for 1 h to give an initial grain size of ∼1 mm.

The processing by ECAP was performed at room temperature using a pressing speed of ∼7 mm s−1 and a solid die having an internal channel with an angle of Φ = 90° and with an outer arc of curvature

Microstructural features after 1–12 passes of ECAP

An OIM image of the unprocessed high purity aluminum is shown in Fig. 1(a) where the grain colors are determined by the orientation of each grain as depicted in the unit triangle. Thus, different colors in neighboring grains correspond to misorientations between these two grains of more than 2°. In this OIM image, and in all subsequent OIM images, the grain boundaries are denoted either by black lines corresponding to low-angle misorientations where the angle of misorientation, θ, is between 2°

General characteristics of processing through 1–12 passes

This investigation provides a comprehensive set of experimental results documenting the microstructural evolution occurring in aluminum having a purity level of 99.99% when processing by ECAP using a die with a channel angle of Φ = 90°. The results match an earlier report for aluminum of the same purity [4] but they provide additional information in two important respects. First, they extend the results from 4 to 12 passes of ECAP. Second, they provide quantitative information on the

Summary and conclusions

  • 1.

    High purity (99.99%) aluminum was successfully pressed through 1–12 passes using an ECAP die with an internal channel angle of 90°. The as-pressed billets were examined using OIM to provide a detailed characterization of microstructural evolution during ECAP.

  • 2.

    The results show there are two separate processing regimes with the division between these regimes occurring at 4 passes. When pressing up to 4 passes, the microstructure evolves from an array of elongated cells or subgrains to a reasonably

Acknowledgements

We thank Dr. Naoki Takata (formerly of Kyushu University, now at Tokyo Institute of Technology) for experimental assistance in performing the EBSD analysis. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan in the Priority Area “Giant Straining Process for Advanced Materials Containing Ultra-High Density Lattice Defects,” in part by the Kyushu University Interdisciplinary Programs in

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