Microstructures and microhardness of an aluminum alloy and pure copper after processing by high-pressure torsion

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Abstract

Experiments were conducted on an Al–3 wt.% Mg–0.2 wt.% Sc alloy and pure Cu to evaluate the effect of high-pressure torsion (HPT). The results show that very substantial grain refinement is achieved in both materials with as-strained grain sizes of ∼150 and ∼140 nm, respectively. Microhardness measurements demonstrate increases in the hardness near the edges of the disks by factors of approximately three and two over the solution-treated conditions for these two materials, respectively. It is shown in experiments on the Al–Mg–Sc alloy that no additional hardening is achieved by reversing the direction of the torsional straining during the HPT processing. In addition, experiments on pure Cu confirm that the high values of hardness are not retained when samples are subjected to short-term anneals for 15 s or 3 min at a temperature of 473 K.

Introduction

Considerable interest has developed over the last decade in processing materials through the application of severe plastic deformation (SPD) in order to achieve grain sizes at the submicrometer or nanometer level [1]. Although several different SPD processing techniques are available, the most promising procedures appear to be equal-channel angular pressing (ECAP) [2], [3], [4] and high-pressure torsion (HPT) [5], [6]. Experiments have shown that HPT is especially effective in producing extremely small grain sizes. For example, there are experimental reports of grain sizes of ∼270 nm after ECAP [7] and ∼90 nm after HPT [8] in an Al–3% Mg alloy, ∼300 nm after ECAP and ∼100 nm after HPT in an Al–5% Fe alloy [9] and ∼350 nm after ECAP and ∼170 nm after HPT in high purity Ni [10] all compositions are expressed in wt.%. In addition, experiments have shown a grain size of ∼300 nm produced by ECAP in pure Ti may be further refined to ∼200 nm by using HPT [11] and a higher fraction of high-angle boundaries is present after HPT of high purity Ni by comparison with the same material processed by ECAP [10], [12]. All of these results demonstrate the viability of using HPT for the production of ultrafine-grained materials.

An earlier report described the processing of an Al–3% Mg–0.2% Sc alloy by HPT [13]. In these experiments, the grain size was refined to ∼150 nm and it was shown that the microstructures of the HPT samples were inhomogeneous with a central core region having a relatively coarse and ill-defined microstructure and an outer shell where the microstructure was reasonably homogeneous. The measured hardness increased with increasing distance from the center and stabilized in this outer region at a value that was about three times larger than in the solution-treated condition.

The present investigation was conducted with two specific objectives. First, to extend these earlier results by conducting new experiments to examine the influence of the direction of torsional straining on the microstructure produced in the Al–Mg–Sc alloy. Second, to evaluate the potential for using HPT with samples of pure Cu and, in addition, to evaluate the stability of the as-strained microstructure in subsequent short-term annealing treatments. As in the earlier study [13], and following an early suggestion in HPT processing [14], the strain is expressed by specifying the number of revolutions imposed on the sample, N.

Section snippets

Experimental materials and procedures

Two different materials were used for these experiments. First, experiments were conducted with the same Al–3% Mg–0.2% Sc alloy used in the earlier study [13]. This material was produced from 99.99% purity Al, 99.9% purity Mg and 99.999% Sc and the composition is expressed in wt.%. The alloy was prepared as an ingot, homogenized in air at 753 K for 24 h, swaged into rods with a diameter of 10 mm and solution treated in air at 873 K for 1 h to give an initial grain size of ∼0.5 mm. Second, additional

Al–Mg–Sc alloy

Fig. 2 shows a TEM image of the structure in the outer region of the HPT disk for the Al–Mg–Sc alloy after five turns at a pressure of 6 GPa. It is apparent that this region contains ultrafine and reasonably equiaxed grains with an average size of ∼150 nm: an identical grain size was reported in the earlier study [13]. In addition, the presence of rings in the SAED pattern indicates that many of the grain boundaries have high angles of misorientation. The highly deformed nature of the

Summary and conclusions

  • 1.

    Processing by high-pressure torsion (HPT) of an Al–3% Mg–0.2% Sc alloy and pure Cu leads to significant grain refinement with grain sizes of ∼150 and ∼140 nm in these two materials, respectively. There are corresponding increases in the values of the microhardness at the edges of the HPT disks of these materials by factors of approximately three and two, respectively.

  • 2.

    Experiments with the Al–Mg–Sc alloy show that, for totals of two revolutions, less hardening is achieved if the direction of

Acknowledgements

We are grateful to Mr. G. Sakai, Mr. K. Kishikawa and Mr. Y. Hisatsune for their assistance. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan, in part by the Light Metals Educational Foundation of Japan and in part by the National Science Foundation of the United States under Grant No. DMR-0243331.

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