Elsevier

Materials Science and Engineering: A

Volume 529, 25 November 2011, Pages 345-351
Materials Science and Engineering: A

The development of hardness homogeneity in pure aluminum and aluminum alloy disks processed by high-pressure torsion

https://doi.org/10.1016/j.msea.2011.09.039Get rights and content

Abstract

Processing by high-pressure torsion was conducted on four different materials: high-purity (99.99%) aluminum, commercial purity (99.5%) aluminum, an Al-1% Mg solid solution alloy and a commercial aluminum Al-6061 alloy. Disks of each material were processed through 1/4, 1 and 5 turns and detailed microhardness measurements were recorded to permit the construction of color-coded hardness maps and three-dimensional representations of the hardness distributions. There are significant differences between these four materials. Whereas the hardness is initially high in the centers of the HPT disks of high-purity aluminum, the hardness is initially low in the centers of the disks for the other three materials. The hardness achieves saturation values after 5 turns in high-purity Al and the commercial purity Al but more torsional strain is required to achieve homogeneity in the other two alloys. There is evidence that it is difficult to achieve a well-defined saturation hardness in the Al-6061 alloy.

Highlights

► The Vickers microhardness distributions were measured on four different materials processed by high-pressure torsion. ► The hardness is initially high in the centers of the HPT disks of high-purity aluminum. ► The hardness is initially low in the centers of the disks for the other three materials. ► The hardness achieves saturation values after 5 turns in high-purity Al and the commercial purity Al. ► More torsional strain is required to achieve homogeneity in the other two alloys.

Introduction

The processing of bulk metals through the application of severe plastic deformation (SPD) provides an opportunity for achieving remarkable grain refinement to the submicrometer or even the nanometer level [1]. High-pressure torsion (HPT) is an especially attractive processing technique because it is easy to conduct and generally it leads to exceptional grain refinement [2].

The principle of HPT processing is that a sample, usually in the form of a thin disk, is placed between massive anvils and then subjected to a high applied pressure and concurrent torsional straining. The equivalent von Mises strain imposed on the disk, ɛeq, is given by a simple relationship of the form [3], [4], [5]:εeq=2πNrh3where N is the number of imposed turns and r and h are the radius and height (or thickness) of the disk, respectively. It should be noted that Eq. (1) includes only the effect of the torsional straining whereas in practice there is also an additional strain that arises directly from the very high compressive pressure, P. It was shown in recent experiments on an Al-6061 alloy that this applied pressure leads to a significant increase in the values of hardness within the disk even in the absence of any torsional straining [6].

It follows from Eq. (1) that the imposed strain reaches a maximum around the edge of the disk and it is equal to zero in the center of the disk where r = 0. Therefore, Eq. (1) suggests that processing by HPT will introduce very significant inhomogeneity into the material so that both the microstructures and the measured microhardness values will be extremely inhomogeneous. Contrary to this prediction, very early experiments on disks of nickel demonstrated that there was a gradual evolution towards a homogeneous structure by increasing either the applied pressure, P, and/or the total numbers of revolutions, N [7], [8]. There are now numerous reports confirming this gradual evolution towards homogeneity in several different materials [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. A simple model was proposed to account for this evolution based on the development and propagation of deformation in a repetitive manner throughout the disk [8] and later the evolution was modeled successfully using strain gradient plasticity theory [22]. The evolution of homogeneity in the values of hardness is matched also by a corresponding saturation in the minimum grain size attained by HPT processing and this has led to much speculation on the significance and limits associated with this grain refinement [23], [24].

Sufficient results are now available to show that there are significant differences in the evolution towards homogeneity even in relatively similar materials such as pure aluminum and aluminum-based alloys. Accordingly, the present research was conducted to compare results on a range of materials with the objective of examining the factors influencing the development of homogeneity across the HPT disks.

Section snippets

Experimental materials and procedures

Four different materials were selected for this research where these materials were chosen because experimental data are already available and it was necessary only to supplement these earlier studies by conducting HPT processing under specific conditions in order to bring all experimental results into a similar format. The four materials are described in the following paragraph and the subsequent experimental results are presented exclusively in the form of measurements of the Vickers

High-purity (99.99%) aluminum

The results of the hardness measurements for the high purity aluminum are shown in Fig. 1, Fig. 2. After N = 1/4 turn, the pure Al shows a doughnut-like pattern of a hardness distribution in Fig. 1(a) with an outer peripheral ring of hardness where Hv  40, a large inner ring of higher hardness where the values lie in the range of ∼45–55 and then a very small central region of lower hardness where Hv  30. These differences in the hardness values across the diameter of the disk are clearly depicted

Discussion

The results from these measurements reveal significant differences between the various materials examined in this research. The most important difference lies in the high hardness values recorded in the central regions of the disks at very low numbers of turns for the high-purity aluminum which contrast with the other three materials where the hardness values at the centers of the disks are significantly lower. The lower hardness values at the centers of the disks are consistent with Eq. (1)

Summary and conclusions

  • 1.

    Processing by high-pressure torsion was conducted through 1/4, 1 and 5 turns and detailed microhardness measurements were recorded on four different materials: high-purity (99.99%) aluminum, commercial purity (99.5%) aluminum Al-1050, an Al-1% Mg solid solution alloy and a commercial aluminum Al-6061 alloy.

  • 2.

    The results show significant differences between these four materials. In high-purity aluminum, the hardness is initially high in the centers of the HPT disks but decreases with torsional

Acknowledgement

This work was supported by the National Science Foundation of the United States under grant no. DMR-0855009.

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