Tensile and compressive deformation behaviors of commercially pure Al processed by equal-channel angular pressing with different dies

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Abstract

An investigation was conducted on the tensile and compressive properties of commercially pure Al processed by equal-channel angular pressing (ECAP) with two different dies. The specimens along two different directions were cut from the ECAPed billets to study the effect of orientation on the compressive yield strength and the surface deformation morphologies along normal direction (ND) and extrusion direction (ED). It was found that the yield strength of the specimens along ED under tension was lower than that along ED under compression, but higher than that along ND under compression. The surface observations showed that there were some differences in the deformation morphologies under compression along two different directions. Based on the mechanical tests and observations of surface morphologies of ECAPed pure Al, the relationship between shear deformation behavior and tension/compression asymmetry was discussed.

Introduction

In the past decade, the technique of equal-channel angular pressing (ECAP) has attracted much attention as a method of severe plastic deformation (SPD) to produce ultrafine-grained (UFG) or submicron-grained (SMG) materials [1], [2], [3], [4], [5], [6]. Up to now, many metals and alloys have been fabricated into UFG materials with high strength or superplasticity via ECAP technique, albeit their ductility is decreased in comparison with that in their coarse-grained counterparts [7], [8], [9]. In general, ECAP can apply a high shear strain to materials through a specially designed die having two equally sized channels connected at a finite angle [1], [10]. It is noted that the high shear strain often induces shear flow lines or textures in the ECAPed materials [1], [11], [12], [13], [14], leading to a significant anisotropy in strength [11], [12], [13]. Although the mechanical properties of ECAPed materials have been extensively investigated, there are few reports about their anisotropic properties [13].

Tabachnikova et al. [11] studied the tensile and compressive properties of commercially pure Ti and UFG Ti at ambient temperature processed by ECAP, and found that the stress differential (SD) effect values (anisotropic degree in strength) are significantly different for samples strained along the directions parallel and perpendicular to the ECAP axis. Han et al. [12] investigated the anisotropic compressive properties and shear deformation mechanism of pure iron subjected to ECAP with single-pass, suggesting that the shear plane induced by the first pass of ECAP is a relatively weak plane to resist subsequent shear deformation. In addition, the effects of strain path on the flow stress anisotropy and Bauschinger effect (BE) in UFG Cu were investigated by Haouaoui et al. [13]. They found that the SD ratio was always negative along the extrusion direction, regardless of the route and number of ECAP pass. They argued that the change of SD ratio could be affected not only by the crystallographic texture and grain size but also by the grain morphology and grain boundary characters. Recently, Yu et al. [14] reported that the yield strength under compression was about 20% higher than that under tension for SMG Al processed by ECAP over a grain size range of 350–750 nm, which disappeared for their counterparts with larger grain size. For ECAPed materials, the plastic deformation often localizes into some shear bands [1], [15]. The shear localization was observed in compression tests in ECAPed samples [14], but the strength asymmetry between tension and compression in ECAPed Al and its alloys has not been paid much attention to establish the relationship between the shear localization and strength asymmetry. In particular, most researches ignored the differences in the shear deformation behaviors on the specimens compressed along different directions [11], [13].

In general, most ECAP process are conducted using a die angle of 90°, applying the equivalent strain of about 1 per pass [16]. But some researches have also performed ECAP tests in the dies having different channel angles in order to reveal its deformation mechanism. Furukawa et al. [6] studied the shearing patterns during ECAP under three different routes using die angles of 90° and 120°, respectively, and discussed the microstructure evolution in different routes. Furuno et al. [17] observed the microstructures and texture of the ECAPed pure Al and Al–1%Mg–0.2%Sc alloy with a die having an internal channel angle of 60°. In addition, Zheng et al. [18] investigated the structure and mechanical properties of 7050 and 2224 Al alloys with ultrafine grains produced by ECAP using a die angle of 120°. Though pure Al has been studied by many researchers as a model material to reveal the deformation mechanism of ECAP in dies having different channel angles, the anisotropic properties and the strength asymmetry under tensile and compressive loading have not been systematically investigated for pure Al processed by ECAP using dies with different channel angles. In the present work, we studied the mechanical properties of commercially pure Al processed by ECAP for one or two passes using dies with 90° and 120° channel angles, respectively. The strength asymmetry under tension and compression is discussed in terms of their surface deformation morphologies.

Section snippets

Experimental procedure

The experimental material in the present work is 1060 commercially pure Al. Firstly, the Al plate was annealed at 390 °C for 0.5 h, gaining the average grain size of about 20 μm. Secondly, the plate was made into some billets with a dimension of 8 mm × 8 mm × 70 mm by spark cutting technique. Then, ECAP was conducted at room temperature (RT) to extrude the Al billets using two kinds of dies having different intersecting channel angles, which one is Φ = 90° and Ψ = 90° and the other is Φ = 120°and Ψ = 0°. The

Tensile deformation and fracture behaviors

The tensile stress–strain curves of commercially pure Al before and after ECAP are shown in Fig. 3. Fig. 3(a) demonstrates that commercially pure Al has been significantly strengthened after ECAP, but its elongation decreases drastically. For example, its yield strength is more than doubled after extrusion only for one pass, from 41 to 108 MPa, but the uniform elongation decreases from about 40% to less than 3%. After ECAP for two passes, the increase in strength is less significant, indicating

Conclusions

  • (1)

    The tensile stress–strain curves of the specimens pressed by a 120° die is much scatter than that by a 90° die, which should be highly associated with much non-uniform shear deformation during pressing.

  • (2)

    There are mainly large and deep dimples in the fracture zone of the annealed tensile specimens. With increasing the number of ECAP pass, the amount of small dimples increases, and the dimples become shallow and more uniform.

  • (3)

    When compressing along different directions, the specimens displayed

Acknowledgements

The authors would like to thank W. Gao, H.H. Su, G. Yao and M.J. Zhang for the assistance in the sample preparation, mechanical tests, SEM and TEM observations. This work was financially supported by “Hundred of Talents Project” by Chinese Academy of Sciences and the National Outstanding Young Scientist Foundation under Grant No. 50625103.

References (35)

  • V.M. Segal

    Mater. Sci. Eng. A

    (1995)
  • V.M. Segal

    Mater. Sci. Eng. A

    (1999)
  • R.Z. Valiev et al.

    Prog. Mater. Sci.

    (2006)
  • R.Z. Valiev et al.

    Acta Metall. Mater.

    (1994)
  • M.A. Muñoz-Morris et al.

    Scripta Mater.

    (2003)
  • M. Furukawa et al.

    Mater. Sci. Eng. A

    (2002)
  • Z.C. Wang et al.

    Mater. Sci. Eng. A

    (2002)
  • W.J. Kim et al.

    Acta Mater.

    (2003)
  • T.L. Tsai et al.

    Mater. Sci. Eng. A

    (2003)
  • T. Aida et al.

    Scripta Mater.

    (2001)
  • E.D. Tabachnikova et al.

    Mater. Sci. Eng. A

    (2001)
  • M. Haouaoui et al.

    Acta Mater.

    (2006)
  • C.Y. Yu et al.

    Scripta Mater.

    (2005)
  • Y. Wu et al.

    Scripta Mater.

    (1997)
  • Y. Iwahashi et al.

    Scripta Mater.

    (1996)
  • K. Furuno et al.

    Acta Mater.

    (2004)
  • R.Z. Valiev

    Mater. Sci. Eng. A

    (1997)
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