Microstructure and texture evolution in ECAE processed A5056

https://doi.org/10.1016/S0921-5093(99)00657-7Get rights and content

Abstract

Equal channel angular extrusion (ECAE) represents an effective means of microstructural refinement through severe plastic deformation. In aluminum based alloys microstructures obtained by this process generally result in interesting and remarkable combinations of mechanical properties, such as for instance enhanced strength together with high levels of ductility. In the present investigation, the evolution of the tensile properties, microstructures and crystallographic textures during ECAE has been studied for an Al-Mg based workhardening type alloy (A5056) by metallographic observation, transmission electron microscopy (TEM) and X-ray diffraction. The mechanical and microstructural characteristics of these alloys have been compared to extrudates of the same material prepared by conventional extrusion. The results show that dynamic recovery and the development of subgrains, mainly separated from each other by low angle grain boundaries, play an important role with respect to the formation of the ultrafine microstructures observed. No evidence for recrystallization and the nucleation of new grains occured up to pressing temperatures of 300°C and very sharp deformation textures were found to develop. Similar to rolling deformation a transition from a pure metal type texture to an alloy type texture in function of the deformation degree occurs. The formation of these ECAE-textures is discussed on the basis of the Taylor theory of polycrystalline deformation.

Introduction

Fig. 1 shows schematically the principle of Equal Channel Angular Extrusion (ECAE), as originally developed by Segal et al. [1], [2], [3]: during the ECAE process the material is pressed through a die, that consists of two extrusion channels with identical cross-sections intersecting at a given angle. If this angle is 90°, as shown in Fig. 1, a true strain of 1.17 per pass of ECAE [1], [2] is introduced into the material. However, since the geometrical dimensions of the billet passing the deformation tool virtually do not change, the same procedure may be repeated several times and high levels of mechanical strain are accumulated. As a result a very strong refinement of the microstructure down to the submicrometer or even nanometer scale may be achieved [4]. Recently [5], [6] intense plastic straining by ECAE has been demonstrated to lead to the formation of ultrafine microstructures in a large variety of commercial aluminum alloys. These materials show a substantial increase in mechanical strength, whereas ductility remains almost unchanged with increasing number of ECAE passes. Besides this, also many alloys on aluminum basis, which have been severly deformed by ECAE, reveal superplastic flow at relatively low temperatures and/or at high strain rates [7], [8], [9], [10], [11], [12], [13]. Both these quite particular mechanical characteristics originate from the very unique microstructure that evolves during the ECAE process. Many efforts have been made in the past in order to characterize and to explain this microstructural development [14], [15], [16], [17], [18], [19]. Still, however, the mechanisms by which ultrafine grains are formed during ECAE are not fully understood. At present it is widely accepted that highly strained aluminium alloys contain a major fraction of large angle grain boundaries and that the misfit angle of these grain boundaries increases with progressing deformation [16], [17], [20]. A high resolution TEM-study by Furukawa et al. on a ECAE processed Al-3Mg revealed the presence of non-equilibrium atomic structures within these large angle grain boundaries. Although many aspects of microstructural development during ECAE of Aluminum based alloys have been explained, very little is actually known on the evolution of the crystallographic texture. Behind this background the present paper reports, how and what kind of preferential grain orientations are formed during ECAE of an Al-Mg based workhardening type alloy (A5056). The results on texture evolution are presented together with the development of the tensile properties and the microstructures during ECAE of A5056. Furthermore the mechanical and some of the microstructural properties of the material processed by ECAE are compared to extrudates of the same alloy obtained by conventional direct extrusion.

Section snippets

Experimental procedure

Prior to deformation either by ECAE or conventional direct extrusion the billets of A5056 (Al-4.65Mg-0.10 Si -0.16Fe)1 have been annealed for 16 h at 425°C and subsequently quenched in water. The resulting initial microstructure is shown in Fig. 2 and consists of recrystallized, almost equiaxed grains with an average grain size of approximately 100 μm. In the following thermomechanical treatment the ECAE billets were deformed at 1 mm s−1 and at temperatures between

Results and discussion

Fig. 3 shows the dependence of 0.2% proof stress and total elongation until failure of ECAE processed A5056 in function of the number of passes performed. The results show that at low deformation temperatures below 300°C the strength of the alloy progressively increases, whereas ductility does not change significantly. A comparison of 0.2% proof stress and total elongation for ECAE deformed samples to the values obtained for billets, that have been prepared by conventional extrusion, at various

Summary and conclusions

ECAE has been shown to considerably enhance the mechanical strength of A5056 without any significant loss in ductility of the alloy compared to conventional extrudates of the same material. The development of the microstructures at moderate temperatures up to 200°C is widely controlled by dynamic recovery even up to 8 passes of ECAE. The onset of dynamic recrystallization was only observed above 300°C. At lower temperatures the ECAE process yields in a strongly refined microstructure, in which

References (25)

  • V.M. Segal

    Mater. Sci. Eng A

    (1995)
  • R.Z. Valiev

    Mater. Sci. Eng A

    (1997)
  • A.H. Chokshi et al.

    Mater. Sci. Eng A

    (1993)
  • R.Z. Valiev et al.

    Scr. Mater.

    (1997)
  • K. Nakashima et al.

    Acta Mater.

    (1998)
  • Y. Iwahashi et al.

    Acta Mater.

    (1998)
  • O.V. Mishin et al.

    Scr. Mater.

    (1996)
  • J. Hirsch et al.

    Acta metall.

    (1988)
  • J. Hirsch et al.

    Acta metall.

    (1988)
  • V.M. Segal et al.

    Russ. Metall.

    (1981)
  • V.M. Segal, in: H. Henein and T. Oki (Eds.), Proceedings of the First International Conference on Processing Materials...
  • Z. Horita, T. Fujinami, M. Nemoto, T.G. Langdon: in: T. Sato, S. Kumai, T. Kobayashi and Y. Murakami (Eds), Proceedings...
  • Cited by (0)

    View full text