Microstructure and texture evolution in ECAE processed A5056
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
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