MATERIALS SEIENEE ENGINEERING A ELSEVIER Materials Science and Engineering A260(1999)275-283 Microstructural evolution over a large strain range in aluminium deformed by cyclic-extrusion-compression M.Richert a,Q.Liu b.*,N.Hansenb Department of Structure and Mechanics of Solids,University of Mining and Metallurgy,PL-059,Krakow,Poland Materials Research Department,Riso National Laboratory.DK-4000.Roskilde,Denmark Received 20 April 1998:received in revised form 28 July 1998 Abstract Polycrystalline pure aluminium (99.99%)has been deformed at room temperature by the Cyclic-Extrusion-Compression (CEC)-method to strains in the range 0.9-60(1-67 cycles).At different strains,the microstructure and local crystallography have been characterised in particular by transmission electron microscopy.It has been found that the microstructure develops from a cell block structure into an almost equiaxed structure of cells and subgrains,that the spacing between the boundaries subdividing the structure is almost unaffected by the strain and that the misorientation across these boundaries increases with the strain over the whole strain range.At the largest strain,the average misorientation across the deformation induced boundaries is ~25.The flow stress in compression is measured after the cyclic deformation and it is found that the flow stress increases with strain towards a saturation level which is reached at a relatively low strain.The discussion comprises the effect of deformation mode and plastic strain over a large strain range on the microstructural evolution and mechanical behaviour of aluminium.1999 Elsevier Science S.A.All rights reserved. Keywords:Cyclic-extrusion-compression;Aluminium;Microstructure;Plasticity;Large-strain deformation 1.Introduction X-rays,the flow stress (0.2%offset)has been measured in compression and the microhardness has been mea- The microstructural and mechanical response to sured on the deformed specimens.Some of these results large strain deformation has been studied over many will be recapitulated in this paper together with new years [1-7].This refects that large strain behaviour is results mainly describing the effect of strain on mi- of both fundamental and technological importance [6]. crostructures and local crystallography,especially the Most of these studies have been carried out on speci- angle of misorientation across dislocation boundaries mens deformed in monotonic loading over a strain and grain boundaries.The principal experimental tech- range extending up to 6-7 [4].It is of interest to extend nique has been TEM including Kikuchi pattern analy- this strain range by introducing alternative deformation sis.The material investigated has been polycrystalline modes.This has been done in the present work concen- pure aluminium (99.99%).The specimens have been trating on specimens deformed by the Cyclic-Extru- deformed at room temperature over a very large strain sion-Compression(CEC)-method [5]. range of :=0.9-60,thereby allowing a comparison The CEC-method was invented to allow arbitrarily with previous experiments [5,8,9]. large strain deformation of a sample with the preserva- tion of the original sample shape and the deformed specimens to be examined extensively [5,8,9].For exam- 2.Experimental procedures ple,the microstructure evolution has been studied by optical metallography and Transmission Electron Mi- 2.1.Deformation method croscopy (TEM),the texture has been measured by *Corresponding author.Tel:+45-4677-5808;fax:+45-4677- The CEC-method is a combination of extrusion and 5758;e-mail:qing.liu@risoe.dk. compression which alternate (Fig.1).The sample is 0921-5093/99/S-see front matter 1999 Elsevier Science S.A.All rights reserved. P:S0921-5093(98)00988-5Materials Science and Engineering A260 (1999) 275–283 Microstructural evolution over a large strain range in aluminium deformed by cyclic-extrusion–compression M. Richert a , Q. Liu b,*, N. Hansen b a Department of Structure and Mechanics of Solids, Uni6ersity of Mining and Metallurgy, PL-059, Krakow, Poland b Materials Research Department, Risø National Laboratory, DK-4000, Roskilde, Denmark Received 20 April 1998; received in revised form 28 July 1998 Abstract Polycrystalline pure aluminium (99.99%) has been deformed at room temperature by the Cyclic-Extrusion–Compression (CEC)-method to strains in the range 0.9–60 (1–67 cycles). At different strains, the microstructure and local crystallography have been characterised in particular by transmission electron microscopy. It has been found that the microstructure develops from a cell block structure into an almost equiaxed structure of cells and subgrains, that the spacing between the boundaries subdividing the structure is almost unaffected by the strain and that the misorientation across these boundaries increases with the strain over the whole strain range. At the largest strain, the average misorientation across the deformation induced boundaries is 25°. The flow stress in compression is measured after the cyclic deformation and it is found that the flow stress increases with strain towards a saturation level which is reached at a relatively low strain. The discussion comprises the effect of deformation mode and plastic strain over a large strain range on the microstructural evolution and mechanical behaviour of aluminium. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Cyclic-extrusion–compression; Aluminium; Microstructure; Plasticity; Large-strain deformation 1. Introduction The microstructural and mechanical response to large strain deformation has been studied over many years [1–7]. This reflects that large strain behaviour is of both fundamental and technological importance [6]. Most of these studies have been carried out on speci￾mens deformed in monotonic loading over a strain range extending up to 6–7 [4]. It is of interest to extend this strain range by introducing alternative deformation modes. This has been done in the present work concen￾trating on specimens deformed by the Cyclic-Extru￾sion–Compression (CEC)-method [5]. The CEC-method was invented to allow arbitrarily large strain deformation of a sample with the preserva￾tion of the original sample shape and the deformed specimens to be examined extensively [5,8,9]. For exam￾ple, the microstructure evolution has been studied by optical metallography and Transmission Electron Mi￾croscopy (TEM), the texture has been measured by X-rays, the flow stress (0.2% offset) has been measured in compression and the microhardness has been mea￾sured on the deformed specimens. Some of these results will be recapitulated in this paper together with new results mainly describing the effect of strain on mi￾crostructures and local crystallography, especially the angle of misorientation across dislocation boundaries and grain boundaries. The principal experimental tech￾nique has been TEM including Kikuchi pattern analy￾sis. The material investigated has been polycrystalline pure aluminium (99.99%). The specimens have been deformed at room temperature over a very large strain range of o=0.9–60, thereby allowing a comparison with previous experiments [5,8,9]. 2. Experimental procedures 2.1. Deformation method The CEC-method is a combination of extrusion and compression which alternate (Fig. 1). The sample is * Corresponding author. Tel.: +45-4677-5808; fax: +45-4677- 5758; e-mail: qing.liu@risoe.dk. 0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0921-5093(98)00988-5