Acta Materialia 55 (2007) 2013–2024 www.actamat-journals.com
Microstructure and texture gradient in copper deformed by equal channel angular pressing Werner Skrotzki
a,*
, Nils Scheerbaum b, Carl-Georg Oertel a, Roxane Arruffat-Massion c, Satyam Suwas d, La´szlo´ S. To´th c
a
c
Institut fu¨r Strukturphysik, Abteilung Metallphysik, Technische Universita¨t Dresden, D-01062 Dresden, Germany b IFW Dresden, Institut fu¨r Metallische Werkstoffe, P.O. Box 270116, D-01171 Dresden, Germany Laboratoire de Physique et Me´canique des Mate´riaux, ISGMP, Universite´ de Metz, Ile du Saulcy, F-57045 Metz Cedex 01, France d Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India Received 18 May 2006; received in revised form 3 November 2006; accepted 5 November 2006 Available online 25 January 2007
Abstract The aim of this paper was to examine the gradient in the microstructure and texture that develops during equal channel angular pressing (ECAP). Copper of 4N purity was subjected to ECAP deformation at room temperature by using three passes of Route A. The local microstructure and texture were investigated by orientation imaging microscopy and high-energy synchrotron radiation, respectively. The refined microstructure consists of elongated grains inclined in the direction of extrusion. The texture is characterized by typical shear components of face-centred cubic metals which deviate from their ideal positions. Inclination of the grain long axis and the texture components with respect to the extrusion direction depend on the distance from the top of the extruded bar and change from pass to pass. The texture gradient is modelled with the flow line model using the viscoplastic self-consistent polycrystal approach. Microstructure formation and reasons for the texture gradient are discussed. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Copper; ECAP; Microstructure; Texture; Heterogeneity; Flow line model
1. Introduction Since the spurt in research activities pertaining to nanotechnology, much interest has arisen in processes involving severe plastic deformation. Equal channel angular pressing (ECAP) in particular has drawn considerable attention due to its potential to produce ultrafine-grained or in some cases nanometer grain size materials in bulk form [1]. It is now well known that materials with such small grain sizes have extraordinary properties [2–4], such as simultaneous ultrahigh strength and high ductility as well as the capability of superplastic forming. They therefore have great potential for technological applications. *
Corresponding author. Tel.: +49 351 463 35144; fax: +49 351 463 37048. E-mail address:
[email protected] (W. Skrotzki).
In ECAP, a billet is deformed in a narrow deformation zone at the plane of intersection of two die channels whose cross sections are of equal area, and the strain mode approximates closely to simple shear [1] (ideal case, Fig. 1). As the overall billet geometry remains nearly constant during ECAP, multiple passes and different routes [5] are possible without any reduction in cross-sectional area. This allows materials to be deformed to very high plastic strains that cannot be readily obtained via more conventional manufacturing processes, such as rolling or extrusion. However, there are certain limitations with materials processed by this method. Because a complex deformation mode is operating in the process, there exists a gradient from the top to the bottom part of the ECAPdeformed billet. Consequently, microstructure and texture formation is non-uniform across the billet. This has been demonstrated by the present authors [6–13] in some
1359-6454/$30.00 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.11.005
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W. Skrotzki et al. / Acta Materialia 55 (2007) 2013–2024
Fig. 1. (a) Volume element ABCD deformed by simple shear in the intersection plane during ECAP and coordinate systems used: xS ; y S ; zS ¼ simple shear system; x; y; z ¼ ECAP system. (b and c) Scheme of sample sectioning for the study. (d) Schematics of texture measurement with synchrotron radiation (simplified).
ECAP-deformed face-centred cubic (fcc) metals including copper, and also in models based on viscoplasticity [14– 17], finite element methods (FEM), both in two and three dimensions [14,15,18–27], and by slip line theory [28,29], which all provide evidence of inhomogeneous deformation during ECAP. In general, plastic deformation takes place in a broad zone the shape of which sensitively depends on factors such as contact friction, material flow response, backpressure and die design. It appears that the simple shear viewpoint is approached when the inner and outer corners of the die are sharp, there is no friction at the die walls, backpressure is applied, and the material behaves in a rigid plastic fashion. In practice, however, these conditions may only be partly fulfilled. Therefore, to optimize the properties of ECAP-deformed materials with regard to technological consequences, the inhomogeneity of deformation has to be studied in more detail experimentally and compared with modelling of microstructure and texture.
This is a very important aspect of the ECAP process, and has rarely been addressed. The present investigation is a step forward in this direction: pure copper has been subjected to Route A of ECAP, and the resulting heterogeneities concerning microstructural evolution as well as texture formation have been studied. The number of passes was limited to three in order to keep the uniformly deformed billet length long enough, and thus not complicate the basic study by introducing end effects. A detailed paper on this aspect has already been published recently by Li et al. [24] for the case of a rounded die, where the authors compared the experimental textures measured by orientation imaging microscopy (OIM) with those obtained by simulation using two-dimensional (2-D) FEM following the Taylor and visco-plastic self-consistent (VPSC) polycrystal approach. However, in the present investigation, a more thorough experimental characterization of the microstructure and texture has been carried out for the case of a non-rounded die. The reason for selecting pure copper and following route A of ECAP is to compare the results with the existing literature on copper, and to analyse the significance of the additional but crucial information regarding heterogeneity. In particular, the texture gradient has been measured in a statistically much more reliable way by diffraction of high-energy synchrotron radiation. Texture measurement by this method has an advantage over other methods of local texture measurement due to the fact that high-energy synchrotron radiation is capable of measuring the texture from an appreciable volume, in contrast to electron backscatter diffraction (EBSD), where the penetration depth of the electron beam is too low. On the other hand, the global textures have been measured using neutron diffraction in order to provide information regarding the bulk texture of the coarse-grained starting material. Textures have been represented using the orientation distribution function (ODF) method, which provides a better resolution of texture components than pole figures. This is particularly relevant keeping the goals of the present investigation in mind, as one needs to address and account for small shifts in location of texture components, while describing the heterogeneities. Texture modelling has been carried out using To´th’s flow line model of ECAP [30], an analytical approach, where non-uniform deformation can be easily incorporated by changing the shape of the flow lines. This attribute of the model makes it one of the most suitable models for studying texture heterogeneities. 2. Experimental Three billets with dimensions of 9:9 9:9 100 mm3 were cut by spark erosion from extruded 4N copper rods (Goodfellow GmbH). All further cuttings for sample extraction were done in the same way. To obtain a homogeneous, strain-free starting microstructure the billets were annealed for 1 h at 450 C. ECAP was carried out with one, two and three passes using Route A [5]. The channels of the
W. Skrotzki et al. / Acta Materialia 55 (2007) 2013–2024
ECAP die had a square cross section of 10 10mm2 meeting at an angle U ¼ 90 . All corners and edges of the die were sharp. The ECAP was done at room temperature ðhomologous temperature ¼ 0:22Þ with a crosshead speed of 1 mm s1 using MoS2 as lubricant. To characterize the microstructure, in particular with respect to inhomogeneity, all samples were analyzed in cross (yz) and longitudinal section (xy) by EBSD (Fig. 1b). The texture was measured at five positions along the y-axis, as schematically shown in Fig. 1c, and the basic geometry for synchrotron irradiation for the texture measurement is shown in Fig. 1d. Samples for EBSD were first mechanically ground on wet SiC paper (grit 220–2400) with Struers LaboPol-21 followed by electropolishing using Struers Lectropol-5 at 15 C and electrolyte D2 at a polishing voltage of 24 V. The electropolishing was repeated twice for 20 s each time. Finally, the samples were rinsed for 1–3 s in an ultrasonic bath containing ethanol. The EBSD measurements were carried out on a LEO 1530 scanning electron microscope equipped with a field emission gun. Automated orientation analyses of the Kikuchi patterns were performed with the CHANNEL 5 software package produced by HKL Technology. Several orientation maps were measured by EBSD on the yz and xy planes of the billets. From these orientation maps parameters like grain size (diameter of a circle with equivalent area), aspect ratio (ratio between long and short axis of an ellipse fitted to grain shape and area) and inclination of the grain long axis (mean inclination for aspect ratios larger than 3) as well as misorientation distribution have been obtained. Since the orientation maps cover only a small area within the billet the microstructural parameters measured display local properties. Texture measurements on the coarse-grained starting material were done by neutron diffraction on a cube of about 10 mm (Fig. 1b) length at the GKSS research centre in Geesthacht, Germany. As the neutron texture represents the entire cube, it is therefore referred to as global texture. In order to examine texture heterogeneity after ECAP deformation, the local texture was analyzed by diffraction of high-energy synchrotron radiation using beam lines BW5 (100 keV) and W2 (50 keV) at DESY-HASYLAB in Hamburg, Germany. The sample for local texture measurements with synchrotron radiation was a pin of 1 1 9:9 mm3 taken from the centre of the billet with the long sample axis parallel to the y-axis. The texture was measured at five positions along the y-axis, as schematically shown in Fig. 1c, and the basic geometry for texture measurements using high-energy synchrotron radiation is shown in Fig. 1d. Details about the synchrotron texture measurements are given in Refs. [31,32]. Three pole figures, namely (2 0 0), (2 2 0) and (1 1 1), were used to calculate the orientation distribution functions (ODFs) with the spherical harmonic technique [33]. The sample coordinate system to calculate the ODFs was chosen with x, y and z parallel to the extrusion direction, normal and transverse direction, respectively.
2015
3. Texture modeling The flow line model proposed by To´th and co-workers [30,34] was considered for the deformation mode within the ECAP die. The following function was employed: n
n
/ ¼ ðd xÞ þ ðd yÞ ¼ ðd x0 Þ
n
ð1Þ
where d is the diameter of the die, x0 defines the incoming (and outgoing) position of the flow line and n is a parameter. That value of n was used which led to the best agreement between simulated and experimental textures. The accumulated von Mises equivalent strain in one pass only depends on n and is given by: 2 pðn 1Þ e ¼ pffiffiffi 2 : 3 n sinðp=nÞ
ð2Þ
In the present simulations, n values between 3 and 12 were employed representing a variation of the equivalent strain in one pass between e ¼ 0:907 and e ¼ 1:15, respectively. Details of the simulation procedure employed in this work have been published in Ref. [34]. The velocity gradient corresponding to the flow line function in Eq. (1) was introduced into the VPSC code. The experimental textures were discretized to 3000 grains. The strain rate sensitivity index of crystallographic slip was chosen to be 0.05. The 12f1 1 1gh1 1 0i slip systems of FCC metals were employed neglecting hardening. The ‘‘texture-corrected’’ technique [34] was employed for these simulations, meaning that the experimental texture was used as input in subsequent passes. 4. Results 4.1. Microstructure The microstructure of the starting material is fully recrystallized with the grain size being quite inhomogeneous (Fig. 2a). The mean grain size is 21 lm. The grains are nearly equiaxed in the yz-plane and slightly elongated in the x-direction. Grain boundaries are of high angle type (misorientation >15) with first-order twins (R ¼ 3; 60 about h1 1 1i) and second-order twins (R ¼ 9; 38:9 about h1 1 0i) dominating (Fig. 3). During ECAP the grains become sheared, resulting in an oblate grain structure with the grain long axis inclined to the extrusion axis (Fig. 2b and c). Moreover, grain fragmentation sets in, leading to a misorientation distribution which is dominated by low-angle grain boundaries (subgrain boundaries, misorientations
