Materials Science Forum Vols. 654-656 (2010) pp 1219-1222 © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.654-656.1219
Enhancing Ductility of ECAP Processed Metals Llorca-Isern Núria1,a, Grosdidier Thierry2,3,b, Cabrera Jose Maria4,c 1
Department of Materials Science and Metallurgical Engineering, Facultat de Química, University of Barcelona, C/Martí Franqués 1-11, 08028 Barcelona, Spain 2 Laboratoire d’Etude des Textures et Application aux Matériaux (LETAM), CNRS 3143, Université Paul Verlaine - Metz, Ile du Saulcy, 57045 Metz Cedex 01, France 3 State Key Laboratory of Materials Modification & Department of Materials Engineering, Dalian University of Technology, Dalian 116024, China 4 Department of Materials Science, ETSEIB, Universitat Politècnica de Catalunyaist, Av. Diagonal 647, 08028 Barcelona, Spain a
[email protected],
[email protected],
[email protected]
Keywords: Severe plastic deformation, ductility, ECAP, Cu
Abstract. Mechanical properties such as hardness, mechanical strength or fatigue resistance are by far the most successful material behaviour improved by the ECAP processed. However, the lack of ductility is the most critical negative effect. Combination of multimodal grain size is one of the most promising solutions to improve the strength/ductility balance. Different characterisation techniques are used here to analyse the properties of ECAP Cu thermal treatments and their mechanical properties influence were also investigated. Introduction Severe plastic deformation of bulk microstructured materials produces nanostructured materials showing a very high strength but a limited ductility. It is thus required to compromise both properties in order to manufacture real products. Recently, several authors [1-9] have studied different ways to enhance ductility in nanostructured materials. One of the methods is to get bimodal or multi-modal grain size distributions [1]. Even if the number fraction of larger grains in the nanostructure is low, their volume fraction can be sufficiently high to contribute to dislocationbased plasticity in the material [1]. Therefore, both thermomechanical and powder metallurgy approaches have been tested over the last years to produce this type of hybrid microstructures [4 – 7]. In many of the tested processes, the intrinsic heterogeneity of the recrystallisation process was used to introduce some large recrystallized grains within a heavily deformed structure. The difficulty however lies in the exact control of the thermo-mechanical sequence to generate the suitable mixture in a fairly reproducible manner. Thus, alternative methods for processing materials with bimodal or broad grain size distributions such as electric current assisted sintering (ECAS) of milled powders [7, 8] or direct current electro-deposition [9] have also been recently proposed. In the present work, pure copper samples processed by ECAP and having multimodal grain microstructure were investigated. Experimental Procedures High-purity copper (99.98% by mass) was selected for the present study. ECAP was conducted at room temperature using a die with a channel angle of 90° and an angle of 20° for the outer arc of curvature. Samples were pressed by 1 to 16 passes through route BC, i.e. the sample was rotated about the longitudinal axis by 90° between passes. The total equivalent strain introduced in this ECAP process was ~1 in each pass reaching a strain ~16 at the 16th pass [10]. Samples for heat treatments were obtained from all ECAPed specimens. Isothermal heat treatments were conducted
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at 100, 200, 300, 400, 500, 600 ºC for 30 minutes in a tubular furnace under Ar atmosphere. After a holding time, samples were quenched in water to retain the microstructure. Results and Discussion An effective route to create grain size heterogeneities, as illustrated in Fig.1, is to use recrystallization induced during the high straining process. Fig. 1 illustrates, by longitudinal EBSD analysis, the microstructure evolution of a commercial purity Cu alloy processed by ECAP. As already demonstrated in many metals produced by ECAP, the grain size reduction can be roughly described by the “fragmentation” of the initial grains and the creation of sub-grains having an increased amount of misorientation with increasing strain; thus leading to ultrafine grains separated by high misorientation boundaries (Fig. 1a to 1c). In the case of this Cu, the samples processed for 12 passes and more, retained a very heterogeneous structure consisting of a mixture of patches having different microstructures. They were broadly made of 3 types of grains: ultrafine equiaxed (similar to those in Fig. 1c), coarse and fairly equiaxed (some are visible in Fig. 1d) and medium elongated (similar to those in Fig. 1b). In short, the microstructure is formed as a result of a complex combination of dynamic and fairly static recrystallisation occurring at different stages of the ECAP process.
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Figure 1 Evolution of the microstructure with increasing number of passes (route Bc) of ECAP: (a) 1 pass, (b) 2 passes, (c) 8 passes and (d) 12 passes. Interestingly however, the microstructure was fairly similar after 12 or 16 passes. This indicates that pressing rods for many passes – at a controlled temperature depending on the deformed metal – can induce a balance between recovery/recrystallization and deformation processes to produce a multi-modal grain size distribution including a significant fraction of ultrafine grains.
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Figure 2 a) Microtensile test representation for ECAPed and starting material to fracture, b) enlarged initial part of the stress-strain curves. Figure 2 shows some tensile engineering stress-strain curves recorded on the ECAP-Cu samples deformed at different number of passes. It is clear from Fig. 2a that the strength of the material increases gradually with the number of passes from 1 to 8. However, these curves peak soon after yielding, in sharp contrast to the behavior of the initial coarse grain material. Such very limited strain hardening is common to severely plastically deformed Cu due to the low capacity of dislocation storage [11, 12]. Comparatively, as more clearly visible in the enlarged image of Fig. 2b, the strain hardening capability could be improved for the case of the heterogeneous microstructure obtained with increasing number of passes (12 and 16); low strain in the case when the grain size was increased with higher ECAP-induced strain leading to an improved strength/ductility balance. It is also interesting to notice in Fig. 2a that, despite different number of passes, the highly strained samples (12 and 16) presented very similar mechanical response.
Figure 3 Microstructures for ECAP-8 passes and heat treatment at 100º C (upper left), 200º C (upper right), 300º C (lower left) and 400º C (lower right). (White bar corresponds to 20 microns)
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It is worth noting that the recrystallized grain size diminished when increasing the number of ECAP passes for temperatures ranging from 100 to 300 ºC. At 400 ºC an apparent random behaviour is observed for low strain for finally increase the grain size with higher previous strain. After 200 ºC recrystallization and a grain growth process are present in all samples. Most grains contain annealing twins, providing evidence for recrystalization.
Figure 4 Stress-strain curves after ECAP- 8 passes and heat-treatment at 100º and 200ºC Figure 4 shows the stress-strain curves for ECAP-8 passes samples heat treated at 100ºC and at 200ºC. As can be seen the curve for the samples heat treated at 100ºC shows a more pronounced strain hardening as compared to the specimen without heat treatment. Conclusions Multimodal grain size distribution induced either by accumulated strain or by thermal recrystallization enhances ductility in severely plastically deformed metals. Acknowledgements NLLI and JMC wish to thank the CICYT (Spanish Government), project MAT2008-06793-C0201/02 for financial support; thanks are also due to the Serveis Cientifictècnics (University of Barcelona) for their collaboration. References [1] [2] [3] [4]
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