Materials Science Forum Vols. 584-586 (2008) pp 393-398 online at http://www.scientific.net © (2008) Trans Tech Publications, Switzerland Online available since 2008/Jun/17
GRAIN REFINEMENT OF PURE COPPER BY ECAP N. Lugo 1, a, J.M. Cabrera 1, 2,b, N. Llorca 3,c, C.J. Luis 4,d, R. Luri4, e, J. León4, f, I. Puertas4,g 1
Department of Materials Science and Metallurgical Engineering ETSEIB, Polytechnic University of Catalonia (UPC). Av Diagonal 647, 08023 Barcelona, Spain. 2 3
CTM Technical Centre, Av. Bases de Manresa 1, 08242, Manresa, Spain.
Department of Materials Science and Metallurgical Engineering, University of Barcelona (UB). Barcelona, Spain.
4
Mechanical, Energetics and Materials Engineering Department, Public University of Navarra, Pamplona, Spain. a
[email protected],
[email protected],
[email protected], d
[email protected],
[email protected],
[email protected], g
[email protected] Keywords: Severe Plastic Deformation (SPD), Equal Channel Angular Pressing (ECAP), Ultrafine Grain (UFG).
Abstract. Pure commercial Cu of 99,98 wt % purity was processed at room temperature by EqualChannel Angular Pressing (ECAP) following route Bc. Heavy deformation was introduced in the samples after a considerable number of ECAP passes, namely 1, 4, 8, 12 and 16. A significant grain refinement was observed by transmission electron microscopy (TEM). Tensile and microhardness tests were also carried out on the deformed material in order to correlate microstructure and mechanical properties. Microhardness measurements displayed a quite homogeneous strain distribution. The most significative microstructural and mechanical changes were introduced in the first ECAP pass although a gradual increment in strength and a slight further grain refinement was noticed in the consecutive ECAP passes. Introduction. In the past two decades different Severe Plastic Deformation (SPD) processes has been proposed. All SPD processes introduce large deformation and interesting results have been reported in the production of ultrafine grain size (UFG) in the micrometric and nanometric ranges. However some SPD processes have serious limitation for possible industrial applications due to the few quantities of materials obtained and the presence of defects such as no homogenous deformation and porosity. Many studies [1] have shown that Equal-Channel Angular Pressing (see figure 1a) is one of the SPD technique with more attractive potential to obtain new materials with this kind of microstructure. The main characteristic of ECAP is not only the mechanical properties acquired, but the possibility to obtain relatively large amounts of bulk materials processed with homogenous deformation within the sample [2, 3]. In the ECAP process, samples with cross section of a square or circular shape can be deformed without undergoing significant changes in the dimensions, enabling significant deformations through an unlimited number of passes within a die. The ECAP die contains two channels with an equal cross section intersecting at an angle that can range typically from 90° to 135°. Dies of 90º impart a strain ~1 in each pass and, after some ECAP passes, a high fraction of high-angle boundaries (>15º) is produced which contributes to a significant improvement of the mechanical properties [1-6].
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Figure 1. a) Sketch of the equal-channel angular pressing (ECAP) process. b) Processing route BC, 90º rotation after each pass [5]. It is well known that grain size refinement is obtained at increasing ECAP deformation passes. According to the Hall-Petch law, this grain size diminution should promote increasing yield stresses. However, most ECAP studies are limited to no more than eight passes, and relatively few data are available for further passes. In consequence, the present study aims to analyse if further refinement is promoted at very large strains. For this purpose, a case study material has been processed by ECAP up to 16 passes. Correlation between microstructure and mechanical properties has been carried out as well. On the other hand few studies are focused on the necessary force to pass the sample through the die [8] and its possible relation with the microstructure evolution in the sample. This aspect has been also considered in this work. Experimental Procedure. High-purity copper (99.98% in mass) was used in this study. This copper was supplied in a cold-drawn state. An annealing treatment was performed to the asreceived material (600ºC for two hours), giving a grain size of ~60 µm and a hardness of 65 HV. Bars of the as-received material were then machined to obtain cylindrical samples with 10 mm in diameter and 80 mm in length for ECAP processing. 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 at strain rate of 50mm/min. Samples were pressed from 1, 4, 8, 12 and 16 passes through route BC (see figure 1b), i.e. the sample was rotated along the longitudinal axis by 90° after each single pass [6]. The total equivalent strain introduced in this ECAP process was ~1 in each pass [7]. The ECAP samples were processed in an ECAP system [8] able to register the force and displacement applied in every moment. Microstructures were observed through optical and transmission electron microscopy (TEM) using specimens taken from sliced discs from the ECAPed samples. Optical samples were prepared following the grinding sequence 400, 600, 1200, and then electro-polished and electro-etched with a solution of 30% nital and 70% methanol. These samples were observed through an optical microscope Olympus GX51. TEM samples were prepared using a Precision Ion Polishing System (Model 691 PIPS, GATAN Inc.) and observed in a High Resolution TEM (300 kV HRTEM Phillips). Mechanical properties at room temperature were evaluated in all specimens by microhardness and microtensile tests. The microhardness test was carried out in an AKASHI hardness-testing machine with an applied load of 50 g for 15 seconds. Tensile specimens were extracted from extrusion direction. They were pulled with an initial strain rate of 3.3x10-3 s-1 at room temperature using an INSTRON machine. Extrusion Force. During ECAP the materials are put under severe deformations which increase the materials strength. For such reason it is supposed that the extrusion force should increase at increasing ECAP passes. In many ECAP systems the force is applied on the plunger by means
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simple hydraulic press without chance to measure the real force applied during the process. One of the present research groups has developed an ECAP system in which the extrusion force can be monitored, among other variables [8]. This equipment has been used in this work for processing the parts and to monitor the evolution of the extrusion force with the number of ECAP passes, from 1 to 16 passes. 100
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Fig, 2. Force applied to pass the sample through of the die. a) 1 pass, b) 1, 4, 8, 12 and 16 passes. As shown in figure 2 the first pass is clearly defined by three different zones. Zone I can be associated to the geometric accommodation of the sample to the shearing channel region. In zone II a strong increment of force is noticed with no almost plunger displacement. In this moment, the material starts to be heavily deformed by shearing. This force still increases slightly in region III as a consequence of both the friction and the diameter of the exit channel which has been built slightly lower than the entrance one. In the subsequent passes the same type of curves is noticed, although zone I is shifted towards higher force due to the strain-hardening already introduced in the first pass. An interesting observation is the lack of difference between extrusion curves after 4 passes, being an indication that a sort of steady state is being reached in the microstructure of the extruded copper samples. Microstructures. Optical and TEM observations were carried out for all samples along the longitudinal section. After the first pass through the ECAP die (fig 3a), the grains are severely deformed according to the shear plane direction (at approximately 45º in the first pass) as clearly shown in both Optical and TEM images. In the consecutive passes, as the deformation is increased, the microstructure becomes finer and the observation by optical microscope is not any more feasible. However some large grains are still present in the 4th pass as illustrated in the optical and TEM pictures of fig 3b. Further passes promotes an extremely distorted microstructure, and it is almost impossible to clearly differentiate the morphology of individual grains by optic microscopy. Nevertheless TEM observations display a significant decrease in the grain size with regard to the annealed one as shown in figures 3c, 3d and 3e. The strong grain refinement observed after the 4th pass (see table 1) is in good agreement with extrusion forces illustrated in figure 2. As well, this figure is also indicating that a steady state grain has been attained at the fourth pass, and no further microstructural refinement should be observed. This again agrees with the grain size measurements listed in table I.
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a) 1 pass
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0.5 µm d) 12 passes
e) 16 passes
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Fig, 3. Optical and TEM observation of the microstructure. a) 1 pass, b) 4 passes, c) 8 passes, d)12 passes and e) 16 passes. Mechanical Properties. Microhardness measurements were scanned along the longitudinal section in order to check if there was a homogeneous strain distribution. Results are plotted in figure 4. After the first pass an important change in the hardness value is observed in good agreement with
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the tensile test results that will be shown later. In the subsequent passes the hardness increment is produced more gradually with respect the first pass. Theses increments are in accordance with the grain size. A homogenous distribution in the hardness surface is obtained up to the 8th pass plotted (see figure 4d). However, after this pass a heterogeneous distribution starts to appear at 12 and 16 passes. This fact has been associated to the apparition of bimodal grain size structure (see figures 3d and 3e). Table 1. Grain size for the ECAP sample. Grain Size Sample Standard Deviation (µm) Annealed 65 5 1 pass 15 10 4 passes 0.237 0.101 8 passes 0.197 0.103 12 passes 0.141 0.054 16 passes 0.156 0.039
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Fig, 4. Diagrams of Microhardness scanning measurements. a) 1 pass, b) 4 passes, c) 8 passes, d) 12 passes and e) 16 passes. ED: Extrusion direction, ND: Normal direction
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Microtensile Test. Additional evaluation of the mechanical properties of the ECAP-processed samples was undertaken by microtensile tests. Yield strength and ultimate tensile strength are in good agreement with the microstructure and hardness values reported in the previous paragraph. Flow curves are plotted in figure 5. It can be noticed that the largest strength was obtained in the sample deformed by ECAP. In terms of ductility, samples showed similar values of deformation (ε∼0.2) which is almost half of the annealed material. However, the samples undergoing 12 and 16th passes show better ductility behaviour, which again can be associated to the bimodal grain size distribution.
True Stres (MPa)
600 500 Anneled 1 pass 4 passes 8 passes 12 passes 16 passes
400 300 200 100 0
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Fig, 5. Tensile test curves.
Conclusion. The extrusion forces needed to pass the sample through the die displayed three different zones as a consequence of the die geometry. Heavy deformations are introduced in all ECAP passes but the most important changes are produced in the first pass, and noticed more gradually for the subsequent passes. Refinement in the grains size and increment in the mechanical properties are mainly produced after the first pass. Acknowledgements. Thanks are given to the Ministry of Science and Technology of Spain for supporting the project DPI2005-09324-C02-01/02, and to the MAT 2006-14341-C02-02. NL thanks the scholarship (BES2003-2754) granted by the Ministry of Science and Technology of Spain. References. [1] R. Z. Valiev, T. G. Langdon. 2006.. Progress in Materials Science; 51, (2006) 88. [2] V.M. Segal, V.I. Reznikov, A.E. Drobyshevskiy and V.I. Kopylov: Russian Metall., (1981) 99. [3] N. Lugo, N. Llorca, J.M. Cabrera, Z. Horita, Mat. Sci. Eng. A.; 477 (2008) 366. [4] R.Z. Valiev, N.A. Krasilinikov, N.K. Tsenev, Mat. Sci. Eng. A 137 (1991) 35. [5] K. Nakashima, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater.; 46 (1998) 1589. [6] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater.; 46 (1998) 3317. [7] M. Furukawa, Z. Horita, N. Nemoto, T.G. Langdon J. Materials Science. 36 (2001) 2835. [8] J. Leon, C. J. Luis, R. Luri, B. Huarte, I. Puertas, Current Nanoscience. 3 (2007) 241.
