Effect of the Velocity of Equal-Channel Angular Pressing on the

ISSN 0031-918X, The Physics of Metals and Metallography, 2008, Vol. 106, No. 4, pp. 411–417. © Pleiades Publishing, Ltd., 2008. Original Russian Text © K.V. Ivanov, E.V. Naidenkin, 2008, published in Fizika Metallov i Metallovedenie, 2008, Vol. 106, No. 4, pp. 426–432.

STRENGTH AND PLASTICITY

Effect of the Velocity of Equal-Channel Angular Pressing on the Formation of the Structure of Pure Aluminum K. V. Ivanov and E. V. Naidenkin Institute of Strength Physics and Materials Science, Siberian Branch, Russian Academy of Sciences, pr. Akademicheskii 2/1, Tomsk, 634021 Russia Received July 24, 2007

Abstract—Transmission electron microscopy, X-ray diffraction, and electron back-scattering pattern analysis have been used to investigate the effect of the speed of equal-channel angular pressing (ECAP) at room temperature on the formation of ultrafine-grained structure in pure aluminum. It has been established that eight ECAP passages with a pressing speed of 3.3 × 10–2 mm/s results in the formation in aluminum of a substantially inhomogeneous grain structure with a grain size in the range of 1–27 µm (average size 3.0 µm). An increase in the speed of pressing by an order of magnitude leads to an increase in the level of internal stresses and dislocation density, an increase in the upper boundary of the interval of the grain-size distribution and in the average grain size (to 3.4 µm), and a decrease in the number of boundaries with high-angle misorientations. It is assumed that these changes are connected with the fact that processes of dislocation-structure relaxation have no time to occur during the ECAP at high pressing speeds. PACS numbers: 62.20.Fe, 81.40.Lm DOI: 10.1134/S0031918X08100116

INTRODUCTION In recent years, a great attention of researchers is paid to ultrafine-grained (UFG) metallic materials in which the structure refinement is achieved by severe plastic deformation [1–3]. The interest in such materials is caused by their unique physical and mechanical properties. In particular, many fundamental, even usually structure-insensitive characteristics, such as the elastic moduli, the Curie and Debye temperatures, saturation magnetization, etc., can change in these materials. They, as a rule, possess high strength, retain satisfactory plasticity, and manifest (under some conditions) low-temperature and/or high-strain-rate superplasticity [1, 2]. The equal-channel angular pressing (ECAP), developed by Segal et al. [4], is at present one of the basic methods for the realization of severe plastic deformation. The method makes it possible to obtain massive nonporous work pieces with an ultrafine grain size in the submicrocrystalline (for pure metals) or nanosized (most frequently, for alloys and intermetallic compounds) range. The works of the domestic and foreign authors made it possible to establish that, in obtaining UFG structures with properties attractive from a practical viewpoint, of large importance are many characteristics of the material to be deformed, e.g., the type of crystal lattice, elemental composition, the presence of impurities, and the parameters of the ECAP process, such as the angle of channel intersection, the number of passages of the sample through the channels, the angle

of rotation of the sample between consecutive passages, the temperature and the deformation rate, the equipment geometry and the lubricants used. The selection of optimum deformation conditions for obtaining the desired parameters of the structures arising during ECAP is a complex problem. There is a number of works that describe in detail the influence of the degree of deformation [5, 6], type of route [6], channel intersection angle [7, 8], speed of pressing [9], and temperature [10] on the forming microstructure. For example, the works of Japanese researchers [6, 8, 9, 11] examine in detail the evolution of the structure of pure aluminum during ECAP. It has been established in those works that a single passage results in a banded structure with predominantly low-angle subgrain boundaries. The misorientation at the low-angle boundaries grows with increasing number of passages; this growth is most rapid if we rotate the work piece after each passage about the longitudinal axis by 90° (route B) [6]. After four passages, the size of grains or subgrains in aluminum lies in the interval of 0.5–1.5 µm. The experimental results in the works cited were obtained using transmission electron microscopy (TEM), including the high-resolution technique. Despite the fact that TEM is a powerful and reliable research method, it is necessary to use some additional procedures for obtaining absolutely reliable experimental data. This especially refers to the materials obtained by severe plastic deformation, which possess a very complex structure difficult for studying. For example, the application of the method of analysis of diffraction patterns of back-scattered elec-

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Z X

Y

37°

Fig. 1. Equal-channel angular pressing (schematic). The structure of the sample was investigated in the plane Y.

trons allowed the authors of [12] to reveal a number of specific features of the aluminum microstructure after ECAP, and the authors of [13] drew conclusions about the optimum routes of ECAP, which differed from those cited above. In connection with the above, we in this work set a task to use several mutually complementing experimental procedures to perform, on the example of pure aluminum, a comprehensive study of the influence of one of the least studied (in our opinion) parameters of ECAP, namely, the deformation rate, on the nature of the arising structure (size of grains and of the elements of grain–subgrain structure; relationship between high-angle and low-angle boundaries; etc.). EXPERIMENTAL As the material for studies, pure aluminum (99.99%) was used. To facilitate cutting work pieces for ECAP, the aluminum plate was rolled by 30%, after which it was subjected to recrystallization annealing at a temperature of 500°ë for 1 h. The grain size in the initial (prior to ECAP) aluminum was several millimeters. The work pieces of size 12 × 12 × 40 mm were subjected to ECAP at room temperature in a tool that contained two channels of square section intersecting at right angle. The external angle of channel intersection was ψ = 37°. The degree of true deformation per pass estimated by the expression given in [2] was 1. All samples were extruded eight times; after each passage they were turned about the longitudinal axis by 90° (route Bc). In one case the speed of motion of the press plunger was 3.3 × 10–2 mm/s (batch 1); in the other case, 3.3 × 10–1 mm/s (batch 2). The structure of the samples obtained was investigated by the methods of X-ray diffraction, transmission electron microscopy, and electron back-scatter diffrac-

tion (EBSD) analysis (orientation imaging microscopy (OIM) [14]). The samples for structural studies were cut out by the electroerosion method on the Y plane parallel to the axes of the intersecting channels (Fig. 1). The damaged surface layer was moved away by grinding using an emery paper and a diamond paste. Before studying by the OIM method, the samples were electrolytically polished in an electrolyte containing 20% perchloric acid and 80% ethanol at –25°C and a voltage of 45 V. The foils for transmission electron microscopy were prepared by the method of jet electropolishing on a Micron-104 device in an electrolyte of composition of 25% HNO3 + 75% CH3OH at +5°ë and a voltage of 12 V. The X-ray diffraction analysis was carried out on a Shimadzu XRD-6000 diffractometer in Cu Kα radiation using the Bragg–Brentano focusing geometry. The dislocation density was estimated from the profiles of the Bragg maxima by the formula [2] ρ = 2 3 〈 ε hkl〉 2

1/2

/ ( D hkl b ),

1/2

where Dhkl and 〈 ε hkl〉 are the volume-averaged values of the coherent-domain sizes and microstresses in the direction perpendicular to the hkl plane, and b is the Burgers vector of dislocations (for aluminum, b = 0.286 nm [15]). The electron-microscopic examination of thin foils by TEM was conducted using an EM-125K electron microscope equipped with a goniometer, at an accelerating voltage of 100 kV. The size of the element of the grain–subgrain structure (hereinafter, a (sub)grain) was estimated as the maximum spacing between the boundaries of regions with an identical contrast. The crystallographic characteristics of the microstructure were determined using a special attachment for the analysis of EBSD patterns (Pegasus) to a Quanta 2

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(a)

2 µm

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2 µm

(b)

Fig. 2. Structure of aluminum after ECAP: (a) batch 1 and (b) batch 2.

200 3D (scanning electron microscope + focused ion beam microscope) work station with a tungsten thermionic cathode at an accelerating voltage of 30 kV and working distance of 15 mm. The microstructure regions suitable for a study were selected using scanning electron-microscopic images at magnifications from 2000 to 5000. The sizes of regions of scanning were 30 × 30, 60 × 60, and 100 × 100 µm with a step of scanning of 0.1, 0.25, and 0.6 µm, respectively. The fraction of the unindexed points, located predominantly in the vicinity of boundaries, did not exceed 15%. The size of the region which forms the diffraction pattern of the backscattered electrons, according to estimations made in [14] for aluminum, lies in the range of 0.2–0.5 µm. The Kikuchi patterns formed by the back-scattered electrons were indexed automatically by a TSL OIM datacollection program. The accuracy of determining orientation angles by this method is 1° [14]. The processing of the data files was conducted with the use of the TSL OIM-analysis software. To the unindexed points, the values of the orientation angles of the nearest neighbors were assigned. When constructing OIM images, the regions of a specific color corresponded to regions inside which the orientation of the crystal lattice changed less than by the so-called critical angle. It was assumed that at the value of the critical angle of 15° the OIM image corresponded to a grained structure; at 2°, to a grain–subgrain ((sub)grain) structure. RESULTS AND DISCUSSION The TEM studies showed that in the samples of batch 1 the ECAP process leads to a considerable refinement of microstructure. It is seen from the brightfield images of structure (Fig. 2a) that the size of THE PHYSICS OF METALS AND METALLOGRAPHY

(sub)grains lies in the range from 0.5 to 5.0 µm. The majority of (sub)grains is free of dislocations; in some (sub)grains, isolated dislocations are observed, which do not form pileups, tangles, dislocation walls, or other formations. A significant part of grain boundaries exhibit fringe contrast characteristic of equilibrium boundaries. Extinction contours connected with the presence of internal stresses are observed only in a limited number of (sub)grains. According to X-ray diffraction data, the dislocation density is 1 × 109 cm–2, which corresponds to values characteristic of metals subjected to severe plastic deformation [2]. It should be noted that in [16] the dislocation density in aluminum after ECAP was greater by approximately three orders of magnitude as compared to our work. It is highly improbable that such a substantial difference could arise because of the difference in the purity of the material or the type of route that was used (in [16], aluminum of 99.9% purity was deformed by route A). The origin of this discrepancy requires additional studies. The authors of a number of works [6, 17] noted analogous features of the aluminum structure after ECAP under coinciding conditions but reported a somewhat smaller size of (sub)grains. This difference is likely to be caused by the higher value of the external angle of the rounding of the intersecting channels (ψ = 37°) in the tool used in this work in comparison with the angle used in the works cited (ψ = 20°). Figure 3a displays OIM images of the structures constructed for the values of the critical angle of 2°. It is evident that the structure is formed by equiaxed or slightly elongated (sub)grains with sizes in the range from 1.2 to 7.0 µm (average size 2.5 µm). The corresponding size distribution of (sub)grains (Fig. 4a) Vol. 106

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(c)

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35 µm

(b)

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(d)

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Fig. 3. Structure of aluminum after ECAP: an OIM image constructed for the values of critical angle of (a, b) 2° and (c, d) 15°; (a, c) batch 1; and (b, d) batch 2.

shows that the interval indicated is shifted toward larger sizes in comparison with the (sub)grain sizes obtained with the aid of TEM. This can be due to the fact that the different contrast on TEM bright-field images is formed by regions with a misorientation angle greater than 0.5°, whereas the value of the critical angle assigned for the OIM analysis (based on the considerations of the accuracy of the method) is 2°. Taking into account this remark, it can be concluded that both procedures lead to close results in the estimation of (sub)grain sizes. In the case of grains, i.e., regions bounded by highangle boundaries, the size distribution interval is substantially wider (Fig. 3c). Note that the lower boundary of the interval coincides with the lower size of (sub)grains, and a significant (to 65%) fraction of the volume is occupied by grains which lie within the interval of sizes characteristic of (sub)grains. At the same time, grains with sizes that are several times greater than the sizes of (sub)grains are observed. It is seen from the corresponding OIM image that these grains are elongated along the direction of shear and form bands in which no transverse high-angle boundaries arise. The average size of grains is equal to 3.0 µm. Fig-

ure 5a shows the distribution of boundaries over the angles of misorientation without taking into account low-angle boundaries with misorientations less than