diency of cryogenic deformation for refinement of the structure, it is ... Key words: severe plastic deformation, cryogenic deformation, copper, structure, texture.
ISSN 0031918X, The Physics of Metals and Metallography, 2010, Vol. 109, No. 2, pp. 171–176. © Pleiades Publishing, Ltd., 2010. Original Russian Text © T.N. Kon’kova, S.Yu. Mironov, A.V. Korznikov, 2010, published in Fizika Metallov i Metallovedenie, 2010, Vol. 109, No. 2, pp. 184–189.
STRUCTURE, PHASE TRANSFORMATIONS, AND DIFFUSION
Severe Cryogenic Deformation of Copper T. N. Kon’kova, S. Yu. Mironov, and A. V. Korznikov Institute for Metals Superplasticity Problems, Russian Academy of Sciences, ul. St. Khalturina 39, Ufa, 450001 Russia Received May 21, 2009; in final form, August 3, 2009
Abstract—The method of automated analysis of electron backscatter diffraction (EBSD) patterns has been employed for the characterization of the structure of copper obtained using severe plastic deformation by shear under high pressure under cryogenic conditions. It has been established that severe cryogenic deforma tion leads to a considerable refinement of the structure to a grain size of 0.2 μm. Based on an analysis of the texture data and misorientation spectrum, it has been concluded that it is the {111}具110典 dislocation slip that was the main mechanism of plastic flow and that the contribution from twinning was very small. It has been shown that the evolution of the grain structure was mainly determined by elongation of initial grains in the direction of macroscopic shear and their subsequent fragmentation. Key words: severe plastic deformation, cryogenic deformation, copper, structure, texture DOI: 10.1134/S0031918X10020092
INTRODUCTION The constant interest in submicrocrystalline and nanocrystalline materials is one of the stable trends in the development of modern materials science [1–5]. At present, the nanocrystalline materials are pro duced predominantly by the methods of powder met allurgy, crystallization of amorphous alloys, or deposi tion on a substrate [6]. These methods, however, are characterized by a number of disadvantages, among which the limited volume of the material being obtained is one of the most substantial drawbacks. In this connection, the use of the methods of severe plas tic deformation (SPD) that enable massive materials to be produced is practiced more and more widely [7]. The main SPD methods are equalchannel angular (ECA) pressing [8], multiaxial deformation [9], screw pressing [10], and accumulative rollbonding (ARB) [11]. However, the minimum grain size attainable in the course of SPD of some aluminum alloys is, as a rule, markedly larger than 0.1 μm [11, 12]. In recent years, with the aim of grain refinement the SPD is actively employed under the conditions of cryogenic temperatures, which supposedly suppress the processes of recovery [13–17]. In most of these works copper served as the material for investigations. It is shown that the cryogenic conditions of defor mation make it possible to decrease the grain size to 0.2 μm when using ECA pressing and rolling in the course of SPD [13–15] and to less than 0.1 μm in some cases [14, 15]. The twins detected in these mate rials testify that twinning can operate as an additional mechanism of refinement of the structure [14, 15]. Probably, it is the cryogenic conditions that favor the
activation of alternative mechanisms of deformation, because copper is universally thought to be not prone to deformation twinning. In order to clarify the expe diency of cryogenic deformation for refinement of the structure, it is necessary to carry out systematic micro structural investigations. At present, structural investi gations in this field were mainly performed using transmission electron microscopy (TEM). Along with evident advantages, this method is characterized by a number of serious limitations that do not allow one to conduct a complete characterization of the structure. A high labor intensity of measurements, especially of crystallographic measurements, and their extremely low statistic sampling should be mentioned among the most important limitations. In this connection, the purpose of this work was to study in detail the micro structure with the use of a relatively new method of automated analysis of electron backscatter diffraction (EBSD) patterns [18]. EXPERIMENTAL A commercially pure (99.9%) copper of the M1 grade was employed as the material for the investiga tion. The initial hotrolled rod was cut into parts 40 mm in diameter and 70 mm long and was subjected to multiaxial deformation in an air atmosphere in a temperature range of 500–300°С [9]. The samples were forged by approximately 40% in height along three orthogonal axes. Owing to the deformation, the material produced had an average size of fragments of ~0.65 μm. This state was accepted as the initial.
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The cryogenic deformation was effected using the method of severe plastic deformation by highpressure 1
torsion. Samples in the form of a disk 10 mm in diam eter and 2 mm thick were cut from the central, most deformed part of the forged workpiece and were deformed on a Schenck universal testing machine at an applied pressure of 4.5 GPa. The deformation was of a reversive character; the sample was deformed using sequential rotation of the anvil clockwise and counterclockwise through an angle of 45°. The total deformation was equivalent to ten revolutions of the anvil, which corresponds to the degree of shear defor mation γ = 628 according to the formula [7, 19] γ = 2πRN/l, used in the case of conventional torsion deformation for calculation of the degree of shear deformation at a distance R from the axis of the sample in the form of a disk; N is the number of revolutions; and l is the sam ple thickness. Before deformation, the samples were cooled in a container with liquid nitrogen to a temperature of –196 ± 10°С. To prevent the rapid warmingup of cop per to room temperature in the course of deformation, we used block heads of an alloy ZhS6U that were also cooled in the container with liquid nitrogen. The time interval between the withdrawal of the samples and block heads from the container and the onset of defor mation was, on the average, about 40 s; and the period of a deformation cycle was on the order of 2–3 min. Preliminary experiments showed that during the first three minutes after taking the block heads out of the container, their temperature increased only by 6 K above the temperature of liquid nitrogen. Thus, it is supposed that the cryogenic conditions of deforma tion were maintained on the whole. The deformed plates were electrolytically polished in a 7% solution of orthophosphoric acid H3PO4 in distilled water at room temperature at a voltage of 10 V. The microstructural investigations on the midradius of the deformed workpiece were performed by the methods of TEM and EBSD. For TEM investigations, we employed a Phillips CM 30 electron microscope operating at an accelerat ing voltage of 300 kV. The EBSD analysis was carried out using a TSL OIMTM software installed on a Hita chi S4300SE scanning electron microscope with a field cathode. The EBSD scanning was conducted with the help of an automated motion of an electron beam over the hexagonal grid. So, we obtained EBSD maps consisting of 200 000–500 000 pixels and con taining from 726 to 18 120 grains. The scanning step (pixel size) varied from 25 to 100 nm. To minimize errors, each electron diffraction pattern was automat ically indexed on the basis of seven Kikuchi lines. The fraction of indexed electron diffraction patterns was 1 This
experiment was Karlsruhe, Germany.
performed
in
Forshungszentrum
~99.5% of their total number. The average value of the coefficient characterizing the correctness of indexing of electron diffraction patterns (so called «confidential index» [20]) ranged for various EBSD maps from 0.16 to 0.18. For comparison, it is thought that if this coef ficient exceeds 0.1, then the fraction of correctly indexed electron diffraction patterns is 95% [20]. As a rule, the nonindexed electron diffraction patterns and those with a low confidential index corresponded to regions near grain boundaries. The EBSD data were subjected to automatic correcting; all fine grains con sisting of 3 pixels and less were automatically elimi nated from the EBSD maps. When calculating the misorientation, from all its descriptions that are crys tallographically equivalent we used a description with the minimum angle. The misorientation was calcu lated between neighboring (adjacent) points of scan ning. In view of an experimental error of the EBSD method, all lowangle boundaries with the misorien tation smaller than 2° were eliminated from consider ation. As a criterion for distinction of lowangle and highangle boundaries, the misorientation of 15° was used. The distribution of boundaries over misorienta tion angles was effected according to their length. The size of structure elements (grains/subgrains) was determined by the intercept method. RESULTS AND DISCUSSION Morphology of the Structure and Grain Size Electronmicroscopic investigation. Typical TEM images of the microstructure are shown in Fig. 1. It is seen that the structure consists of approximately equi axed fragments whose size reaches 0.2 μm. Visually the dislocation density is rather high. The angles in tri ple junctions are close to 120°. An analysis of the structure at high magnifications (Fig. 1b) revealed the presence of thin (~30 nm) plates inside some fragments (shown by arrows in Fig. 1b). According to morphological signs, these plates can be related to twins. Mechanical twinning in the course of cryogenic rolling of copper was noted in a number of works [14, 15]. It should be emphasized, however, that the specific (rectangular) shape of the twins indicated in Fig. 1b allows one to classify them preferably as annealing twins. On the other hand, this type of twin ning under cryogenic conditions of deformation seems to be relatively unlikely. Analysis of electron backscatter diffraction patterns (EBSD analysis). EBSD maps of the microstructure are shown in Fig. 2. In these maps the lowangle boundaries are given by thin gray lines; the highangle boundaries, by thick black lines. The maps are ori ented so that the vertical axis is parallel to the radial direction of the sample, whereas the horizontal axis is parallel to the tangential direction, i.e., the direction of shear upon torsion.
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100 nm
500 nm (b)
Fig. 1. Microstructure of the deformed material: (a) darkfield image at small magnification and (b) brightfield image at high magnification.
5 μm
2 μm RD
RD TD (a)
TD (b)
Fig. 2. EBSD maps of the microstructure taken with a scanning step of (a) 50 and (b) 25 nm. RD and TD designate the radial and tangential direction of the sample, respectively.
In view of a rather large statistic sampling, the EBSD map shown in Fig. 2a represents the grain structure sufficiently reliably. The microstructure is macroscopically homogeneous and consists of approximately equiaxed grains whose average grain size is ~0.25 μm (average subgrain size is ~0.11 μm). It is interesting that some grains revealed in the structure are markedly elongated in the direction of THE PHYSICS OF METALS AND METALLOGRAPHY
shear (the example is outlined in Fig. 2b). In such grains the subboundaries are oriented, on the average, in the transverse direction. It is suggested that the for mation of the final equiaxed structure is caused by the preliminary elongation of initial grains in the shear direction according to the geometrical conditions of deformation and by their subsequent transverse divi sion via the formation of boundaries of deformation origin. Vol. 109
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(b)
111
16 Specific fraction, %
14 12 10
Experimental distribution Arbitrary distribution
8 6 4 2 0
5 10 15 20 25 30 35 40 45 50 55 60 65 Misorientation angle, deg
001
101
Fig. 3. Distribution of boundaries over (a) misorientation angles and (b) misorientation axes.
Misorientation Spectrum Figure 3 displays the misorientation spectrum obtained with the help of EBSD analysis. Distributions of boundaries over misorientation angles and axes are shown in Figs. 3a and 3b. respectively. For comparison, an arbitrary distribution of misorientations is also given in Fig. 3a. This distribution is a theoretically calculated spectrum of boundaries in a polycrystal with chaotically misoriented grains (so called Mackenzie distribution [21]). To calculate it, a set of random computergener ated misorientations was used. It is obvious that the experimental distribution over misorientation angles differs rather substantially from the arbitrary distribution (Fig. 3a), which manifests itself in the presence of a lowangle maximum in the real misorientation spectrum. This peak reflects a developed network of lowangle boundaries inside grains (Fig. 2b). Furthermore, there is noted a sharp drop in the fraction of lowangle and highangle boundaries (indicated by an arrow in Fig. 3b). The cause for this can be a reversive character of the load ing used in this work. Remind that highpressure tor sion was effected via sequential rotations through an angle of 45° in reciprocal directions. In other words, it is possible that in the course of the reverse rotation of the block head the substructure formed during the pre ceding cycle of deformation was partly destroyed and this factor prevented the formation of highangle boundaries. Such an effect was described in [22]. The weak peak in the vicinity of 60° and the cluster of misorientation axes near the 〈111〉 pole conjugated with these boundaries are caused by twin misorienta tions. Thus, the EBSD analysis confirms the presence of twinning upon cryogenic deformation of copper (Fig. 2b), although the total fraction of twin bound aries in the structure is only about 2.5% and, hence,
twinning is unlikely to play a significant role in the structure formation. The distribution of boundaries over misorientation axes is additionally characterized by the presence of clusters near poles 〈001〉 and 〈101〉 (Fig. 3b) caused by the 5°–15° and 50°–55° boundaries, respectively. Texture The scheme of deformation in the course of high pressure torsion is close to simple shear, and the plane of shear is coincident with the sample surface. Corre spondingly, the direction of the normal to the shear plane is perpendicular to this surface, and the direc tion of shear is tangential to this surface. The components of the ideal shear texture in fcc metals are shown in Fig. 4a [23]. The experimental pole figure {111} obtained by the EBSD analysis and reconstructed in the coordinate system of simple shear (the normal direction to the shear plane is the shear direction) is presented in Fig. 4b. It is seen that the texture formed in the material can be described to some approximation as a superposition of the ideal components Β and B of the texture of simple shear (Figs. 4a, 4b). The texture maxima in the real pole fig ure are somewhat shifted from their ideal positions. This effect is most likely to be connected with a devia tion of the real plane and shear direction from the sample surface and tangential direction, respectively. Probably, the real texture also includes some of А com ponents of the texture of simple shear (Figs. 4a, 4b). The formation of a texture of simple shear indicates that, apparently, it is the dislocation slip {111}〈110〉 that is the main mechanism of deformation. The tex ture data together with a very low content of twin boundaries support the assumption that the contribu tion from twinning to the microstructure formation is
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(a)
{111}
B
{111}
SD
B
{111}
ND
(b)
175
SD
〈110〉 A*1 A A*1
A*2
A
A*2 A A
B
B
max = 2.564 2.191 1.873 1.601 1.369 1.170 1.000 0.855 min = 0.196
C
Fig. 4. (a) The ideal texture of simple shear in fcc metals and (b) the experimental direct pole figure {111}. ND and SD designate the normal direction to the shear plane and the shear direction, respectively.
insignificant. Thus, neither cryogenic conditions of deformation nor initial submicrocrystalline structure do lead to a change in the main mechanism of defor mation. The texture formed is very weak; the maximum intensity is only ~2.5% over the background level. It is possible that this effect, as well as the high fraction of lowangle boundaries in the structure, is connected with the reversive character of loading, as was already discussed above. CONCLUSIONS In this work, we investigated the efficiency of cryo genic deformation in the formation of nanocrystalline structure in commercially pure copper. Based on this work, the following conclusions can be drawn. (1) Cryogenic deformation led to an appreciable refinement of the microstructure (to a grain size of 0.25 μm). (2) The main mechanism of plastic flow is disloca tion slip, as evidenced by the formation of a simple shear texture and by the presence in the structure of only 2.5% twin boundaries among the total number of grain boundaries. ACKNOWLEDGMENTS We thank Prof. G.A. Salishchev for the idea of this study. We are grateful to R.M. Galeev and O.R. Valia khmetov for the help in obtaining the initial state of the sample to be studied and to Forshungszentrum Karlsruhe (Germany) personal, namely, to Prof. H.J. Fecht, Dr. Julia Ivanisenko, and L. Kurma THE PHYSICS OF METALS AND METALLOGRAPHY
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