Hiroyuki Miyamoto 1, Alexei Vinogradov 2, Satoshi Hashimoto 2 and Rika Yoda 3

Transcription

1 Materials Transactions, Vol. 50, No. 8 (2009) pp to 1929 #2009 The Japan Institute of Metals Formation of Deformation Twins and Related Shear Bands in a Copper Single Crystal Deformed by Equal-Channel Angular Pressing for One Pass at Room Temperature Hiroyuki Miyamoto 1, Alexei Vinogradov 2, Satoshi Hashimoto 2 and Rika Yoda 3 1 Department of Mechanical Engineering, Doshisha University, Kyoto , Japan 2 Department of Intelligent Materials, Osaka City University, Osaka , Japan 3 Kobelco Research Institute Inc., Kobe , Japan Deformation twinning, which is extremely difficult to produce in face-centered cubic (FCC) crystals with medium to high stacking fault energy (SFE), was observed in pure copper single crystals with an appropriate crystallographic orientation subjected to equal-channel angular pressing (ECAP) for one pass at room temperature. Dense shear bands were also observed, delineated by large-angle grain boundaries, and with close to a twinning relation with the matrix, suggesting the role of deformation twinning at their nucleation sites. The activation of deformation twinning is rationalized by a favorable crystallographic orientation and predominant dislocation density in a primary slip system. [doi: /matertrans.m ] (Received February 13, 2009; Accepted May 14, 2009; Published July 8, 2009) Keywords: deformation twinning, deformation structure, copper, single crystal, equal-channel angular pressing (ECAP), shear band 1. Introduction It is generally considered that deformation twins are extremely difficult to form in FCC metals with medium to high stacking fault energy (SFE) such as pure aluminum, copper and nickel at room temperature and low strain-rate. 1 3) The majority of the deformation twinning in pure copper reported in the literature was observed at extremely low temperature and/or a very high strain-rate. 4 10) A summary of the observations of deformation twinning are provided in Table 1 in ref. 11). A constitutive equation developed by Meyer et al. based on dislocation dynamics predicted that the deformation twinning in copper can be expected at extremely low temperature and high strain rate. 3) Shear bands are a form of plastic instability, which occurs in many metals and alloys including copper. The formation mechanism of shear bands in pure copper has been intensively studied, 1,4,11) and frequently attributed to deformation twinning without clear evidence. 1,11) However, there are limited studies where deformation twinning was observed in copper deformed by cold rolling, 11) cross-rolling, or a combination of compression and rolling. 12,13) In our previous research, we observed deformation twinning and associated shear bands metallographically in copper single crystals that had a specific initial orientation after one pass of equalchannel angular pressing (ECAP). 14) Supporting evidence was obtained by electron back-scattering diffraction (EBSD), which indicated splitting into two orientations in a twinning relationship. In addition to ECAP, 14,15) deformation twins were observed in pure copper subjected to high pressure torsion at room temperature and a low strain-rate. 16) These recent studies suggest that the formation of deformation twinning may be possible in pure copper at room temperature and low strain-rate if a certain amount of plastic strain of apploximately 1 is applied in an appropriate crystallographic orientation. This work is an extension of a previous study, 14) which discuss the possibility of deformation twinning in pure copper single crystals and their role in shear band formation. 2. Experimental Copper single crystals of commercial purity (99.95 mass%) were grown by the Bridgman method. The crystallographic orientations of specimens were analyzed by the X- ray Laue back-reflection method. As shown in Fig. 1, we used a die that had a channel with a square cross-section, measuring f 4:1 4:1 mm 2, and a channel angle,, of90. These specimens were cut by an electric-spark discharge facility so that () became parallel to the shear plane and ½112Š was parallel to the shear direction. Throughout this paper, crystallographic orientations are expressed by (hkl)[uvw], where (hkl) refers to a crystallographic plane parallel to the shear plane and [uvw] refers to a crystallographic direction parallel to the shear direction in simple shear deformation in accordance with the shear texture representation. ECAP was carried out for only one pass using a compression test machine (Shimazu; AG-1) at room temperature and at a pressing speed of 10 mm/min. MoS 2 was used as a lubricant. The deformed structures were examined using optical microscopy (OPTIPHOT-100), a scanning electron microscope with a field emission gun (FE-SEM; XL30FE and XL30S-FEG) equipped with electron back scattering diffraction (EBSD), and transmission electron microscopy (H-8100 operated at 200 kv). All observations were carried out from the TD axis. 3. Results The macroscopic structure revealed by chemical etching in nitric acid and {} pole figures indicating local textures of the half-pressed specimen are shown in Fig. 2. Shear bands

2 Formation of Deformation Twins and Related Shear Bands in a Copper Single Crystal Deformed by Equal-Channel Angular Pressing 1925 TD press () ID shear 45 ED [112] Fig. 1 Schematic illustration of ECAP dies and a billet with the coordinate axes ID, TD and ED. can be clearly recognized metallographically, and they are inclined by 45 to ED. The rough surface after chemical etching revealed a strong heterogeneity in the crystallographic orientation or in the dislocation density. Such a heterogeneous band-like structures were also observed in our previous research. 14,17) The pole figures indicated the initial AðÞ½112Š orientation was split into the initial AðÞ½112Š and a newly formed AðÞ½112Š. Note that AðÞ½112Š becomes AðÞ½112Š by 70.5 rotation in a counter-clockwise direction around the TD axis, and these two distinct orientations are in a twinning relationship. These counterclockwise rotations during ECAP were observed in previous experimental and numerical analyses ) For example, Fukuda et al. traced the crystal orientation in an aluminum single crystal billet subjected to ECAP, and found that the initial orientation rotated about 60 around the TD axis. 19) They explained that these rotations occurred because shearing does not take place along a simple shear plane, but rather over an extended shear zone. Figure 3 shows an OIM image and {} pole figure. The red and blue areas of the OIM image correspond to those of two primary orientations in the {} pole figure. The micro shear bands lying at 20 with a width less than 10 mm have an AðÞ½112Š orientation while the matrix has an AðÞ½112Š orientation. The shear bands, which are lying at 50 macroscopically, were revealed as clusters of distorting micro shear bands, and refer to macro shear bands. An OIM image including one micro shear hand at a higher resolution of 50 nm is shown in Fig. 4(a), and reveals that the micro shear band is clearly delineated by large-angle grain boundaries. The same area is shown in an image quality map with computer generated grain boundaries defined as 3, as shown in Fig. 4(b). The -value is the reciprocal density of the coincident site of two crystals on both sides superimposed imaginary. In FCC metals, symmetrical twin boundaries are typical 3 grain boundaries. One can see that parts of the mm A()[112] A()[112] Fig. 2 Macroscopic appearance of specimens after pressing half-way though the ECAP die, and {} pole figures associated with the points labeled 1 4 in the upper cross-section.

3 1926 H. Miyamoto, A. Vinogradov, S. Hashimoto and R. Yoda 50 µ m Fig. 3 EBSD analysis of macro shear band and corresponding {} pole figure. The macro shear band is a cluster of micro shear bands. Note the shear bands and matrix in close relation to twinning. (a) (b) ID Σ3 boundaries Fig. 4 EBSD analysis of a shear bands embedded in a matrix; (a) inverse pole figure map of ID and (b) image quality map indicating 3 grain boundaries by red lines. large-angle grain boundaries delineating the shear band are defined as 3 boundaries, suggesting a possible link between micro shear bands formation and deformation twinning. The TEM micrograph in Fig. 5 shows a micro shear band constituting a macro shear band lying at 50 to the ED axis. A micro shear band with a width of 4 to 5 mm is visible as a clustering of smaller microbands less than 0.5 mm in width. The SADP shows that both the orientations of the matrix and shear band are in a near twinning relationship, are consistent with the OIM in Figs. 3 and 4. Parallel sets of deformation

4 Formation of Deformation Twins and Related Shear Bands in a Copper Single Crystal Deformed by Equal-Channel Angular Pressing 1927 a a Micro shear band b b matrix 1µm Twin relation a b Fig. 5 TEM micrograph and SAD patterns in a shear band and matrix. Note the shear band and matrix are in twinning relationship. twins were observed in the matrix shown in Fig. 6(a). Their thickness ranges from 20 to 100 nm, which is consistent with the literature. 15) The SADP in Fig. 6(b) reveals duplicated orientations in the twinning relationship. As shown in Fig. 6(c), the twin is lenticular and thus tapered to a sharp edge at its periphery. This growth form is a typical morphology of deformation twins, and can be assumed to be related to the shear accommodation process during deformation, such as the pole mechanism proposed by Venables. 21) 4. Discussion It is important to note that the initial AðÞ½112Š is unfavorable for nucleation of twinning, whereas AðÞ½112Š is favorable in a directional sense. In other words, AðÞ½112Š can twin to AðÞ½112Š, but not vice versa. It can be assumed that AðÞ½112Š rotated to AðÞ½112Š in the counter-clockwise direction. Then, parts of the AðÞ½112Š orientation twins to AðÞ½112Š. Shear bands having a AðÞ½112Š in twinning relationship with the matrix may have formed from the matrix rotated to AðÞ½112Š, with subsequent activation of twinning to AðÞ½112Š orientation. Plastic deformations by slip and twinning are considered competitive mechanisms. 21) Twinning starts when twinning stress becomes less than the stress for slip. At room temperature and low strain rate, the stress for a thermally activated slip in fcc metals is much smaller than the twinning stress. With increasing dislocation density and strain hardening, stress for slip becomes higher and twinning tends to be activated. Critical dislocation density may exist, over which deformation twinning replaces slip as the major deformation mechanism. 2) Mayers et al. calculated both the stress for slip and twinning empirically taking into account the athermal and thermal components of stress, and suggested that both plastic strain larger than 0.8 and strain-rate higher than 200 s 1 are required for deformation twinning to occur in copper with a grain size of 10 mm. 3) According to the pole mechanism proposed by Venables, 21) a dislocation having the Burgers vector, 1=2a½110Š dissociated into the Schokley partial dislocation, 1=6a½112Š and Frank sessile dislocation, 1=3a½Š, ð1=2a½110š ¼1=6a½112Šþ1=3a½ŠÞ between two forest (pole) dislocations. Twins nuclei are formed by a subsequent helical movement of the Schokley partial dislocation on the stacking () planes. 21) Local twinning stress to activate twinning is a function of the stacking fault energy. Since local shear stress to activate twinning can be concentrated by a dislocation pile-up in front of the pole, a high dislocation density as well as a crystallographic orientation of the

5 1928 H. Miyamoto, A. Vinogradov, S. Hashimoto and R. Yoda (a) (c) 100nm 100nm (b) Fig. 6 TEM micrographs of (a) parallel deformation twins; (b) a tip of a deformation twin and (c) duplicated SAD patterns of the deformation twin and matrix in micrograph (a). twinning plane and direction with respect to the applied shear stress may be crucial factors for the formation of deformation twinning. In simple shear deformation, such as occurs in ECAP, dislocations of primary slip system parallel to the shear plane were predominant in contrast to very scarce forest dislocations in other slip systems. Thus, the larger interspace between forest (pole) dislocations in addition to the dense dislocations in twinning plane may diminish the applied stress required for twinning. Such unique slip patterns in ECAP may facilitate the nucleation of deformation twins. It is commonly understood that shear bands form as a result of plastic instability under certain conditions, and the imposed strain tends to localize in these shear bands. 22) Although, the shear banding mechanism in copper is not yet completely understood, it has frequently been attributed to the existence of a layered substructure such as mechanical twin lamellae and microbands in metals with low to medium SFE. 1,4,23) Wagner et al. 23) attributed shear bands formation and the associated orientation splitting around the TD axis to mechanical twinning, although deformation twinning was not observed. In the lamellae structure, since dislocation glide across the lamellae is impeded, the strain cannot be accommodated by a uniform slip, thus resulting in shear band formation. It should be mentioned that deformation twins were observed in the matrix, but not in shear bands in this study. As pointed out by Hirsch et al., 1) it might be explained that deformation twins form in smaller strain followed by the shear band formation, and they might become nucleation sites for the formation of shear bands. A larger volume of shear bands might grow by the coalescence of separately nucleated deformation twins. Thus, shear bands continuously replaced the twinned structure and inherited the orientation of twinned region. 1) More concrete evidence for the role of twinning in initiating shear bands should be obtained in the future. 5. Conclusions A pure copper single crystal with an appropriate crystallographic orientation was subjected to ECAP for one pass at room temperature. The following conclusions were drawn. (1) The crystallographic orientation was split into two orientations as a result of shear band formations, one remaining at the initial orientation and the other orientation was rotated around the TD axis. Both orientations have a {} plane and h112i direction parallel to the macroscopic shear plane and shear direction of ECAP, respectively. (2) The above mentioned orientations of shear bands and the matrix were in a twinning relationship, and a large fraction of the interface between them comprised 3 grain boundaries. (3) Deformation twins were observed in the matrix, and they were parallel to the primary slip system plane parallel to the macroscopic shear plane of ECAP. This suggests that deformation twinning plays an important role as nucleation sites of shear bands. (4) Deformation twinning tends to occur in ECAP even in pure copper because the dislocation density was concentrated in the primary slip system parallel to the shear plane of ECAP resulting in scarce forest dislocations. Thus, twinning stress became smaller than that required for slip.

6 Formation of Deformation Twins and Related Shear Bands in a Copper Single Crystal Deformed by Equal-Channel Angular Pressing 1929 REFERENCES 1) J. Hirsch, K. Lucke and M. Hatherly: Acta Metall. 36 (1988) ) E. El-Danaf, S. R. Kalidindi and R. D. Doherty: Metal. Mater. Trans. 30A (1999) ) M. A. Meyers, O. Vohringer and V. A. Lubarda: Acta Mater. 49 (2001) ) K. Morii, H. Mecking and Y. Nakayama: Acta Metall. 33 (1985) ) A. Malin, J. Huber and M. Z. Hatherly: Metallkde 72 (1981) ) Y. Wang, T. Jiao and E. Ma: Mater. Trans. 44 (2003) ) G. T. Gray III, P. S. Follansbee and C. E. Frantz: Mater. Sci. Eng. A (1989) ) T. H. Blewitt, R. R. Coltman and J. K. Redman: J. Appl. Phys. 28 (1957) ) O. Johari and G. Thomas: Acta Metall. 12 (1964) ) M. A. Meyers, U. R. Andrate and A. H. Chokshi: Metal. Mater. Trans. 26A (1995) ) G. D. Kohlhoff, A. S. Malin, K. Lucke and M. Hatherly: Acta Metall. 36 (1988) ) S. Dymek and M. Wrobel: Mater. Chem. Phys. 81 (2003) ) M. Wrobel, S. Dymek and M. Blicharski: Scr. Mater. 35 (1996) ) H. Miyamoto, U. Erb, T. Koyama, T. Mimaki, A. Vinogradov and S. Hashimoto: Phil. Mag. Lett. 84 (2004) ) C. X. Huang, K. Wang, S. D. Wu, Z. F. Zhang, G. Y. Li and S. X. Li: Acta Mater. 54 (2006) ) Y. H. Zhao, X. Z. Liao, Y. T. Xhu, Z. Horita and T. G. Langdon: Mater. Sci. Eng. A (2005) ) H. Miyamoto, J. Fushimi, T. Mimaki, A. Vinogradov and S. Hashimoto: Mater. Sci. Eng. A 405 (2005) ) M. Furukawa, Y. Kawasaki, Y. Miyahara, Z. Horita and T. G. Langdon: Mater. Sci. Eng. A (2005) ) Y. Fukuda, K. Oh-ishi, M. Furukawa, Z. Horita and T. G. Langdon: Acta Mater. 52 (2004) ) S. Li, I. J. Beyerlein, C. T. Necker, D. J. Alexander and M. Bourke: Acta Mater. 52 (2004) ) J. A. Venables: Phil. Mag. 63 (1961) ) F. J. Humphreys and M. Hatherly: Recrystallization and related annealing phenomena, second ed., (Elsevier UK, 2004). 23) P. Wagner, O. Engler and K. Lucke: Acta Metall. 43 (1995)