research papers Separating the recrystallization and deformation texture components by high-energy X-rays

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1 Journal of Applied Crystallography ISSN Received 11 March 2002 Accepted 27 August 2002 Separating the recrystallization and deformation texture components by high-energy X-rays Y. D. Wang, a * X.-L. Wang, a,b A. D. Stoica, a J. D. Almer, c U. Lienert c and D. R. Haeffner c a Spallation Neutron Source, 701 Scarboro Road, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA, b Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA, and c Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA. Correspondence wangy@ornl.gov # 2002 International Union of Crystallography Printed in Great Britain ± all rights reserved High-energy synchrotron diffraction offers great potential for experimental study of recrystallization kinetics. Measurements on partially recrystallized samples using a monochromatic high-energy synchrotron beam show that recrystallized grains generate sharp diffraction spots, whereas the intensity from the deformed matrix varies smoothly along the Debye±Scherrer rings. Based on these observations, a method has been developed to separate the recrystallization texture components from those originating from the deformation matrix. The validity of this method is demonstrated with partially recrystallized interstitial-free steel. 1. Introduction The ability to separate the deformation and recrystallization texture components in partially recrystallized samples is of critical importance in the study of recrystallization kinetics of deformed metals or alloys (Hutchinson, 1996). Traditional measurement techniques using X-ray or neutron diffraction give the average texture within the scattering volume (usually on the millimetre length scale). With these techniques, it is not possible to determine separately the volume fraction of individual texture components originating from deformation or recrystallization. Although the electron back-scattering pattern (EBSP) technique has been used to distinguish the recrystallization components from the deformed matrix by a proper choice of the tolerance value of the image quality (Wright, 1999), there are arguments on how to separate the recrystallization and recovery components, especially during the nucleation of new grains. Furthermore, in situ measurements to trace the changes of texture during recrystallization are still a challenge with the EBSP technique. High-energy X-rays provide high penetration depths (over a millimetre in most materials) and have been increasingly used for bulk and local texture measurements (Margulies et al., 2001; Wcislak et al., 2002). Previous investigations (Lauridsen et al., 2000) with a monochromatic beam showed the potential of using a high-energy synchrotron beam for in situ studies of the growth kinetics during recrystallization of individual nuclei in bulk materials. The high spatial resolution allows the separation of diffraction spots originating from distinct grains. However, because of the extremely small incident-beam divergence from a synchrotron source, the number of recorded grains at a given sample orientation is limited. Laue diffractometry with a polychromatic beam (Ice & Larson, 2000; Larson et al., 2002) would give diffraction information from far more grains. However, while this method has been demonstrated for thin foils using X-rays up to 30 kev, a demonstration with high-energy X-rays for bulk materials is still lacking. With a monochromatic beam, the individual grains deliver diffraction spots when the Bragg condition is ful lled, and each spot is a diffraction image of a grain carrying information about the size and the sub-microstructure. When the grain size is suf ciently small and the X-ray beam is illuminating a high number of grains, the individual grain contribution becomes indistinguishable and smooth Debye±Scherrer rings are observed. Thus, the beam cross section can be set to obtain either spotty or continuous diffraction rings depending on the grain size. Hirsch & Kellar (1952) and Hirsch (1952) rst used the microbeam X-ray with various sizes, produced by a glass capillary collimator and a high-intensity rotating anode X-ray tube, for studying the grain size of cold-worked metals. Synchrotron radiation microbeams are more suitable to illuminate a small number of grains (down to a single grain) and modern image detectors are able to record the diffraction patterns in real time to observe the structural evolution in situ. The choice of an adequate value for the beam cross section is crucial to trace the recrystallization process. The grains (or sub-grains) of the deformed matrix are small (considerably less than 1 mm) and contain a large density of crystallographic defects, such as dislocations and staking faults. On the other hand, the recrystallized grains are large [usually much larger than 1 mm (Cahn, 1996)] and contain almost no defects. Taking into account these factors, the best way to distinguish these two types of grains is to increase the beam size until the diffraction pattern delivered by the deformed matrix becomes smooth, but the recrystallized grains still show individual diffraction spots. 684 Y. D. Wang et al. Recrystallization and deformation texture J. Appl. Cryst. (2002). 35, 684±688

2 In our studies of partially recrystallized samples using a monochromatic high-energy X-ray beam, we have observed a number of high-intensity spikes on top of the smooth Debye± Scherrer rings. These spikes are indications of the formation of new grains, the intensity of which showed considerable growth during the recrystallization process. In other words, the spikes result from the recrystallized texture components, while the smooth Debye±Scherrer rings arise from the retained deformed matrix. Although the grains giving rise to the spikes are not necessarily in the optimal diffraction condition, the number of the spikes is a statistical representation of the grain orientation probability. Thus, by separating the intensity of the spikes from that of the smooth Debye±Scherrer rings, it is possible to determine the recrystallization and remaining deformation texture components in partially recrystallized samples, respectively. In this paper, the validity of this method is demonstrated with partially recrystallized interstitial-free (IF) steel. 2. Experimental The experiment was performed at the 1-ID beamline of the Advanced Photon Source (APS), Argonne National Laboratory, using a setup shown schematically in Fig. 1. A monochromatic X-ray beam with energy of kev ( = A Ê ) was obtained using two meridionallly bent Laue silicon crystals (Shastri et al., 2001), located upstream of a 3.3 cm undulator operated at a gap of 11 mm. The incident-beam cross section was mm with a ux of photons s 1. A Mar345 on-line image-plate scanner was used to collect the diffraction patterns. The detector is 345 mm in diameter and has 100 mm 2 pixels; it was located 779 mm from the sample. With this geometry, low-index Debye±Scherrer rings for the body-centred cubic (b.c.c.) iron sample are collected simultaneously. The material was commercial Ti-stabilized IF steel, with the following composition (wt%): C, 0.15 Mn, 0.01 P, S, N, Nb, Si, Al, Ti and the remaining Fe. The as-received steel sheet was 1.2 mm thick, Figure 1 Schematic of the experimental setup, including the de nition of 2 (Bragg angle),! (sample rotation) and (azimuthal angle). which was obtained by cold-rolling a hot-rolled sheet of thickness 3.7 mm using a laboratory mill. A mm coupon was cut from the cold-rolled sheet and mounted in a vacuum furnace, with its normal direction (ND) parallel to the incident beam. To optimize the coverage of the pole gures, the rolling direction (RD) and the transverse direction (TD) were mounted at a 45 inclination to the rotation axis,! (Fig. 1). The texture in cold-rolled steel exhibits orthorhombic sample symmetry, so by varying the rotation angle from 0 to 60 in 5 steps, partial pole gures (with polar angles covering 30±90 ) were measured. Benchmarking against literature data con rmed that those partial pole gures were suf cient to generate the three-dimensional orientation distribution function (ODF). The specimen was in situ annealed at 823 K for 2.5 h. Measuring the change of Debye±Scherrer rings at a xed! position as a function of time provides information about the grain growth kinetics. Currently, although only 2 s exposure time is enough at each!, reading the data off the image plate still takes about 2 min, which prevents the in situ determination of the full texture during annealing. The ODFs presented below were determined from data after cooling to room temperature. 3. Results and discussion The Debye±Sherrer rings measured at! = 0 at various annealing times are shown in Fig. 2. Smooth Debye±Sherrer rings are clearly evident at the beginning of recrystallization (Fig. 2a). When the annealing time reaches 78 min, some small spots start to appear on the {200} rings (Fig. 2b). With increasing annealing time, the number of spots and the intensity of the spots that appeared earlier begin to increase (Figs. 2c and 2d). The sample volume illuminated by the X-ray beam, the distance from the sample to the detector, and the width of the Debye±Sherrer ring on detector determine the critical grain size for which the individual spots can be resolved. This size was estimated to be about several micrometres according to Lauridsen et al. (2000). The full Debye±Scherrer rings at! =0 for the partially recrystallized sample after annealing are shown in Fig. 3(a). Again, sharp diffraction spots, from the recrystallized grains, are seen. Fig. 3(b) shows the intensity of the {110} pole [the inner ring in Fig. 3(a)] as a function of the azimuthal angle, which was obtained from the raw data by integrating the detector channels along the radial direction. It can be seen from Fig. 3(b) that a number of high intensity and sharp peaks are superimposed on top of the smooth Debye±Scherrer rings. As each sharp peak is distributed over only a few pixels in the azimuthal direction, it is fairly easy to separate the peaks (dashed line) from the background (solid line) by rst calculating the derivative of the whole curve and then discarding those points with exceedingly large derivatives. After the smooth background was determined for all! positions, an ODF analysis of the retained deformation regions was carried out using a method similar to those used for solving the traditional pole gure inversion. In this paper, a modi ed maximum-entropy method (Wang et al., 1997), with J. Appl. Cryst. (2002). 35, 684±688 Y. D. Wang et al. Recrystallization and deformation texture 685

3 Figure 2 The Debye±Scherrer rings measured at! = 0 with a variation of annealing time at 823 K: (a) 60 min; (b) 78 min; (c) 84 min; (d) 88 min. L max = 22, was used to calculate the ODF of the retained deformation regions from the measured pole- gure data. To enable a comprehensive analysis of all seven measured Debye±Scherrer rings, some improvements were made in the algorithm of our ODF analysis software. First, the absolute scale factor for each hkl plane was determined. This allows the volume fraction of the retained deformation regions (and hence the recrystallized regions) to be estimated by comparing the scale factors of the cold-rolled and partially recrystallized samples. Second, a correction is allowed for small misalignments of the sample by minimizing the residual errors between the experimental pole gures and the recalculated ones. The asymmetry in Fig. 3(a) indicates a small tilt of our sample (about 5 ) with respect to the! axis. In principle, the misalignment of a sample could be corrected experimentally by rocking one of the arc axes. In the present study, this tilt was taken into account and corrected in the ODF analysis. Aside from a quantitative analysis of the texture of the retained deformed regions, qualitative information about the orientations of the recrystallized grains can also be obtained from the experimental data. This was achieved by rst determining the orientation of the spike in the sample coordinates and then superimposing the spikes on the corresponding pole gures. Metallography analysis con rmed that partial recrystallization took place in the sample under investigation. By comparing the scale factors for the cold-rolled and partially recrystallized samples, we have estimated that the recrystallized fraction is about 26%. Figs. 4(a) and 4(b) give the ODF (' =45 section) of the cold-rolled and the retained deformed regions, respectively, of the partially recrystallized samples. Fig. 4(c) shows the difference ODF plot obtained using the following equation: f ˆ vcold-rolled f cold-rolled f retained v retained v recrytallized ; 1 Figure 3 (a) Full Debye±Scherrer rings of partially recrystallized IF steel measured with an image plate after annealing at 823 K for 2.5 h. (b) The corresponding intensity distribution of the {110} pole [the inner ring in (a)] as a function of the azimuthal angle. The solid line is the retained deformation components and the dashed line includes both the retained deformation components and the recrystallized components. 686 Y. D. Wang et al. Recrystallization and deformation texture J. Appl. Cryst. (2002). 35, 684±688

4 Figure 4 ODF (' =45 section) of (a) the cold-rolled sample, (b) the retained deformed matrix after partial recrystallization, (c) the difference calculated using equation (1), and (d) the principal ideal texture components. The scales in (a), (b) and (c) indicate multiples of a random distribution (MRD). where v and f are, respectively, the volume fraction and ODF of the regions of interest. For the present specimen, v cold-rolled = 100%, v retained = 74%, v recrystallized = 26%. To facilitate discussions, the principal ideal texture components at the ' =45 section are depicted in Fig. 4(d). The major texture components in cold-rolled IF steel consist of two texture bers (Ray et al., 1994): the - ber with RD h110i and the - ber with ND h111i. After partial recrystallization, the - ber showed little change while the - ber decreased. This is best seen in the difference ODF plot (Fig. 4c), which shows the texture component that disappeared during recrystallization. The decrease of the - ber compo- Figure 5 Orientation of recrystallized grains superimposed on the pole gures of the cold-rolled sample: (a) {200} pole; (b) {110} pole. The - ber texture component with a 10 scattering bank is also marked in dashed lines in the corresponding pole gures. The scales indicate MRDs. The 55, 45 and 90 marked in the pole gures are the corresponding polar angles of the {111}- ber texture component. J. Appl. Cryst. (2002). 35, 684±688 Y. D. Wang et al. Recrystallization and deformation texture 687

5 nent during recrystallization has been reported previously by Hutchinson (1996) using the EBSP technique. The present study, with high-energy X-rays, however, showed better statistics than the EBSP technique and could be conducted in situ. The bright spots in Fig. 4(c) indicate that the disappearance of various texture components along the - ber is not quite homogeneous. If the recrystallization in IF steel is indeed controlled by nucleation, as suggested by Magnusson et al. (2001), then Fig. 4(c) provides evidence that the nucleation and/or growth in IF steel is inhomogeneous. While the difference ODF plots show texture components that disappeared during recrystallization, analysis of the spike distribution indicates the growth orientation of the recrystallized texture components. In Figs. 5(a) and 5(b), the orientation of recrystallized grains for (200) and (110) planes is superimposed on top of the corresponding pole gures. As only 100 recrystallized grains were recorded by the image plate, it is not possible to conduct a quantitative analysis of the orientation distribution of the new grains using pole- gure inversion. Nevertheless, the distribution of the new grains in the pole gures indicates again a - ber texture (note in particular =55 for the {200} pole and = 45 and 90 for the {110} pole). When Fig. 4(c) and Figs. 5(a) and 5(b) are combined together, a picture of the recrystallization process emerges. In the early stage of recrystallization (up to 26 vol.%), certain orientations of the - ber nucleate and grow. As a result, a new - ber is formed in the recrystallized region. This picture is consistent with EBSP studies of the microtexture by Hutchinson (1996), which showed that the two - bers are related, i.e. the recrystallized - ber developed by consuming the - ber in the retained deformed matrix. For a quantitative analysis of the recrystallized texture components, the number of measured grains must be increased. This can be achieved by sampling at different locations within the same sample and/or by rotating the! in ner steps (5 in the present experiment). The number of sampled grains will also increase if the divergence of incident beam can be relaxed. In many cases, the ODF measured with laboratory X-ray or neutron diffraction, which exhibits appreciable incident beam divergence, represents the texture of the entire sample. If this holds true, the ODF of the recrystallized components may also be determined by subtracting the ODF of retained deformation regions from that of the entire sample. Experimental work is underway to explore these ideas. 4. Concluding remarks A method to separate the deformation and the recrystallization texture in a partially recrystallized sample is demonstrated using high-energy synchrotron X-rays. With this method, the ODF of the retained and disappearing deformation texture components can be determined quantitatively, whereas the type of textures of the new grains can be identi- ed. The experimental results for partially recrystallized IF steel are consistent with EPSP studies. Once fully developed, the present method will provide a fast and ef cient way to study the kinetics of recrystallization processes. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the US Department of Energy under contract DE- AC05-00R The authors are grateful to Dr Y. H. Sha with Northeastern University (China) for his help with preparation of IF steel samples. This research was supported in part by an appointment (YDW) to the Oak Ridge National Laboratory Postdoctoral Research Associates Program administered by the Oak Ridge Institute for Science and Education and Oak Ridge National Laboratory. References Cahn, R. W. (1996). Recovery and Recrystallization, in Physical Metallurgy. 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