Evolution of Grain-Boundary Microstructure and Texture in Interstitial-Free Steel Processed by Equal-Channel Angular Extrusion

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1 Evolution of Grain-Boundary Microstructure and Texture in Interstitial-Free Steel Processed by Equal-Channel Angular Extrusion AYAN BHOWMIK, SOMJEET BISWAS, SATYAM SUWAS, R.K. RAY, and D. BHATTACHARJEE The equal-channel angular extrusion (ECAE) of Ti-bearing interstitial-free (IF) steel was performed following two different routes, up to four passes, at a temperature of 300 C. The ECAE led to a grain refinement to submicron size. After the second pass, the grain size attained saturation thereafter. The microstructural analysis indicated the presence of coincident-site lattice (CSL) boundaries in significant fraction, in addition to a high volume fraction of highangle random boundaries and some low-angle boundaries after the deformation. Among the special boundaries, R3 and R13 were the most prominent ones and their fraction depended on the processing route followed. A deviation in the misorientation angle distribution from the Mackenzie distribution was noticed. The crystallographic texture after the first pass resembled that of simple shear, with the {112}, {110}, and {123} aligned to the macroscopic shear plane. DOI: /s Ó The Minerals, Metals & Materials Society and ASM International 2009 I. INTRODUCTION OVER the past few years, tailoring microstructures with ultrafine grain sizes in bulk materials has triggered much interest among the scientific community. This is due to the fact that grain size strengthening is one of the few mechanisms that lead to improvement in the strength of materials, retaining an appreciable amount of ductility and flow properties. It has now been well established that ultrafine grain sizes can be achieved in bulk materials by severe plastic deformation, which involves imposing extremely large plastic strains during deformation processing without changing the dimensions of the workpiece significantly. [1,8] A number of innovative and nonconventional processes of fabrication have emerged following this philosophy, namely highpressure torsion, [2,3] multiaxial forging, [4,5] accumulative roll bonding, [6,7] and equal-channel angular extrusion (ECAE). [8 13] Of all these techniques, ECAE, developed originally by Segal, [8] is the most widely investigated and has been successfully applied to materials with different crystal structures. [14 23] During ECAE, a billet of square or circular cross section is passed through two channels having the same area of cross section, inclined at an angle, generally 90 or 120 deg. Because the cross section of the billet remains the same after every pass, the AYAN BHOWMIK, Master s Student, SOMJEET BISWAS, Doctoral Student, and SATYAM SUWAS, Assistant Professor, are with the Department of Materials Engineering, Indian Institute of Science, Bangalore , India. Contact satyamsuwas@ gmail.com R.K. RAY, Visiting Scientist, and D. BHATTACHARJEE, Chief, are with the R&D Division, TATA Steel, Jamshedpur, Jharkhand , India. Manuscript submitted September 11, Article published online August 22, 2009 ECAE process can be repeated for any given number of passes. [24] The ECAE process, in principle, can have three variants, which gives rise to so-called routes based on the rotation about the longitudinal axis of the billet, as given during the following consecutive passes: (1) route A, without any rotation; (2) route B, rotation of 90 deg; and (3) route C, rotation of 180 deg. A further subdivision is introduced in route B, by changing the sense of rotation. If the rotation is in the opposite sense in the consecutive passes, the resulting is called route B A. On the other hand, if the rotation is introduced in the same sense, this gives rise to route B C. It has been proven through experiments that route B C is the most efficient route for grain refinement, followed by route A. Because ECAE involves imparting large plastic strains on materials in a monotonic or changing strain path mode, it is inevitable that such materials would develop a characteristic crystallographic texture. [13] The evolution of microstructure and texture during ECAE has been investigated by many researchers; however, most of the studies were carried out on fcc materials and also on hcp materials. Relatively fewer studies have been aimed at bcc materials. The issue of grain refinement and the consequent strengthening has been quite actively pursued in the pretext of steels using many approaches. The need for lighter and stronger steels for automotive applications has been a requirement for quite some time now. The ECAE appears to be a viable technique, due to possible strengthening in the interstitial-free (IF) steel due to its grain refinement capability (it is a material that is inherently ductile and has relatively lower density compared to other alloyed steels). It is to be mentioned here that it is not only grains surrounded by high-angle boundaries but also subgrains that contribute to the strengthening. [25 27] METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 40A, NOVEMBER

2 A few reports on the ECAE of IF and low-carbon steels [28 31] are available, and a few of them adequately address the issue of the processing-microstructure correlation; the others are aimed at examining the processing-texture relationship. However, none of these articles addresses the gross microstructural evolution, including the evolution of the grain-boundary structure and its relation with the texture. The present work is an effort to bridge this gap. It is to be mentioned here that the texture in long products, as obtained through ECAE, can be suitably tailored to obtain a property advantage along the axis of the billet. The IF steel has been processed by the two important routes of ECAE, namely route A, which does not involve a change in the strain path, and route B C, which involves an orthogonal change in the strain path. The as-processed materials have been subjected to extensive characterization of the following: (1) the grain-boundary microstructure and (2) the crystallographic texture. Thereafter, an attempt has been made to explore a possible correlation between the same. II. EXPERIMENTAL PROCEDURE A. Material The IF steel investigated in the present study was a titanium-stabilized IF steel obtained from Tata Steel (Jamshedpur, India). The chemical composition of the material is presented in Table I. The microstructure of the as-received material consisted of very coarse grains (average grain diameter ~225 lm) (Figure 1). Table I. Chemical Composition of IF Steel C Mn S P Si Al Ti N Fe bal B. Processing Billets with dimensions mm in cross section and 100 mm in length were machined from the as-received IF steel material. The ECAE experiments were carried out using a die heated to a temperature of 300 ± 10 C with a constant crosshead speed of 1 mms 1 in a hydraulic press. The die setup used for the experiments was similar to the one proposed by Mathieu et al. [32] As illustrated in Figure 2, the die set had sharp inner and outer corners with an interchannel die angle, F = 90 deg, without any rounding off, with the von Mises strain per pass e vm ~ [33] In the schematic diagram, x and y denote the shear direction (SD) and normal-to-shear plane (SPN), respectively. These correspond to the specimen reference system, according to the convention for simple shear texture. [29] The x and y stand for the extrusion direction (ED) and normal direction (ND), respectively. Both the sample and tooling were lubricated using 5 pct MoS 2, in order to reduce the friction. The billets were subjected to ECAE following routes A and B C. C. Characterization All the samples processed by ECAE were subjected to microstructural and textural characterization. The specimens were sectioned as per the scheme shown in Figure 2. The microstructural and textural characterizations were carried out on the transverse direction (TD) plane. After conventional electropolishing, the samples were subjected to an electron backscattering diffraction (EBSD) investigation using a field emission gun scanning electron microscope (SEM) operated at 20 kv. In every scan that was set, more than 1000 grains were sampled, with approximately 98 pct of the data points properly indexed, showing distinct Kikuchi patterns. The image quality (IQ) and inverse pole figure (IPF) maps were obtained from the analysis of the scanned data using TSL 4.6 software (AMETEK Inc., Paoli, PA). The texture developed in the material was analyzed by plotting pole figures as well as by orientation distribution functions (ODFs). The ODFs were calculated by the discrete binning method, [34] using an ADC algorithm. [35] The texture components were analyzed with the help of LaboTex 3.0 software (Labosoft, Cracow, Centrum, Poland). Fig. 1 Optical micrographs of the as-received IF steel; etchant is Marshall s reagent, 10 to 15 s. Fig. 2 Schematic representation of the ECAE die and process; x and y represent the SD and SPN, respectively. The adjoining figure shows the direction of shear acting on plane of intersection of die channels VOLUME 40A, NOVEMBER 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A

3 III. RESULTS A. Microstructural Observation and Analysis A thorough analysis of the microstructure was carried out with EBSD-generated maps. Although microstructures were recorded at each stage of the ECAE, only a few representative microstructures are shown in Figure 3. The images have been presented as superpositions of the IPF map over the IQ map, in order to reveal the substructure as well as the orientation. Fig. 3 IPF map superimposed on pattern quality maps of (a) route A, pass 2; (b) route B C, pass 2; (c) route A, pass 4; and (d) route B C, pass 4. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 40A, NOVEMBER

4 A drastic reduction in the grain size was observed after the first pass of the ECAE. The average grain size reduced from 225 lm in the starting material to approximately 8 lm after the first pass. A further reduction in the mean grain size to ~0.4 lm was observed after the second pass (e vm = 2.3), following which there was almost a saturation in the grain size. Beyond this level of deformation, the changes in the microstructure were reflected in terms of the misorientation distribution. The misorientation distribution (correlated) plots for the samples after various passes following both routes are shown in Figure 4. All these figures show a strong peak in the low-angle regime and a diffused population of peaks at higher angles. In order to substantiate the results on the misorientation distribution, the grainboundary character distribution (GBCD) was also plotted (Figure 5) for both routes. For this, the boundaries have been classified into the following three categories: (1) low-angle grain boundaries (LAGBs) with a misorientation angle less than 15 deg, (2) highangle grain boundaries (HAGBs) with a misorientation greater than 15 deg, and (3) special or coincident-site Fig. 4 Misorientation angle distribution for both the routes after first, second, and fourth passes VOLUME 40A, NOVEMBER 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A

5 Fig. 5 GBCD for routes (a) A and (b) B C. Fig. 6 Misorientation distribution plotted against cumulative probability of grains having them for route (a) A and (b) B C. The dashed line represents the random Mackenzie distribution. lattice (CSL) boundaries, which are again HAGBs. In the case of route A, the fraction of high-angle random boundaries (HAGBs) increases with the decrease in LAGBs from the first to the fourth pass. The CSL boundaries remain almost constant during ECAE by this route. However, in the case of the B C route, there is a decrease in the random HAGBs up to the fourth pass and the LAGBs increase up to the second pass and then decrease somewhat after the fourth pass. However, it can be seen that the fraction of CSL boundaries increases with the number of passes to nearly double compared to route A. Although an increase in LAGBs is observed in the case of route B C, the total fraction of the CSL plus random HAGBs remained high as compared to the LAGBs in both cases after four passes. To carry out an elaborate analysis of the misorientation angle distribution, the cumulative frequency of the uncorrelated misorientation angle distribution was plotted (Figure 6). The dashed line in the figure represents the Mackenzie distribution, the uncorrelated misorientation angle distribution for randomly oriented grains. A distinct deviation in the statistical grain orientation from the classical Mackenzie distribution was observed for both routes. This is a characteristic of a textured grain structure. [36] After the first pass, the curve exhibits an almost horizontal line in the range of 15 to 30 deg, indicating a very low fraction of boundaries in the intermediate misorientation range. With the increase in METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 40A, NOVEMBER

6 needed for route-b C -processed materials. In this case, a much higher fraction of R3 and R13 boundaries (over their normal fraction in a random polycrystalline material) is clearly discernible. For route A, a comparatively high share of R29 boundaries is seen. An elaborate analysis was, therefore, carried out to understand the phenomenon. The number fraction of grains as a function of the deviation from the ideal coincidence angle value was plotted (Figure 9), as per Brandon s criterion, for the cases of the R3 and R13 boundaries. This analysis was carried out for the results obtained from the samples processed by route B C,because these yielded a higher proportion of CSLs. The maximum value of Dh c for R3, h111i/60deg,is8.66degand that for the R13 is 4.16 deg. It is evident from Figure 9(a) that in most of the cases, there is no significant deviation from the ideal coincidence angle for R3 boundaries. The distribution of R13a shows a monotonic increasing trend with an increasing deviation angle, while that for R13b exhibits a Gaussian-type distribution with a peak at approximately 2 deg. It is interesting that these observations were consistent for all the passes. Fig. 7 Misorientation plot normalized to the scaling factor h/h av drawn for routes (a) A and (b) B C. the number of passes, the shape of the curve changes and becomes more like what is expected for the Mackenzie distribution for both the cases; however, there remained a distinct deviation. This systematic trend was noticed for both the routes. However, the deviation in the case of route B C is more due to the presence of the high fraction of LAGBs. In the plot of Figure 7, in which the misorientation angles normalized to their average value for every pass plotted against the probability frequency distribution of grains, a similar trend was observed for both routes A and B C independent of the imposed strain (pass number). Referring to Figure 5 again, the fractions of CSL boundaries were, however, different for both the routes. The fraction of CSL boundaries in the microstructure of the route-b C -processed material was almost double that for route A, for any given ECAE pass. The spectrum of CSL boundaries was separately plotted and the types of these boundaries were identified. The same is presented in Figure 8. It can be seen from the figure that a similar empirical distribution existed for both the routes. A special mention in this regard is B. Texture Figure 10 shows the experimental (110) pole figures obtained after the first, second, and fourth passes, following the two routes of ECAE. For the reader s understanding, a key pole figure has been given in Figure 11. It is to be noted that in the present investigation, by virtue of a different specimen frame of reference, the texture components were located at different positions in the Euler s space; they yielded the same shear plane and direction obtained by Li et al. [30] In order to avoid confusion, they have been designated differently. It is to be mentioned here that the disposition of these components in the pole figure can be obtained by a rotation of simple shear texture components through an angle of 45 deg, which is generally considered the shear plane. [37] The components in the pole figure are rotated in an anticlockwise direction by 45 deg, the rotation needed to bring the two reference systems, namely ND-ED and SPN-SD, in exact coincidence. The experimental pole figure of the material subjected to one pass of ECAE shows the presence of partial h111i h and {110} h fibers. A gradual reduction in the intensity of the texture components (in terms of multiples of random distribution) occurred after four passes. The extent of weakening was more for route B C than for route A. In order to carry out a quantitative texture analysis, ODFs were calculated. Figure 12 shows the u 2 = 0 and 45 deg sections of the experimental ODFs after the first, second, and fourth passes for route A, while Figure 13 displays the same for route B C. A key figure depicting the positions of the ideal components for the u 2 = 0 and 45 deg sections is also given along with the ODFs. Table II provides the locations of the ideal orientations of the texture components in the orientation space and the corresponding Miller indices of the ND, ED, and TD. The respective equivalent component on the SPN-SD reference system as obtained by a counterclockwise rotation about the TD axis is also 2734 VOLUME 40A, NOVEMBER 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A

7 Fig. 8 Frequency distribution of CSL boundaries obtained for route A after (a) pass 1, (b) pass 2, (c) pass 3, and (d) pass 4, and also for route B C after (e) pass 1, (f) pass 2, (g) pass 3, and (h) pass 4. given. While the Miller indices of the two reference axes TD and ND are obtained directly from the LaboTex software, the third axis was obtained by a simple cross product of the two. The relevant ODF sections of the first-pass ECAE-deformed sample is shown in Figure 12(a). When compared with the key figure, the ODF clearly reveals the presence of D ah components in both the u 2 sections at almost the exact location. Most of these components show a significant amount of spread of approximately 10 deg from the highest intensity peak. The component D bh is also discernible in the u 2 = 0 deg section, however, with a prominent shift. The J h component was observed in the u 2 =45deg section with a small shift toward the negative u 1 along a / = 45 deg line. Another component, F h, which appears in both the u 2 sections was also detected, however, with a shift of approximately 15 deg from the ideal location to the positive u 1 direction, along the /-axis. A more careful observation revealed the presence of two other new components that have been designated by us as R ah and R bh. The former one was detected with a slight deviation from the location in which it is exactly expected to be present in the u 2 = 45 deg section, as per the combination of the shear plane and SD. The ideal locations of both the R ah and R bh components are included in Table II. The intensity of the R ah component was quite high and has a considerable spread. The R bh component also exhibits a strong intensity peak. Figures 12(b) and (c) display the ODFs of samples subjected to further ECAE passes through route A. The ODF clearly reveals the existence of only the D ah component after the second pass with some degree of spread, which kept continuing to increase until the end of the fourth pass. After the fourth pass, the maximum intensity of the ODF was the least, compared to all other passes. In the case of route B C, Figures 13(b) and (c) reveal the presence of a prominent D bh component after the second pass. A gradual weakening of the overall texture intensity was noticed in the samples processed by route B C, leading to a relatively weaker texture after the fourth pass. On comparing the maximum intensity of the ODFs of both the routes, a greater METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 40A, NOVEMBER

8 Fig. 9 Number fraction of grains plotted against the deviation from ideal coincidence value for different CSL boundaries: (a) R3, (b) R13a, and (c) R13b, obtained in route B C. degree of weakening of the texture is observed in route B C compared to route A, after any given number of passes. IV. DISCUSSION Microstructural characterization of the ECAEprocessed materials showed a distinct pattern of evolution of the grain-boundary character as well as the crystallographic texture, in addition to the expected grain refinement, which depended on the processing route. The relevant results are discussed under the following subheadings. A. Grain Refinement and Evolution of GBCD As stated in Section III A, substantial grain refinement was achieved in the first pass itself. In the second pass, the grains were refined to submicron size, which was almost the saturation level in the grain refinement. Further microstructural changes took place in the form of an alteration/modification in the GBCD. The saturation in the grain refinement can be understood from the following discussion. It is known that during a single ECAE pass, the material undergoes all the stages of work-hardening behavior. However, stage III sets in even before the completion of one pass (it is to be noted that the total deformation in one pass is equivalent to a strain of ~115 pct). As the ECAE process starts, the prior dislocations present in the material or the ones formed due to the operation of Frank Reed or other sources begin to move along the SD (easy glide). At relatively low strains, as the dislocations are generated in profuse amounts, they get aligned in a low-energy configuration, forming either incidental dislocation boundaries or geometrically necessary boundaries. [38,39] This is aided by the thermal energy due to warm deformation during the present experiments. In Figure 5(a), i.e., in route A, 2736 VOLUME 40A, NOVEMBER 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A

9 Fig. 10 Experimental pole figures obtained for (a) pass 1 (n = 1) and other passes (n = 2, 3, 4) from routes (b) A and (c) B C, shown with maximum of random distribution. the increase in HAGB and decrease in LAGB fractions, with no further decrease in the average grain size, indicates that after a critical strain, the annihilation of dislocations occurs by dynamic recovery. On the other hand, for route B C, with the increase in the number of passes, the random HAGBs decrease while the fraction of CSL boundaries increases. The LAGBs increase up to the second pass and thereafter slightly decrease after the fourth pass. An increase in the overall HAGBs (random HAGB + CSL) from the second to fourth pass, with a decrease in LAGBs without a decrease in grain size, shows that even in this case, during the fourth pass, dynamic recovery sets in. However, from the first to the second pass in route B C, a decrease in the overall HAGBs with an increase in LAGBs was observed. The constant fluctuation observed in the GBCD in the route-b C -processed samples with ECAE passes could be due to orthogonal changes in the strain path in route B C, which may lead to the frequent intersection of planes containing dislocations, leading in turn to an increased tendency toward dislocation interaction, causing either their annihilation or multiplication. [40] A decrease in the grain size from the first to the second pass in route B C was observed along with a proportional increase in the fraction of HAGBs, but an increase in LAGBs was disproportionately larger. The cumulative frequency of the uncorrelated misorientation angle distribution (Figure 6) shows that in both the routes, an increase in HAGBs and decrease in LAGBs with the increase in the number of passes occurred. This led to a distribution that is more like the Mackenzie plot. The deviation from the Mackenzie curve is more pronounced in the case of route B C (Figure 5(b)), due to an increase in LAGBs (correlated)). An increase in the CSL boundaries in route B C could be related to orthogonal changes in the strain path in the presence of high stored (strain) energy, due to the pre-ecae pass and temperature. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 40A, NOVEMBER

10 It should also be noted that with the increasing number of ECAE passes and, hence, the applied strains, more and more dislocations are pumped into the materials. Therefore, the mean-free path of the dislocations gets reduced before the dislocations get trapped in low-angle boundaries. It is quite possible, then, that the addition of new dislocations would increase the misorientation angle across the already-formed LAGB instead Fig. 11 Ideal (110) pole figure after first pass of ECAE obtained by Li et al. [27] of creating new boundaries that may lead to further grain refinement. This observation has been corroborated by X-ray line profile analysis studies of this material, which are reported elsewhere. [41] The type of discrete distribution of misorientations shown in Figure 4, over low and high angles, could be due to two possible reasons. [38,42] First, a dislocationmediated mechanism could give rise to a peak at a lower range of angles, typically less than 10 to 15 deg, while the peak at higher misorientation values could be attributed to the evolution of the texture. The importance of texture in the formation of HAGBs is well known, in general. [39] Due to the ECAE, the various parts of an already subdivided grain are subjected to different operative slip systems that rotate differently toward their respective end orientations. These may differ by very large misorientations, leading to the formation of HAGBs. The trend in misorientation development, as shown in Figure 7, corroborates earlier studies on the empirical misorientation distribution. [43 45] It is found to be an invariant of the choice of strain, material, temperature, and other deformation conditions. The only difference observed in the distribution obtained in our experiments is the absence of data points at low misorientation angles. This is largely due to the limitation of SEM- EBSD, as compared to transmission electron microscopy, to index points having a mutual misorientation angle less than 1 deg and, hence, discarding many boundaries below this cutoff limit. Fig. 12 The experimental ODFs calculated for passes, n = 1, 2, and 4 in route A showing two u 2 sections of (a) 0 deg and (b) 45 deg, along with key figures with the maximum intensity values of the ODFs given within the braces VOLUME 40A, NOVEMBER 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A

11 Fig. 13 The experimental ODFs calculated for passes n = 1, 2, and 4 in route B C, showing two u 2 sections of (a) 0 deg and (b) 45 deg, along with key figures with the maximum intensity values of the ODFs given within the braces. Table II. Positions of Ideal Orientations of All Texture Components for a Bcc (IF Steel) Material Following First Pass of ECAE Euler Angles Miller Indices Component * D 1h * D 2h * J h * F h ** R 1h ** R 2h *Ref. 38. **This work. / 1 / / 2 ND TD ED 9.74/ [011] / [110] / [011] / [110] /225/ [111] / [110] / / [112] / ½531Š [112] (Shear Plane) [SD] ½11 1Þ ½001Š B. Origin of CSL Boundaries For some special misorientation angles and axes, there occurs a high coincidence of a large fraction of lattice points from two misoriented single crystals. [46] It is well known [47] that the coincidence value, R, is the reciprocal of the fraction of lattice points in each crystal that are in good coincidence with each other. Thus, the smaller the value of R, the higher the fraction of atoms in a good match. However, there is a maximum in the deviation angle, Dh c, that can be accommodated in a R boundary, according to Brandon s criterion, [48] as Dh c = 15 deg (R) ½. A grain boundary is regarded as random if Dh > Dh c and ordered (coincident) if Dh Dh c. When two adjacent crystals are deviated by the Dh misorientation, in order to conserve the good fit and low energy of the boundary, a number of secondary dislocations are introduced, forming a sub-boundary. Thus, a CSL boundary is essentially comprised of the preliminary coincidence geometry and a dislocation substructure. These boundaries have somewhat higher misorientation values but are of low energy. Generally, special boundaries are observed in iron and its alloys are subjected to cold working and full annealing. [49 51] However, the presence of such boundaries has been METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 40A, NOVEMBER

12 reported recently by Saha and Ray [52] during the severe cold rolling of IF steels. The present investigation provides additional evidence of this, however, during a different mode of deformation. As seen in Figure 8, a similar empirical distribution is observed in both the routes; however, the corresponding percentage of the CSL boundaries is higher in route A than in route B c. A high percentage of these CSL boundaries have been reported earlier in iron alloys and steels. [49,52] The profile of the CSL boundaries is greatly influenced by the induction of the driving force for migration, which may be provided by the grain rotation that occurs during deformation or by heat treatment. [53] This drives the grain boundaries to tend toward equilibrium. Owing to the high coincidence of atoms, the R3 boundary has low energy and mobility. In the present experiment, the combined effect of the severe deformation and heat treatment triggers the driving force for grain rotation, which in turn may cause the formation of R3 boundaries during ECAE. In an early work, Vitek [54] had proposed a spatial model of polycrystal built of compact polyhedral groups of atoms, with the separation of the atoms in the cluster corresponding to first- and second-nearest-neighbors in a bcc crystal structure. Using this model, he concluded that the symmetry in the grain-boundary structure is inherently higher in bcc than in fcc and the existence of coincidence atoms in boundaries is much more frequent. He also showed that R13 boundaries in bcc have one of the lowest energy values and that they may occur when the material tries to minimize its energy once deformed. 1. Possible role of texture Texture could also play an important role in the formation of particular types of CSL boundaries. [55 57] It has been suggested that R3 boundaries are found to be associated with a strong h110i and h111i fiber, while R13 boundaries are associated with a h100i- and h111i-type fiber texture. In the present investigation, the ECAEprocessed samples exhibited a strong h111i-type fiber texture just after the first pass, which would favor the formation of both R3andR13 boundaries. However, of the two possible types of R13 boundaries, namely R13a having a coincidence angle of deg about the h100i axis and R13b with a coincidence angle of deg about h111i; the fraction of the latter was much higher. This result may also be explained by the presence of predominant texture components corresponding to a h111i fiber axis. Thus, energetically and also from the perspective of texture, the presence of CSL boundaries, in particular, R3 and R13, could be understood, although the reason for the higher fraction of them in route B C over route A is still obscure. 2. CSL distribution The distribution of deviation from the ideal coincidence angle Dh for the R3 and the R13 boundaries shows a distinct behavior (Figure 9). A peak at the low value of the Dh for R3 indicates that most of the grains with such a boundary are having an almost perfect fit, with not much deviation from the ideal angle of coincidence. For all the passes, the distribution of R13a and R13b boundaries shows different trends. The R13a displays a monotonic increasing trend with an increasing deviation angle. From this observation, it can be speculated that these types of boundaries are liable to get converted to high-angle boundaries by the absorption of more dislocations with rising strains, when the Dh value exceeds the maximum deviation angle, Dh c. On the other hand, R13b boundaries exhibit a Gaussian-type distribution, with the maximum number of grains showing a deviation around a particular angle, which is approximately 2 deg. As has been stated earlier, the CSL boundary is comprised of a dislocation substructure, with the spacing between the dislocations being inversely proportional to the misorientation angle between the grains. On the other hand, in the case of R13b (Figure 9(c)), the peak at a particular value denotes that most of the grains having this type of boundary have a particular value of the dislocation spacing that corresponds to the deviation at which the peak occurs. C. Texture Evolution The texture of the ECAE-processed samples closely resembles that of simple shear. It is to be noted that the pole figure presented in Figure 10 was derived for a die having an interchannel angle of 90 deg with sharp inner and outer corners. The die used in our experiments had the same geometry; however, the specimen frame of reference was different. Therefore, the ideal pole figure of Li et al. [30] has been rotated about the TD axis by 90 deg, to match our frame of reference. A careful analysis of the (110) pole figure for the first-pass ECAEdeformed material shows the presence of h111i h fiber aligned at 45 deg with respect to the TD axis through a counterclockwise rotation. This indicates negative simple shear [58] taking place on the macroscopic shear plane. Some component belonging to the {110} h fiber can also be observed; however, the exact components cannot be identified. The pole figures from route A show a monoclinic sample symmetry, while no such symmetry is observed in route B C. This is because of the nature of the deformation imposed during the two routes. While in route A, the texture components obtained after a certain pass are related to those in its successive pass by monoclinic symmetry with TD as the dyad axis; no such symmetry is observed in route B C. As presented in Table II, the ED-ND axes for both the components, D ah and D bh, gives rise to a {112} h111i-type texture along the shear plane and the SD, respectively, at an angle of 45 deg away from the ND in an anticlockwise sense. In the case of the J h and F h components, the {110} planes get aligned along the shear plane, while the h112i- and h001i-type directions, respectively, are aligned along the direction of shear. Most of these components in the texture of the ECAEprocessed IF steel in our experiments have been in agreement with the investigation of Li et al. [29,30] This component has been found to lie more or less at its expected position. The two new components can be obtained by the rotation of the ED-ND axes by ~45 deg about the TD axis; that brings {123} planes and h412i 2740 VOLUME 40A, NOVEMBER 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A

13 directions parallel to the macroscopic shear plane and direction, respectively. It is well known that during any deformation, the rotation of grains or parts of a grain occurs in a microscopic scale in such a way that the crystallographically favorable glide planes and directions in a crystal are oriented along the macroscopic deformation plane and direction. A close examination of all the identified texture components after the first pass of ECAE indicates that either the closest-packed plane or the direction or both are oriented along the shear plane (i.e., the plane of the intersection of the two channels in the die) and direction (Figure 2). For bcc materials, the slip planes consists of {110}-, {112}-, and {123}-type planes. Previously, Li et al. reported that only {110} and {112} planes lie along the shear plane. However, in the present investigation, in addition to these two set of planes, the planes of type {123}, which is the third-closest-packed plane for the bcc structure, also gets aligned along the shear plane. This difference could be due to the fact that in the present experiments, the ECAE was carried out at a higher temperature (300 ± 10 C), while in the work of Li et al., the ECAE of the IF steel was done at room temperature. This indicates that the increase in the temperature of deformation contributed to the slip that occurred on the {123} planes alongside the {110} and {112} planes, which get operative even at room temperature. V. CONCLUSIONS The ECAE was carried out using a Ti-bearing IF steel until four passes following two important routes: A and B C. Based on observations pertaining to the development of the microstructure and texture, the following conclusions could be drawn. 1. The microstructure of the ECAE-deformed IF steel was composed of grains of a submicron size elongated along the macroscopic shear plane and direction. The grain size attained saturation after the second pass. 2. The GBCD analysis revealed the presence of lowangle boundaries, high-angle random boundaries, and special (CSL) boundaries, their proportion being dependent on process variables such as the processing routes, amount of strain, etc. 3. A major fraction of the boundaries in all the cases was observed to be high-angle boundaries. A lesser fraction of low-angle boundaries was also present. A significant degree of deviation in the misorientation angle distribution between routes A and B C was observed, due to orthogonal changes in the strain path in case of route B C. 4. A notable amount of special (CSL) boundaries was found in the microstructure obtained after both the routes of ECAE, with route B C showing the higher fraction of the two, increasing with the amount of deformation. Among the special boundaries, the fraction of R3andR13 boundaries appeared to be high. Each of these showed a distinct trend in the plots of deviation from the ideal coincidence angle vs the number fraction of the grains. 5. The texture obtained after the first pass had a close resemblance to simple shear texture. It was observed that either the crystallographic slip plane or direction or both coincided with the macroscopic shear plane and direction. The activation of slip in the {123} plane was also observed along with the {110}- and {112}-type planes, which are the most common slip planes in bcc materials. ACKNOWLEDGMENTS The authors duly acknowledge Tata Steel (Jamshedpur, India) for providing the financial grant (Grant No /102) and the material to carry out the experiments. One of the authors (AB) gratefully acknowledges the prolific discussions with Prof. T. Watanabe (Retd. Professor, Tohuku University, Sendai, Japan) during the preparation of this article. 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