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The application of equal-channel angular pressing to an aluminum single crystal
Acta Materialia 52 (2004) 1387–1395 www.actamat-journals.com
The application of equal-channel angular pressing to an aluminum single crystal Yukihide Fukuda a, Keiichiro Oh-ishi a, Minoru Furukawa b, Zenji Horita a, Terence G. Langdon c,* a
c
Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 812-8581, Japan b Department of Technology, Fukuoka University of Education, Munakata, Fukuoka 811-4192, Japan Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA Received 15 October 2003; accepted 25 November 2003
Abstract An investigation was conducted to examine the nature of the deformed microstructure when an aluminum single crystal of known orientation is subjected to equal-channel angular pressing (ECAP). The experiment was performed using a single crystal that was initially oriented within the entrance channel of the die so that the (1 1 1) slip plane was parallel to the theoretical shear plane and the [1 1 0] slip direction lay parallel to the direction of shear. The crystal was subjected to a single pass at room temperature and then examined using various microscopic techniques including orientation imaging microscopy and transmission electron microscopy. It is shown that the detailed experimental observations are fully consistent with the expectations from crystallographic considerations except only in the vicinity of the lower die wall where frictional effects are present. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Aluminum; Equal-channel angular pressing; Severe plastic deformation; Single crystals; Texture
1. Introduction The processing of metals through the application of severe plastic deformation (SPD) has attracted much attention within the last decade primarily because the use of SPD procedures provides the potential for achieving very significant grain refinement, typically to the submicrometer or nanometer range, in bulk polycrystalline samples [1,2]. In practice, the most versatile SPD technique appears to be equal-channel angular pressing (ECAP) in which a sample is machined to fit within a channel contained within a die and it is then pressed through the die until it emerges at the exit point [3,4]. Since the cross-section of the sample remains unchanged in ECAP, it is easy to perform repetitive pressings of the same sample in order to impose very high total strains. In practice, it was recognized in early *
Corresponding author. Tel.: +1-213-740-0491; fax: +1-213-7408071. E-mail address: [email protected] (T.G. Langdon).
work that different shearing systems may be activated during ECAP by rotating the sample between each individual pass [3] and it was shown more recently that, by varying the processing route and the total imposed strain, it is possible to obtain samples having different grain boundary character distributions ranging from a high fraction of low-angle boundaries to a high-fraction of high-angle boundaries [5]. Processing through the use of ECAP is especially attractive for two reasons. First, the process may be readily scaled up for the production of fairly large bulk samples [6]. Second, the concept of ECAP processing can be incorporated easily into conventional industrial rolling operations as in the conshearing [7] or the continuous confined strip shearing [8] processes. Numerous experiments have been conducted to date on a very wide range of materials in order to evaluate the microstructures introduced into polycrystalline specimens when they are subjected to processing by ECAP. In practice, much of this work has concentrated on the potential for achieving a submicrometer or
1359-6454/$30.00 Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2003.11.028
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nanometer grain size in the as-processed condition. Careful experiments on samples of polycrystalline aluminum have demonstrated there is excellent agreement between the shearing characteristics and shearing patterns predicted theoretically [9,10] and the microstructures observed experimentally at both the microscopic [11] and the macroscopic [12] levels. Although a very extensive set of experimental data is now available for materials processed by ECAP, these results relate almost exclusively to tests conducted using polycrystalline samples. By contrast, no fundamental investigation has been undertaken to date to examine the deformation structures introduced when ECAP is applied to single crystals having well-defined and specific initial orientations. Thus, the present investigation was initiated specifically to address this deficiency. It is important to note that the use of single crystals provides three significant advantages in any fundamental studies of the ECAP process. First, it is possible to select the initial crystallographic orientation of the sample with respect to the shearing plane in the ECAP die. Second, no adverse artifacts are introduced into the as-pressed deformation structures as a consequence of the presence of adjacent grain boundaries. Third, the use of single crystals provides a unique opportunity to attain a direct evaluation of the degree of homogeneity that is developed in the as-pressed microstructure due to ECAP processing and especially to examine the significance of any inhomogeneities that may arise due to frictional effects in the vicinities of the die walls. This latter evaluation may be readily undertaken by examining the aspressed deformation microstructures both in the central area of the sample and in the regions immediately adjacent to the die walls. In the present study, the experiments were conducted using a single crystal of high-purity aluminum having a well-defined initial orientation with respect to the shearing plane, where pure aluminum was selected to provide a direct comparison with the extensive earlier data reported for the ECAP of polycrystalline aluminum [11,13]. As will be demonstrated, the deformation structure which develops in the central area of the single crystal by ECAP is fully consistent, at least in terms of the slip planes and slip directions, with the expectations arising from simple crystallographic considerations that incorporate the development of texture in ECAP processing.
orientation of the single crystal was determined using the Laue back-reflection method. An ECAP sample was cut from the rod to the desired orientation using an electric-spark discharge facility. This sample had a square cross-section with dimensions of 4 4 mm2 and a total length of 30 mm. Prior to processing by ECAP, the four longer faces of the sample were prepared to a mirror-like finish by grinding on abrasive paper and then electro-polishing. Fig. 1 depicts a section through the vertical ECAP die and illustrates the orientation selected for use in this investigation. Using the conventional terminology adopted for ECAP [4], this schematic illustration shows an ECAP die in which the angle U between the two parts of the channel is 90° and, for simplicity in the illustration, the two parts of the channel meet at an abrupt corner so that the angle representing the outer arc of curvature where the two parts of the channel intersect is given by W ¼ 0°. In practice, the experimental facility used in this investigation had a channel angle of U ¼ 90° and an outer arc of curvature given by W 30°. It is important to note that, as shown in model experiments [14] and in experiments on polycrystalline samples [15], the use of ECAP dies where W 20° to 30° leads to no significant deformation inhomogeneities in ECAP processing because in practice there is a ‘‘dead zone’’ in the vicinity of this outer corner where the sample is no longer in contact with the die wall [16–18]. Three orthogonal directions are defined in Fig. 1, where X is the transverse plane perpendicular to the direction of flow, Y is the flow plane parallel to the side face at the point of exit from the die and Z is the longitudinal plane parallel to the top face at the point of exit, respectively. The two solid arrows at the top and on the right-hand edge of Fig. 1 denote the direction of pressing and therefore the direction of the sample as it
2. Experimental material and procedures The experiments were conducted using a single crystal of high-purity (99.999%) aluminum grown from the melt using the vertical Bridgeman method. This crystal was in the form of a rod with a diameter of 20 mm and a length of 70 mm. The initial crystallographic
Fig. 1. Schematic illustration of a cross-section through the ECAP die showing the three orthogonal axes and the initial orientation of the 0° specimen.
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passes through the die. The theoretical shear plane lies at 45° to the X-direction at the intersection of the two parts of the channel. Since slip occurs in face-centered cubic metals on the f1 1 1gh1 1 0i system, the single crystal was oriented so that the (1 1 1) slip plane lay parallel to the theoretical shear plane, the [1 1 0] slip direction lay parallel to the direction of shear and the Ydirection therefore corresponds to the [1 1 2] direction. For simplicity, this orientation, in which the slip direction is coincident with the shearing direction, is henceforth designated the 0° orientation. In order to deduce the magnitudes of the resolved shear stresses associated with this orientation, Fig. 2 shows a schematic illustration of the relationship between the slip plane and the slip direction in the single crystal and the shear plane and the shear direction in the ECAP die. Noting that the specimen is pressed through the die in a vertical sense as indicated by the upper vertical arrow in Fig. 2, and defining F as the force in the shear direction and A as the area of the shear plane, it follows that the shear stress, s, operating on the slip plane in the slip direction is given by a relationship of the following type for each possible slip system: F s¼ cos h cos k; ð1Þ A where h is the angle subtended between the two normals to the shear and the slip planes and k is the angle between the shear direction and the slip direction, respectively. The magnitude of the angular term fcos h cos kg thus represents the shear factor and, when considering all possible slip systems, this term will have values lying within the range from 0 to 1. It follows, therefore, that the preferential slip system may be predicted by esti-
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mating the values of the shear factors for all possible slip systems and then selecting the system for the shear factor having the highest value. In order to undertake this calculation, Fig. 3 shows the relationship between the crystallographic orientation and the vertical pressing direction for the 0° specimen where the three orthogonal X, Y and Z axes are depicted in the small illustration at the lower right. Using these stereographic projections, it follows that it is possible to construct a tabulation of values for the various shear factors with respect to different slip systems as given in Table 1 where the shear factors are
Fig. 3. Stereographic projection illustrating the relationship between the crystallographic orientation and the vertical pressing direction for the 0° specimen.
Table 1 Estimations of the shear factors, and their relevant importance, for various slip systems in the 0° specimen
Fig. 2. Schematic illustration of the procedure for estimating the magnitudes of the resolved shear stresses for different slip systems through the use of Eq. (1).
Slip plane
Slip direction
0° specimen Shear factor
Order of shear factor
1 1 1
1 1 0 1 0 1 0 1 1
1.00 0.50 0.50
(1) (2) (2)
1 1 1
1 0 1
1 0 1 1 01
0.00 0.17 0.17
(11) (5) (5)
1 1 1
1 1 0 1 0 1 011
0.00 0.17 0.17
(11) (5) (5)
1 1 1
1 1 0 011 101
0.33 0.17 0.17
(4) (5) (5)
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listed in the third column and the numbers in parentheses in the fourth column denote the ordering of these values in terms of their overall magnitudes. It is apparent that the primary slip system is ð 1 1 1Þ½1 1 0 which is listed as (1) in the fourth column of Table 1 and the secondary slip systems are ð 1 1 1Þ½ 1 0 1 and ð1 1 1Þ½0 1 1 where both systems have the same shear factors and they are listed as (2) in Table 1. Processing by ECAP was performed at room temperature using a split die fabricated from tool steel and with the single crystal coated in an MoS2 lubricant prior to pressing. It can be shown from first principles that the strain imposed in a single pass through a die having internal angles of U ¼ 90° and W 30° is 1 [19]. In the present investigation, the single crystal was pressed partially through the die in a single pass and the pressing was terminated when the crystal was essentially at the mid-point of the die. The ECAP die was then opened and the sample carefully removed and sectioned longitudinally on the Y-plane into two approximately equal pieces. One of these pieces was used for observations on the Y-plane using optical microscopy (OM), scanning electron microscopy (SEM) and orientation imaging microscopy (OIM) and the second piece was further cut on the Y-plane into two separate sections for examination on the Y-planes using transmission electron microscopy (TEM): it is important to note that all observations, and all photomicrographs in this report, relate to the Y-plane as depicted in Fig. 1 where the X-direction points horizontally towards the right. For inspection by OM, the surface was prepared using abrasive paper followed by an alumina paste and then electro-polishing using a solution of 10% HClO4 , 20% C3 H5 (OH)3 and 70% C2 H5 OH. The crystal surface was etched using a solution of 5% HBF4 in water. A JSM5600 scanning electron microscope equipped with an OIM capability was used for SEM and OIM. For OIM, the image area was set at 150 150 lm2 and individual measurements were taken periodically at incremental steps of 1.5 lm. A JEM-2000FX transmission electron microscope was used for the TEM observations and selected area electron diffraction (SAED) patterns were recorded using a beam diameter of 12.3 lm.
3. Experimental results 3.1. Observations by OM and OIM The 0° specimen was pressed to the mid-point of the die, removed for inspection, and the upper portion of Fig. 4 shows the section on the Y-plane where the unsheared portion lies vertically on the left, the sheared portion lies horizontally on the right and the arc of curvature within the die of W 30° is clearly revealed in the corner at the lower left. The microstructures on the
Fig. 4. The 0° specimen after pressing approximately half-way through the ECAP die: the four optical photomicrographs were taken at the points labeled 1–4 on the cross-section of the specimen.
polished surface were examined at various positions using OM and Table 2 provides a detailed summary of the experimental observations associated with the four selected positions labeled 1–4 in the upper photomicrograph where the relevant images for these positions are shown in the lower portion of Fig. 4. All of the characteristics summarized in Table 2 are visible through close inspection of the optical micrographs in Fig. 4 where attention is especially directed to the angular relationships for the various sets of striations recorded in Table 2. It is apparent that deformation bands develop close to the shear plane in the upper portion of the deformed region at position 1 and there are unusual sharp striations in the deformed portion in the vicinity of the lower die wall at position 3. It is reasonable to conclude that the different shearing characteristics occurring at position 3 are due to the influence of frictional effects in the vicinity of the die wall. The corresponding OIM images and the (1 1 1) pole figures are shown in Fig. 5 where the images labeled 1–4 refer to the same positions labeled 1–4 in Fig. 4 and each microstructure is shown in color where the colors relate to the crystallographic orientations depicted in the stereographic triangle on the left. Inspection of the relevant pole figure shows that at position 1, located close to the shear plane, the structure remains a single crystal but the
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Table 2 Shearing characteristics in the 0° specimen at the positions labeled 1–4 in Fig. 4 Position
Distance from top Approximate location surface (mm)
Shearing characteristics:all angles are measured with respect to the horizontal X-direction
1
0.7
Close to shear plane
2 3
2 3
Central portion of deformed crystal In vicinity of bottom surface
4
2.2
Within deformed region and 2 mm beyond position 2
Long horizontal striations or deformation bands at low magnifications; additional thin striations at 30° and 60° at high magnifications Prominent striations at 20°; very thin striations at 50° Very sharp striations at 15°; these sharp striations contain thin striations at 50° Very prominent striations or shear bands at 65°; additional thick striations at 20°
Fig. 5. Images taken using orientation imaging microscopy and the corresponding (1 1 1) pole figures for the points labeled 1–4 in Fig. 4, where the colors relate to the crystallographic orientations shown in the stereographic triangle on the left.
crystallographic orientation has been rotated by 60° around the Y-axis where this is equivalent to a counterclockwise rotation around the [1 1 2] direction. At position 2 in the deformed region, the pink colored band lying at 20° to the X-axis corresponds to the striations visible in Fig. 4 and the (1 1 1) pole figure reveals two distinct orientations where there is a strong reflection representing an initial rotation of 60° around the Y-axis and a weak reflection corresponding to the initial orientation of the single crystal: this initial orientation is shown in light purple in the OIM image. At position 3 near the lower die wall, the (1 1 1) pole figure shows the presence of two crystallographic orientations given by the initial orientation and a rotation by 60° around the Y-axis: these are separately revealed as the purple and blue bands in the OIM image. The same two crystallographic orientations are also present in area 4 where it becomes apparent that the shear bands visible in Fig. 4, as documented in Table 2, correspond to the initial orientation of the crystal and the remaining area corresponds to a counter-clockwise rotation by 60° around the Y-axis. To evaluate the precise nature of the deformation introduced by shearing as the single crystal passes
through the shear plane, an additional detailed examination was conducted in the vicinity of the shear plane at the positions labeled from 1 to 10 in Fig. 6: the corresponding pole figures for each of these points are also shown in the lower portion of Fig. 6. It is immediately apparent from inspection of these two rows of pole figures that they have been placed so that the upper and lower sets are reasonably similar: thus, the pole figures labeled 1 and 6 are both in the initial orientation, 2 and 7 show evidence for some scattering, 3 and 8 reveal a distinct elongation in a counter-clockwise sense, 4 and 9 depict two poles, and 5 and 10 reveal the presence of both the initial orientation and the orientation corresponding to a counter-clockwise rotation by 60° around the Y-axis. It can be shown by careful analysis that there are slight differences between the two pole figures at positions 5 and 10: thus, there are two well-defined orientations at position 5 which is well-removed from the shear plane but at position 10, which is located within the ill-defined area of shearing associated with the arc of curvature of the die at the lower left, the two orientations correspond to the initial orientation and a rotation by an initial increment of only 50° around the Y-axis.
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Fig. 6. Pole figures associated with the points labeled 1–10 in the upper cross-section: the pole figures are arranged in two rows so that the upper and lower sets are reasonably similar.
3.2. Observations by TEM The deformed part of the sample was inspected by TEM with the observations taken on the Y-plane at points remote from the shear plane and generally close to the center of the sheared material. These inspections revealed two distinct types of microstructure and Fig. 7 shows an example where both of these microstructures, labeled A and B, appear in adjacent and contiguous regions: the corresponding SAED patterns for these two regions are shown on the right in Fig. 7. The upper region A contains well-defined slip traces along a direction at 45° to the X-axis where this direction corresponds to the shear plane in ECAP and the trace of the primary ð 1 1 1Þ slip plane; the SAED pattern for region A corresponds to the initial crystallographic orientation with the Y-direction along [1 1 2] and the shearing direction along ð 1 1 1Þ. By contrast, the lower region B contains slip traces along a direction at 10° with respect to the X-axis and the SAED pattern for this region is then rotated by 60° in a counter-clockwise sense from the SAED pattern shown for region A. The effect of making a counter-clockwise rotation by 60° around the [1 1 2] direction is depicted schematically in the two lower illustrations in Fig. 7 where the initial orientation is on the left and the orientation after a 60° rotation is on the right. It is apparent from this schematic illustration that the slip traces at B now lie fairly close to the trace of the ð 1 1 1Þ slip plane which, after rotation, is inclined at an angle of )7° from the X-axis.
Fig. 7. Photomicrograph taken by TEM in the central portion of the deformed material showing the presence of two distinct types of microstructure labeled A and B: the corresponding SAED patterns are shown on the right and the effect of a counter-clockwise rotation by 60° is depicted schematically in the two lower illustrations.
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In addition, it can be seen from the photomicrograph in Fig. 7 that there is a well-defined border delineating the transition between regions A and B and this border lies along a direction oriented close to 15° from the X-axis. This direction is therefore coincident with the very sharp striations visible at position 3 in Fig. 4 and with the banded structure visible at position 3 in Fig. 5. Measurements using TEM showed the widths of the slip traces were of the order of 1.3 lm in both regions A and B of Fig. 7 and this is consistent with experimental observations on pure polycrystalline Al where the final equiaxed grain size of the as-pressed material was reported as 1.2–1.3 lm [11,12,20]. It is apparent from the SAED patterns pertaining to regions A and B of Fig. 7 that the microstructures in both regions consist of bands of subgrains having boundaries with low-angles of misorientation and this is consistent with the low imposed strain of 1 since it is well-documented for polycrystalline Al [20] and for various Al alloys [21] that high imposed strains are required in order to achieve grain structures with boundaries having high angles of misorientation. Finally, and in contrast to the microstructures widely reported for polycrystalline materials after ECAP [22–24], the photomicrograph in Fig. 7 reveals the presence of only a relatively small number of dislocations in regions A and B either within the welldefined subgrains or along the subgrain boundaries. This observation suggests that dislocation re-arrangement probably occurs more easily during the application of ECAP to single crystal specimens and this is reasonable when it is noted that there are no constraining effects due to the presence of grain boundaries. 4. Discussion The results obtained in this investigation provide a clear demonstration of the potential for using single crystals to conduct fundamental studies of the flow be-
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havior produced by ECAP processing and the consequent deformation structures. In general terms, the TEM photomicrograph shown in Fig. 7 is consistent with the extensive data reported earlier for the development of deformation structures in polycrystalline Al [11,12,20]. By pressing a single crystal with a well-documented initial orientation, designated a 0° specimen, it is shown through examination by optical microscopy in Fig. 4 that the specimen contains different sets of striations in the pressed condition. This initial orientation was selected specifically because the primary (1 1 1) slip plane is coincident with the shear plane and thus at 45° with the X-axis. After pressing, examination shows there is a very wide area within the pressed material where the initial crystallographic orientation is rotated by 60° in a counter-clockwise sense around the Y-axis but there is an area immediately adjacent to the lower edge of the specimen where the local shearing appears to be influenced by frictional effects at the die wall. It is apparent from the TEM observations, as documented in Fig. 7, that the subgrain bands lie essentially parallel to the primary slip planes both before and after any rotation has taken place. Thus, the elongated subgrains are formed so that their longer sides lie on the primary slip planes. This suggests a simple mechanism for substructural development in ECAP whereby glissile dislocations are generated on these planes during pressing and these dislocations then combine to create the subgrain boundaries. It has been shown that the subgrain widths produced in aluminum and Al-based alloys during ECAP processing are essentially identical to the grain sizes produced in conventional cold-working through compression and extrusion [25]. This similarity in size suggests, therefore, that the subgrain widths are essentially dictated by the development of a balance between the generation and recovery processes. It is instructive to consider the nature of the texture introduced into the single crystal by ECAP. Fig. 8(a)
Fig. 8. (a) The (1 1 1) pole figure reported for a sample of pure polycrystalline Al subjected to 12 passes of ECAP processing using route BC [20] and (b) a conventional B-type shear texture [26].
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shows a schematic illustration of the (1 1 1) pole figure reported for a sample of pure polycrystalline Al subjected to 12 passes using processing route BC in which the sample is rotated by 90° in the same sense between each pass [20]: the shear direction is also indicated at upper right. This pole figure shows two kinds of texture with both having a common (1 1 1) plane normal to the Y-axis but with a relative rotation of 60° around the Yaxis. This type of texture corresponds to the conventional B-type rolling texture in cold-rolled face-centered cubic metals in which there are two variants, B1 and B2 , having a common (1 1 0) plane and existing alternately. For comparison purposes, Fig. 8(b) shows a schematic illustration of a conventional B-type shear texture where the (1 1 0) orientation tends to align with the shear direction and RD and ND represent the rolling direction and the normal direction, respectively [26]. A comparison with the pole figures in Fig. 6 shows that Figs. 8(a) and (b) are very similar to the pole figure obtained at position 5 which is located at a point in the deformed material within the exit channel. The development of this type of texture provides a direct explanation for the experimental results obtained in this investigation including the two distinct orientations that are clearly visible through the TEM observations in Fig. 7. Finally, it is apparent that rotation occurs consistently in the deformed structure in a counter-clockwise sense when viewed on the Y-plane, as can be seen in Fig. 6. This sense of rotation is a natural consequence of the construction of the ECAP die and the presence of an outer arc of curvature at the point where the two channels intersect, as can be seen at the lower left corner in Fig. 6. The presence of an arc of curvature leads to the development of shearing within a zone extending through an arc of 30° so that there is an inhomogeneous strain distribution across the width of the specimen: these distributions have been modeled in several studies using finite element analysis [16,18,27– 30]. Furthermore, the presence of a shearing zone leads also to a tensile component of stress within the shear zone and the nature of this stress is such that, as noted in a detailed analysis [31], there will be a counterclockwise rotation around the Y-axis when viewed as depicted in Fig. 1. Thus, the present observations in the bulk of the deformed crystal are fully consistent with the expectations from crystallographic considerations.
5. Summary and conclusions 1. A single crystal of aluminum was subjected to equalchannel angular pressing at room temperature after orienting the crystal so that the (1 1 1) slip plane was parallel to the theoretical shear plane and the
[1 1 0] slip direction lay parallel to the direction of shear. The pressing was terminated in the first pass with the specimen pressed to approximately the mid-point of the die. 2. Detailed microscopic examination of the specimen after pressing revealed the occurrence of shearing patterns that are consistent with crystallographic expectations including the development of a B-type rolling texture. Different shearing patterns were present in the vicinity of the lower die wall where frictional effects are present. 3. The experimental observations suggest a simple mechanism for substructural development in ECAP. Glissile dislocations form on the primary slip plane and combine to create subgrain boundaries and arrays of subgrains. These subgrains are oriented with their longer sides lying on the primary slip plane.
Acknowledgements This work was supported in part by the Light Metals Educational Foundation of Japan and in part by the National Science Foundation of the United States under Grant No. DMR-0243331.
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The application of equal-channel angular pressing to an aluminum single crystal Yukihide Fukuda a, Keiichiro Oh-ishi a, Minoru Furukawa b, Zenji Horita a, Terence G. Langdon c,* a
c
Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 812-8581, Japan b Department of Technology, Fukuoka University of Education, Munakata, Fukuoka 811-4192, Japan Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA Received 15 October 2003; accepted 25 November 2003
Abstract An investigation was conducted to examine the nature of the deformed microstructure when an aluminum single crystal of known orientation is subjected to equal-channel angular pressing (ECAP). The experiment was performed using a single crystal that was initially oriented within the entrance channel of the die so that the (1 1 1) slip plane was parallel to the theoretical shear plane and the [1 1 0] slip direction lay parallel to the direction of shear. The crystal was subjected to a single pass at room temperature and then examined using various microscopic techniques including orientation imaging microscopy and transmission electron microscopy. It is shown that the detailed experimental observations are fully consistent with the expectations from crystallographic considerations except only in the vicinity of the lower die wall where frictional effects are present. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Aluminum; Equal-channel angular pressing; Severe plastic deformation; Single crystals; Texture
1. Introduction The processing of metals through the application of severe plastic deformation (SPD) has attracted much attention within the last decade primarily because the use of SPD procedures provides the potential for achieving very significant grain refinement, typically to the submicrometer or nanometer range, in bulk polycrystalline samples [1,2]. In practice, the most versatile SPD technique appears to be equal-channel angular pressing (ECAP) in which a sample is machined to fit within a channel contained within a die and it is then pressed through the die until it emerges at the exit point [3,4]. Since the cross-section of the sample remains unchanged in ECAP, it is easy to perform repetitive pressings of the same sample in order to impose very high total strains. In practice, it was recognized in early *
Corresponding author. Tel.: +1-213-740-0491; fax: +1-213-7408071. E-mail address: [email protected] (T.G. Langdon).
work that different shearing systems may be activated during ECAP by rotating the sample between each individual pass [3] and it was shown more recently that, by varying the processing route and the total imposed strain, it is possible to obtain samples having different grain boundary character distributions ranging from a high fraction of low-angle boundaries to a high-fraction of high-angle boundaries [5]. Processing through the use of ECAP is especially attractive for two reasons. First, the process may be readily scaled up for the production of fairly large bulk samples [6]. Second, the concept of ECAP processing can be incorporated easily into conventional industrial rolling operations as in the conshearing [7] or the continuous confined strip shearing [8] processes. Numerous experiments have been conducted to date on a very wide range of materials in order to evaluate the microstructures introduced into polycrystalline specimens when they are subjected to processing by ECAP. In practice, much of this work has concentrated on the potential for achieving a submicrometer or
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nanometer grain size in the as-processed condition. Careful experiments on samples of polycrystalline aluminum have demonstrated there is excellent agreement between the shearing characteristics and shearing patterns predicted theoretically [9,10] and the microstructures observed experimentally at both the microscopic [11] and the macroscopic [12] levels. Although a very extensive set of experimental data is now available for materials processed by ECAP, these results relate almost exclusively to tests conducted using polycrystalline samples. By contrast, no fundamental investigation has been undertaken to date to examine the deformation structures introduced when ECAP is applied to single crystals having well-defined and specific initial orientations. Thus, the present investigation was initiated specifically to address this deficiency. It is important to note that the use of single crystals provides three significant advantages in any fundamental studies of the ECAP process. First, it is possible to select the initial crystallographic orientation of the sample with respect to the shearing plane in the ECAP die. Second, no adverse artifacts are introduced into the as-pressed deformation structures as a consequence of the presence of adjacent grain boundaries. Third, the use of single crystals provides a unique opportunity to attain a direct evaluation of the degree of homogeneity that is developed in the as-pressed microstructure due to ECAP processing and especially to examine the significance of any inhomogeneities that may arise due to frictional effects in the vicinities of the die walls. This latter evaluation may be readily undertaken by examining the aspressed deformation microstructures both in the central area of the sample and in the regions immediately adjacent to the die walls. In the present study, the experiments were conducted using a single crystal of high-purity aluminum having a well-defined initial orientation with respect to the shearing plane, where pure aluminum was selected to provide a direct comparison with the extensive earlier data reported for the ECAP of polycrystalline aluminum [11,13]. As will be demonstrated, the deformation structure which develops in the central area of the single crystal by ECAP is fully consistent, at least in terms of the slip planes and slip directions, with the expectations arising from simple crystallographic considerations that incorporate the development of texture in ECAP processing.
orientation of the single crystal was determined using the Laue back-reflection method. An ECAP sample was cut from the rod to the desired orientation using an electric-spark discharge facility. This sample had a square cross-section with dimensions of 4 4 mm2 and a total length of 30 mm. Prior to processing by ECAP, the four longer faces of the sample were prepared to a mirror-like finish by grinding on abrasive paper and then electro-polishing. Fig. 1 depicts a section through the vertical ECAP die and illustrates the orientation selected for use in this investigation. Using the conventional terminology adopted for ECAP [4], this schematic illustration shows an ECAP die in which the angle U between the two parts of the channel is 90° and, for simplicity in the illustration, the two parts of the channel meet at an abrupt corner so that the angle representing the outer arc of curvature where the two parts of the channel intersect is given by W ¼ 0°. In practice, the experimental facility used in this investigation had a channel angle of U ¼ 90° and an outer arc of curvature given by W 30°. It is important to note that, as shown in model experiments [14] and in experiments on polycrystalline samples [15], the use of ECAP dies where W 20° to 30° leads to no significant deformation inhomogeneities in ECAP processing because in practice there is a ‘‘dead zone’’ in the vicinity of this outer corner where the sample is no longer in contact with the die wall [16–18]. Three orthogonal directions are defined in Fig. 1, where X is the transverse plane perpendicular to the direction of flow, Y is the flow plane parallel to the side face at the point of exit from the die and Z is the longitudinal plane parallel to the top face at the point of exit, respectively. The two solid arrows at the top and on the right-hand edge of Fig. 1 denote the direction of pressing and therefore the direction of the sample as it
2. Experimental material and procedures The experiments were conducted using a single crystal of high-purity (99.999%) aluminum grown from the melt using the vertical Bridgeman method. This crystal was in the form of a rod with a diameter of 20 mm and a length of 70 mm. The initial crystallographic
Fig. 1. Schematic illustration of a cross-section through the ECAP die showing the three orthogonal axes and the initial orientation of the 0° specimen.
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passes through the die. The theoretical shear plane lies at 45° to the X-direction at the intersection of the two parts of the channel. Since slip occurs in face-centered cubic metals on the f1 1 1gh1 1 0i system, the single crystal was oriented so that the (1 1 1) slip plane lay parallel to the theoretical shear plane, the [1 1 0] slip direction lay parallel to the direction of shear and the Ydirection therefore corresponds to the [1 1 2] direction. For simplicity, this orientation, in which the slip direction is coincident with the shearing direction, is henceforth designated the 0° orientation. In order to deduce the magnitudes of the resolved shear stresses associated with this orientation, Fig. 2 shows a schematic illustration of the relationship between the slip plane and the slip direction in the single crystal and the shear plane and the shear direction in the ECAP die. Noting that the specimen is pressed through the die in a vertical sense as indicated by the upper vertical arrow in Fig. 2, and defining F as the force in the shear direction and A as the area of the shear plane, it follows that the shear stress, s, operating on the slip plane in the slip direction is given by a relationship of the following type for each possible slip system: F s¼ cos h cos k; ð1Þ A where h is the angle subtended between the two normals to the shear and the slip planes and k is the angle between the shear direction and the slip direction, respectively. The magnitude of the angular term fcos h cos kg thus represents the shear factor and, when considering all possible slip systems, this term will have values lying within the range from 0 to 1. It follows, therefore, that the preferential slip system may be predicted by esti-
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mating the values of the shear factors for all possible slip systems and then selecting the system for the shear factor having the highest value. In order to undertake this calculation, Fig. 3 shows the relationship between the crystallographic orientation and the vertical pressing direction for the 0° specimen where the three orthogonal X, Y and Z axes are depicted in the small illustration at the lower right. Using these stereographic projections, it follows that it is possible to construct a tabulation of values for the various shear factors with respect to different slip systems as given in Table 1 where the shear factors are
Fig. 3. Stereographic projection illustrating the relationship between the crystallographic orientation and the vertical pressing direction for the 0° specimen.
Table 1 Estimations of the shear factors, and their relevant importance, for various slip systems in the 0° specimen
Fig. 2. Schematic illustration of the procedure for estimating the magnitudes of the resolved shear stresses for different slip systems through the use of Eq. (1).
Slip plane
Slip direction
0° specimen Shear factor
Order of shear factor
1 1 1
1 1 0 1 0 1 0 1 1
1.00 0.50 0.50
(1) (2) (2)
1 1 1
1 0 1
1 0 1 1 01
0.00 0.17 0.17
(11) (5) (5)
1 1 1
1 1 0 1 0 1 011
0.00 0.17 0.17
(11) (5) (5)
1 1 1
1 1 0 011 101
0.33 0.17 0.17
(4) (5) (5)
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listed in the third column and the numbers in parentheses in the fourth column denote the ordering of these values in terms of their overall magnitudes. It is apparent that the primary slip system is ð 1 1 1Þ½1 1 0 which is listed as (1) in the fourth column of Table 1 and the secondary slip systems are ð 1 1 1Þ½ 1 0 1 and ð1 1 1Þ½0 1 1 where both systems have the same shear factors and they are listed as (2) in Table 1. Processing by ECAP was performed at room temperature using a split die fabricated from tool steel and with the single crystal coated in an MoS2 lubricant prior to pressing. It can be shown from first principles that the strain imposed in a single pass through a die having internal angles of U ¼ 90° and W 30° is 1 [19]. In the present investigation, the single crystal was pressed partially through the die in a single pass and the pressing was terminated when the crystal was essentially at the mid-point of the die. The ECAP die was then opened and the sample carefully removed and sectioned longitudinally on the Y-plane into two approximately equal pieces. One of these pieces was used for observations on the Y-plane using optical microscopy (OM), scanning electron microscopy (SEM) and orientation imaging microscopy (OIM) and the second piece was further cut on the Y-plane into two separate sections for examination on the Y-planes using transmission electron microscopy (TEM): it is important to note that all observations, and all photomicrographs in this report, relate to the Y-plane as depicted in Fig. 1 where the X-direction points horizontally towards the right. For inspection by OM, the surface was prepared using abrasive paper followed by an alumina paste and then electro-polishing using a solution of 10% HClO4 , 20% C3 H5 (OH)3 and 70% C2 H5 OH. The crystal surface was etched using a solution of 5% HBF4 in water. A JSM5600 scanning electron microscope equipped with an OIM capability was used for SEM and OIM. For OIM, the image area was set at 150 150 lm2 and individual measurements were taken periodically at incremental steps of 1.5 lm. A JEM-2000FX transmission electron microscope was used for the TEM observations and selected area electron diffraction (SAED) patterns were recorded using a beam diameter of 12.3 lm.
3. Experimental results 3.1. Observations by OM and OIM The 0° specimen was pressed to the mid-point of the die, removed for inspection, and the upper portion of Fig. 4 shows the section on the Y-plane where the unsheared portion lies vertically on the left, the sheared portion lies horizontally on the right and the arc of curvature within the die of W 30° is clearly revealed in the corner at the lower left. The microstructures on the
Fig. 4. The 0° specimen after pressing approximately half-way through the ECAP die: the four optical photomicrographs were taken at the points labeled 1–4 on the cross-section of the specimen.
polished surface were examined at various positions using OM and Table 2 provides a detailed summary of the experimental observations associated with the four selected positions labeled 1–4 in the upper photomicrograph where the relevant images for these positions are shown in the lower portion of Fig. 4. All of the characteristics summarized in Table 2 are visible through close inspection of the optical micrographs in Fig. 4 where attention is especially directed to the angular relationships for the various sets of striations recorded in Table 2. It is apparent that deformation bands develop close to the shear plane in the upper portion of the deformed region at position 1 and there are unusual sharp striations in the deformed portion in the vicinity of the lower die wall at position 3. It is reasonable to conclude that the different shearing characteristics occurring at position 3 are due to the influence of frictional effects in the vicinity of the die wall. The corresponding OIM images and the (1 1 1) pole figures are shown in Fig. 5 where the images labeled 1–4 refer to the same positions labeled 1–4 in Fig. 4 and each microstructure is shown in color where the colors relate to the crystallographic orientations depicted in the stereographic triangle on the left. Inspection of the relevant pole figure shows that at position 1, located close to the shear plane, the structure remains a single crystal but the
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Table 2 Shearing characteristics in the 0° specimen at the positions labeled 1–4 in Fig. 4 Position
Distance from top Approximate location surface (mm)
Shearing characteristics:all angles are measured with respect to the horizontal X-direction
1
0.7
Close to shear plane
2 3
2 3
Central portion of deformed crystal In vicinity of bottom surface
4
2.2
Within deformed region and 2 mm beyond position 2
Long horizontal striations or deformation bands at low magnifications; additional thin striations at 30° and 60° at high magnifications Prominent striations at 20°; very thin striations at 50° Very sharp striations at 15°; these sharp striations contain thin striations at 50° Very prominent striations or shear bands at 65°; additional thick striations at 20°
Fig. 5. Images taken using orientation imaging microscopy and the corresponding (1 1 1) pole figures for the points labeled 1–4 in Fig. 4, where the colors relate to the crystallographic orientations shown in the stereographic triangle on the left.
crystallographic orientation has been rotated by 60° around the Y-axis where this is equivalent to a counterclockwise rotation around the [1 1 2] direction. At position 2 in the deformed region, the pink colored band lying at 20° to the X-axis corresponds to the striations visible in Fig. 4 and the (1 1 1) pole figure reveals two distinct orientations where there is a strong reflection representing an initial rotation of 60° around the Y-axis and a weak reflection corresponding to the initial orientation of the single crystal: this initial orientation is shown in light purple in the OIM image. At position 3 near the lower die wall, the (1 1 1) pole figure shows the presence of two crystallographic orientations given by the initial orientation and a rotation by 60° around the Y-axis: these are separately revealed as the purple and blue bands in the OIM image. The same two crystallographic orientations are also present in area 4 where it becomes apparent that the shear bands visible in Fig. 4, as documented in Table 2, correspond to the initial orientation of the crystal and the remaining area corresponds to a counter-clockwise rotation by 60° around the Y-axis. To evaluate the precise nature of the deformation introduced by shearing as the single crystal passes
through the shear plane, an additional detailed examination was conducted in the vicinity of the shear plane at the positions labeled from 1 to 10 in Fig. 6: the corresponding pole figures for each of these points are also shown in the lower portion of Fig. 6. It is immediately apparent from inspection of these two rows of pole figures that they have been placed so that the upper and lower sets are reasonably similar: thus, the pole figures labeled 1 and 6 are both in the initial orientation, 2 and 7 show evidence for some scattering, 3 and 8 reveal a distinct elongation in a counter-clockwise sense, 4 and 9 depict two poles, and 5 and 10 reveal the presence of both the initial orientation and the orientation corresponding to a counter-clockwise rotation by 60° around the Y-axis. It can be shown by careful analysis that there are slight differences between the two pole figures at positions 5 and 10: thus, there are two well-defined orientations at position 5 which is well-removed from the shear plane but at position 10, which is located within the ill-defined area of shearing associated with the arc of curvature of the die at the lower left, the two orientations correspond to the initial orientation and a rotation by an initial increment of only 50° around the Y-axis.
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Fig. 6. Pole figures associated with the points labeled 1–10 in the upper cross-section: the pole figures are arranged in two rows so that the upper and lower sets are reasonably similar.
3.2. Observations by TEM The deformed part of the sample was inspected by TEM with the observations taken on the Y-plane at points remote from the shear plane and generally close to the center of the sheared material. These inspections revealed two distinct types of microstructure and Fig. 7 shows an example where both of these microstructures, labeled A and B, appear in adjacent and contiguous regions: the corresponding SAED patterns for these two regions are shown on the right in Fig. 7. The upper region A contains well-defined slip traces along a direction at 45° to the X-axis where this direction corresponds to the shear plane in ECAP and the trace of the primary ð 1 1 1Þ slip plane; the SAED pattern for region A corresponds to the initial crystallographic orientation with the Y-direction along [1 1 2] and the shearing direction along ð 1 1 1Þ. By contrast, the lower region B contains slip traces along a direction at 10° with respect to the X-axis and the SAED pattern for this region is then rotated by 60° in a counter-clockwise sense from the SAED pattern shown for region A. The effect of making a counter-clockwise rotation by 60° around the [1 1 2] direction is depicted schematically in the two lower illustrations in Fig. 7 where the initial orientation is on the left and the orientation after a 60° rotation is on the right. It is apparent from this schematic illustration that the slip traces at B now lie fairly close to the trace of the ð 1 1 1Þ slip plane which, after rotation, is inclined at an angle of )7° from the X-axis.
Fig. 7. Photomicrograph taken by TEM in the central portion of the deformed material showing the presence of two distinct types of microstructure labeled A and B: the corresponding SAED patterns are shown on the right and the effect of a counter-clockwise rotation by 60° is depicted schematically in the two lower illustrations.
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In addition, it can be seen from the photomicrograph in Fig. 7 that there is a well-defined border delineating the transition between regions A and B and this border lies along a direction oriented close to 15° from the X-axis. This direction is therefore coincident with the very sharp striations visible at position 3 in Fig. 4 and with the banded structure visible at position 3 in Fig. 5. Measurements using TEM showed the widths of the slip traces were of the order of 1.3 lm in both regions A and B of Fig. 7 and this is consistent with experimental observations on pure polycrystalline Al where the final equiaxed grain size of the as-pressed material was reported as 1.2–1.3 lm [11,12,20]. It is apparent from the SAED patterns pertaining to regions A and B of Fig. 7 that the microstructures in both regions consist of bands of subgrains having boundaries with low-angles of misorientation and this is consistent with the low imposed strain of 1 since it is well-documented for polycrystalline Al [20] and for various Al alloys [21] that high imposed strains are required in order to achieve grain structures with boundaries having high angles of misorientation. Finally, and in contrast to the microstructures widely reported for polycrystalline materials after ECAP [22–24], the photomicrograph in Fig. 7 reveals the presence of only a relatively small number of dislocations in regions A and B either within the welldefined subgrains or along the subgrain boundaries. This observation suggests that dislocation re-arrangement probably occurs more easily during the application of ECAP to single crystal specimens and this is reasonable when it is noted that there are no constraining effects due to the presence of grain boundaries. 4. Discussion The results obtained in this investigation provide a clear demonstration of the potential for using single crystals to conduct fundamental studies of the flow be-
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havior produced by ECAP processing and the consequent deformation structures. In general terms, the TEM photomicrograph shown in Fig. 7 is consistent with the extensive data reported earlier for the development of deformation structures in polycrystalline Al [11,12,20]. By pressing a single crystal with a well-documented initial orientation, designated a 0° specimen, it is shown through examination by optical microscopy in Fig. 4 that the specimen contains different sets of striations in the pressed condition. This initial orientation was selected specifically because the primary (1 1 1) slip plane is coincident with the shear plane and thus at 45° with the X-axis. After pressing, examination shows there is a very wide area within the pressed material where the initial crystallographic orientation is rotated by 60° in a counter-clockwise sense around the Y-axis but there is an area immediately adjacent to the lower edge of the specimen where the local shearing appears to be influenced by frictional effects at the die wall. It is apparent from the TEM observations, as documented in Fig. 7, that the subgrain bands lie essentially parallel to the primary slip planes both before and after any rotation has taken place. Thus, the elongated subgrains are formed so that their longer sides lie on the primary slip planes. This suggests a simple mechanism for substructural development in ECAP whereby glissile dislocations are generated on these planes during pressing and these dislocations then combine to create the subgrain boundaries. It has been shown that the subgrain widths produced in aluminum and Al-based alloys during ECAP processing are essentially identical to the grain sizes produced in conventional cold-working through compression and extrusion [25]. This similarity in size suggests, therefore, that the subgrain widths are essentially dictated by the development of a balance between the generation and recovery processes. It is instructive to consider the nature of the texture introduced into the single crystal by ECAP. Fig. 8(a)
Fig. 8. (a) The (1 1 1) pole figure reported for a sample of pure polycrystalline Al subjected to 12 passes of ECAP processing using route BC [20] and (b) a conventional B-type shear texture [26].
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shows a schematic illustration of the (1 1 1) pole figure reported for a sample of pure polycrystalline Al subjected to 12 passes using processing route BC in which the sample is rotated by 90° in the same sense between each pass [20]: the shear direction is also indicated at upper right. This pole figure shows two kinds of texture with both having a common (1 1 1) plane normal to the Y-axis but with a relative rotation of 60° around the Yaxis. This type of texture corresponds to the conventional B-type rolling texture in cold-rolled face-centered cubic metals in which there are two variants, B1 and B2 , having a common (1 1 0) plane and existing alternately. For comparison purposes, Fig. 8(b) shows a schematic illustration of a conventional B-type shear texture where the (1 1 0) orientation tends to align with the shear direction and RD and ND represent the rolling direction and the normal direction, respectively [26]. A comparison with the pole figures in Fig. 6 shows that Figs. 8(a) and (b) are very similar to the pole figure obtained at position 5 which is located at a point in the deformed material within the exit channel. The development of this type of texture provides a direct explanation for the experimental results obtained in this investigation including the two distinct orientations that are clearly visible through the TEM observations in Fig. 7. Finally, it is apparent that rotation occurs consistently in the deformed structure in a counter-clockwise sense when viewed on the Y-plane, as can be seen in Fig. 6. This sense of rotation is a natural consequence of the construction of the ECAP die and the presence of an outer arc of curvature at the point where the two channels intersect, as can be seen at the lower left corner in Fig. 6. The presence of an arc of curvature leads to the development of shearing within a zone extending through an arc of 30° so that there is an inhomogeneous strain distribution across the width of the specimen: these distributions have been modeled in several studies using finite element analysis [16,18,27– 30]. Furthermore, the presence of a shearing zone leads also to a tensile component of stress within the shear zone and the nature of this stress is such that, as noted in a detailed analysis [31], there will be a counterclockwise rotation around the Y-axis when viewed as depicted in Fig. 1. Thus, the present observations in the bulk of the deformed crystal are fully consistent with the expectations from crystallographic considerations.
5. Summary and conclusions 1. A single crystal of aluminum was subjected to equalchannel angular pressing at room temperature after orienting the crystal so that the (1 1 1) slip plane was parallel to the theoretical shear plane and the
[1 1 0] slip direction lay parallel to the direction of shear. The pressing was terminated in the first pass with the specimen pressed to approximately the mid-point of the die. 2. Detailed microscopic examination of the specimen after pressing revealed the occurrence of shearing patterns that are consistent with crystallographic expectations including the development of a B-type rolling texture. Different shearing patterns were present in the vicinity of the lower die wall where frictional effects are present. 3. The experimental observations suggest a simple mechanism for substructural development in ECAP. Glissile dislocations form on the primary slip plane and combine to create subgrain boundaries and arrays of subgrains. These subgrains are oriented with their longer sides lying on the primary slip plane.
Acknowledgements This work was supported in part by the Light Metals Educational Foundation of Japan and in part by the National Science Foundation of the United States under Grant No. DMR-0243331.
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