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The evolution of homogeneity in processing by high-pressure torsion
Acta Materialia 55 (2007) 203–212 www.actamat-journals.com
The evolution of homogeneity in processing by high-pressure torsion Cheng Xu a, Zenji Horita b, Terence G. Langdon
a,*
a
b
Departments of Aerospace and Mechanical Engineering and Materials Science, University of Southern California, 3650 McClintock Avenue, Los Angeles, CA 90089-1453, USA Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan Received 3 April 2006; received in revised form 26 July 2006; accepted 26 July 2006 Available online 17 October 2006
Abstract Disks of high-purity aluminum were processed by high-pressure torsion (HPT) at room temperature under different conditions of imposed pressure and numbers of turns. Measurements were taken of the microhardness values both along diameters in each disk and following a rectilinear grid pattern to give color-coded maps of the hardness distributions. The results show the hardness increases by a factor of 2 in the first turn of HPT but the microhardness distribution is inhomogeneous because higher values of hardness are recorded in the central regions of the disks. This central region of inhomogeneity decreases with increasing numbers of turns so that the hardness distribution becomes essentially homogeneous after five turns. The results are different from earlier reports in HPT where the central regions of the disks have a lower hardness. The results are interpreted using a model in which the degree of hardness depends upon the rate of recovery in the material. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Hardness; High-pressure torsion; Homogeneity; Severe plastic deformation; Ultrafine grains
1. Introduction Processing through the application of severe plastic deformation (SPD) is now an accepted procedure for producing significant grain refinement in bulk crystalline solids [1,2]. The two SPD processing techniques of equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) are currently receiving considerable attention, and it is well established that both procedures are effective in reducing the grains to at least the submicrometer level. However, processing by HPT is an especially attractive procedure because, by comparison with ECAP, it leads both to smaller grains and to a higher fraction of high-angle grain boundaries [3,4]. The principle of HPT is based on the procedures first developed by Bridgman [5]. A sample, generally in the form of a thin disk, is placed between two large anvils and subjected to a high pressure and concurrent torsional strain*
Corresponding author. Tel: +1 213 740 0491; fax: +1 213 740 8071. E-mail address: [email protected] (T.G. Langdon).
ing. Thus, the two important parameters in HPT are the magnitude of the imposed pressure, P, and the number of revolutions applied to the sample, N. An important limitation in HPT is that the imposed strain varies across the sample and, in principle at least, the strain is reduced to zero in the center of the disk. As a consequence of this variation, it is reasonable to anticipate that the microstructures produced by HPT will be extremely inhomogeneous. Nevertheless, the experimental data available to date reveal a clear dichotomy. First, there are reports for austenitic steel [6], Cu [7] and high-purity Ni [8] of significant variations in the values of the microhardness across the diameters of disks processed by HPT, with lower hardness values in the centers of the disks and higher values in the peripheral regions. Second, there are reports for commercial purity Al [9], an Al–Mg–Sc alloy [10], Cu [11] and high-purity Ni [12,13] showing that, although the microhardness is lower in the central region in the early stages of deformation, the microstructures become reasonably homogeneous across the disks when torsional
1359-6454/$30.00 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.07.029
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straining is continued to a sufficiently high total strain under a high imposed pressure. All of the results available to date have relied upon taking measurements of the microhardness values along diameters of the disks after HPT processing. However, an alternative approach was developed recently for evaluating microstructural inhomogeneities in samples processed by ECAP [14,15]. In this procedure, microhardness values are recorded in a rectilinear grid pattern across the sectioned planes of samples processed by ECAP and the results are plotted in the form of color-coded contour maps to give a clear and visual presentation of the local variations in hardness over the sections. The present investigation was initiated in order to undertake similar measurements on disks processed by HPT under different conditions of pressure and numbers of revolutions, with the overall objective of determining whether the microstructures generated by HPT evolve into truly homogeneous arrays at sufficiently high strains and pressures. As will be demonstrated, the results of this study provide important new information on the development of homogeneity during HPT processing by showing there is a direct link between microstructural evolution in HPT and the rate of recovery within the material. 2. Experimental materials and procedures The experiments were conducted using pure aluminum of 99.99% purity where this material was selected because it was used earlier both for an extensive study of microstructural evolution in ECAP [16,17] and for the development of color-coded maps showing the microhardness distributions after processing by ECAP [14,15]. An aluminum ingot was rolled at room temperature into a plate having a thickness of 25 mm. It was then cut into a block with a cross-section of 25 · 25 mm2 and swaged into cylindrical rods of 10 mm diameter. Samples were obtained for HPT by slicing the rods into disks 0.85 mm thick. All of the disks were annealed at 773 K for 1 h prior to HPT to give an initial grain size of 1 mm. The HPT processing was performed at room temperature (298 K) using a facility described elsewhere [10]. Briefly, the facility consisted of an upper and lower anvil made from high-strength YXR3 tool steel with nitrided surfaces. A spherical depression was machined at the center of each anvil to a depth of 0.25 mm and with a diameter of 10 mm. For testing, the Al disk was placed in the depression on the lower anvil, the two anvils were brought into position to impose a pressure on the disk and the disk was then concurrently compressed and torsional straining was achieved through rotation of the lower anvil at a speed of 1 r.p.m. Because of the high pressure, there was a small reduction in the thickness of the disk during processing: measurements showed the thickness was reduced after the first turn and there was an additional reduction in thickness during torsional straining up to a total of five turns. Three different
applied loads were used in these experiments: 10, 20 and 48 tons, corresponding to imposed pressures, P, of 1.25, 2.5 and 6.0 GPa, respectively. The disks were subjected to HPT under these various pressures for totals of one, three or five turns. Following HPT, the disks were mounted and carefully polished to a mirror-like surface. All of the microhardness measurements were taken using an FM-1e microhardness tester equipped with a Vickers indenter. Two different procedures were used for taking measurements of the Vickers microhardness, Hv. First, individual values of the microhardness were measured across the diameter of each disk in incremental steps as depicted schematically for half of the diameter in Fig. 1. Measurements were taken to determine the average values of Hv at positions separated by 0.3 mm up to a distance of 1.2 mm on either side of the center of each disk; at greater distances from the center the measurements were taken at increments of 0.6 mm. It should be noted that special attention was given to the central region because this is the critical area where there is uncertainty in the published data. At each selected point, shown by the small open circle in Fig. 1, the hardness was estimated by averaging the measurements recorded for four separate points uniformly positioned around the selected point at a distance of 0.15 mm. These measurements provided detailed information on the variation of Hv across the diameter of each disk after testing under different conditions. Second, individual measurements of Hv were recorded on the surface of each disk following a regular grid pattern with a spacing of 0.3 mm between each separate point. The individual values of Hv were then plotted as color-coded contour maps depicting the variations in the local hardness across the surface of each sample. The internal microstructures of the disks were examined by transmission electron microscopy (TEM) after processing by HPT. After torsional straining, the disks were ground mechanically to a thickness of 100 lm, punched into disks with diameters of 3 mm and then thinned to perforation using a twin-jet electropolishing facility with a solution of HClO4, C6H14O2 and C2H5OH at a temperature of 278 K. All TEM observations were made using a Philips 420 microscope operating at 120 kV. The microstructures were observed after HPT both in the central regions of the disks and near the edges of the as-processed disks. The grain sizes were estimated from direct measurements of individual grains in the microscope.
Fig. 1. Schematic illustration of procedure for taking microhardness measurements along the diameter of the sample after HPT.
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3. Experimental results 3.1. The variation of average microhardness across the disk diameter Following the procedure illustrated in Fig. 1, the values of the Vickers microhardness were measured across the diameter of each separate disk and the results are summarized in Table 1, where the error bars denote the 95% confidence limit. For comparison purposes, the upper row gives the hardness values, and the associated error bars, for measurements taken on an as-received disk prior to HPT. It is apparent from inspection of the results in Table 1 that both the values of Hv and the error bars vary in a systematic and consistent way for all of the disks. Several trends are visible. First, the error bars are highest in the centers of the disk for all samples subjected to HPT and they become significantly lower at the periphery. Second, the error bars in the central region decrease with increasing numbers of turns, thereby indicating a greater microstructural uniformity after larger numbers of revolutions. Third, the average values recorded for Hv in the peripheral regions are reasonably similar for all of the disks processed by HPT despite significant differences in the imposed pressure, P, and the number of turns, N. The data are presented pictorially in Fig. 2, where Hv is plotted against the distance from the center and data are shown for three different experimental conditions: (a) for P = 1.25 GPa for one, three and five turns; (b) for P = 6.0 GPa for one, three and five turns; and (c) for five turns at pressures of 1.25, 2.5 and 6.0 GPa, where the lower solid points correspond to the as-received unprocessed condition. The first important conclusion is that, for all testing conditions, the hardness increases significantly after one turn of HPT by a factor which is slightly less than 2. This increase in hardness is comparable to the increase in strength typically recorded in a range of aluminum-based alloys after processing through one pass in ECAP [18]. The second important conclusion is that, contrary to the results described earlier for several different materials [6–13], the centers of the disks for pure aluminum exhibit values of Hv which are higher, rather than lower, than the surrounding areas. Furthermore, there is a general con-
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sistency in the results since similar high values of Hv are visible in the central region at N = 1 turn for pressures of both 1.25 GPa in Fig. 2a and 6.0 GPa in Fig. 2b. The third conclusion is that the values of Hv at the outer periphery undergo no significant change either as a function of increasing number of turns in Fig. 2a and b or as a function of increasing pressure in Fig. 2c. By contrast, the relatively high values recorded for Hv in the central region decrease in subsequent passes so that ultimately, after five turns, the values of Hv are reasonably similar across all of the disks. The fourth conclusion from Fig. 2 is that the sample processed at 1.25 GPa exhibits a lower central peak after one turn and ultimately, after five turns, this sample shows less variation in Hv than the two samples tested to five turns at the higher pressures. 3.2. The variation of microhardness across the disk surface All of the microhardness measurements taken from the rectilinear grid patterns at incremental spacings of 0.3 mm were plotted as color-coded contour maps to provide pictorial displays of the distributions of the individual hardness values for each testing condition. These plots are shown in Fig. 3 for a pressure of 1.25 GPa, in Fig. 4 for a pressure of 6.0 GPa and in Fig. 5 for the sample pressed to five turns under a pressure of 2.5 GPa, where X and Y denote two arbitrary orthogonal axes marked in mm so that the position (0, 0) is at the mid-point of each disk. The distributions of the individual microhardness values are represented pictorially through a series of colors which define, in incremental values of Hv = 5, the values of Hv in the range from 30 to 60: the scale for these colors are the same in Figs. 3–5 and they are shown in the small insets inserted in each display. For the samples subjected to HPT at a pressure of 1.25 GPa, Fig. 3 shows the individual distributions of Hv after one, three and five turns. Inspection shows there is a relatively large area in the center of the disk after one turn, extending to a width of 4 mm, where the hardness values are exceptionally high. Thus, there is significant inhomogeneity in this condition but, unlike the results reported earlier for several metals [6–13], the hardness in the central region is higher than around the edge. Increasing the strain
Table 1 Summary of microhardness values and error bars at the 95% confidence level along the radius of pure Al processed by HPT HPT condition
Distance from center (mm)
P (GPa)
0
N (turns)
0.3
0.6
0.9
1.2
1.8
2.4
3.0
3.6
4.2
4.8
As-received
25.5 ± 0.6
25.3 ± 0.4
25.4 ± 0.6
25.4 ± 0.5
25.1 ± 0.7
24.4 ± 0.8
25.0 ± 0.6
25.4 ± 0.3
25.0 ± 0.4
25.6 ± 0.6
–
1.25
1 3 5
44.1 ± 7.5 46.0 ± 5.8 38.6 ± 1.8
44.4 ± 3.0 44.5 ± 2.8 39.2 ± 0.8
47.5 ± 2.3 39.8 ± 3.8 39.2 ± 0.6
48.8 ± 1.5 39.7 ± 2.8 39.2 ± 0.8
47.7 ± 2.7 39.5 ± 1.0 39.5 ± 0.8
44.4 ± 2.1 41.8 ± 1.2 39.1 ± 0.3
40.7 ± 0.7 39.7 ± 0.9 39.9 ± 0.6
39.8 ± 1.1 41.0 ± 1.0 39.8 ± 0.8
39.3 ± 0.8 40.7 ± 1.1 40.4 ± 0.6
40.0 ± 0.7 40.3 ± 2.0 41.6 ± 3.1
39.8 ± 0.7 40.3 ± 1.8 40.2 ± 1.5
2.5
5
40.9 ± 4.7
39.7 ± 1.0
39.5 ± 1.0
38.3 ± 2.9
40.4 ± 1.0
40.4 ± 0.7
40.9 ± 0.8
40.9 ± 1.3
41.4 ± 0.5
41.0 ± 1.4
40.9 ± 0.9
6.0
1 3 5
45.5 ± 10.5 41.7 ± 6.3 41.7 ± 3.9
47.2 ± 6.0 43.5 ± 3.5 42.1 ± 2.8
47.8 ± 6.0 42.5 ± 3.3 41.5 ± 2.7
46.7 ± 5.4 39.9 ± 1.2 40.3 ± 0.7
44.3 ± 5.2 40.3 ± 0.7 39.9 ± 0.6
40.3 ± 1.7 40.6 ± 0.7 40.8 ± 0.6
40.7 ± 0.6 41.2 ± 0.9 40.7 ± 1.0
39.8 ± 0.8 40.8 ± 1.1 39.4 ± 2.8
40.3 ± 0.7 40.4 ± 0.7 41.4 ± 2.9
39.6 ± 0.9 41.1 ± 1.4 42.4 ± 0.7
39.8 ± 1.1 41.3 ± 1.1 41.7 ± 0.8
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imposed on the sample decreases the size of the region of high hardness so that it is reduced to a diameter of <0.5 mm at three turns and disappears after five turns. Thus, the results in Fig. 3 provide unambiguous evidence for the gradual development of a homogeneous microstructure in pure Al with increasing numbers of turns. After five turns, the display in Fig. 3 shows a fully homogeneous microstructure except for some very minor regions of slightly higher hardness at various randomly located points within a region of 1 mm from the edge of the disk. Similar displays are given in Fig. 4 for disks subjected to a pressure of 6.0 GPa and again there is a region of higher hardness in the center of the disk after one turn, but at this increased pressure the region of higher hardness is restricted to a smaller region with a diameter of 1.2 mm. This area of high hardness is again reduced with additional straining to the extent that it covers a region having a diameter of 0.3 mm after three turns, and it is further reduced almost to a point after five turns. A comparison of the displays after five turns for pressures of 1.25 and 6.0 GPa in Figs. 3 and 4 suggests that the development of a homogeneous structure occurs equally rapidly, and possibly even more rapidly, at the lower pressure. The display in Fig. 5 for five turns under an imposed pressure of 2.5 GPa is consistent with these trends because the microstructure is reasonably homogeneous, there is an extremely small region with higher hardness immediately in the center of the disk and there is a general degree of homogeneity which is intermediate between the two displays for disks taken through five turns presented in Figs. 3 and 4. This display also confirms that the presence of some degree of inhomogeneity around the periphery of the disk becomes more apparent with increasing pressure. 3.3. Microstructural observations after HPT
Fig. 2. The average Vickers microhardness, Hv, versus distance from the center of the disk after processing by HPT: (a) after various turns with a pressure of 1.25 GPa, (b) after various turns with a pressure of 6.0 GPa and (c) using various pressures after five turns.
In the early stages of processing, the present results show that the microhardness values vary with the distance from the center of the disk whereas at higher strains, associated with a total of at least N = 5 turns, the hardness values become essentially homogeneous. These results are consistent with earlier studies in demonstrating a dependence on position within the disk in the earlier stages of HPT processing but they are different because all previous investigations have reported regions of lower hardness near the disk centers at the lower strains [6,13]. Furthermore, in many of the earlier studies the hardness results were supported by extensive microstructural observations, usually by TEM, showing the presence of smaller grains at the edges of the disks [8–13]. Accordingly, it was important in this investigation to inspect the microstructures both in the central regions and near the edges of the disks. Fig. 6 shows representative microstructures taken on the sample processed by one turn at a pressure of 1.25 GPa in two different positions: (a) near the center of the disk and (b) near the edge of the disk. It is immediately apparent that these two areas have very different microstructures.
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Fig. 3. Color-coded contour maps showing the Vickers microhardness across the surface of the disks processed by HPT at a pressure of 1.25 GPa for: (a) one, (b) three and (c) five turns. The significance of the colors is shown by the small inset.
First, the grains are visibly smaller in the central area by comparison with the edge, and measurements gave average grain sizes of 0.8 lm in the center and 1.2 lm at the periphery. Second, the appearance of the grains is different in these two areas. Detailed inspection showed that in the central region the boundaries were irregular, there were distortions and bending of the lattice fringes, and there was a high density of intragranular dislocations. All of these observations are consistent with the presence of highenergy non-equilibrium boundaries as reported earlier after processing by ECAP [19,20] and HPT [21]. By contrast, the grains were clear and almost free of intragranular dislocations near the edge of the disk, the grain boundaries were
well defined and the overall appearance was consistent with a low-energy microstructure after significant recovery. 4. Discussion 4.1. The development of homogeneity during HPT The results from these measurements reveal two trends which are important for materials processed by HPT. First, and consistent with the trends suggested in earlier studies of processing by HPT [9–13], the values of the local microhardness evolve during HPT processing and ultimately, after five turns for pure Al, the hardness values
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Fig. 4. Color-coded contour maps showing the Vickers microhardness across the surface of the disks processed by HPT at a pressure of 6.0 GPa for: (a) one, (b) three and (c) five turns. The significance of the colors is shown by the small inset.
are essentially identical throughout the disk. This evolution suggests, therefore, the gradual development of a homogeneous microstructure. Furthermore, this evolution is independent of the imposed pressure, at least in the range from 1.25 to 6.0 GPa, although the results suggest that the homogeneity may evolve more rapidly at the lower pressures. The ability to achieve homogeneity in HPT, despite the variation in the local strain imposed by the torsional rotation, is probably due to a combination of the shearing strain achieved in torsion and the compressive strain introduced by the imposed pressure. However, more detailed studies are needed to obtain a complete and definitive understanding of the nature of grain refinement in HPT.
Second, and contrary to earlier studies [6–13], the values of the microhardness are higher in the central region in the early stages of processing. The hardness in this region subsequently decreases and becomes equal to the level of hardness at the edge of the disk. This result is unexpected because, although there is general agreement that the straining induced in the disk samples by HPT processing varies with position within the disk, all previous reports documented examples where the hardness values in the central regions were exceptionally low [6–13]. Before attempting to understand these results, it is first necessary to determine whether the microhardness
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the evolution in the color-coded contour maps with increasing numbers of turns visible in Figs. 3 and 4, suggests that the data probably reflect genuine microstructural features of the as-processed disks. To evaluate the possibility of the occurrence of some recovery in the post-HPT period immediately after processing, two additional disks were pressed through one and five turns using a pressure of P = 1.25 GPa and microhardness measurements were taken immediately following the straining operation. The results for these two tests yielded data that were essentially identical, at least within the levels of the experimental errors, with the results shown for one and five turns in Fig. 2a, thereby confirming that postHPT recovery plays little or no significant role in the present investigation. This conclusion is consistent also with published data for commercial-purity aluminum, where a slightly smaller grain size was measured in the central region of a disk subjected to N = 8 turns with a pressure of P = 1 GPa due, it was suggested, to the onset of recovery in the highly strained outer peripheral region [9]. Fig. 5. Color-coded contour map showing the Vickers microhardness across the surface of the disk processed by HPT at a pressure of 2.5 GPa for five turns. The significance of the colors is shown by the small inset.
4.2. Factors influencing the development of homogeneity in HPT
measurements documented in Figs. 2–5 truly represent the microstructural characteristics introduced into the disks during torsional straining or whether instead they are a consequence of significant recovery occurring in the highly strained outer regions of the disks during the post-HPT period which elapsed between performing the straining and taking the measurements. In this respect, it is important to note that, although no attempt was made in this investigation to control precisely the period of time between torsional straining and measuring the microhardness values, the general overall consistency of all of the experimental data presented in Figs. 2–5, and especially
Two factors influence the development of homogeneity in HPT: the values of the imposed pressure and the values of the imposed strain, where the latter is represented by the numbers of turns. The effect of the number of turns, N, is easily seen using Figs. 3 and 4. When the number of turns is increased, the inhomogeneity in the central region is gradually removed. This suggests that the microstructure around the periphery is in a reasonably equilibrated condition such that there is little effect of additional straining. The effect of the imposed pressure, P, is shown in Fig. 2c and through a comparison of the color-coded contour maps for N = 5 in Figs. 3–5.
Fig. 6. Representative microstructures recorded by TEM after processing for one turn with a pressure of 1.25 GPa: (a) near the center of the disk and (b) near the edge of the disk.
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These maps show that greater homogeneity is attained at the lowest pressure of P = 1.25 GPa. In examining these data, it is important to note that the high values recorded for Hv in the vicinity of the centers of the disks are mutually consistent with the smaller grain sizes and with the more highly deformed structures occurring in this region, as shown in Fig. 6a. Similarly, the earlier reports of lower values of Hv in the central region were consistent with TEM observations showing larger grains in this region by comparison with the edges of the disks [8– 13]. Thus, all of the experimental observations reported to date are mutually consistent but there is a very clear dichotomy between the earlier data reported for several materials and the present results obtained on high-purity aluminum. In order to understand the reason for this difference, it is instructive to compare the present results with data reported earlier for the same pure aluminum when processed by ECAP [15]. 4.3. The interrelationship between HPT and ECAP Inspection of Fig. 6 shows that the microstructures are markedly different in the central region and at the edge of the disk. Near the center, in Fig. 6a, the highly deformed microstructure is similar to that observed in a very wide range of ultrafine-grained materials produced through processing by ECAP or HPT [19,21–29]. At the edge, shown in Fig. 6b, the microstructure is similar to a fully annealed condition after the occurrence of extensive recovery. These differences, when combined with the microstructural observations made immediately after HPT processing as described in Section 4.1, suggest that in situ recovery plays an important role in microstructural evolution during HPT processing. Furthermore, it is well known that high-purity aluminum has a very high rate of recovery by comparison with Cu, steel and most other materials. It is possible to make a direct comparison between HPT and ECAP by recognizing that, although in HPT the imposed strain is reduced to zero at the center of the disk, a comparison is possible by making separate determinations of the approximate levels of the imposed strains in the HPT disks and in billets processed by ECAP. Fig. 7 depicts a typical disk of radius r and height h used in HPT. On torsional straining, the incremental shear strain, dc, is given by d‘/h, where d‘ is the total displace-
Fig. 7. The principle for estimating the strain on a disk of thickness h and radius r.
ment, and this strain may be expressed as rdh/h, where dh is the angular rotation around the mid-point [6,13]. Since h is given by 2pN, the equivalent von Mises strain, eeq, is given by [30] 2pNr eeq ¼ pffiffiffi h 3
ð1Þ
In the present investigation for the disks used to construct Figs. 2–5, the initial disk radii were 5 mm and the initial thicknesses were of the order of 0.85 mm. Thus, considering a disk subjected to a single turn so that N = 1, it follows that the equivalent strain imposed within an annulus located at a distance of 1 mm from the center of the disk is eeq 4.3. By comparison, for samples processed by ECAP using a conventional die with an angle of 90 between the two parts of the channel, it can be shown that an equivalent strain of 1 is imposed in each pass through the die [31] and therefore a total strain of 4 is imposed on the billet after four passes. Inspection of the earlier data for the development of homogeneity in samples of pure aluminum processed by ECAP showed that the measured value of Hv increases abruptly in the first pass of ECAP when the imposed strain is 1, it increases further by a minor amount in the second pass when the imposed strain is 2 and thereafter it decreases by a very minor amount up to a total of eight passes and an imposed strain of 8 [15]. In addition, an earlier examination by TEM revealed the development of a homogeneous microstructure of equiaxed grains after four passes of ECAP, equivalent to a strain of 4, with an average grain size of 1.3 lm [16,17]. Combining these earlier observations for samples processed by ECAP with the present calculations for HPT, it is reasonable to anticipate that HPT will lead to a region of inhomogeneity near the center of the disk after N = 1 turn, with the inhomogeneity extending outwards from the center to a radius of 1 mm. Inspection shows this prediction is in good agreement with the general appearance of the color-coded maps in Figs. 3a and 4a. In addition, the measured average grain size of 1.2 lm in the outer region of the disk in Fig. 6b after HPT is consistent with the equilibrium grain size of 1.3 lm recorded for samples pressed through four passes in ECAP [16,17]. The preceding analysis leads to the conclusion that materials having very high rates of recovery, such as high-purity Al, will achieve a saturation condition at the edge of the disks within a single turn of HPT. This conclusion is supported by the values of the microhardness after HPT which, according to Table 1, are 40–41 at the edge of the disks after one to five turns, a value similar to the average value of Hv 42 reported earlier for the same material processed through ECAP for four passes [15]. Although there is a general agreement between the earlier results obtained from ECAP [15] and the present data from HPT, it is important to recognize that the comparison is incomplete because the imposed pressure, which leads to an immediate reduction in disk thickness prior to straining,
C. Xu et al. / Acta Materialia 55 (2007) 203–212
is not incorporated in the analysis. Measurements show the thickness of the disk decreases during the initial application of pressure in the absence of any torsional straining and the thickness further decreases in the subsequent straining. For example, measurements on a disk with an initial thickness of 0.850 mm showed that the instantaneous thickness was reduced to 0.805 mm on application of a pressure of P = 1.25 GPa and was further reduced to 0.745 mm after torsional straining through N = 1 turn. It is concluded that the presence of a high applied pressure causes straining in the central region of the disk and, because the strain at the center is necessarily lower in magnitude than in the peripheral region, microstructural evolution occurs in pure aluminum more slowly near the center of the disk than at the edge. This means in practice that the recovery processes in the central region are delayed until the disk has gone through larger numbers of turns, and this is consistent with the gradual evolution in the values of Hv to a steady-state condition across the disks at large numbers of turns as demonstrated for five turns in Fig. 2c. This approach also provides a qualitative explanation for the experimental results shown in Figs. 3 and 4 for N = 1 turn, where the width of the central region of higher microhardness covers a larger area in the disk subjected to a pressure of P = 1.25 GPa than in the disk subjected to a pressure of P = 6.0 GPa. Thus, the compressive strain is smaller under the lower applied pressure and the sample is therefore at an earlier stage in the evolutionary cycle. Finally, despite the similarity in the grain sizes measured at the edge of the disk after HPT and after processing by ECAP through four passes, there is general agreement that HPT leads consistently to grain sizes which are smaller than in ECAP and this trend is supported by the observation that samples processed by ECAP may experience additional grain refinement by subsequent processing using HPT [4,32,33].
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Fig. 8. Schematic illustration of the variation of microhardness with strain in HPT or ECAP for materials having either slow (curve A) or fast (curve B) rates of recovery.
increase with increasing strain in HPT or ECAP until a saturation strain is reached at a significantly higher strain than in pure aluminum; this condition is shown by curve A in Fig. 8. Examples of these two conditions were given earlier when experimental measurements of Hv showed that the high-purity aluminum used in the present experiments followed curve B and a commercial Al-6061 alloy followed curve A [15]. For materials where the stacking fault energies are low and the recovery processes are generally slow, a higher hardness will be reached in the peripheral region in the early stages of HPT testing and this means that, as reported in numerous experiments [6–13], the hardness in HPT disks will be initially higher at the edge and lower in the center. The results from this analysis are depicted schematically for low total strains in Fig. 9 in terms of the distributions
4.4. The evolution of microstructures in different materials processed by HPT High-purity aluminum has a high stacking fault energy and it is well established that recovery occurs very easily. By contrast, other materials, such as Ni, steel and aluminum-based alloys, have lower stacking fault energies and/ or slower recovery rates. This means in practice that microstructural evolution occurs at a slower rate in these materials in HPT and higher imposed strains are needed in order to achieve microstructural homogeneity. The situation is illustrated schematically for different materials in Fig. 8 where the Vickers microhardness is depicted as a function of the imposed strain in either HPT or ECAP. For a material having a rapid recovery rate such as high-purity Al, the microhardness increases initially, remains almost constant after the saturation strain of 4 and thereafter remains reasonably constant; this is shown by curve B in Fig. 8. For a material having a slow recovery rate, the microhardness continues to
Fig. 9. Schematic illustration of the variation of the Vickers microhardness across the disk at low total strains in HPT for materials having either slow (lower) or fast (upper) rates of recovery.
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of microhardness values along the diameters of disks processed by HPT for two different materials having slow and fast recovery rates corresponding to low and high stacking fault energies. For a material where recovery is rapid, as in the pure aluminum used in this investigation, the hardness is higher in the center of the disk in the initial stages of HPT and there is a more refined and deformed microstructure in the central region. By contrast, for a material where recovery is slow, the hardness is initially lower in the center but gradually the microstructure evolves into a homogeneous condition. An example of this latter evolution is given by data reported for high-purity Ni [12,13]. For both types of material, it is anticipated the microstructures will ultimately evolve and become essentially homogeneous throughout the disks at high imposed strains. 5. Summary and conclusions 1. Samples of high-purity aluminum were processed by HPT at room temperature for up to five turns under pressures of 1.25, 2.5 and 6.0 GPa. Measurements of the Vickers microhardness, Hv, were recorded across the diameters of each disk and on polished surfaces following a rectilinear grid pattern. The latter results are presented as color-coded maps depicting the distributions of the hardness values across the surfaces. 2. After a single turn in HPT, the overall average hardness increases by a factor of 2, but the hardness values tend to be low around the edge of the disk and high in the central region. The inhomogeneity in the central region decreases with increasing numbers of turns so that ultimately, after five turns, the hardness distribution are essentially homogeneous. 3. The results show the hardness distributions depend upon both the numbers of turns and the imposed pressure. For pure Al, optimal homogeneity is achieved after five turns when using the lowest pressure of 1.25 GPa. 4. These results contrast with earlier reports in HPT where the central regions of the disks generally have a lower hardness than in the peripheral region. This difference is explained through a model in which the degree of hardness is dependent upon the rate of recovery and thus upon the value of the stacking fault energy in the material. Using this model for HPT processing, it is predicted there will be significant differences in the microstructures observed across the disks after low numbers of turns for materials having fast and slow rates of recovery. Acknowledgements This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan and in part by the Na-
tional Science Foundation of the United States under Grant No. DMR-0243331. References [1] Valiev RZ, Islamgaliev RK, Alexandrov IV. Prog Mater Sci 2000;45:103. [2] Valiev RZ, Estrin Y, Horita Z, Langdon TG, Zehetbauer MJ, Zhu YT. JOM 2006;58(4):33. [3] Zhilyaev AP, Kim BK, Nurislamova GV, Baro´ MD, Szpunar JA, Langdon TG. Scripta Mater 2002;46:575. [4] Zhilyaev AP, Kim BK, Szpunar JA, Baro´ MD, Langdon TG. Mater Sci Eng 2005;A391:377. [5] Bridgman PW. Studies in large plastic flow and fracture. New York, NY: McGraw-Hill; 1952. [6] Vorhauer A, Pippan R. Scripta Mater 2004;51:921. [7] Jiang H, Zhu YT, Butt DP, Alexandrov IV, Lowe TC. Mater Sci Eng 2000;A290:128. [8] Yang Z, Welzel U. Mater Lett 2005;59:3406. [9] Zhilyaev AP, Oh-ishi K, Langdon TG, McNelley TR. Mater Sci Eng 2005;A410–411:277. [10] Sakai G, Horita Z, Langdon TG. Mater Sci Eng 2005;A393:344. [11] Horita Z, Langdon TG. Mater Sci Eng 2005;A410–411:422. [12] Zhilyaev AP, Lee S, Nurislamova GV, Valiev RZ, Langdon TG. Scripta Mater 2001;44:2753. [13] Zhilyaev AP, Nurislamova GV, Kim BK, Baro´ MD, Szpunar JA, Langdon TG. Acta Mater 2003;51:753. [14] Xu C, Langdon TG. Scripta Mater 2003;48:1. [15] Xu C, Furukawa M, Horita Z, Langdon TG. Mater Sci Eng 2005;A398:66. [16] Iwahashi Y, Horita Z, Nemoto M, Langdon TG. Acta Mater 1997;45:4733. [17] Iwahashi Y, Horita Z, Nemoto M, Langdon TG. Acta Mater 1998;46:3317. [18] Horita Z, Fujinami T, Nemoto M, Langdon TG. Metall Mater Trans 2000;31A:691. [19] Wang J, Horita Z, Furukawa M, Nemoto M, Tsenev NK, Valiev RZ, et al. J Mater Res 1993;8:2810. [20] Oh-ishi K, Horita Z, Smith DJ, Langdon TG. J Mater Res 2001;16:583. [21] Horita Z, Smith DJ, Furukawa M, Nemoto M, Valiev RZ, Langdon TG. J Mater Res 1996;11:1880. [22] Valiev RZ, Krasilnikov NA, Tsenev NK. Mater Sci Eng 1991;A137:35. [23] Valiev RZ, Korznikov AV, Mulyukov RR. Mater Sci Eng 1993;A168:141. [24] Wang J, Iwahashi Y, Horita Z, Furukawa M, Nemoto M, Valiev RZ, et al. Acta Mater 1996;44:2973. [25] Furukawa M, Horita Z, Nemoto M, Valiev RZ, Langdon TG. Acta Mater 1996;44:4619. [26] Horita Z, Smith DJ, Nemoto M, Valiev RZ, Langdon TG. J Mater Res 1998;13:446. [27] Chang CP, Sun PL, Kao PW. Acta Mater 2000;48:3377. [28] Valiev RZ, Alexandrov IV, Zhu YT, Lowe TC. J Mater Res 2002;17:5. [29] Zhao YH, Liao XZ, Jin Z, Valiev RZ, Zhu YT. Acta Mater 2004;52:4589. [30] Wetscher F, Vorhauer A, Stock R, Pippan R. Mater Sci Eng 2004;A387–389:809. [31] Iwahashi Y, Wang J, Horita Z, Nemoto M, Langdon TG. Scripta Mater 1996;35:143. [32] Stolyarov VV, Zhu YT, Lowe TC, Islamgaliev RK, Valiev RZ. Nanostruct Mater 1999;11:947. [33] Zhilyaev AP, Baro´ MD, Langdon TG, McNelley TR. Rev Adv Mater Sci 2004;7:41.
The evolution of homogeneity in processing by high-pressure torsion Cheng Xu a, Zenji Horita b, Terence G. Langdon
a,*
a
b
Departments of Aerospace and Mechanical Engineering and Materials Science, University of Southern California, 3650 McClintock Avenue, Los Angeles, CA 90089-1453, USA Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan Received 3 April 2006; received in revised form 26 July 2006; accepted 26 July 2006 Available online 17 October 2006
Abstract Disks of high-purity aluminum were processed by high-pressure torsion (HPT) at room temperature under different conditions of imposed pressure and numbers of turns. Measurements were taken of the microhardness values both along diameters in each disk and following a rectilinear grid pattern to give color-coded maps of the hardness distributions. The results show the hardness increases by a factor of 2 in the first turn of HPT but the microhardness distribution is inhomogeneous because higher values of hardness are recorded in the central regions of the disks. This central region of inhomogeneity decreases with increasing numbers of turns so that the hardness distribution becomes essentially homogeneous after five turns. The results are different from earlier reports in HPT where the central regions of the disks have a lower hardness. The results are interpreted using a model in which the degree of hardness depends upon the rate of recovery in the material. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Hardness; High-pressure torsion; Homogeneity; Severe plastic deformation; Ultrafine grains
1. Introduction Processing through the application of severe plastic deformation (SPD) is now an accepted procedure for producing significant grain refinement in bulk crystalline solids [1,2]. The two SPD processing techniques of equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) are currently receiving considerable attention, and it is well established that both procedures are effective in reducing the grains to at least the submicrometer level. However, processing by HPT is an especially attractive procedure because, by comparison with ECAP, it leads both to smaller grains and to a higher fraction of high-angle grain boundaries [3,4]. The principle of HPT is based on the procedures first developed by Bridgman [5]. A sample, generally in the form of a thin disk, is placed between two large anvils and subjected to a high pressure and concurrent torsional strain*
Corresponding author. Tel: +1 213 740 0491; fax: +1 213 740 8071. E-mail address: [email protected] (T.G. Langdon).
ing. Thus, the two important parameters in HPT are the magnitude of the imposed pressure, P, and the number of revolutions applied to the sample, N. An important limitation in HPT is that the imposed strain varies across the sample and, in principle at least, the strain is reduced to zero in the center of the disk. As a consequence of this variation, it is reasonable to anticipate that the microstructures produced by HPT will be extremely inhomogeneous. Nevertheless, the experimental data available to date reveal a clear dichotomy. First, there are reports for austenitic steel [6], Cu [7] and high-purity Ni [8] of significant variations in the values of the microhardness across the diameters of disks processed by HPT, with lower hardness values in the centers of the disks and higher values in the peripheral regions. Second, there are reports for commercial purity Al [9], an Al–Mg–Sc alloy [10], Cu [11] and high-purity Ni [12,13] showing that, although the microhardness is lower in the central region in the early stages of deformation, the microstructures become reasonably homogeneous across the disks when torsional
1359-6454/$30.00 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.07.029
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straining is continued to a sufficiently high total strain under a high imposed pressure. All of the results available to date have relied upon taking measurements of the microhardness values along diameters of the disks after HPT processing. However, an alternative approach was developed recently for evaluating microstructural inhomogeneities in samples processed by ECAP [14,15]. In this procedure, microhardness values are recorded in a rectilinear grid pattern across the sectioned planes of samples processed by ECAP and the results are plotted in the form of color-coded contour maps to give a clear and visual presentation of the local variations in hardness over the sections. The present investigation was initiated in order to undertake similar measurements on disks processed by HPT under different conditions of pressure and numbers of revolutions, with the overall objective of determining whether the microstructures generated by HPT evolve into truly homogeneous arrays at sufficiently high strains and pressures. As will be demonstrated, the results of this study provide important new information on the development of homogeneity during HPT processing by showing there is a direct link between microstructural evolution in HPT and the rate of recovery within the material. 2. Experimental materials and procedures The experiments were conducted using pure aluminum of 99.99% purity where this material was selected because it was used earlier both for an extensive study of microstructural evolution in ECAP [16,17] and for the development of color-coded maps showing the microhardness distributions after processing by ECAP [14,15]. An aluminum ingot was rolled at room temperature into a plate having a thickness of 25 mm. It was then cut into a block with a cross-section of 25 · 25 mm2 and swaged into cylindrical rods of 10 mm diameter. Samples were obtained for HPT by slicing the rods into disks 0.85 mm thick. All of the disks were annealed at 773 K for 1 h prior to HPT to give an initial grain size of 1 mm. The HPT processing was performed at room temperature (298 K) using a facility described elsewhere [10]. Briefly, the facility consisted of an upper and lower anvil made from high-strength YXR3 tool steel with nitrided surfaces. A spherical depression was machined at the center of each anvil to a depth of 0.25 mm and with a diameter of 10 mm. For testing, the Al disk was placed in the depression on the lower anvil, the two anvils were brought into position to impose a pressure on the disk and the disk was then concurrently compressed and torsional straining was achieved through rotation of the lower anvil at a speed of 1 r.p.m. Because of the high pressure, there was a small reduction in the thickness of the disk during processing: measurements showed the thickness was reduced after the first turn and there was an additional reduction in thickness during torsional straining up to a total of five turns. Three different
applied loads were used in these experiments: 10, 20 and 48 tons, corresponding to imposed pressures, P, of 1.25, 2.5 and 6.0 GPa, respectively. The disks were subjected to HPT under these various pressures for totals of one, three or five turns. Following HPT, the disks were mounted and carefully polished to a mirror-like surface. All of the microhardness measurements were taken using an FM-1e microhardness tester equipped with a Vickers indenter. Two different procedures were used for taking measurements of the Vickers microhardness, Hv. First, individual values of the microhardness were measured across the diameter of each disk in incremental steps as depicted schematically for half of the diameter in Fig. 1. Measurements were taken to determine the average values of Hv at positions separated by 0.3 mm up to a distance of 1.2 mm on either side of the center of each disk; at greater distances from the center the measurements were taken at increments of 0.6 mm. It should be noted that special attention was given to the central region because this is the critical area where there is uncertainty in the published data. At each selected point, shown by the small open circle in Fig. 1, the hardness was estimated by averaging the measurements recorded for four separate points uniformly positioned around the selected point at a distance of 0.15 mm. These measurements provided detailed information on the variation of Hv across the diameter of each disk after testing under different conditions. Second, individual measurements of Hv were recorded on the surface of each disk following a regular grid pattern with a spacing of 0.3 mm between each separate point. The individual values of Hv were then plotted as color-coded contour maps depicting the variations in the local hardness across the surface of each sample. The internal microstructures of the disks were examined by transmission electron microscopy (TEM) after processing by HPT. After torsional straining, the disks were ground mechanically to a thickness of 100 lm, punched into disks with diameters of 3 mm and then thinned to perforation using a twin-jet electropolishing facility with a solution of HClO4, C6H14O2 and C2H5OH at a temperature of 278 K. All TEM observations were made using a Philips 420 microscope operating at 120 kV. The microstructures were observed after HPT both in the central regions of the disks and near the edges of the as-processed disks. The grain sizes were estimated from direct measurements of individual grains in the microscope.
Fig. 1. Schematic illustration of procedure for taking microhardness measurements along the diameter of the sample after HPT.
C. Xu et al. / Acta Materialia 55 (2007) 203–212
3. Experimental results 3.1. The variation of average microhardness across the disk diameter Following the procedure illustrated in Fig. 1, the values of the Vickers microhardness were measured across the diameter of each separate disk and the results are summarized in Table 1, where the error bars denote the 95% confidence limit. For comparison purposes, the upper row gives the hardness values, and the associated error bars, for measurements taken on an as-received disk prior to HPT. It is apparent from inspection of the results in Table 1 that both the values of Hv and the error bars vary in a systematic and consistent way for all of the disks. Several trends are visible. First, the error bars are highest in the centers of the disk for all samples subjected to HPT and they become significantly lower at the periphery. Second, the error bars in the central region decrease with increasing numbers of turns, thereby indicating a greater microstructural uniformity after larger numbers of revolutions. Third, the average values recorded for Hv in the peripheral regions are reasonably similar for all of the disks processed by HPT despite significant differences in the imposed pressure, P, and the number of turns, N. The data are presented pictorially in Fig. 2, where Hv is plotted against the distance from the center and data are shown for three different experimental conditions: (a) for P = 1.25 GPa for one, three and five turns; (b) for P = 6.0 GPa for one, three and five turns; and (c) for five turns at pressures of 1.25, 2.5 and 6.0 GPa, where the lower solid points correspond to the as-received unprocessed condition. The first important conclusion is that, for all testing conditions, the hardness increases significantly after one turn of HPT by a factor which is slightly less than 2. This increase in hardness is comparable to the increase in strength typically recorded in a range of aluminum-based alloys after processing through one pass in ECAP [18]. The second important conclusion is that, contrary to the results described earlier for several different materials [6–13], the centers of the disks for pure aluminum exhibit values of Hv which are higher, rather than lower, than the surrounding areas. Furthermore, there is a general con-
205
sistency in the results since similar high values of Hv are visible in the central region at N = 1 turn for pressures of both 1.25 GPa in Fig. 2a and 6.0 GPa in Fig. 2b. The third conclusion is that the values of Hv at the outer periphery undergo no significant change either as a function of increasing number of turns in Fig. 2a and b or as a function of increasing pressure in Fig. 2c. By contrast, the relatively high values recorded for Hv in the central region decrease in subsequent passes so that ultimately, after five turns, the values of Hv are reasonably similar across all of the disks. The fourth conclusion from Fig. 2 is that the sample processed at 1.25 GPa exhibits a lower central peak after one turn and ultimately, after five turns, this sample shows less variation in Hv than the two samples tested to five turns at the higher pressures. 3.2. The variation of microhardness across the disk surface All of the microhardness measurements taken from the rectilinear grid patterns at incremental spacings of 0.3 mm were plotted as color-coded contour maps to provide pictorial displays of the distributions of the individual hardness values for each testing condition. These plots are shown in Fig. 3 for a pressure of 1.25 GPa, in Fig. 4 for a pressure of 6.0 GPa and in Fig. 5 for the sample pressed to five turns under a pressure of 2.5 GPa, where X and Y denote two arbitrary orthogonal axes marked in mm so that the position (0, 0) is at the mid-point of each disk. The distributions of the individual microhardness values are represented pictorially through a series of colors which define, in incremental values of Hv = 5, the values of Hv in the range from 30 to 60: the scale for these colors are the same in Figs. 3–5 and they are shown in the small insets inserted in each display. For the samples subjected to HPT at a pressure of 1.25 GPa, Fig. 3 shows the individual distributions of Hv after one, three and five turns. Inspection shows there is a relatively large area in the center of the disk after one turn, extending to a width of 4 mm, where the hardness values are exceptionally high. Thus, there is significant inhomogeneity in this condition but, unlike the results reported earlier for several metals [6–13], the hardness in the central region is higher than around the edge. Increasing the strain
Table 1 Summary of microhardness values and error bars at the 95% confidence level along the radius of pure Al processed by HPT HPT condition
Distance from center (mm)
P (GPa)
0
N (turns)
0.3
0.6
0.9
1.2
1.8
2.4
3.0
3.6
4.2
4.8
As-received
25.5 ± 0.6
25.3 ± 0.4
25.4 ± 0.6
25.4 ± 0.5
25.1 ± 0.7
24.4 ± 0.8
25.0 ± 0.6
25.4 ± 0.3
25.0 ± 0.4
25.6 ± 0.6
–
1.25
1 3 5
44.1 ± 7.5 46.0 ± 5.8 38.6 ± 1.8
44.4 ± 3.0 44.5 ± 2.8 39.2 ± 0.8
47.5 ± 2.3 39.8 ± 3.8 39.2 ± 0.6
48.8 ± 1.5 39.7 ± 2.8 39.2 ± 0.8
47.7 ± 2.7 39.5 ± 1.0 39.5 ± 0.8
44.4 ± 2.1 41.8 ± 1.2 39.1 ± 0.3
40.7 ± 0.7 39.7 ± 0.9 39.9 ± 0.6
39.8 ± 1.1 41.0 ± 1.0 39.8 ± 0.8
39.3 ± 0.8 40.7 ± 1.1 40.4 ± 0.6
40.0 ± 0.7 40.3 ± 2.0 41.6 ± 3.1
39.8 ± 0.7 40.3 ± 1.8 40.2 ± 1.5
2.5
5
40.9 ± 4.7
39.7 ± 1.0
39.5 ± 1.0
38.3 ± 2.9
40.4 ± 1.0
40.4 ± 0.7
40.9 ± 0.8
40.9 ± 1.3
41.4 ± 0.5
41.0 ± 1.4
40.9 ± 0.9
6.0
1 3 5
45.5 ± 10.5 41.7 ± 6.3 41.7 ± 3.9
47.2 ± 6.0 43.5 ± 3.5 42.1 ± 2.8
47.8 ± 6.0 42.5 ± 3.3 41.5 ± 2.7
46.7 ± 5.4 39.9 ± 1.2 40.3 ± 0.7
44.3 ± 5.2 40.3 ± 0.7 39.9 ± 0.6
40.3 ± 1.7 40.6 ± 0.7 40.8 ± 0.6
40.7 ± 0.6 41.2 ± 0.9 40.7 ± 1.0
39.8 ± 0.8 40.8 ± 1.1 39.4 ± 2.8
40.3 ± 0.7 40.4 ± 0.7 41.4 ± 2.9
39.6 ± 0.9 41.1 ± 1.4 42.4 ± 0.7
39.8 ± 1.1 41.3 ± 1.1 41.7 ± 0.8
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C. Xu et al. / Acta Materialia 55 (2007) 203–212
imposed on the sample decreases the size of the region of high hardness so that it is reduced to a diameter of <0.5 mm at three turns and disappears after five turns. Thus, the results in Fig. 3 provide unambiguous evidence for the gradual development of a homogeneous microstructure in pure Al with increasing numbers of turns. After five turns, the display in Fig. 3 shows a fully homogeneous microstructure except for some very minor regions of slightly higher hardness at various randomly located points within a region of 1 mm from the edge of the disk. Similar displays are given in Fig. 4 for disks subjected to a pressure of 6.0 GPa and again there is a region of higher hardness in the center of the disk after one turn, but at this increased pressure the region of higher hardness is restricted to a smaller region with a diameter of 1.2 mm. This area of high hardness is again reduced with additional straining to the extent that it covers a region having a diameter of 0.3 mm after three turns, and it is further reduced almost to a point after five turns. A comparison of the displays after five turns for pressures of 1.25 and 6.0 GPa in Figs. 3 and 4 suggests that the development of a homogeneous structure occurs equally rapidly, and possibly even more rapidly, at the lower pressure. The display in Fig. 5 for five turns under an imposed pressure of 2.5 GPa is consistent with these trends because the microstructure is reasonably homogeneous, there is an extremely small region with higher hardness immediately in the center of the disk and there is a general degree of homogeneity which is intermediate between the two displays for disks taken through five turns presented in Figs. 3 and 4. This display also confirms that the presence of some degree of inhomogeneity around the periphery of the disk becomes more apparent with increasing pressure. 3.3. Microstructural observations after HPT
Fig. 2. The average Vickers microhardness, Hv, versus distance from the center of the disk after processing by HPT: (a) after various turns with a pressure of 1.25 GPa, (b) after various turns with a pressure of 6.0 GPa and (c) using various pressures after five turns.
In the early stages of processing, the present results show that the microhardness values vary with the distance from the center of the disk whereas at higher strains, associated with a total of at least N = 5 turns, the hardness values become essentially homogeneous. These results are consistent with earlier studies in demonstrating a dependence on position within the disk in the earlier stages of HPT processing but they are different because all previous investigations have reported regions of lower hardness near the disk centers at the lower strains [6,13]. Furthermore, in many of the earlier studies the hardness results were supported by extensive microstructural observations, usually by TEM, showing the presence of smaller grains at the edges of the disks [8–13]. Accordingly, it was important in this investigation to inspect the microstructures both in the central regions and near the edges of the disks. Fig. 6 shows representative microstructures taken on the sample processed by one turn at a pressure of 1.25 GPa in two different positions: (a) near the center of the disk and (b) near the edge of the disk. It is immediately apparent that these two areas have very different microstructures.
C. Xu et al. / Acta Materialia 55 (2007) 203–212
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Fig. 3. Color-coded contour maps showing the Vickers microhardness across the surface of the disks processed by HPT at a pressure of 1.25 GPa for: (a) one, (b) three and (c) five turns. The significance of the colors is shown by the small inset.
First, the grains are visibly smaller in the central area by comparison with the edge, and measurements gave average grain sizes of 0.8 lm in the center and 1.2 lm at the periphery. Second, the appearance of the grains is different in these two areas. Detailed inspection showed that in the central region the boundaries were irregular, there were distortions and bending of the lattice fringes, and there was a high density of intragranular dislocations. All of these observations are consistent with the presence of highenergy non-equilibrium boundaries as reported earlier after processing by ECAP [19,20] and HPT [21]. By contrast, the grains were clear and almost free of intragranular dislocations near the edge of the disk, the grain boundaries were
well defined and the overall appearance was consistent with a low-energy microstructure after significant recovery. 4. Discussion 4.1. The development of homogeneity during HPT The results from these measurements reveal two trends which are important for materials processed by HPT. First, and consistent with the trends suggested in earlier studies of processing by HPT [9–13], the values of the local microhardness evolve during HPT processing and ultimately, after five turns for pure Al, the hardness values
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C. Xu et al. / Acta Materialia 55 (2007) 203–212
Fig. 4. Color-coded contour maps showing the Vickers microhardness across the surface of the disks processed by HPT at a pressure of 6.0 GPa for: (a) one, (b) three and (c) five turns. The significance of the colors is shown by the small inset.
are essentially identical throughout the disk. This evolution suggests, therefore, the gradual development of a homogeneous microstructure. Furthermore, this evolution is independent of the imposed pressure, at least in the range from 1.25 to 6.0 GPa, although the results suggest that the homogeneity may evolve more rapidly at the lower pressures. The ability to achieve homogeneity in HPT, despite the variation in the local strain imposed by the torsional rotation, is probably due to a combination of the shearing strain achieved in torsion and the compressive strain introduced by the imposed pressure. However, more detailed studies are needed to obtain a complete and definitive understanding of the nature of grain refinement in HPT.
Second, and contrary to earlier studies [6–13], the values of the microhardness are higher in the central region in the early stages of processing. The hardness in this region subsequently decreases and becomes equal to the level of hardness at the edge of the disk. This result is unexpected because, although there is general agreement that the straining induced in the disk samples by HPT processing varies with position within the disk, all previous reports documented examples where the hardness values in the central regions were exceptionally low [6–13]. Before attempting to understand these results, it is first necessary to determine whether the microhardness
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209
the evolution in the color-coded contour maps with increasing numbers of turns visible in Figs. 3 and 4, suggests that the data probably reflect genuine microstructural features of the as-processed disks. To evaluate the possibility of the occurrence of some recovery in the post-HPT period immediately after processing, two additional disks were pressed through one and five turns using a pressure of P = 1.25 GPa and microhardness measurements were taken immediately following the straining operation. The results for these two tests yielded data that were essentially identical, at least within the levels of the experimental errors, with the results shown for one and five turns in Fig. 2a, thereby confirming that postHPT recovery plays little or no significant role in the present investigation. This conclusion is consistent also with published data for commercial-purity aluminum, where a slightly smaller grain size was measured in the central region of a disk subjected to N = 8 turns with a pressure of P = 1 GPa due, it was suggested, to the onset of recovery in the highly strained outer peripheral region [9]. Fig. 5. Color-coded contour map showing the Vickers microhardness across the surface of the disk processed by HPT at a pressure of 2.5 GPa for five turns. The significance of the colors is shown by the small inset.
4.2. Factors influencing the development of homogeneity in HPT
measurements documented in Figs. 2–5 truly represent the microstructural characteristics introduced into the disks during torsional straining or whether instead they are a consequence of significant recovery occurring in the highly strained outer regions of the disks during the post-HPT period which elapsed between performing the straining and taking the measurements. In this respect, it is important to note that, although no attempt was made in this investigation to control precisely the period of time between torsional straining and measuring the microhardness values, the general overall consistency of all of the experimental data presented in Figs. 2–5, and especially
Two factors influence the development of homogeneity in HPT: the values of the imposed pressure and the values of the imposed strain, where the latter is represented by the numbers of turns. The effect of the number of turns, N, is easily seen using Figs. 3 and 4. When the number of turns is increased, the inhomogeneity in the central region is gradually removed. This suggests that the microstructure around the periphery is in a reasonably equilibrated condition such that there is little effect of additional straining. The effect of the imposed pressure, P, is shown in Fig. 2c and through a comparison of the color-coded contour maps for N = 5 in Figs. 3–5.
Fig. 6. Representative microstructures recorded by TEM after processing for one turn with a pressure of 1.25 GPa: (a) near the center of the disk and (b) near the edge of the disk.
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C. Xu et al. / Acta Materialia 55 (2007) 203–212
These maps show that greater homogeneity is attained at the lowest pressure of P = 1.25 GPa. In examining these data, it is important to note that the high values recorded for Hv in the vicinity of the centers of the disks are mutually consistent with the smaller grain sizes and with the more highly deformed structures occurring in this region, as shown in Fig. 6a. Similarly, the earlier reports of lower values of Hv in the central region were consistent with TEM observations showing larger grains in this region by comparison with the edges of the disks [8– 13]. Thus, all of the experimental observations reported to date are mutually consistent but there is a very clear dichotomy between the earlier data reported for several materials and the present results obtained on high-purity aluminum. In order to understand the reason for this difference, it is instructive to compare the present results with data reported earlier for the same pure aluminum when processed by ECAP [15]. 4.3. The interrelationship between HPT and ECAP Inspection of Fig. 6 shows that the microstructures are markedly different in the central region and at the edge of the disk. Near the center, in Fig. 6a, the highly deformed microstructure is similar to that observed in a very wide range of ultrafine-grained materials produced through processing by ECAP or HPT [19,21–29]. At the edge, shown in Fig. 6b, the microstructure is similar to a fully annealed condition after the occurrence of extensive recovery. These differences, when combined with the microstructural observations made immediately after HPT processing as described in Section 4.1, suggest that in situ recovery plays an important role in microstructural evolution during HPT processing. Furthermore, it is well known that high-purity aluminum has a very high rate of recovery by comparison with Cu, steel and most other materials. It is possible to make a direct comparison between HPT and ECAP by recognizing that, although in HPT the imposed strain is reduced to zero at the center of the disk, a comparison is possible by making separate determinations of the approximate levels of the imposed strains in the HPT disks and in billets processed by ECAP. Fig. 7 depicts a typical disk of radius r and height h used in HPT. On torsional straining, the incremental shear strain, dc, is given by d‘/h, where d‘ is the total displace-
Fig. 7. The principle for estimating the strain on a disk of thickness h and radius r.
ment, and this strain may be expressed as rdh/h, where dh is the angular rotation around the mid-point [6,13]. Since h is given by 2pN, the equivalent von Mises strain, eeq, is given by [30] 2pNr eeq ¼ pffiffiffi h 3
ð1Þ
In the present investigation for the disks used to construct Figs. 2–5, the initial disk radii were 5 mm and the initial thicknesses were of the order of 0.85 mm. Thus, considering a disk subjected to a single turn so that N = 1, it follows that the equivalent strain imposed within an annulus located at a distance of 1 mm from the center of the disk is eeq 4.3. By comparison, for samples processed by ECAP using a conventional die with an angle of 90 between the two parts of the channel, it can be shown that an equivalent strain of 1 is imposed in each pass through the die [31] and therefore a total strain of 4 is imposed on the billet after four passes. Inspection of the earlier data for the development of homogeneity in samples of pure aluminum processed by ECAP showed that the measured value of Hv increases abruptly in the first pass of ECAP when the imposed strain is 1, it increases further by a minor amount in the second pass when the imposed strain is 2 and thereafter it decreases by a very minor amount up to a total of eight passes and an imposed strain of 8 [15]. In addition, an earlier examination by TEM revealed the development of a homogeneous microstructure of equiaxed grains after four passes of ECAP, equivalent to a strain of 4, with an average grain size of 1.3 lm [16,17]. Combining these earlier observations for samples processed by ECAP with the present calculations for HPT, it is reasonable to anticipate that HPT will lead to a region of inhomogeneity near the center of the disk after N = 1 turn, with the inhomogeneity extending outwards from the center to a radius of 1 mm. Inspection shows this prediction is in good agreement with the general appearance of the color-coded maps in Figs. 3a and 4a. In addition, the measured average grain size of 1.2 lm in the outer region of the disk in Fig. 6b after HPT is consistent with the equilibrium grain size of 1.3 lm recorded for samples pressed through four passes in ECAP [16,17]. The preceding analysis leads to the conclusion that materials having very high rates of recovery, such as high-purity Al, will achieve a saturation condition at the edge of the disks within a single turn of HPT. This conclusion is supported by the values of the microhardness after HPT which, according to Table 1, are 40–41 at the edge of the disks after one to five turns, a value similar to the average value of Hv 42 reported earlier for the same material processed through ECAP for four passes [15]. Although there is a general agreement between the earlier results obtained from ECAP [15] and the present data from HPT, it is important to recognize that the comparison is incomplete because the imposed pressure, which leads to an immediate reduction in disk thickness prior to straining,
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is not incorporated in the analysis. Measurements show the thickness of the disk decreases during the initial application of pressure in the absence of any torsional straining and the thickness further decreases in the subsequent straining. For example, measurements on a disk with an initial thickness of 0.850 mm showed that the instantaneous thickness was reduced to 0.805 mm on application of a pressure of P = 1.25 GPa and was further reduced to 0.745 mm after torsional straining through N = 1 turn. It is concluded that the presence of a high applied pressure causes straining in the central region of the disk and, because the strain at the center is necessarily lower in magnitude than in the peripheral region, microstructural evolution occurs in pure aluminum more slowly near the center of the disk than at the edge. This means in practice that the recovery processes in the central region are delayed until the disk has gone through larger numbers of turns, and this is consistent with the gradual evolution in the values of Hv to a steady-state condition across the disks at large numbers of turns as demonstrated for five turns in Fig. 2c. This approach also provides a qualitative explanation for the experimental results shown in Figs. 3 and 4 for N = 1 turn, where the width of the central region of higher microhardness covers a larger area in the disk subjected to a pressure of P = 1.25 GPa than in the disk subjected to a pressure of P = 6.0 GPa. Thus, the compressive strain is smaller under the lower applied pressure and the sample is therefore at an earlier stage in the evolutionary cycle. Finally, despite the similarity in the grain sizes measured at the edge of the disk after HPT and after processing by ECAP through four passes, there is general agreement that HPT leads consistently to grain sizes which are smaller than in ECAP and this trend is supported by the observation that samples processed by ECAP may experience additional grain refinement by subsequent processing using HPT [4,32,33].
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Fig. 8. Schematic illustration of the variation of microhardness with strain in HPT or ECAP for materials having either slow (curve A) or fast (curve B) rates of recovery.
increase with increasing strain in HPT or ECAP until a saturation strain is reached at a significantly higher strain than in pure aluminum; this condition is shown by curve A in Fig. 8. Examples of these two conditions were given earlier when experimental measurements of Hv showed that the high-purity aluminum used in the present experiments followed curve B and a commercial Al-6061 alloy followed curve A [15]. For materials where the stacking fault energies are low and the recovery processes are generally slow, a higher hardness will be reached in the peripheral region in the early stages of HPT testing and this means that, as reported in numerous experiments [6–13], the hardness in HPT disks will be initially higher at the edge and lower in the center. The results from this analysis are depicted schematically for low total strains in Fig. 9 in terms of the distributions
4.4. The evolution of microstructures in different materials processed by HPT High-purity aluminum has a high stacking fault energy and it is well established that recovery occurs very easily. By contrast, other materials, such as Ni, steel and aluminum-based alloys, have lower stacking fault energies and/ or slower recovery rates. This means in practice that microstructural evolution occurs at a slower rate in these materials in HPT and higher imposed strains are needed in order to achieve microstructural homogeneity. The situation is illustrated schematically for different materials in Fig. 8 where the Vickers microhardness is depicted as a function of the imposed strain in either HPT or ECAP. For a material having a rapid recovery rate such as high-purity Al, the microhardness increases initially, remains almost constant after the saturation strain of 4 and thereafter remains reasonably constant; this is shown by curve B in Fig. 8. For a material having a slow recovery rate, the microhardness continues to
Fig. 9. Schematic illustration of the variation of the Vickers microhardness across the disk at low total strains in HPT for materials having either slow (lower) or fast (upper) rates of recovery.
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of microhardness values along the diameters of disks processed by HPT for two different materials having slow and fast recovery rates corresponding to low and high stacking fault energies. For a material where recovery is rapid, as in the pure aluminum used in this investigation, the hardness is higher in the center of the disk in the initial stages of HPT and there is a more refined and deformed microstructure in the central region. By contrast, for a material where recovery is slow, the hardness is initially lower in the center but gradually the microstructure evolves into a homogeneous condition. An example of this latter evolution is given by data reported for high-purity Ni [12,13]. For both types of material, it is anticipated the microstructures will ultimately evolve and become essentially homogeneous throughout the disks at high imposed strains. 5. Summary and conclusions 1. Samples of high-purity aluminum were processed by HPT at room temperature for up to five turns under pressures of 1.25, 2.5 and 6.0 GPa. Measurements of the Vickers microhardness, Hv, were recorded across the diameters of each disk and on polished surfaces following a rectilinear grid pattern. The latter results are presented as color-coded maps depicting the distributions of the hardness values across the surfaces. 2. After a single turn in HPT, the overall average hardness increases by a factor of 2, but the hardness values tend to be low around the edge of the disk and high in the central region. The inhomogeneity in the central region decreases with increasing numbers of turns so that ultimately, after five turns, the hardness distribution are essentially homogeneous. 3. The results show the hardness distributions depend upon both the numbers of turns and the imposed pressure. For pure Al, optimal homogeneity is achieved after five turns when using the lowest pressure of 1.25 GPa. 4. These results contrast with earlier reports in HPT where the central regions of the disks generally have a lower hardness than in the peripheral region. This difference is explained through a model in which the degree of hardness is dependent upon the rate of recovery and thus upon the value of the stacking fault energy in the material. Using this model for HPT processing, it is predicted there will be significant differences in the microstructures observed across the disks after low numbers of turns for materials having fast and slow rates of recovery. Acknowledgements This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan and in part by the Na-
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