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Dynamic recrystallization and dynamic recovery in pure aluminum at 583K
Acta metall, mater. Vol. 43, No. 2, pp. 723 729, 1995
Pergamon
0956-7151(94)00267-3
Copyright (<, 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0956-7151/95 $9.50 + 0.00
DYNAMIC RECRYSTALLIZATION AND DYNAMIC R E C O V E R Y IN P U R E A L U M I N U M AT 583 K H. YAMAGATA Yamaha Motor Co. Ltd, R & D Div. 2, Iwata, Shizuoka 438, Japan (Received 8 April I994) Abstract--In situ observations using transmission Laue X-ray photographs taken during 583 K compression tests were used to compare the restorative process of 99.999 mass% aluminum to that of 99.99 mass% aluminum. A regular multipeak stress oscillation, typical of dynamic recrystallization, was generated in 99.999 mass% aluminum, while monotonous work hardening, typical of dynamic recovery, were present in 99.99 mass% aluminum. Spot patterns as well as coarse recrystallized grains containing subgrains confirmed dynamic recrystallization in the 99.999 mass% aluminum. The dynamic recovery of 99.99 mass% aluminum was confirmed by asterism patterns and equiaxed subgrains. Dynamic recrystallization was found to take place only in the high purity range above 99.999 mass%. This dynamic recrystallization was considered to be the results of discontinuous dynamic recrystallization which is well known in Cu, Ni and 7-Fe.
1. INTRODUCTION Cu, Ni, and 7-Fe have dynamic recrystallization (DRX) [1-3] as a restorative process at elevated temperatures. DRX is normally accompanied by regular multipeak stress oscillations on stress-strain curves (DRX-type stres~strain curves) resulting from tests with constant strain rates or constant displacement speeds. In creep testing, DRX is accompanied by regular periodic variations in the strain rate [4, 5]. Rapidly cooling the specimens after testing yields typical DRX grains [1 3] containing both subgrains and high dislocation densities. Previous studies [1-3, 6,7] in these metals conclude that low stacking fault energy suppresses dynamic recovery (DRV) to effectively accumulate the strains required for the nucleation of new grains. In A1, despite extensive study [8, 9], multipeak stress oscillation has not been reported, and moreover, DRX grains generally have not been observed. These preceding studies have, therefore, concluded that the high stacking fault energy eliminates the necessary force driving DRX [2, 3, 68]. This conclusion, first proposed over 30 years ago [4, 10], is now established as the accepted theory. This has been reconfirmed even more recently as typified by the following two reports: Kassner & McMahon [11] did not observe multipeak stress oscillation in torsion testing at strains ranging from 0.5 to 16 in 99.999 mass% AI polycrystals (at 644 K and a strain rate of 5.04 x 10-4/s). After testing, subgrains in the elongated grains were observed. Furthermore, multitWhen the chemical composition of AI in the original paper is not described in either mass% or at.%, then only % is used to indicate purity in this report. AM 432
peak stress oscillation did not occur in torsion testing for single crystal specimens [12] (test conditions were identical to those of the polycrystal tests). Subgrains and deformation bands were observed. Dadson and Doherty [13] reported monotonous work hardening curves in compression tests up to a strain of 0.5 in 99.9999% tAl polycrystals (at 573 K and 673 K at a strain rate of 4.61 x 10 4/s); only subgrain structures were observed. As mentioned previously, without DRX, stress-strain curves exhibit monotonous work hardening (DRV-type stres~strain curves) followed by a steady state under testing with conditions of constant strain rate or constant displacement speed. Additionally, all of the above referenced papers reported similar DRV-type stress-strain behavior, Also noteworthy is that regular periodic strain rate variation has not been reported in creep testing as well [14, 15]. A few reports suggest that A1 shows DRX: Ravichandran and Prasad [17] have reported DRX grains in 99.999% AI samples during compression tests up to a strain of 0.6 (at 573 and 673 K in the strain rate range of 10 3-10/s). However, the stress-strain curves were characteristic of DRV-type curves. Excepting this, there are some reports advocating the existence of DRX in AI [18, 19] according to recrystallized grains observed after deformation. This in itself does not constitute strong evidence for DRX because static recrystallization (SRX) occurs rapidly after the deformation [20]. With the lack of definite evidence for DRX, pure AI has been classified [4] as a metal whose restorative process is limited to only DRV. Against the accepted theory, the present author happened to discover regular multipeak stress
U
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YAMAGATA:
DYNAMIC RECRYSTALL1ZAT1ON IN PURE AI
oscillations during elevated temperature deformation of zone refined 99.999 mass% AI [21]. Microstructural observations after compression tests revealed, in numerous instances, mottled recrystallized grains containing subgrains. In situ detection of D R X using the transmission Laue method has recently strengthened the case for multipeak stress oscillations caused by D R X [22]. Given this background, restorative processes were compared in 99.999 and 99.99 mass% AI at 583 K to reveal the effect of impurities. In situ observations were employed as in the previous study. Also, discussions pertaining to the topics of McQueen et al. [23] are covered in this report. 2. EXPERIMENTAL
2. I. Speeimen For in situ observation, a rectangular single crystal specimen (2 x 3 x 6 ram) was cut from a single crystal bar of 99.999 mass% purity Al (99.999 AI). The compression axis is shown in the stereographic triangle of Fig. 1. Trace impurities were identical to those of the previous testing [21]. A rectangular polycrystal specimen (the same dimensions as the single crystal specimen) was cut from a polycrystal ingot of 99.99 mass% purity AI (99.99 AI). Specifically, the impurities consist mainly of 26 mass ppm Si, 36 mass ppm Fe, 10 mass ppm Cu, and other trace elements. The specimens (7 x 7 x 10 mm) for optical microscopy observation were cut from the single crystal bar or the polycrystal ingot. All the specimens were chemically polished and annealed for 10.8 ks at 473 K in an oil bath. After this first annealing, the 99.99 AI polycrystal specimen had an average grain diameter of 2 mm. 2.2. Compression test X-ray diffraction patterns were in situ photographed during the hot compression tests. The compression equipment has already been explained in detail in the previous paper [22]. Specimens were heated with hot CO_, gas and compressed at a constant speed of 0 . 0 6 m m / m i n (1.67 × 10 4/s in the initial strain rate). The specimen temperature was controlled at 5 8 3 + 5 K. A MoSz lubricant was utilized to reduce friction. An Instron testing machine (type 1185) equipped with a furnace was used in air to prepare specimens for optical microscopy observations. The specimens were compressed up to a strain of 0.6 under a constant compression speed of 0.1 mm/min (1.67 × 10 a/s in the initial strain rate). A stress relaxation test in 99.99 AI was carried out for 480 s to obtain the SRX microstructure as shown in Fig. 7. Stress relaxation was implemented by stopping the crosshead after compression up to a 0.6 strain (at a constant compression speed of 0.1 mm/min). The temperature was controlled at 583 _+ 2.5 K. The spec-
imens were then quenched with water at r o o m temperature within 5 s after the termination of the compression test. 2.3. X-ray d(~kaetion pattern A 50 k w 4 0 mA white X-ray beam with a Cu target was used. The incident beam diameter was 2 r a m with a pin hole collimater. The distance between the film and the center of the specimen was 35 ram. The exposure periods of 60 or 300s were used. Laue-spots appear on specimens that have grains with fewer substructures. If D R X grains appear during the exposure period and the grains are not so strained, Laue-spots appear in the photographs instead of asterism of strained crystals. D R X is detected, then, as a spot pattern. The detailed background of the photographic techniques and interpretation has been given in a previous paper [22]. 2.4. Microstructural obserration After hot compression testing or stress relaxation testing, the quenched samples were immediately electro-polished with an ethanol and perchloric acid solution. The samples were then anodized with a Barker solution [24]. The cross-sectional surface horizontal to the compressive axis was observed with a polarized light optical microscope. With this technique, fine subgrains cannot be distinguished [25] as readily as using the techniques of the transmission electron microscope or the technique of channeling contrast imaging with the scanning electron microscope. But, the chosen technique was convenient for rapid specimen preparation. 3. RESULTS 3.1. D R X q f 99.999 A1 Figure 1 shows a true stress strain curve for a 99.999 A1 single crystal. Two large peaks are observed up to a strain of 0.6. The diffraction patterns in Fig. 2(a-g) correspond to the start of exposure for the strain values " a " to " g " indicated on the curve
15
8 7.5 &
g b
04
T-11
I I I [ I I I I I I I I I [ I 0.5 True strain Fig. 1. True stress true strain curve in a single crystal of 99.999 A1. The compression axis is indicated in the stereographic triangle. 0
YAMAGATA: DYNAMIC RECRYSTALLIZATION IN PURE Al
725
Fig. 3. DRX grains with (A) or without (B) subgrains at a strain of 0.6.
exposed just after unloading stress "g". The pattern of Fig. 2(g) is composed of both spots and streaks. The pattern of Fig. 2(g) indicates that coarse D R X grains have appeared in the deformed matrix. This deduction is supported by Fig. 3. The microstructure is composed of coarse D R X grains with or without subgrains. These results were also confirmed at 632 K [22]. 3.2. D R V of 99,99 Al
Fig. 2. Diffraction patterns in 99.999 A1. The exposure time in each photograph is 300 s.
of Fig. I. The characteristics in sequence are as follows: The initial diffraction pattern in Fig. 2(a) has a typical elliptical arrangement of Laue-spots+ in undeformed crystals. An asterisrn appears [Fig. 2(b)] instead of a spot pattern. The streaks in the asterism elongate [Fig. 2(c)] with increasing strain up to the first peak stress of Fig. 1. After the first peak stress, spots being to appear [Fig, 2(d)]. The pattern of Fig. 2(d) similarly diffuses with increasing strain [Fig. 2(e)], At around the second peak stress, spots appear again [Fig. 2(f)]. Finally, Fig. 2(g) was
Figure 4 shows a true stress-strain curve in the polycrystal 99.99 A1. The curve shows m o n o t o n o u s work hardening in contrast to the stress oscillation of 99.999 AI. The corresponding diffraction pattern change is given in Figs. 5(a-g). Figure 5(a) shows the initial coarse grain structure. The spots begin to diffuse with increasing strain, The photographs
15_
efl_f
7.5 *~
_
¢
-
0
~L I I I ~-I
0 ?Since the photographs in Fig. 2 were printed directly from film negatives, weak spots are less distinct due to the high contrast inherent in this technique. Compare this to the results of using a scanner [22].
g
I [ I I I I [ I ~
0.5 True strain
Fig. 4. True stress true strain curve for coarse grained 99.99 AI.
726
YAMAGATA:
DYNAMIC RECRYSTALLIZATION IN PURE A1
Fig. 6. Subgrains in the initial grains at a strain of 0.6. The original grain boundary (indicated by the arrow) is serrated.
4. DISCUSSION 4.1. Impurity effect on D R X and D R V The diffraction pattern change of 99.999 AI, as in Fig. 2, was similarly observed at 632 K in a previous paper [22]. The characteristics of 99.999 A1 can be summarized from this and the previous paper: asterisms appear in the work hardening stages, while spot patterns occur in the work softening stages.
Fig. 5. Diffraction patterns in 99.99 Al. The exposure time for d and g is 60s, and for the rest is 300s.
following Fig. 5(c) only show streaks regardless of the exposure period. Finally, Fig. 5(g), photographed just after unloading stress " g " , also consists of streaks. This sequence demonstrates that D R X does not take place at all in 99.99 AI. This result is also deduced from Fig. 6. The original grain boundary, though serrated, still establishes at the initial position, and the grain interiors are filled with subgrains. Figure 6 is indicative of a typical D R V structure usually reported in A1. Figure 7 shows the microstructure after stress relaxation testing. The exposure is composed of equiaxed SRX grains without subgrains. The SRX structure of Fig. 7 is completely different from either the D R V structure of Fig. 6 or the D R X structure of Fig. 3.
Fig. 7. Equiaxed SRX grains without subgrains alter stress relaxation (for 480 s after straining up to 0.6).
YAMAGATA: DYNAMIC RECRYSTALL1ZATION IN PURE AI The patterns consist of both spots and streaks at high strains. In 99.99 AI, however, the spot patterns do not appear at all. This definite difference in pattern change (Figs 2 and 5) and microstructures (Figs 3 and 6) demonstrates that DRX takes place in 99.999 AI while only DRV occurs in 99.99 A1. Although Fig. 4 was taken from polycrystal 99.99 A1, monotonous work hardening is not due to the polycrystal structure of the specimen. In 99.999 A1, multipeak stress oscillations appear not only in single crystals but also in polycrystals [21]. Thus, the relatively higher impurity content of 99.99 AI is thought to cause the DRV-type stress-strain curve. Recently, Tanaka et al. [26] revealed the effect of trace Si constituents. Using [111] single crystals of zone refined AI base alloys, stress-strain curves were compared at 533 K under constant compression speed (in the initial strain rate of 1.67 x 10-4/s), DRX-type stress-strain curves were observed in 2 mass ppm Si alloy, while DRV-type curves were found in the 25 mass ppm Si alloy. Their results have also supported the bound at which the restorative mode changes from DRV to DRX: around 99.999 AI. 4.2. Microstructural el~olution
The restorative processes in pure A1 have been well researched [8, 9] and accorded extensive study. The conclusions of these studies can be summarized as follows: the stress-strain curve shows work hardening up to a strain of around 2 followed by work softening as a steady state is reached. The grains elongate. The grain boundaries are trapped by equiaxed subgrains, and are serrated. The subgrain size remains constant at steady state, This is typical of the DRV structure at high strains. At ultra high strain ranges (above 20) [27-31], the equiaxed subgrain form is maintained. The original grain boundaries are elongated to the order of subgrain size, and 30% of the grain boundaries are composed of high angle boundaries. The grain structure at this strain level is formed through a process that is different from the continuous DRX process. In continuous DRX, high angle grain boundaries are formed through a progressive increase in the misorientation angle between the subgrains. The process is also different from discontinuous DRX where new grains grow at the expense of the surrounding subgrains. This process is called geometric DRX by McQueen et al. [29]; however, geometric DRX should be classified as DRV because multipeak stress oscillation and migration of grain boundaries over long distances are not observable. DRX discovered in 99.999 AI is quite different from geometric DRX ,but has much the same characteristics as discontinuous DRX commonly observed in Ca, Ni and 7-Fe. Since DRX is found to take place even in pure AI, the accepted theory holding that the high stacking fault energy eliminates DRX fails,
4.3. Why multipeak stress observed in previous papers
oscillations
727 were not
Only the present author has reported DRX-type stress-strain curves in pure AI. The reason for this can be understood by an examination of papers in this area [32-38]. Tensile testing [34, 35] cannot deform specimens to the high strains that are necessary for multipeak stress oscillation to appear; large work softening by DRX causes plastic instability. Torsion or compression testing could deform the specimens to high strains; however, excessive strain rate [36], unfavorable temperature range [37], or low purity A1 samples [38] may have precluded multipeak stress oscillation. Though detailed chemical analyses were not listed in papers [32, 33], the experiments are thought to have been carried out using less pure metals. Recent experimentation has not shown evidence of DRX-type stress-strain curves in 99.999 mass% [12] and 99.9999% [13] AI. These studies only report DRV microstructures. Ravichandran and Prasad [17] have reported that they have observed DRX grains under optical microscopy. However, the stress-strain curves were of a DRV-type. In these reports, the purity is indicated as 99.999 or 99.9999%, and thus the actual impurity content is unclear. If the trace impurities in their AI specimens were the same as in the present 99.999 AI, it is not clear why the DRXtype stress-strain curve was not observed. Their test conditions were the same in almost every respect to those in the present report. The form of stress oscillation varies with the orientation of the 99.999 AI single crystals. Recently, Tanaka et al. [39] have revealed this relationship using three multiple-glide-orientation single crystals ([111], [100] and [l 10]) under compression testing at 600 K. The first peak stress of the [111] single crystal appeared at the lowest strain. The work hardening rate up to the first peak stress was highest in [lll]. The flow stress at high strains, however, converged into nearly the same value for the three orientations. The form of regular stress oscillations taking place in both single and polycrystals definitely depends on the orientation as well as the strain rate and temperature. It is not the result of an irregular stress fluctuation due to an experimental technique. While a well-defined steady state was not apparent in the stress strain curve of [21], this was not attributable to the increasing friction load of the graphite lubricant [23]. Rather, the phenomenon was a result of a gradual increase in the true strain rate caused by operating an Instron testing machine at a constant crosshead speed, 4.4. Dynamic grain growth
McQueen et al. [23] also suggested that the stress oscillations reported by the present author are related to dynamic grain growth (DGG), irregular grain boundary-sliding, or -migration. D G G was named by
728
YAMAGATA:
DYNAMIC RECRYSTALLIZATION IN PURE Al
Straub and Blum [40]. They found sudden increases in strain rate during compression creep tests at 923 K for coarse grained (grain size: 3.5 mm) 99.99 mass% AI. Since the grain interior was composed of subgrains, they suggested that the sudden strain rate increases are attributable to softening due to grain growth. Sudden strain rate increases did not appear in 99.5 mass% A1. The reason for this was explained to be the presence of impurities preventing D G G . They did not detect regular periodic oscillations in the strain rate. Their observations must have been irregular strain rate fluctuations due to D G G . The opinion that the stress oscillations are caused by D G G , irregular grain boundary-sliding, or -migration might hold in polycrystals. This does not hold in single crystals however: D G G requires pre-existing high angle grain boundaries in the original single crystals. This condition would not be fulfilled without the nucleation of new grains through DRX. Sudden strain rate increases in creep tests correspond to rapid softening in constant compression speed tests. Since rapid grain boundary migration will eliminate dislocations markedly, rapid grain growth after D R X will be a necessary condition for pronounced stress decreases in constant compression speed tests. We can infer from Figs 2 and 3 that fairly coarse grains containing fewer subgrains were rapidly built up in the deformed crystal. Because the newly created grains by D R X must be small, the grain boundaries will migrate rapidly in the work softening stages, thus producing coarse grains. Geometric D R X may take place at ultra high strains, when the grain growth at low strains is prevented by trace impurities. The opinion [23] that the observed recrystallized structure is due to SRX fails, because a mottled substructure consisting of subgrains, typical of D R X , has been detected in many instances (Fig. 3 as well as [21]). The D R X structure is clearly distinct from the SRX structure which has grains free of substructures (Fig. 7). D R X appears in Cu [41] and Ni [42] even though the purity is low. D R X does not appear in low purity AI as observed in the present experiment. If high purity AI exhibits much the same D R X process as in Cu or Ni, it would be interesting to study the relation between the nucleation-growth mechanism of the new grains and the trace impurities as well as the influence of stacking fault energy. 5. CONCLUSION In situ observations of X-ray diffraction patterns during 583 K compression tests were used to compare the restorative processes of 99.999 and 99.99 mass% aluminum. A regular multipeak stress oscillation was generated in 99.999 mass% aluminum while m o n o t o n o u s work hardening occurred in 99.99 mass% aluminum. D R X in the 99.999 mass%
aluminum was confirmed by spot patterns and by the presence of recrystallized grains containing subgrains. In contrast, 99.99 mass% aluminum experienced D R V evident in asterism patterns and subgrains. D R X takes place only in the high purity range above 99.999 mass%. This D R X process was discussed in relation to the established mechanisms. Acknowledgements--The author is grateful to Professor M.
Otsuka and Mr K. Tanaka of Shibaura Institute of Technology for their valuable discussions. The author, unfamiliar with these topics, also wishes to thank Dr H. J. McQueen and co-authors [23] who have offered their insightful and critical comment. REFERENCES
1. T. Sakai, Bull. Japan Inst. Metals 22, 1036 (1983). 2. T. Sakai and J. J. Jonas, Aeta metall. 32, 189 (1984). 3. H. J. McQueen, E. Evangelista and N. D. Ryan, Recrystallization '90, p. 89. The Minerals, Metals and Materials Society, Warrendale, Pa (1990). 4. D. Hardwick, C. M. Sellars and W. J. McG. Tegart, J. Inst. Metals 90, 21 (1961 62). 5. G. Gottstein, Metal Sei. 17, 497 (1983). 6. G. Gottstein and U. F. Kocks, Acta metall. 31, 175 (1983). 7. B. Derby and M. F. Ashby, Seripta metall. 21, 879 (1987). 8. H. J. McQueen and K. Conrad, Microstruetural Control in Al Alloys, p. 197. Met. Soc. AIME., Warrendale, Pa (1986). 9. M. E. Kassner, M. M. Mysblyaef and H. J. McQueen, ,Mater. Sci. Engng. A 108, 45 ([989). I0. H. J. McQueen, W. A. Wong and J. J. Jonas, Can. J. Phys. 45, 1225 (1967). I1. M. E. Kassner and M. E. McMahon, Metall. Trans. 18A, 835 (1967). 12. M. E. Kassner, Metall. Trans. 20A, 2182 (1989). 13. A. B. C. Dadson and R. D. Doherty, Acta metall, mater. 39, 2589 (1991). 14. I. S. Servi and N. J. Grant, Trans. Am. Inst. Min. Engrs 10, 909 (1951). 15. D. McLean, J. Inst. Metals 80, 507 (1951 52). 16. G. A. Henshhall, M. E. Kassner and H. J. McQueen, Metall. Trans. 23A, 88l (1992). 17. N. Ravichandran and Y. V. R. K. Prasad, Metall. Trans. 22A, 2339 (1991). 18. L. Styczynski, W. Pachla and S. Wojciechowski, Metal Sei. 16, 525 (1982). 19. S. P. Belyayev, V. A. Likhachev, M. M. Myshlyayev and O. N. Senkov, Phys. Metals Metallogr. 52, 143 (198l). 20. J. J. Jonas, C. M. Sellars and W. J. McG. Tegart, Metal/. Re~'. 14, 1 (1969). 21. H. Yamagata, Scripta metall, mater. 27, 20I, 727, 1157 (1992). 22. H. Yamagata, Seripta metall, mater. 30, 411 (1994). 23. H. J. McQueen, W. Blum, S. Straub and M. E. Kassner, Scripta metall, mater. 28, 1299 (1993). 24. Ch. Perdrix, F. Montheillet, G. Wyon and F. Weill, Metallography 13, 289 (1980). 25. J. K. Solberg, H. J. McQueen, N. Ryun and E. Nes, Phil. Mag. 60, 447 (1989). 26. K. Tanaka, M. Otsuka and H. Yamagata, Proc. ll4th Spring Meeting of Japan Inst. ~?/ Metals, p. 369 (1994). 27. Ch. Perdrix, M. Y. Perrin and F. Montheillet, Mbm. scient. Revue MOtall. 78, 309 (1981).
YAMAGATA:
D Y N A M I C RECRYSTALLIZATION IN PURE AI
28. F. Montheillet, M. Cohen and J. J. Jonas, Acla metall. 32, 2077 (1984). 29. H. J. McQueen, O. Kunstad, N. Ryum and J. K. Solberg, Scripta metall. 19, 73 (1985). 30. D. Hardwick and W. J. MeG. Tegart, M~;m. seient. Revue M?tall. LVIII 7 (1961). 31. H. P. St/iwe, Z. Metallk. 56, 633 (1965). 32. C. M. Sellars and W. J. McG. Tegart, Acta metall. 14, 1136 (1966). 33. M. M. Farag, C. M. Sellars and W. J. McG. Tegart, Deformation under Hot Working Conditions, p. 60. Iron and Steel Institute, London (1968). 34. S. Howe and C. Elbaum, Phil. Mag. 8, 37 (196l). 35. P. Olla and P. F. Virdes, Metall. Trans. 18A, 293 (1987).
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36. H. J. McQueen and N. Ryum, Stand. J. Metal 14, 183 (1985). 37. H. Luthy, A. K. Miller and O, D. Sherby, Acta metall. 28, 169 (1980). 38. H. Ormerod and W. J. McG. Tegart, J. Metals 89, 94 (1960 61). 39, K. Tanaka, M. Otsuka, H. Fukutomi and H. Yamagata, Proc. 84th Spring Meeting of Japan Inst. of Light Metals, p. 143 (1993). 40. S. Straub and W. Blum, Scripta metall, mater. 24, 1837 (1990). 41. N. Furushiro, Y. Okada and S. Hori. J. Soc. Mater. Sci. Japan 29, 776 (1980). 42. M. Ohashi, T. Endo and T. Sakai, J. Japan Inst. Metals 54, 43 (1990).
Pergamon
0956-7151(94)00267-3
Copyright (<, 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0956-7151/95 $9.50 + 0.00
DYNAMIC RECRYSTALLIZATION AND DYNAMIC R E C O V E R Y IN P U R E A L U M I N U M AT 583 K H. YAMAGATA Yamaha Motor Co. Ltd, R & D Div. 2, Iwata, Shizuoka 438, Japan (Received 8 April I994) Abstract--In situ observations using transmission Laue X-ray photographs taken during 583 K compression tests were used to compare the restorative process of 99.999 mass% aluminum to that of 99.99 mass% aluminum. A regular multipeak stress oscillation, typical of dynamic recrystallization, was generated in 99.999 mass% aluminum, while monotonous work hardening, typical of dynamic recovery, were present in 99.99 mass% aluminum. Spot patterns as well as coarse recrystallized grains containing subgrains confirmed dynamic recrystallization in the 99.999 mass% aluminum. The dynamic recovery of 99.99 mass% aluminum was confirmed by asterism patterns and equiaxed subgrains. Dynamic recrystallization was found to take place only in the high purity range above 99.999 mass%. This dynamic recrystallization was considered to be the results of discontinuous dynamic recrystallization which is well known in Cu, Ni and 7-Fe.
1. INTRODUCTION Cu, Ni, and 7-Fe have dynamic recrystallization (DRX) [1-3] as a restorative process at elevated temperatures. DRX is normally accompanied by regular multipeak stress oscillations on stress-strain curves (DRX-type stres~strain curves) resulting from tests with constant strain rates or constant displacement speeds. In creep testing, DRX is accompanied by regular periodic variations in the strain rate [4, 5]. Rapidly cooling the specimens after testing yields typical DRX grains [1 3] containing both subgrains and high dislocation densities. Previous studies [1-3, 6,7] in these metals conclude that low stacking fault energy suppresses dynamic recovery (DRV) to effectively accumulate the strains required for the nucleation of new grains. In A1, despite extensive study [8, 9], multipeak stress oscillation has not been reported, and moreover, DRX grains generally have not been observed. These preceding studies have, therefore, concluded that the high stacking fault energy eliminates the necessary force driving DRX [2, 3, 68]. This conclusion, first proposed over 30 years ago [4, 10], is now established as the accepted theory. This has been reconfirmed even more recently as typified by the following two reports: Kassner & McMahon [11] did not observe multipeak stress oscillation in torsion testing at strains ranging from 0.5 to 16 in 99.999 mass% AI polycrystals (at 644 K and a strain rate of 5.04 x 10-4/s). After testing, subgrains in the elongated grains were observed. Furthermore, multitWhen the chemical composition of AI in the original paper is not described in either mass% or at.%, then only % is used to indicate purity in this report. AM 432
peak stress oscillation did not occur in torsion testing for single crystal specimens [12] (test conditions were identical to those of the polycrystal tests). Subgrains and deformation bands were observed. Dadson and Doherty [13] reported monotonous work hardening curves in compression tests up to a strain of 0.5 in 99.9999% tAl polycrystals (at 573 K and 673 K at a strain rate of 4.61 x 10 4/s); only subgrain structures were observed. As mentioned previously, without DRX, stress-strain curves exhibit monotonous work hardening (DRV-type stres~strain curves) followed by a steady state under testing with conditions of constant strain rate or constant displacement speed. Additionally, all of the above referenced papers reported similar DRV-type stress-strain behavior, Also noteworthy is that regular periodic strain rate variation has not been reported in creep testing as well [14, 15]. A few reports suggest that A1 shows DRX: Ravichandran and Prasad [17] have reported DRX grains in 99.999% AI samples during compression tests up to a strain of 0.6 (at 573 and 673 K in the strain rate range of 10 3-10/s). However, the stress-strain curves were characteristic of DRV-type curves. Excepting this, there are some reports advocating the existence of DRX in AI [18, 19] according to recrystallized grains observed after deformation. This in itself does not constitute strong evidence for DRX because static recrystallization (SRX) occurs rapidly after the deformation [20]. With the lack of definite evidence for DRX, pure AI has been classified [4] as a metal whose restorative process is limited to only DRV. Against the accepted theory, the present author happened to discover regular multipeak stress
U
723
724
YAMAGATA:
DYNAMIC RECRYSTALL1ZAT1ON IN PURE AI
oscillations during elevated temperature deformation of zone refined 99.999 mass% AI [21]. Microstructural observations after compression tests revealed, in numerous instances, mottled recrystallized grains containing subgrains. In situ detection of D R X using the transmission Laue method has recently strengthened the case for multipeak stress oscillations caused by D R X [22]. Given this background, restorative processes were compared in 99.999 and 99.99 mass% AI at 583 K to reveal the effect of impurities. In situ observations were employed as in the previous study. Also, discussions pertaining to the topics of McQueen et al. [23] are covered in this report. 2. EXPERIMENTAL
2. I. Speeimen For in situ observation, a rectangular single crystal specimen (2 x 3 x 6 ram) was cut from a single crystal bar of 99.999 mass% purity Al (99.999 AI). The compression axis is shown in the stereographic triangle of Fig. 1. Trace impurities were identical to those of the previous testing [21]. A rectangular polycrystal specimen (the same dimensions as the single crystal specimen) was cut from a polycrystal ingot of 99.99 mass% purity AI (99.99 AI). Specifically, the impurities consist mainly of 26 mass ppm Si, 36 mass ppm Fe, 10 mass ppm Cu, and other trace elements. The specimens (7 x 7 x 10 mm) for optical microscopy observation were cut from the single crystal bar or the polycrystal ingot. All the specimens were chemically polished and annealed for 10.8 ks at 473 K in an oil bath. After this first annealing, the 99.99 AI polycrystal specimen had an average grain diameter of 2 mm. 2.2. Compression test X-ray diffraction patterns were in situ photographed during the hot compression tests. The compression equipment has already been explained in detail in the previous paper [22]. Specimens were heated with hot CO_, gas and compressed at a constant speed of 0 . 0 6 m m / m i n (1.67 × 10 4/s in the initial strain rate). The specimen temperature was controlled at 5 8 3 + 5 K. A MoSz lubricant was utilized to reduce friction. An Instron testing machine (type 1185) equipped with a furnace was used in air to prepare specimens for optical microscopy observations. The specimens were compressed up to a strain of 0.6 under a constant compression speed of 0.1 mm/min (1.67 × 10 a/s in the initial strain rate). A stress relaxation test in 99.99 AI was carried out for 480 s to obtain the SRX microstructure as shown in Fig. 7. Stress relaxation was implemented by stopping the crosshead after compression up to a 0.6 strain (at a constant compression speed of 0.1 mm/min). The temperature was controlled at 583 _+ 2.5 K. The spec-
imens were then quenched with water at r o o m temperature within 5 s after the termination of the compression test. 2.3. X-ray d(~kaetion pattern A 50 k w 4 0 mA white X-ray beam with a Cu target was used. The incident beam diameter was 2 r a m with a pin hole collimater. The distance between the film and the center of the specimen was 35 ram. The exposure periods of 60 or 300s were used. Laue-spots appear on specimens that have grains with fewer substructures. If D R X grains appear during the exposure period and the grains are not so strained, Laue-spots appear in the photographs instead of asterism of strained crystals. D R X is detected, then, as a spot pattern. The detailed background of the photographic techniques and interpretation has been given in a previous paper [22]. 2.4. Microstructural obserration After hot compression testing or stress relaxation testing, the quenched samples were immediately electro-polished with an ethanol and perchloric acid solution. The samples were then anodized with a Barker solution [24]. The cross-sectional surface horizontal to the compressive axis was observed with a polarized light optical microscope. With this technique, fine subgrains cannot be distinguished [25] as readily as using the techniques of the transmission electron microscope or the technique of channeling contrast imaging with the scanning electron microscope. But, the chosen technique was convenient for rapid specimen preparation. 3. RESULTS 3.1. D R X q f 99.999 A1 Figure 1 shows a true stress strain curve for a 99.999 A1 single crystal. Two large peaks are observed up to a strain of 0.6. The diffraction patterns in Fig. 2(a-g) correspond to the start of exposure for the strain values " a " to " g " indicated on the curve
15
8 7.5 &
g b
04
T-11
I I I [ I I I I I I I I I [ I 0.5 True strain Fig. 1. True stress true strain curve in a single crystal of 99.999 A1. The compression axis is indicated in the stereographic triangle. 0
YAMAGATA: DYNAMIC RECRYSTALLIZATION IN PURE Al
725
Fig. 3. DRX grains with (A) or without (B) subgrains at a strain of 0.6.
exposed just after unloading stress "g". The pattern of Fig. 2(g) is composed of both spots and streaks. The pattern of Fig. 2(g) indicates that coarse D R X grains have appeared in the deformed matrix. This deduction is supported by Fig. 3. The microstructure is composed of coarse D R X grains with or without subgrains. These results were also confirmed at 632 K [22]. 3.2. D R V of 99,99 Al
Fig. 2. Diffraction patterns in 99.999 A1. The exposure time in each photograph is 300 s.
of Fig. I. The characteristics in sequence are as follows: The initial diffraction pattern in Fig. 2(a) has a typical elliptical arrangement of Laue-spots+ in undeformed crystals. An asterisrn appears [Fig. 2(b)] instead of a spot pattern. The streaks in the asterism elongate [Fig. 2(c)] with increasing strain up to the first peak stress of Fig. 1. After the first peak stress, spots being to appear [Fig, 2(d)]. The pattern of Fig. 2(d) similarly diffuses with increasing strain [Fig. 2(e)], At around the second peak stress, spots appear again [Fig. 2(f)]. Finally, Fig. 2(g) was
Figure 4 shows a true stress-strain curve in the polycrystal 99.99 A1. The curve shows m o n o t o n o u s work hardening in contrast to the stress oscillation of 99.999 AI. The corresponding diffraction pattern change is given in Figs. 5(a-g). Figure 5(a) shows the initial coarse grain structure. The spots begin to diffuse with increasing strain, The photographs
15_
efl_f
7.5 *~
_
¢
-
0
~L I I I ~-I
0 ?Since the photographs in Fig. 2 were printed directly from film negatives, weak spots are less distinct due to the high contrast inherent in this technique. Compare this to the results of using a scanner [22].
g
I [ I I I I [ I ~
0.5 True strain
Fig. 4. True stress true strain curve for coarse grained 99.99 AI.
726
YAMAGATA:
DYNAMIC RECRYSTALLIZATION IN PURE A1
Fig. 6. Subgrains in the initial grains at a strain of 0.6. The original grain boundary (indicated by the arrow) is serrated.
4. DISCUSSION 4.1. Impurity effect on D R X and D R V The diffraction pattern change of 99.999 AI, as in Fig. 2, was similarly observed at 632 K in a previous paper [22]. The characteristics of 99.999 A1 can be summarized from this and the previous paper: asterisms appear in the work hardening stages, while spot patterns occur in the work softening stages.
Fig. 5. Diffraction patterns in 99.99 Al. The exposure time for d and g is 60s, and for the rest is 300s.
following Fig. 5(c) only show streaks regardless of the exposure period. Finally, Fig. 5(g), photographed just after unloading stress " g " , also consists of streaks. This sequence demonstrates that D R X does not take place at all in 99.99 AI. This result is also deduced from Fig. 6. The original grain boundary, though serrated, still establishes at the initial position, and the grain interiors are filled with subgrains. Figure 6 is indicative of a typical D R V structure usually reported in A1. Figure 7 shows the microstructure after stress relaxation testing. The exposure is composed of equiaxed SRX grains without subgrains. The SRX structure of Fig. 7 is completely different from either the D R V structure of Fig. 6 or the D R X structure of Fig. 3.
Fig. 7. Equiaxed SRX grains without subgrains alter stress relaxation (for 480 s after straining up to 0.6).
YAMAGATA: DYNAMIC RECRYSTALL1ZATION IN PURE AI The patterns consist of both spots and streaks at high strains. In 99.99 AI, however, the spot patterns do not appear at all. This definite difference in pattern change (Figs 2 and 5) and microstructures (Figs 3 and 6) demonstrates that DRX takes place in 99.999 AI while only DRV occurs in 99.99 A1. Although Fig. 4 was taken from polycrystal 99.99 A1, monotonous work hardening is not due to the polycrystal structure of the specimen. In 99.999 A1, multipeak stress oscillations appear not only in single crystals but also in polycrystals [21]. Thus, the relatively higher impurity content of 99.99 AI is thought to cause the DRV-type stress-strain curve. Recently, Tanaka et al. [26] revealed the effect of trace Si constituents. Using [111] single crystals of zone refined AI base alloys, stress-strain curves were compared at 533 K under constant compression speed (in the initial strain rate of 1.67 x 10-4/s), DRX-type stress-strain curves were observed in 2 mass ppm Si alloy, while DRV-type curves were found in the 25 mass ppm Si alloy. Their results have also supported the bound at which the restorative mode changes from DRV to DRX: around 99.999 AI. 4.2. Microstructural el~olution
The restorative processes in pure A1 have been well researched [8, 9] and accorded extensive study. The conclusions of these studies can be summarized as follows: the stress-strain curve shows work hardening up to a strain of around 2 followed by work softening as a steady state is reached. The grains elongate. The grain boundaries are trapped by equiaxed subgrains, and are serrated. The subgrain size remains constant at steady state, This is typical of the DRV structure at high strains. At ultra high strain ranges (above 20) [27-31], the equiaxed subgrain form is maintained. The original grain boundaries are elongated to the order of subgrain size, and 30% of the grain boundaries are composed of high angle boundaries. The grain structure at this strain level is formed through a process that is different from the continuous DRX process. In continuous DRX, high angle grain boundaries are formed through a progressive increase in the misorientation angle between the subgrains. The process is also different from discontinuous DRX where new grains grow at the expense of the surrounding subgrains. This process is called geometric DRX by McQueen et al. [29]; however, geometric DRX should be classified as DRV because multipeak stress oscillation and migration of grain boundaries over long distances are not observable. DRX discovered in 99.999 AI is quite different from geometric DRX ,but has much the same characteristics as discontinuous DRX commonly observed in Ca, Ni and 7-Fe. Since DRX is found to take place even in pure AI, the accepted theory holding that the high stacking fault energy eliminates DRX fails,
4.3. Why multipeak stress observed in previous papers
oscillations
727 were not
Only the present author has reported DRX-type stress-strain curves in pure AI. The reason for this can be understood by an examination of papers in this area [32-38]. Tensile testing [34, 35] cannot deform specimens to the high strains that are necessary for multipeak stress oscillation to appear; large work softening by DRX causes plastic instability. Torsion or compression testing could deform the specimens to high strains; however, excessive strain rate [36], unfavorable temperature range [37], or low purity A1 samples [38] may have precluded multipeak stress oscillation. Though detailed chemical analyses were not listed in papers [32, 33], the experiments are thought to have been carried out using less pure metals. Recent experimentation has not shown evidence of DRX-type stress-strain curves in 99.999 mass% [12] and 99.9999% [13] AI. These studies only report DRV microstructures. Ravichandran and Prasad [17] have reported that they have observed DRX grains under optical microscopy. However, the stress-strain curves were of a DRV-type. In these reports, the purity is indicated as 99.999 or 99.9999%, and thus the actual impurity content is unclear. If the trace impurities in their AI specimens were the same as in the present 99.999 AI, it is not clear why the DRXtype stress-strain curve was not observed. Their test conditions were the same in almost every respect to those in the present report. The form of stress oscillation varies with the orientation of the 99.999 AI single crystals. Recently, Tanaka et al. [39] have revealed this relationship using three multiple-glide-orientation single crystals ([111], [100] and [l 10]) under compression testing at 600 K. The first peak stress of the [111] single crystal appeared at the lowest strain. The work hardening rate up to the first peak stress was highest in [lll]. The flow stress at high strains, however, converged into nearly the same value for the three orientations. The form of regular stress oscillations taking place in both single and polycrystals definitely depends on the orientation as well as the strain rate and temperature. It is not the result of an irregular stress fluctuation due to an experimental technique. While a well-defined steady state was not apparent in the stress strain curve of [21], this was not attributable to the increasing friction load of the graphite lubricant [23]. Rather, the phenomenon was a result of a gradual increase in the true strain rate caused by operating an Instron testing machine at a constant crosshead speed, 4.4. Dynamic grain growth
McQueen et al. [23] also suggested that the stress oscillations reported by the present author are related to dynamic grain growth (DGG), irregular grain boundary-sliding, or -migration. D G G was named by
728
YAMAGATA:
DYNAMIC RECRYSTALLIZATION IN PURE Al
Straub and Blum [40]. They found sudden increases in strain rate during compression creep tests at 923 K for coarse grained (grain size: 3.5 mm) 99.99 mass% AI. Since the grain interior was composed of subgrains, they suggested that the sudden strain rate increases are attributable to softening due to grain growth. Sudden strain rate increases did not appear in 99.5 mass% A1. The reason for this was explained to be the presence of impurities preventing D G G . They did not detect regular periodic oscillations in the strain rate. Their observations must have been irregular strain rate fluctuations due to D G G . The opinion that the stress oscillations are caused by D G G , irregular grain boundary-sliding, or -migration might hold in polycrystals. This does not hold in single crystals however: D G G requires pre-existing high angle grain boundaries in the original single crystals. This condition would not be fulfilled without the nucleation of new grains through DRX. Sudden strain rate increases in creep tests correspond to rapid softening in constant compression speed tests. Since rapid grain boundary migration will eliminate dislocations markedly, rapid grain growth after D R X will be a necessary condition for pronounced stress decreases in constant compression speed tests. We can infer from Figs 2 and 3 that fairly coarse grains containing fewer subgrains were rapidly built up in the deformed crystal. Because the newly created grains by D R X must be small, the grain boundaries will migrate rapidly in the work softening stages, thus producing coarse grains. Geometric D R X may take place at ultra high strains, when the grain growth at low strains is prevented by trace impurities. The opinion [23] that the observed recrystallized structure is due to SRX fails, because a mottled substructure consisting of subgrains, typical of D R X , has been detected in many instances (Fig. 3 as well as [21]). The D R X structure is clearly distinct from the SRX structure which has grains free of substructures (Fig. 7). D R X appears in Cu [41] and Ni [42] even though the purity is low. D R X does not appear in low purity AI as observed in the present experiment. If high purity AI exhibits much the same D R X process as in Cu or Ni, it would be interesting to study the relation between the nucleation-growth mechanism of the new grains and the trace impurities as well as the influence of stacking fault energy. 5. CONCLUSION In situ observations of X-ray diffraction patterns during 583 K compression tests were used to compare the restorative processes of 99.999 and 99.99 mass% aluminum. A regular multipeak stress oscillation was generated in 99.999 mass% aluminum while m o n o t o n o u s work hardening occurred in 99.99 mass% aluminum. D R X in the 99.999 mass%
aluminum was confirmed by spot patterns and by the presence of recrystallized grains containing subgrains. In contrast, 99.99 mass% aluminum experienced D R V evident in asterism patterns and subgrains. D R X takes place only in the high purity range above 99.999 mass%. This D R X process was discussed in relation to the established mechanisms. Acknowledgements--The author is grateful to Professor M.
Otsuka and Mr K. Tanaka of Shibaura Institute of Technology for their valuable discussions. The author, unfamiliar with these topics, also wishes to thank Dr H. J. McQueen and co-authors [23] who have offered their insightful and critical comment. REFERENCES
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D Y N A M I C RECRYSTALLIZATION IN PURE AI
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