Restoration mechanisms in large-strain deformation of high purity aluminum at ambient temperature

Pergamon

Scripta Metallurgicaet Materialia, Vol. 31, No. 10, pp. 1331-1336, 1994 Copyright©1994 Elsevier ScienceLtd Printed in the USA. All fights reserved 0956-716X/94$6.00 + 00

RESTORATION MECHANISMS IN LARGE-STRAIN DEFORMATION OF HIGH PURITY ALUMINUM AT AMBIENT TEMPERATURE M.E. Kassner 1, H.J. McQueen 2, J. Pollard 1, E. Evangelista3, and E. Cerri 3 tDepartment of Mechanical Engineering, Oregon State University, Corvallis, OR 97331 2Concordia University, Montreal, Canada 3Dept. of Mechanics, University of Ancona, 1-60131 Ancona, ITALY ( R e c e i v e d May 9, 1994) ( R e v i s e d J u n e 24, 1994) Introduction The objective of this research is to study the dynamic restoration mechanisms associated with large strain deformation of aluminum at ambient temperatures. Specifically, we hope to first determine whether, for high purity aluminum at ambient temperature, deformation to large plastic strains (e.g. > 1) will result in a saturation of the flow stress and, particularly, a mechanical steady state. For steady state, there is a balance between hardening processes and the restoration mechanism of dynamic recovery. Steady state is usually observed in metals and alloys deformed at elevated temperature (e.g., T > 0.5 Tin) but the existence at lower temperatures is uncertain. Saturation may also occur due to dynamic recrvstailization, but this may be followed by softening, which is usually absent in steady-state where dynamic recovery is the only restoration mechanism. It has been shown that many fee and bee metals and alloys do not achieve a saturation or steady state over the large torsional or compressive strain range to failure at ambient temperature (1-2). However, saturation was observed in aluminum, silver, and copper, perhaps exclusively as a result of dynamic recovery (3,4). Shelby and coworkers (3) performed an extensive study of deformation of solid specimens of high-purity (99.999%) aluminum in torsion between -79 and 260°C. Saturation was achieved at all temperatures and the investigators suggested this to be a result of a mechanical steady-state. The more recent results of Hughes and Nix (1), however, place the conclusions of a steady state in some question. Hughes and Nix performed ambient temperature torsion tests on hollgw specimens which may allow for more accurate characterization of stress versus strain behavior. Yamagata (5-7) performed compression tests on 99.999% pure A1 single crystals at ambient and elevated temperatures. Saturation was observed at ambient temperature (softening was not concluded) and believed to be a consequence of dynamic reerystallization initiating at shear bands already at small plastic swains during compression tests. Some difficulties may exist in unambiguously determining whether the reerystallization occurs during deformation or subsequent (static recrystallization) to it, as suggested by Choi et al. (8), who found that only dynamic recovery occurs at ambient temperature.

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We propose to provide additional insight as to whether a saturation in the flow stress and, in particular, whether a mechanical steady-state, exclusively as a result of dynamic recovery, can be achieved in aluminum at ambient temperature. We attempted, in this study, to determine whether saturation or a genuine mechanical steady-state is achievable at low temperatures by deforming high purity aluminum to large strains at ambient temperature by performing tests in f , ~ . Compression tests were performed in this study by repeated straining followed by remachining to eliminate the barreling which results from friction and leads to erroneous uniaxial stress measurements. Although the repeated straining~machining~ straining sequences require interruptions in a largestrain deformation test, the uniaxial data will be unique. The total strain to failure should also be larger than that for hollow torsion specimens and may provide less ambiguous data with regard to the observation of a saturation and a genuine steady-state with solid torsion specimens. In the case of the observation of a saturation, a careful distinguishing between dynamic recrystallization and dynamic recovery would be performed.

Exucrimental Procedure The aluminum used in this study was of 99.999% purity. Cylindrical compression specimens were machined from either a 15.9 mm diameter rod or a 76 mm diameter ingot. Rod specimens were annealed at 425"C for 2 hours resulting in a grain size of about 0.6 mm. The ingot specimens were annealed at 325"C for 4 hours resulting in a grain size of 0.78 ram. Compression tests were performed at a typical starting aspect ratio of 2:1 or 3:1 while remachined specimens used 3:2. A constant true strain rate was used. Determination of the true stress versus strain curves required careful determination of the machine compliance. Specimen ends were lubricated with an aerosol teflon powder. Specimens were strained, re-machined, and re-strained in a repeating process to restore proper cylindrical dimensions and to replace lubrication. Specimens were typically strained to about 0.3-0.5 between machining steps and were eventually deformed to a total strain of approximately 2-3. Significant barrelling as a result of friction was not observed. The specimens were immersed in LN upon completion of each deformation step to reduce the possibility of static recrystallization. Rod specimens were re-machined at ambient temperature, while ingot specimens deformed to large ( > 1)strains were remachined while immersed in a stream of LN. Specimens were resoaked in LN after each machining pass. Final passes removed no more than 0.05 mm from the specimen diameter. TEM thin foil and polarized light optical microscopy specimen preparation was performed at low (about -40"C) temperature so as to reduce the possibility of static recrystallization.

Results and Discussion Figure l(a) reports the results of two compression tests of 99.999% pure, annealed, aluminum deformed to strains of nearly two, at a low strain-rate of 10"Ys-1. Of course, the most interesting finding illustrated in Fig. 1 is that of a saturation. However, after a swain of about one, the stress gradually decreases by approximately 10%. Clearly, our results at ambient temperature and the relatively low strain-rate of 10.5 appear to contrast almost all other work on the large strain deformation of high purity aluminum. The saturation does not appear to be the result of a mechanical steady-state, classically defined as a balance of hardening processes and dynamic recovery. Here, textural softening is not expected and the "rate" of softening is higher than for observed textural softening (9). Rather, the stress versus strain trends of Fig. l(a) suggest that (dynamic) recrystallization is occurring based on the shapes of curves observed at elevated (10) and near-ambient temperature tests on other metals where dynamic recrystallization has been confirmed (4). The strain-hardening character of the aluminum can be described by a classic "hardening-rate" (da/dE = 0) versus stress plot. This is illustrated in Fig. l(b) for 99.999% pure aluminum deformed at ambient temperature at the strain rate of 10-5 s-l, illustrated in Fig. l(a). Figure 10a) shows that the aluminum evinces Stage II and III behavior at the lower stresses (or early plastic strains). Eventually, saturation (0 = 0) is observed. Interestingly, the recrystallization precludes the onset of the Stage IV usually observed in metals deformed to large strains at ambient temperature. Negative hardening rates (softening) are not illustrated. This discontinuity in Stage III probably marks the onset of rccrystallization as suggested by others (10). Presumably, below this stress (or strain) associated with this discontinuity, recrystallization would not be evident in the metal.

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FIG. 1. (a) The stress versus strain behavior of 99.999% pure aluminum deformed at a low strain rate of 10.5 sl in compression (with repeated remachining) at ambient temperature. Softening beyond strains of about 1.0 is clearly evident. (0) The strain-hardening "rate" (0 = d~/de) versus stress for 99.999% pure aluminum deformed in compression at ~ = 10-5 s"! and at ambient temperature. Careful transmission electron microscopy (TEM) was quickly (within about 30-40 min) performed on the liquid nitrogen (LN) quenched compression specimens. About 5 foils were examined, generally from planes parallel to the compression axis. Clear evidence for recrystallization was observed in two foils. The most prominent microstructural feature appeared to be small (0.9 #m), relatively equiaxed subgrains that formed from dislocation reaction during hardening. Within this substructure, relatively large (perhaps over 50 #m) grains have nucleated and, presumably, grown. The observation of recrystailized grains apparently formed during deformation in specimens examined, after the softening observed in the stress versus strain curves, appears consistent with the static recrystallization literature (11-13). Cold-worked aluminum of high purity statically recrystallizes within relatively short time periods (minutes). Additionally, polarized light optical microscopy was performed as an additional cheek for evidence of (discontinuous) dynamic recrystallization. Optical micrographs are illustrated in Fig. 2. This figure illustrates the microstructure of (a) the annealed aluminum and aluminum deformed at 10-5 s"1 to a strain of (0) 0.6 and (c) 1.7. The compression axis is perpendicular to the long axes of the micrographs. Figure 2(a) shows the large equiaxed grains of the starting polyerystal. At a strain of 0.6, according to Figs. 3 (a) and (0), reerystallization is not expected to have commenced, as a deviation from the linear decrease in the hardening rate is not yet reached. That is, 0.6 is less than the strain corresponding to the discontinuity discussed in the previous paragraph. As expected, the micrograph shows no evidence of reerystallization. However, by a strain of 1.7 the micrographs show strong evidence of recrystallization consistent with the mechanical tests and TEM. Roughly, 50 #m-sized grains appear that are over an order of magnitude smaller than the starting grains and much larger than the 1 micron-sized subgrains. Again, extreme precaution was exercised to preclude static recrystallization. After deformation of two specimens to these strain levels (remaehining was not performed over the course of deformation) they were immediately immersed in liquid nitrogen and removed only for short periods (less than that required for static recrystallization (11-13)) for electropelishing and anodizing sequences. Figures 3(a) and (0) report the compressive-stress versus strain behavior of our 99.999% pure aluminum deformed at a higher strain-rate of 10-3 s"1 for small diameter rod and larger ingot. Interestingly, at this higher strain-rate, for the larger diameter specimen, saturation (and softening) is again observed as at 10"~ ~ a relatively large strain of about 3. Deformation to such a large strain level required a relatively large starting sample followed by 15 separate LN remachining operations. The smaller diameter rod could only be deformed to strains less than 2, if 3:2 aspect ratios were maintained, and saturation was not observed although the "bend" in the hardening curve was identical to that of the ingot in Fig. 3(0) suggesting that recrystailization is nonethless occurring. Optical microscopy confirmed that recrystallization occurs. An important question is the failure of others to observe recrystallization in aluminum deformed to a comparable strain as in this study. Hughes and Nix, however, used less pure AI and may have suppressed (dynamic)

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FIG. 2. Polarized light optical micrographs of pure aluminum in the as-annealed condition (a) and deformed in compression at a strain-rate of 10-5 s "1 at ambient temperature to a strain of 0.6 before discontinuity in the hardening rate Co), and to a strain of 1.7, after substantial softening (c). Dynamically recrystallized grains are evident at the higher rate.

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recrystallization. 99.99% pure A1 recrystallizes much more sluggishly than 99.99c2% pure A1 (11). Recent, conflicting, results (5,8) may be the result of less than very accurate stress versus strain measurements and a failure to distinguish static recrystallization, that may occur before microstructural observations, and dynamic recrystallization. The observation of a mechanical steady-state by Sherby and coworkers (99.999% pure A1) may have been the result of recrystallization at the outer portions of the specimens (leading to softening) while the interior portions of the solid specimens experienced hardening not yet sufficient to induce recrystallization. The resulting stress versus strain behavior may appear roughly "flat" within their experimental errors. Our observations of recrystallization at ambient temperature but an absence at higher temperatures (9) may be explained by the dynamic recovery precluding dynamic recrystallization at higher temperature, but as temperature decreases, dynamic recovery is less mitigating. The observations by Yamagata (5) that dynamic recrystallization occurs in single crystals and polycrystals also at elevated temperatures appear inconsistent with our recent work (14,15) that shows no evidence of this at strain-rates comparable to those of Yamagata in polycrystals and single crystals. Our tests ranged from almost 200 to 600"C and used precise mechanical testing that showed no evidence of softening from recrystallization; only very consistent texture softening of 19% over early strains that was independent of temperature and strain rate.

Conclusions 1.

High purity (99.999%) aluminum was deformed at ambient temperature to large strains of 1.5-3.0 in compression at modest strain-rates of 10-3 s°I and 10-5 s-I. Eventually a saturation in the stress versus strain curve is observed, followed by significant softening.

2.

The stress versus strain behavior, which includes analysis of the changes in the hardening rates with flow stress, TEM and optical microscopy suggest that the softening is due to recrystallization during deformation.

3.

Some previous reportings of a mechanical steady-state, defined as a balance between dynamic recovery and hardening, may actually have been a result of recrystallization.

4.

Other reportings of continual (Stage IV) hardening with an absence of a steady-state, even at large strains, may be valid for cases where recrystallization is suppressed, such as with lower purity.

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We find no evidence of dynamic recrystallization at elevated temperatures. Other investigations that conclude dynamic recrystallization in single crystals at high temperatures are inconsistent with the present work by the authors. Acknowledgements

This work was partially supported by the National Science Foundation Grant INT-910 8633, Washington, DC, USA, and the CNR, Rome, Italy. The assistance by A. AUeman with the data analysis and the TEM by Prof. J. Koike are greatly appreciated. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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