Scripta METALLURGICA Vol. 27, pp. IIS7-I160, 1992 Pergamon Press Ltd. et MATERIALIA Printed in the U.S.A. All rights reserved

M I C R O S T R U C T U R A L E V O L U T I O N O F S I N G L E - C R Y S T A L L I N E A L U M I N U M D U R I N G M U L T I P E A K S T R E S S O S C I L L A T I O N A T 623 K

H. Yamagata YAMAHA MOTOR CO., LTD.

Fundamental Research Div. Iwata, Shizuoka 438, Japan

(Received August 25, 1992)

INTRODUCTION

High purity aluminum showed dynamic recrystallization (DRX) with regular multipeak stress oscillations over a wide temperature range from 511 to 820 K (1,2). DRX grains were observed after the test. The observation of a recrystallized structure ldoes not in itself constitute very strong evidence of DRX (5), as it can be argued that the quenching times were insufficiently short to retain a true hot worked structure. This may hold in elevated temperature testings, but the present author detected DRX grains even immediately after compression tests at 296 K (2). The revealed DRX process at 296 K, as well as the regular multipeak stress oscillations at elevated temperatures, is evidence that aluminum shows DRX as a restoration process.

Geometric DRX (GDRX) generated by DRV mechanisms (6) is also reported in less pure aluminum (7-10). Multipeak stress oscillations do not take place in the process. Since DRX and GDRX processes are thought to have some relation, the present experiment was performed to investigate the difference between DRX and GDRX by revealing the DRX process of high purity aluminum at the temperature showing multipeak stress oscillations. The evolution of subgrains and DRX grains were observed with polarized light microscopy.

EXPERIMENTAL

Cubic single-crystalline specimens with one edge length of 15 mm were cut from a single-crystal bar (purity, five nine). The specimen preparation and the hot compression test procedure were the same as for the previous testings (1,2). The compression axis is shown in FIG.1. Constant crosshead speeds of 1 ram/rain (initial strain rate of 1.1 l x l 0 -3) and 10 mm/min (1.1 l x l 0 -2) were applied. The microstructures were compared on one of the four side surfaces of the specimens. The plane direction of the observed surface is also indicated in FIG.1. The specimens, quenched into 296 K water within 5 s, were electropolished and anodized for polarized light microscopy.

RESULTS AND DISCUSSION

The true stress vs. true strain curves at 623 K are shown for two strain rates in FIG.2. The rapid initial rise to the first peak stress and subsequent work-softening is observed in both curves.The microstructural evolution was investigated using five specimens at a crosshead speed of 1 mm/min. The points indicated on the curve correspond to the strain values: A: 0.10; B: 0.17; C: 0.25; D: 0.30; E:0.70. The characteristics of the sequence are as follows where the vertical direction of the reference photographs is parallel to the compressive axis:

1The following two reports on hot compression tests with high purity aluminum have been published recently. A.B.C. Dadson et al. (3) reported that AI (purity, six nine) did not recrystallize during the compression at 573 and 673 K at a true strain rate of 4.61 x 10-4/s. Instead, the specimen exhibited dynamic recovery (DRV) along with the presence of subgrain structures. The stress-strain curves in the report show gradual strain-hardening, typical of DRV, with increasing strain. Similarly, N. Ravichandran et al. (4) also obtained gradual strain- hardenin G curves during the compression of A1 (purity, five nine) at 573 and 673 K at true strain rates ranging from 10 -a to 10/s; however, they reported that DRX grains were observed with optical microscopy. While the purities and the test conditions were much the same as those used by the present author, they did not observe multipeak stress oscillations. The reasons are unclear at present.

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1158 MICROSTRUCTURAL EVOLUTION Vol. 27, No. 9

A. A weak subgrain structure starts to form at a strain of 0.10 (FIG.3). B. The subgrain structure continues to develop at a strain of 0.17 (FIG.4). C. Coarse, rectangular, textile-like subgrains about 20 I.tm in diameter well-developed at a strain of 0.25 (FIG.5). D. DRX grains suddenly appear at a strain of 0.30 after the peak stress (FIG.6). These grains do not yet have a subgrain structure(FIG.6). The recrystallized area is about half of the observed side surface area. Some grains have relatively large diameters of 5 mm. At another location, small DRX grains of the same order of size as the subgrains are observed in the subgrain structure (FIG.7). E. Large DRX grains show definite subgrain structures (FIG.8). Small DRX grains (FIG.9) nucleating in the equiaxed subgrains (FIG. 10) are observed.

It has been pointed out that DRX is similar to static recrystallization (SRX) in its unit mechanism of nucleation and growth (6). F. Inoko and K. Kashihara (11) recently have obtained the following results in the SRX study of 30% tensile-strained aluminum single-crystals (purity, four nine). The recrystallized primary grains are preferentially nucleated in deformation bands. Most of the grain orientations are obtained by rotation about the axis normal to one of four { 111 } planes of the matrix. Less than 10 % of the total grains are formed by the strain induced boundary migration. Similar to their SRX study, a number of nucleated grains appeared in deformation bands at a strain of 0.30 at 296 K(2). It is suggested that the formation of a mobile high angle boundary for DRX requires deformation conditions which favor both dislocation accumulation and heterogeneity(6). The deformation bands were differently colored from the surroundings with polarized light microscopy in the 296 K compressed specimen (2), and thus it was clear that deformation bands serve as nucleation sites for DRX grains. On the other hand, because of the coarse subgrains and the large DRX grains in the 623 K compressed specimen, it was not clear whether or not deformation bands work as nucleation sites. In FIGs.7,9, and 10 subgrains seem to become individual grains with progressive misorientation as proposed by the rotation recrystallization mechanism (12).

The onset of DRX at 623 K is indicated by a sharp stress drop, while DRX at 296 K did not show an accompanying stress drop (2). The rather large grains in the quenched specimen indicate high grain boundary mobility during the stress drop. A sharp stress drop is thought to occur in single-crystals when the two necessary conditions, the local lattice distortion to nucleate grains and the high grain boundary mobility to cause prominent softening, are fulfilled. The absence of stress oscillations at 296 K is attributed to low grain boundary mobility due to a low diffusion rate.

The nucleation of DRX grains at deformation bands and the onset of a sharp stress drop show much the same behavior as DRX in copper single-crystals (13,14) but are different from GDRX. While the high angle grain boundary area increases without changing the total number of grains, these grain boundaries do not migrate a long distance in GDRX. Since the density of high angle grain boundaries is high (10), GDRX is attributed to the low grain boundary mobility due to high impurity concentrations.The restoration process of aluminum is thought to be quite different between four nine and five nine purity.

SUMMARY

Compression tests were carried out on single-crystalline aluminum at 623 K where regular multipeak stress oscillations, typical of dynamic recrystallization, take place. The subgrain structure is definite with polarized light microscopy with increasing strains over the strain range of the rapid initial rise to the first peak stress. A rapid drop in flow stress takes place by dynamic recrystallization. The subgrain shape becomes equiaxed in the strain range over the peak stress. The absence of stress oscillations at 296 K is attributed to low grain boundary mobility due to a low diffusion rate. Geometric dynamic recrystallization is also attributed to low grain boundary mobility due to high impurity concentration.

ACKNOWLEDGEMENT

The author thanks Prof. M. Otsuka of Shibaura Inst. of Technology and Dr. T. Takasugi of Tohoku Univ. for their valuable discussions.

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1. H. Yamagata, Scripta. Metall. Mater.27, 201 (1992). 2. H. Yamagata, Scripta. Metall. Mater. Accepted. 3. A.B.C. Dadson and R.D. Doherty, Acta metall, mater. 39, 2589 (1991). 4. N. Ravichandran and Y.V.R.K. Prasad, Metall. Trans. 22A, 2339 (1991). 5. J.J. Jonas, C.M. Sellars and W.J.McG. Tegart, Metall. Reviews 14, 1 (1969). 6. H.J. McQueen, E. Evangelista and N.D. Ryan, Recrystallization '90, p.89, The Minerals, Metals and

M~,erials Society, Warrendale (1990). 7. S.P. Belyayev, V.A. Likhachev, M.M. Myshlyayev and O.N. Senkov, Phys. Metal Metall. 52, 143

(1981). 8. Ch. Perdrix, M.Y. Perrin and F. Montheillet, Mem. Scient. Revue Metall. 78, 309 (1981). 9. L. Styczynski, W. Pachla and S. Wojciechowski, Metal Sci. 16,525 (1982). 10. J.K. Solberg, H.J, McQueen, N. Ryum and E. Nes, Phil. Mag. 60, 447 (1989). 11. F. Inoko and K. Kashihara, J. Japan Inst. Metals 56,361 (1992). 12. J.P. Poirier, Creep of crystals, p. 181, Cambridge Univ. Press, Cambridge (1985). 13. P. Karduck, G. Gottstein and H. Mecking, Acta Metall. 31, 1525 (1983). 14. V.M. Sample, G.L. Fitzsimons and A.J. DeArdo, Acta Metall. 3.5,367 (1987).

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FIG. 1 The compressive axis and the plane direction of the observed surface. Stereographic projection.

[ I F W I [ I I I I L I I I ~ - 0 . 5 1.0

True strain FIG. 2 The true stress vs. true strain curves. Crosshead speeds of lmm/min (lower), and 10mm/min. (upper).

FIG.3 Subgrains at a strain of 0.10(scale FIG. 4 Subgrains at a strain of 0.17 (scale bar, bar, 200,u m). 100/~ m).

1160 MICROSTRUCTURAL EVOLUTION Vol. 27, No. 9

FIG. 5 Subgrains at a strain of 0.25 (scale bar, 100/z m).

FIG.6 DRX grains consuming subgrains at a strain of 0.30 (scale bar, 200/1 m).

FIG. 7 A small DRX grain in subgrains, indicated by an arrow at a strain of 0.30 (scale bar, 50/~ m).

FIG. 8 Subgrains in DRX grains at a strain of 0.70 (scale bar, 200/1 m).

FIG. 9 Small DRX grains (indicated by arrows) in FIG. 10 Equiaxed subgrains at a strain of 0.70 subgrains at a strain of 0.70 (scale bar, 100/L m). (scale bar, 50/z m).