Texture changes during the tensile deformation of a fine grained aluminium alloy

Scripta

METALLURGICA

Vol. 19, pp. 27-31, 1985 P r i n t e d in the U . S . A .

P e r g a m o n P r e s s Ltd. All r i g h t s r e s e r v e d

TEXTURE CHANGES DURING THE TENSILE DEFORMATION OF A FINE GRAINED ALUMINIUM ALLOY O.A.Ruano and G. Gonzalez Centre National de Investigaciones Metalurgicas Av. de Gregorio del Amo,8 28040 Madrid, Spain ( R e c e i v e d J u l y 3, 1984) ( R e v i s e d O c t o b e r 29, 1984) Introduction Tension tests of single crystals have been widely used for determination of the orientation changes during plastic deformation. The lattice rotations of aluminium single crystals have been determined with the help of stereographic projections (i). Supposing a (Iii) Ii01] slip system, during single slip the tensile axis (TA) rotates toward [i01] which is the slip direction. However when the TA reaches the boundary [001]-[iii] of the stereographic triangle [001]-[011]-[iii], the primary and the conjugate systems are activated and duplex slip should occur. Consequently, the TA should rotate toward [i12]. These considerations would apply to a polycrystalline material only if the individual grains could rotate independently from each other. The individual grains of a polycrystalline material, however, cannot rotate freely due to the interaction between adjacent grains. According to von Mises (2) five independent slip systems are needed for fulfillment of the continuity condition between grains. Taylor (3) selected such systems that require the lowest load during the deformation (principle of minimum work) and made calculations of lattice rotations for axisymmetric tension and { I i ~ I 0 > slip. He predicts three regions of rotation inside the stereographic triangle, where in the region close to [011] the material rotates either to [001] or [Iii]. Taylor's theory for tension and compression has been tested using wire or rod samples by determining the orientation of individual grains within the samples (4) and by wire drawing of single crystals (5). Despite the many successful predictions achieved, this theory cannot completely explain the orientation changes of the grains of all FCC metals (6-11). Stable crystal orientations that are not destroyed during the rolling process have been determined from FCC single crystal rolling experiments. These orientations have been related to the ideal orientations describing textured polycrystalline metals (12). They are the {110}<112> and the {114411>. However, for 70/30 brass and silver no stable orientation was found. Leffers (13) calculated the orientation changes due to rolling for one hundred random distributed grains using a modified Taylor's model and slip taking place on the {iii} system. He obtained an "alloy" texture, as for silver and brass, when the grains could deform like isolated single crystals and a "pure-metal" texture, as for pure aluminium and copper, when Taylor's condition of continuity was partially fulfilled. However, if the rolling process is described by a compressive load in the normal direction (ND) and a tensile load in the rolling direction (RD) it can be shown that the final orientations are {ll0} (14,15). This prediction has only been achieved for silver and s-brass which show a{llO} final stable orient ation and is attributed to a change in texture from the "pure-metal" type due to mechanical twinning (16). In the present study the final stable orientations of a rolled FCC alloy with a well de fined initial texture are investigated by means of tensile tests. The high stacking fault energy material used, should deform as in Leffers' isolated single crystal model: it is an aluminium alloy with a very fine grain size tested in tension at a temperature and strain rate where pure superplastic behavior no longer exists. Under such conditions grain boundary sliding and crystallographic slip are the deformation mechanisms. While pure grain boundary sliding causes a randon grain rotation and therefore the disappearance of a texture (17), slip will stabilize certain orientations. A combination of both mechanisms would stabilize specific orientation as they are reached during grain rotation allowing the sample to be deformed without a strong interaction between grains. This work may shed some light on the nature of the deformation mechanisms involved in the formation of the two types of textures of FCC metals by studying the texture changes after tensile tests.

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Experimental Procedure The AI-5%Ca-5%Zn alloy was processed by Alcan (18). Cast ingots were hot rolled to a slab 6.5 rma thick and then cold rolled 65% to a 2.3 mm thick sheet. Tensile samples of 30 mm gage length were machined out of the as received material at various fixed angles to the RD. All samples were soaked at 500°C for 2 hours to produce the same microstructure. The grain size was about 2 ~m. Tensile testing was performed in an Instron testing machine at 400°C at a constant cross head speed of 10 -2 s-I. After testing, samples obtained far from the fracture regions were first mechanically and then chemically polished to a thickness of i00 ~m. (Iii) pole figures were determined using a Philips goniometer with a pitch of 5 ° using CuK~ radiation. The same sample was used for texture measurements by reflection and transmission. For the calculation of random intensities an overlapping method was followed (19). Results A (Iii) pole figure of the initial texture prior to tensile deformation of the AI-5%Ca-5%Zn alloy is shown in Fig. i. The texture can approximately be described by a (1!2) ~Ii] orientation with the intensity maxima in the rolling direction at the periphery split about i0 °, as indicated by solid triangles. The strongest intensity maxima (nine times random) are 30 ° tilted from the sheet ND toward the RD. The texture is not very different from that Observed for rolled aluminium or copper (20,21) and can be considered as a W e l l defined rolling texture. Fig. 2 shows a (iii) pole figure of a sample deformed 110% at 400°C and 19-2 s-I with the RD parallel to the TA. At this temperature and strain rate the material is at the limit of conditions necessary for superplastic behaviour and grain boundary sliding is no longer the only important deformation mechanism (22). The texture can be described accurately by a (I12) Jill] orientation as indicated by the solid triangles. Furthermore the two strong maxima at the periphery in the RD (23x) indicate that the grains tend to have their [ili] direction parallel to the TA. These maxima may vary with test conditions. Texture is inhibited at low tensile elongations: maxima of only eleven times random intensity were obtained when the sample was deformed 40% at 400 °C and I0 -I s -I. On the other hand, intensity maxima of about 84 times random units were observed depending on temperature and strain rate conditions. If the sample is deformed at 4000C and 10 -2 s-i but with the RD perpendicular to the TA, elongations of 140% are reached. Fig. 3 shows a (iii) pole figure of a sample tested in such conditions. The texture can be described by a (Ii0)[ll~orientation as indicated by the solid squares. This ideal orientation accounts for all of the high intensity areas except for those RD

-TD

FIG.I

F IG. 2

(III) pole figure of AI-5%Ca-5%Zn prior to tensile deformation.

(III) pole figure of a sample deformed at 400°C and i0 -I s -I with the tensile axis in the rolling direction.

•

(1i2)[i11]

• (ii2)[i11]

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RD

9v53. ~A

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"

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753` TD

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7

.

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FIG. 4

(iii) pole figure of sample deformed at 400°C and I0 -I s-I with the tensile axis perpendicular to the rolling direction. ~ ([[O) [i12]

(iii) pole figure of sample deformed at 400°C and i0-I s-i with the tensile axis at 45°from the rolling direction

situated close to the rolling direction in the periphery and is similar to the "alloy"texture The highest intensities correspond to both areas on the equator. This indicates that a direction, which in the figure is parallel to the transverse direction (TD), is again aligned with the TA. Fig. 4 shows a (iii) pole figure of a sample deformed with the RD at 45°from the TA under the same conditions. A 135% elongation was obtained. This texture cannot be described by any low index ideal orientation. The main feature of the pole figure is the presence of high intensity areas close to the TA which indicates a rotation of the grains in order to have a direction parallel to the TA. It is also remarkable that the position of the two highest intensity maxima, tilted about 30°toward the RD, do not change after deformation. Discussion The strongest maxima observed in Fig. 2 and 3 suggest the existence of slip processes which tend to align a direction with the TA. This could be explained by a combined effect of grain boundary sliding and crystallographic slip occurring in the fine grained material. Then, the material behaves as a single crystal with respect to its crystallographic changes. In order to explain the texture changes that occurs during tensile deformation a stereographic projection can be used. Fig. 5 shows a standard (001) stereographic projection of poles and zone circles for a cubic crystal. The unit stereographic triangle 1001]-[011]-[lll] is used to describe the location of the TA. According to Fig. I, the initial orientation can be considered to be close to (1i2)[i11] . If the sample is tested in the RD, the location of the TA inside the stereographic triangle will be close to [iII]. The normal and transverse directions will ther be close to AND and ATD respectively which are marked in Fig. 5. We believe that after deformation is iniciated, the grains rotate to align their Illll direction with the TA, i.e. the TA will move toward [IIi]. Further deformation will cause multiple slip and no rotation should be observed. This agrees with the experimental observation that the sharpness of the intensity maxima, especially of those located in the RD, increases with increasing deformation. The stability of the [iiI] orientation is best explained by deformation on three slip systems of type ~001} . If the sample is tested along the TD, the TA could be represented close to [011] . The RD and the ND will then be located at BRD and BND respectively as shown in Fig. 5. According to Fig. 3, the initial texture changes to (II0)~I12] . The dotted lines inside the stereographic triangle in Fig. 5 represent the path that the TA should follow in order to explain the texture changes, i.e. how Fig. 1 transforms into Fig. 3 after deformation along the TD. First, the grains slip on a {iii} system as for most FCC single crystals. This process rotates the

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FIG.5 Standard (001) stereographic projection for cubic crystals showing changes of orientation with extension.

crystal such that the TA, which is located at [011] , first approaches [I12] as can be seen in Fig. 5. In the same figure can also be followed the variation of the RD and the ND when the TA reaches [I12] . This variation is due to a 30 ° rotation around [iII], i.e. around BRD. Because of this rotation, BRD will remain in the same place and BND will reach [ii0] . If deformation now occurs in the {00~410> slip system, as has been previously observed (8,23), the TA will reach Jill] which is a stable orientation. This variation corresponds to a 19,5 ° rotation around [ii0] parallel to ND. In Fig. 5, BND therefore will not change and BRD will move from [Iii] to [I12] . As a result of these rotations a (ii0)[I12] texture will be obtained. The path for the TA from [011] to Jill] is not unique. Other paths, however, would not explain the (II0)[II~ texture of Fig. 3. If the sample is tested with the RD at 45 ° from the TA, the latter should be located at point C in Fig. 5. The rolling, transverse and normal directions should then be at CRD , CTD and CND, respectively. Although the texture from Fig. 4 exhibits certain tendency for the gralns to align themselves with their [ill] direction parallel to the TA, the maxima that are present cannot be accounted for by any ideal orientation. For this reason it is not possible to draw conclusions about reorientations during deformation from point C to [IIi]. Conclusion I.

The use of fine grained materials tested in tension in the range where grain boundary sliding as well as crystallographic slip are important allows the achievement of sharp intensity maxima and textures which correspond to defined ideal orientations.

2.

The texture development increases with tensile elongation of an AI-5%Ca-5%Zn alloy indicating that the grains tend to rotate toward a final stable orientation.

3.

The deformed samples tend to align a direction with the tensile axis.

Due to this tend-

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ency, a "pure-metal" and an "alloy" texture are obtained in samples tested in the rolling and transverse direction, respectively. 4.

The "pure-metal" and the "alloy" textures can be explained by slip processes, first on the {iii} and then on the ~001} slip systems.

5.

The final stable orientation will depend on the initial texture and on the position of the tensile axis with respect to the initial texture. Sharp textures are obtained when the tensile axis is parallel to a symmetric axis of the initial texture. Acknowledgements

The authors wish to thank M. Torralba for assistance with the texture experiments, G. Frommeyer (MPI-Eisenforschung, West Germany) for valuable advice and G. Caruana for the computer programs. The rolled sheet was kindly supplied by D.M. Moore (Alcan, Canada). References i. 2. 3. 4. 5. 6. 7. 8. 9. I0. Ii. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

G.I. Taylor and C.F. Elam, Proc. Roy. Soc. 108,28 (1925). R. yon Mises, Z. Angew. Math. Mech. 8, 161 (1928). G.I. Taylor, J. Inst. Met. 62, 307 (1938). C.S. Barrett and C.H. Levenson, Trans. AIME. 137, 112 (1940). H.Ahlborn, Z. Metallkde. 56, 205 (1965) G. Mayer and W.A. Backofen, Trans. AIME 242, 1587 (1968). C. Chin, in Textures in Research and Practice, J. Grewen and G. Wassermann, eds., p.51, Springer Verlag (1969). F. Haessner, Z. Metallkde. 54, 98 <1963) I.L. Dillamore and H. Katoh, Met. Sci. 8, 21 (1974). J.S. Kallend and G.J. Davies, Phil. Mag. 25, 471 (1972). G.Y. Chin and B.C. Wonsiewicz, Metall. Trans. i, 551 (1970). R. Penelle, Proc. Sixth Int. Conf. Textures of Mater., p.67, Tokyo (1981). T. Leffers, Phys. Stat. Sol. 25, 337 (1968). E. Schmid and W. Boas, Kristallplastizit~t, Springer Verlag (1936). H.P. StHwe, in Aluminium und Aluminium Legierungen, D. Altenpohl, ed., p.380, Springer Verlag (1965). G. Wassermann, Z. Metallkde. 54, 61 (1963). J.W. Edington, Metall. Trans. 13A, 703 (1982). D.M. Moore and C.R. Morris, Mater. Sci. Eng. 43, 85 (1980). J. Grewen, D. Sauer and H.P. Wahl, Z. Metallkde. 61, 430 (197~). G. Wassermann and J. Grewen, Texturen metalliseher Werkstoffe, Springer Verlag (1962). H. Hu, P.R. Sperry and P.A. Beck, Trans. AIME 194, 76 (1952). P. Ramirez, M.S. Thesis, University of Madrid (1983). R. Karnop and G. Sachs, Z. Phys. 42, 283 (1927).