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Texture gradient evolution in Al-5%Ca-5%Zn sheet alloy after tensile deformation at high superplastic strain rate
Scripta h4aterialia,Vol. 35, No. 12, pp. 14%1460,1996 E1sevie.rScience Ltd Copyright 8 1996 Acta Metallurgica Inc. Printed in the USA. All rights reserved 1359~6462/96 $12.00 + .OO
PI1 S1359-6462(96)00324-7
TEXTURIE GRADIENT EVOLUTION IN Al-S%Ca-5%Zn SHEET ALLSOY AFTER TENSILE DEFORMATION AT HIGH SUPERPLASTIC STRAIN RATE M.T. Perez-Prado, M.C. Cristina, M. Torralba, O.A. Ruano and G. Ghzalez-Doncel Dpto. #deMetalurgia Fisica, Centro National de Investigaciones Metaltigicas, C.S.I.C. Avda. de Gregorio de1 Amo 8,28040 Madrid, Spain (Received June 24, 1996) (Accepted July 22,1996) Introduction
Texture inhomegeneities have been found in many materials (l-9). Given the significant influence of texture in industrial processes like superplastic forming of complex-shaped components, it is important to study the evolution of texture gradients under different testing conditions, particularly at high strain rates. High stra.in rates are interesting for superplastic forming, since they enable faster production rates. The study of texture evolution allows, on the other hand, to go deeper into the microscopic mechanisms that occur during plastic deformation. It has been reported, for example, that initial texture gradients disappear when the material is tested superplastically (1). Grain boundary sliding (GBS) is commonly proposed as the microscopic mechanism responsible for superplastic deformation (10,ll). Therefore, grains would rotate independently of each other leading to a randomization of texture. However, the strengthening of certain preferred orientations during superplastic deformation (2) has suggested that GBS-based models are not always adequate to explain superplasticity. Strong through-thickness texture-gradients have been observed in hot rolled Al alloys (l-7). As a consequence of the severe deformation during the hot rolling process, a well defined Brass texturecomponent (B-orientation) {011}<21 l> develops in the mid layer. The through-thickness texturegradient developed causes occasionally a marked difference in mechanical properties between the outer and the inner regions of rolled materials (8). Although several approaches to understand the appearance of the B-component in hot rolled materials have been proposed (5,6,12), their validity is still to be checked. However, the B-component has also been observed in aluminum alloys after deep drawing (7) and in Al-S%Ca-S%Zn after tensile testing under specific conditions (13). Therefore, it is appropriate to explain the texture gradient and the appearance of the B-component taking into account these later findings. The Al-S%Ca-5%Zn sheet alloy deforms superplastically when tested uniaxially at temperatures ranging from 3’50” to 450°C and at strain rates between lo-‘i’ and lo-*zi’ (14). The B-orientation, however, is not present in the texture of the as-rolled material, but it appears after straining in tension along the transverse direction under certain conditions of temperature and moderately high superplastic strain rates (13). In this work the evolution of the through-thickness texture-gradient in the Al-5%Ca1455
1456
TEXTUREDEVELOPMENT
Vol. 35, No. 12
5%Zn sheet alloy when tested uniaxially in the transverse direction is investigated. Due to the importance of high strain rates in superplastic forming processes, tests at higher strain rates than those usually reported in the literature have been conducted. Current models which predict the appearance of the B-component are criticized on the light of these new findings. Experimental
Procedure
The Al-Swt.%Ca-Swt.%Zn alloy studied was prepared from 99.99% aluminum by the Aluminum Company of Canada. Cast ingots were hot rolled and then cold rolled to a 3.2 mm thick sheet (14). Tensile samples of 10 mm gage length and 5 mm width were machined-out of the as-received material at 90” of the rolling direction (RD), i.e. with the tensile axis parallel to the transverse direction (TD). Some longitudinal samples (tensile axis parallel to RD) were also tested for comparison of the texture developed. Tensile testing was performed in a SERVOSISm testing machine at 10-2s~’and at lo-‘i’, the highest strain rate allowed by the machine. These strain rates are considered high in the superplastic context. Test temperatures ranging from 100°C to 55O’C were used. When possible, strain rate changing tests around 10“s-’ were performed with the aim of going deeper into the microscopic mechanisms which are operative under the different test conditions. Texture measurements were carried out by means of the Schulz reflection method, using a SIEMENSTM diffractometer furnished with a D5000 goniometer and a close Eulerian cradle. The xradiation used was P-filtered CuKa. Direct pole figures and the representation of the threedimensional orientation distribution function (ODF) of the Euler angles 4 ,, a’, and 6, were obtained and, thus, the different components of the texture identified. To investigate the through-thickness texture-gradient, ODFs were obtained both in the external region (near the surfaces of the sheet = 50 pm), zone I, and in the mid layer (~33% vol), zone II. Special care was taken on sample preparation. The surfaces were prepared by abrasion on successively finer silicon carbide papers and then polishing through 1 urn diamond paste. Final surface preparation was performed in Barker’s reagent (4 ml HBR, 200 ml H20) at 18V during 50s. Results and Discussion
The texture of the Al-S%Ca-5%Zn in the as-received condition is slightly inhomogeneous throughout the thickness. Figure 1 shows the (111) direct pole figures corresponding to the outer, Fig. l(a), and the inner zone, Fig.l(b). As can be seen, the texture in both regions is very similar. A more detailed analysis of the texture by means of the ODF, however, allowed to determine the precise ideal orientations in both regions. The texture in the as-received condition comprises three well defined compo-
Figure 1. (111) pole figures showing the texture of the as-received
AI-5%Ca-W’n
sheet alloy in: a) zone I; b) zme II.
Vol. 35, No. 12
TEXTURE DEVELOPMENT
Figure 2. ODF q1,==90’cuts of AI-S%Ca-S%Zn in the as-received condition showing the ideal orientations: a) zone I and b) zone !I.
1457
Figure 3. (111) Pole figures of Al-S%Ca-S%Zn after testing along TD at Louisaand 4OO’C:a) zone I; b) zone II.
nents: in zone I (outer) the main component is { 113}<332>, and there are two other, namely the {01 l} and the {013)<3 lO> components; in zone II (inner) the main component is the {225}<554>, close to {113)<332> (A’P,=O’, A@+“, AV2=Oo).There are also two minor ones, the {011}<100>, adso detected in zone I, and the {014}<410>, close to {013}<310> (A$=5”). The intensity of the minor components in both zones is approximately half that of the main one. Figure 2 shows the (P1=900ODF cut for zones I and II, where the different components of the texture appear. Emphasis in this work will concentrate on the texture changes of the main component. After testing at appropriate strain rates and temperatures, the difference between the textures present in zones I and II was drastically accentuated. As an example, Fig. 3 shows the (111) pole figures corresponding to zones I, Fig.3(a), and II, Fig.3(b), after testing at 400°C and lo-*se’. Under this conditions the Al-S%Ca-5%Zn alloy elongated about 170%. The main component of zone I is the same as that measured before testing, i.e., {113)<332>, whereas the main one in zone II is the (011}<21 l> (Bcomponent), very different from the {225}<554> initial component. Thus the small texture gradient present in the ;as-received material increases dramatically with deformation. This suggests that the deformation of Al-S%Ca-5%Zn under this condition, can not be explained by means of a unique deformation mechanism. The enhancement of the through-thickness texture-gradient after tensile deformation occurs under a wide range of strain rates and temperatures in which the tensile behavior of Al-S%Ca-5%Zn changes dramatically. Figure 4 shows several true strain vs. true stress tensile curves obtained at different temperatures and strain rates. The stress is represented in a logarithmic scale for better comparison of tests at different temperatures. The appearance of the B-component in the mid layer, and thus, the increase in the through-thickness texture-gradient, is observed under all these testing conditions, except in the tests perfbrmed at lOO”C,in which no change in texture occurred, and at 550”C/10~2s~‘,in which the main ideal orientation was neither the {225}<554> nor the (0 11)c2 1l> orientation. As can be seen from Fig. 4, the tensile behaviour differs noticeably, depending upon the test conditions. The elonga-
Figure 4. Stress-strain tensile curves of AI-W&a-5%Zn tested along TD at several temperatures: a) & =lO”s”; b) i: =lOS’s“.
TEXTUREDEVELOPMENT
1458
Vol. 35, No. 12
TABLE 1 Strain Rate Sensitivity Index, m, Measuredat the Different Temperatures VW
200
30
4w
ml
ml
”
0. I
ll.lh-0.2
0.25
0.5
0.5
tion to failure increases and the maximum stress decreases with increasing test temperature. For example, the phenomenon occurs at 200”C/l~2s~’ and at 500°C/10~zs~‘,for which the stresses measured at 0.15 strain were 174 MPa and 6 MPa and the true strains to failure were 0.43 and 1.89, respectively. These differences are also manifested on the strain rate sensitivity index, m, which values, measured at 10‘2s~‘,are shown in Table 1. A dramatic change in the deformation mechanism controlling the behaviour of Al-S%Ca-S%Zn at 200°C (m=O.l) and at 500°C (m=0.5) can be inferred from those values (16). From commonly reported deformation-mechanism equations (16), m=O.l would suggest slip creep at constant structure deformation mechanism (pipe diffusion controlled) whereas m=0.5 would suggest GBS mechanisms as is generally accepted for superplastic deformation. The appearance of the through-thickness texture-gradient during the above testing conditions, however, indicates that the analysis of the high temperature deformation of Al-S%Ca-S%Zn on the basis of single mechanisms is too simplistic. The through-thickness texture-gradient increase, with the appearance of the B-component, adds new insight to the reported literature on textures of rolled aluminum alloys. Several proposals have been drawn to explain the appearance of a through-thickness texture-gradient in some Al alloys after hot rolling (5). Among them, two are considered the most plausible. In a first approach this phenomenon is explained in terms of the through-thickness temperature-gradient originated during hot rolling. The appearance of a texture-gradient in Al-S%Ca-S%Zn under tensile testing, however, questions the validity of this proposal. In a second approach it is suggested that a large amount of&,,-shear-strain, as well as high m values (m>0.2), give rise to a B-orientation in the mid-layer, thus leading to a texture gradient. On the other hand, it is known that in a tensile test no macroscopic shear occurs. Therefore, that large local shear of some grains, resulting in the appearance of the B-component, may have taken place during testing of Al-S%Ca-S%Zn is, at least, controversial. Furthermore, in this work, the phenomenon has taken place in tests for which mc0.2; for example the test performed at 300”C/l~zs~‘. Therefore, the strain rate sensitivity does not seem to have a considerable influence in the appearance of B-component under the testing conditions studied. Since in longitudinal tests neither the Bcomponent nor the through-thickness texture-gradient appear, the origin of this phenomenon should be influenced by the relative orientation between the applied stress and the crystallites, as well as to a different stress distribution over the grains in zones I and II. It is remarkable that the B-component, which appears in the inner region when the material is tested along TD, “remembers” the original rolling frame and not the tensile direction. Also, some other differences between the microstructure of zones I and II resulting from the rolling process (for example, grain size), not yet detected, can account for these phenomena.
Figure 5. Inverse pole figure showing the position of the tensile axis for the main texture components (Zones I and II) of Al5%Ca-5%Zn tested along TD: a) before deformation; b) tier deformation.
Vol. 35, No. 12
TEXTURE DEVELOPMENT
1459
The pronounced increase in the through-thickness texture-gradient and the formation of the Bcomponent in Al-S%Ca-S%Zn can be better appreciated with the aid of the inverse pole figure, as shown in Fig.5. This figure represents the rotation of the tensile axis in the standard stereographic triangle. Before testing, the tensile axis of the crystallites belonging to the main component in both zones I and II is located in the [Ol l] comer of the triangle, Fig.S(a). After deformation, the tensile axis of the crystallites which define the main component in zone II rotates towards the [ 11 l] comer of the standard stereographic triangle (35” rotation), whereas no rotation occurs in the crystallites of zone I, since the tensile axis remains in the [Ol l] direction, Fig.S(b). From this observation it follows that different deformation processes between zones I and II have had to occur and that, despite the extensive tensile elongation achieved in the high temperature range, with m=0.5, (i.e. superplasticity), the activation of slip systems has taken place during deformation. This contrasts with what is observed in the test carried out at 550°C/10”s~‘. In this case, it is also m=0.5 with tensile elongations of 540% (1.86 true strain). However, the texture gradient developed is not so pronounced and the B-component is no longer the main texture component after deformation. It is seen then,, that crystallographic slip is very important in zone II, at least. The slip systems which are operative in zone II during deformation are, on the other hand, very sensitive to the orientation of the crystallites, since the B-component does not appear when testing the material along the longitudinal direction. In zone I, however, the texture basically does not change both when testing along TD and RD. This indicales that the operative slip systems in this zone are not sensitive to the crystallite orientation with respect to the applied stress. On the other hand, since a randomization of texture is not observed after deformation, GBS can not be considered as the main deformation mechanism responsible for superplastic deformation, contrarily to common thinking (11,15) and in agreement with previous research (2,17). The rotation of the grains in zone II is smaller after testing at 55O’C than at 4OO’C despite that the strain to failure is larger in the test at higher temperature. Therefore, another mechanism different from crystallographic slip, e.g. GBS, should increase its contribution to deformation in this temperature range. The fact lhat the contribution of GBS is more important at 550°C than at 400°C is in line with the strain rate sensitivity (m) values measured at both temperatures, which are 0.5 and 0.25, respectively. Conclusions The through-thickness texture-gradient evolution in the Al-S%Ca-S%Zn sheet alloy tested under several conditions of strain rate (1U’s“ and lo-‘s“) and temperature (lOO”C< T ~550°C) has been investigated and the following are the most important conclusions: 1.
2.
The alloy presents a slight through-thickness texture-gradient in the a&received condition: in zone I (outer region), the main texture component is { 113}<332>. In zone II (mid-layer) the main component is the {225}<554>, close to (113}<332> (A(PM’, A4r=5”,A$=O”). I.e., the texture is very similar in both zones. A strong through-thickness texture-gradient develops after testing the material along the transverse direction in a wide range of temperatures and strain rates: in the outer region (66%) the ideal orientation { 113}<332> remains as the main component, while in the central part a strong Bcomponent {0 11) ~2 1l> appears. This suggests that the deformation of Al-S%Ca-S%Zn under the testing conditions investigated, can not be explained by means of a unique deformation mechanism and that GBS is not the only deformation mechanism under the superplastic conditions investigated.
1460
3.
TEXTURE DEVELOPMENT
Crystallographic slip is an important deformation mechanism. The slip systems which are operative in zone II during deformation are very sensitive to crystallites orientation, since the Bcomponent does not appear when testing the material along the longitudinal direction. In zone I, however, the texture basically does not change both when testing along TD and RD. This indicates that the operative slip systems in this region are not sensitive to the crystallite orientation with respect to the applied stress. AcknowledPements
The authors acknowledge CICYT financial support (MAT94-0888). References
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 1s. 16. 17.
Vol. 35, No. 12
P. L. Blackwell and P. S. Bate, Proc. Int. Conf. Superplasticity: 60 years after Pearson, 183-192 (1994) P. L. Blackwell and P. S. Bate, Met. Trans. A 24A, 1085-1093 (1993). 0. Daaland and E. Nes, Acta mater. 44, (4). 1389-1411 (1996). F. Barlat, J. C. Brem, and J. Liu, Scripta metall. 27, 1121-l 126 (1992). 0. Engler, P. Wagner, J. Savoie, D. Ponge, and G. Gottstein, Scriptametall. 28, 1317-1322 (1993). A. W. Bowen, Mat. Sci. Tech. 6, 1068-1071 (1990). J. Hirsch, Mat. Sci. Tech. 6, 1048-1057 (1990) M. Peters, J. Eschweiler, and K. Welpmann, Scriptametall. 20, 259-264 (1986). 0. Engler and K. Ltkcke, Mat. Sci. Eng. A148, 15-23 (1991). 0. D. Sherby and J. Wadsworth, Prog. Mater. Sci. 33 (1989). J. W. Edington, K. N. Melton, and C. P. Cutler, Prog. Mater. Sci. 21, 61 (1976). L. S. Toth, P. Ciilormini, and J. J. Jonas, Acta metall. 26 (2), 3077-3091 (1988). 0. A. Ruano and G. GonzPez-Doncel, Scripta metall. 19,27 (1985). D. M. Moore and L. R. Morris, Mater. Sci. Eng. 43,85 (1980). T. G. Langdon, Mat. Sci. Eng. A174,225-230 (1994). B. Walser and 0. D. Sherby, Scripta metall. 16,213-219 (1982). F. Li, W. T. Roberts, and P. S. Bate, Scripta metall. 29, 875-880 (1993).
PI1 S1359-6462(96)00324-7
TEXTURIE GRADIENT EVOLUTION IN Al-S%Ca-5%Zn SHEET ALLSOY AFTER TENSILE DEFORMATION AT HIGH SUPERPLASTIC STRAIN RATE M.T. Perez-Prado, M.C. Cristina, M. Torralba, O.A. Ruano and G. Ghzalez-Doncel Dpto. #deMetalurgia Fisica, Centro National de Investigaciones Metaltigicas, C.S.I.C. Avda. de Gregorio de1 Amo 8,28040 Madrid, Spain (Received June 24, 1996) (Accepted July 22,1996) Introduction
Texture inhomegeneities have been found in many materials (l-9). Given the significant influence of texture in industrial processes like superplastic forming of complex-shaped components, it is important to study the evolution of texture gradients under different testing conditions, particularly at high strain rates. High stra.in rates are interesting for superplastic forming, since they enable faster production rates. The study of texture evolution allows, on the other hand, to go deeper into the microscopic mechanisms that occur during plastic deformation. It has been reported, for example, that initial texture gradients disappear when the material is tested superplastically (1). Grain boundary sliding (GBS) is commonly proposed as the microscopic mechanism responsible for superplastic deformation (10,ll). Therefore, grains would rotate independently of each other leading to a randomization of texture. However, the strengthening of certain preferred orientations during superplastic deformation (2) has suggested that GBS-based models are not always adequate to explain superplasticity. Strong through-thickness texture-gradients have been observed in hot rolled Al alloys (l-7). As a consequence of the severe deformation during the hot rolling process, a well defined Brass texturecomponent (B-orientation) {011}<21 l> develops in the mid layer. The through-thickness texturegradient developed causes occasionally a marked difference in mechanical properties between the outer and the inner regions of rolled materials (8). Although several approaches to understand the appearance of the B-component in hot rolled materials have been proposed (5,6,12), their validity is still to be checked. However, the B-component has also been observed in aluminum alloys after deep drawing (7) and in Al-S%Ca-S%Zn after tensile testing under specific conditions (13). Therefore, it is appropriate to explain the texture gradient and the appearance of the B-component taking into account these later findings. The Al-S%Ca-5%Zn sheet alloy deforms superplastically when tested uniaxially at temperatures ranging from 3’50” to 450°C and at strain rates between lo-‘i’ and lo-*zi’ (14). The B-orientation, however, is not present in the texture of the as-rolled material, but it appears after straining in tension along the transverse direction under certain conditions of temperature and moderately high superplastic strain rates (13). In this work the evolution of the through-thickness texture-gradient in the Al-5%Ca1455
1456
TEXTUREDEVELOPMENT
Vol. 35, No. 12
5%Zn sheet alloy when tested uniaxially in the transverse direction is investigated. Due to the importance of high strain rates in superplastic forming processes, tests at higher strain rates than those usually reported in the literature have been conducted. Current models which predict the appearance of the B-component are criticized on the light of these new findings. Experimental
Procedure
The Al-Swt.%Ca-Swt.%Zn alloy studied was prepared from 99.99% aluminum by the Aluminum Company of Canada. Cast ingots were hot rolled and then cold rolled to a 3.2 mm thick sheet (14). Tensile samples of 10 mm gage length and 5 mm width were machined-out of the as-received material at 90” of the rolling direction (RD), i.e. with the tensile axis parallel to the transverse direction (TD). Some longitudinal samples (tensile axis parallel to RD) were also tested for comparison of the texture developed. Tensile testing was performed in a SERVOSISm testing machine at 10-2s~’and at lo-‘i’, the highest strain rate allowed by the machine. These strain rates are considered high in the superplastic context. Test temperatures ranging from 100°C to 55O’C were used. When possible, strain rate changing tests around 10“s-’ were performed with the aim of going deeper into the microscopic mechanisms which are operative under the different test conditions. Texture measurements were carried out by means of the Schulz reflection method, using a SIEMENSTM diffractometer furnished with a D5000 goniometer and a close Eulerian cradle. The xradiation used was P-filtered CuKa. Direct pole figures and the representation of the threedimensional orientation distribution function (ODF) of the Euler angles 4 ,, a’, and 6, were obtained and, thus, the different components of the texture identified. To investigate the through-thickness texture-gradient, ODFs were obtained both in the external region (near the surfaces of the sheet = 50 pm), zone I, and in the mid layer (~33% vol), zone II. Special care was taken on sample preparation. The surfaces were prepared by abrasion on successively finer silicon carbide papers and then polishing through 1 urn diamond paste. Final surface preparation was performed in Barker’s reagent (4 ml HBR, 200 ml H20) at 18V during 50s. Results and Discussion
The texture of the Al-S%Ca-5%Zn in the as-received condition is slightly inhomogeneous throughout the thickness. Figure 1 shows the (111) direct pole figures corresponding to the outer, Fig. l(a), and the inner zone, Fig.l(b). As can be seen, the texture in both regions is very similar. A more detailed analysis of the texture by means of the ODF, however, allowed to determine the precise ideal orientations in both regions. The texture in the as-received condition comprises three well defined compo-
Figure 1. (111) pole figures showing the texture of the as-received
AI-5%Ca-W’n
sheet alloy in: a) zone I; b) zme II.
Vol. 35, No. 12
TEXTURE DEVELOPMENT
Figure 2. ODF q1,==90’cuts of AI-S%Ca-S%Zn in the as-received condition showing the ideal orientations: a) zone I and b) zone !I.
1457
Figure 3. (111) Pole figures of Al-S%Ca-S%Zn after testing along TD at Louisaand 4OO’C:a) zone I; b) zone II.
nents: in zone I (outer) the main component is { 113}<332>, and there are two other, namely the {01 l}
Figure 4. Stress-strain tensile curves of AI-W&a-5%Zn tested along TD at several temperatures: a) & =lO”s”; b) i: =lOS’s“.
TEXTUREDEVELOPMENT
1458
Vol. 35, No. 12
TABLE 1 Strain Rate Sensitivity Index, m, Measuredat the Different Temperatures VW
200
30
4w
ml
ml
”
0. I
ll.lh-0.2
0.25
0.5
0.5
tion to failure increases and the maximum stress decreases with increasing test temperature. For example, the phenomenon occurs at 200”C/l~2s~’ and at 500°C/10~zs~‘,for which the stresses measured at 0.15 strain were 174 MPa and 6 MPa and the true strains to failure were 0.43 and 1.89, respectively. These differences are also manifested on the strain rate sensitivity index, m, which values, measured at 10‘2s~‘,are shown in Table 1. A dramatic change in the deformation mechanism controlling the behaviour of Al-S%Ca-S%Zn at 200°C (m=O.l) and at 500°C (m=0.5) can be inferred from those values (16). From commonly reported deformation-mechanism equations (16), m=O.l would suggest slip creep at constant structure deformation mechanism (pipe diffusion controlled) whereas m=0.5 would suggest GBS mechanisms as is generally accepted for superplastic deformation. The appearance of the through-thickness texture-gradient during the above testing conditions, however, indicates that the analysis of the high temperature deformation of Al-S%Ca-S%Zn on the basis of single mechanisms is too simplistic. The through-thickness texture-gradient increase, with the appearance of the B-component, adds new insight to the reported literature on textures of rolled aluminum alloys. Several proposals have been drawn to explain the appearance of a through-thickness texture-gradient in some Al alloys after hot rolling (5). Among them, two are considered the most plausible. In a first approach this phenomenon is explained in terms of the through-thickness temperature-gradient originated during hot rolling. The appearance of a texture-gradient in Al-S%Ca-S%Zn under tensile testing, however, questions the validity of this proposal. In a second approach it is suggested that a large amount of&,,-shear-strain, as well as high m values (m>0.2), give rise to a B-orientation in the mid-layer, thus leading to a texture gradient. On the other hand, it is known that in a tensile test no macroscopic shear occurs. Therefore, that large local shear of some grains, resulting in the appearance of the B-component, may have taken place during testing of Al-S%Ca-S%Zn is, at least, controversial. Furthermore, in this work, the phenomenon has taken place in tests for which mc0.2; for example the test performed at 300”C/l~zs~‘. Therefore, the strain rate sensitivity does not seem to have a considerable influence in the appearance of B-component under the testing conditions studied. Since in longitudinal tests neither the Bcomponent nor the through-thickness texture-gradient appear, the origin of this phenomenon should be influenced by the relative orientation between the applied stress and the crystallites, as well as to a different stress distribution over the grains in zones I and II. It is remarkable that the B-component, which appears in the inner region when the material is tested along TD, “remembers” the original rolling frame and not the tensile direction. Also, some other differences between the microstructure of zones I and II resulting from the rolling process (for example, grain size), not yet detected, can account for these phenomena.
Figure 5. Inverse pole figure showing the position of the tensile axis for the main texture components (Zones I and II) of Al5%Ca-5%Zn tested along TD: a) before deformation; b) tier deformation.
Vol. 35, No. 12
TEXTURE DEVELOPMENT
1459
The pronounced increase in the through-thickness texture-gradient and the formation of the Bcomponent in Al-S%Ca-S%Zn can be better appreciated with the aid of the inverse pole figure, as shown in Fig.5. This figure represents the rotation of the tensile axis in the standard stereographic triangle. Before testing, the tensile axis of the crystallites belonging to the main component in both zones I and II is located in the [Ol l] comer of the triangle, Fig.S(a). After deformation, the tensile axis of the crystallites which define the main component in zone II rotates towards the [ 11 l] comer of the standard stereographic triangle (35” rotation), whereas no rotation occurs in the crystallites of zone I, since the tensile axis remains in the [Ol l] direction, Fig.S(b). From this observation it follows that different deformation processes between zones I and II have had to occur and that, despite the extensive tensile elongation achieved in the high temperature range, with m=0.5, (i.e. superplasticity), the activation of slip systems has taken place during deformation. This contrasts with what is observed in the test carried out at 550°C/10”s~‘. In this case, it is also m=0.5 with tensile elongations of 540% (1.86 true strain). However, the texture gradient developed is not so pronounced and the B-component is no longer the main texture component after deformation. It is seen then,, that crystallographic slip is very important in zone II, at least. The slip systems which are operative in zone II during deformation are, on the other hand, very sensitive to the orientation of the crystallites, since the B-component does not appear when testing the material along the longitudinal direction. In zone I, however, the texture basically does not change both when testing along TD and RD. This indicales that the operative slip systems in this zone are not sensitive to the crystallite orientation with respect to the applied stress. On the other hand, since a randomization of texture is not observed after deformation, GBS can not be considered as the main deformation mechanism responsible for superplastic deformation, contrarily to common thinking (11,15) and in agreement with previous research (2,17). The rotation of the grains in zone II is smaller after testing at 55O’C than at 4OO’C despite that the strain to failure is larger in the test at higher temperature. Therefore, another mechanism different from crystallographic slip, e.g. GBS, should increase its contribution to deformation in this temperature range. The fact lhat the contribution of GBS is more important at 550°C than at 400°C is in line with the strain rate sensitivity (m) values measured at both temperatures, which are 0.5 and 0.25, respectively. Conclusions The through-thickness texture-gradient evolution in the Al-S%Ca-S%Zn sheet alloy tested under several conditions of strain rate (1U’s“ and lo-‘s“) and temperature (lOO”C< T ~550°C) has been investigated and the following are the most important conclusions: 1.
2.
The alloy presents a slight through-thickness texture-gradient in the a&received condition: in zone I (outer region), the main texture component is { 113}<332>. In zone II (mid-layer) the main component is the {225}<554>, close to (113}<332> (A(PM’, A4r=5”,A$=O”). I.e., the texture is very similar in both zones. A strong through-thickness texture-gradient develops after testing the material along the transverse direction in a wide range of temperatures and strain rates: in the outer region (66%) the ideal orientation { 113}<332> remains as the main component, while in the central part a strong Bcomponent {0 11) ~2 1l> appears. This suggests that the deformation of Al-S%Ca-S%Zn under the testing conditions investigated, can not be explained by means of a unique deformation mechanism and that GBS is not the only deformation mechanism under the superplastic conditions investigated.
1460
3.
TEXTURE DEVELOPMENT
Crystallographic slip is an important deformation mechanism. The slip systems which are operative in zone II during deformation are very sensitive to crystallites orientation, since the Bcomponent does not appear when testing the material along the longitudinal direction. In zone I, however, the texture basically does not change both when testing along TD and RD. This indicates that the operative slip systems in this region are not sensitive to the crystallite orientation with respect to the applied stress. AcknowledPements
The authors acknowledge CICYT financial support (MAT94-0888). References
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 1s. 16. 17.
Vol. 35, No. 12
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