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On micro-superplasticity
Acta mater. Vol. 45, No. 9, Pp. 3533-3542, 1997 F 1997 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Bntain PII: S1359-6454(97)00065-7 1359-6454/97 $17.00 + 0.00
Pergamon
ON MICRO-SUPERPLASTICITY M. G. ZELINt Department
of Chemical
Engineering
(Received
and Materials Science, University U.S.A. 2 January
1997; accepted 28 January
of California,
Davis, CA 95616,
1997)
Abstract-Micro-superplasticity phenomenon, i.e. formation of thin fibers, whose appearance suggests extremely high strain in the local regions, has been documented in a number of plastically and superplastically deformed materials, predominantly in aluminum alloys. The fiber formation occurs both at the fracture surface and at the deformed surface. The fiber length increases with the total strain, and it is maximum under the optimum superplastic strain rate. An ultra-fine grain structure, a high concentration of oxygen and often high concentration of the alloy elements have been observed in the fibers. Suggested explanations for fiber formation have been reviewed; the important role of oxidation reactions has been emphasized. c 1997 Acta Metallurgica Inc.
2. EXPERIMENTAL
1. INTRODUCTION
Superplasticity is the ability of crystalline materials to undergo large deformations reaching 100 and 1000%. Since the early studies of superplastic (SP) flow, it has been realized that significant deformations in superplastic materials are achieved due to the high stability of SP flow [l]. Particularly, specimens superplastically deformed in tension do not demonstrate the formation of a fixed localized neck up to the final strains at which failure occurs. Recently [2-161, similar stable plastic flow in micro-volumes has been observed. Thin fibers with diameter from 0.1 to 1 pm have been reported in a number of materials, predominantly, aluminum alloys. Since the individual fibers look like micro-tensile specimens, which have undergone tremendous deformation, Shaw [S] suggested the term micro-superplasticity for this phenomenon of fiber formation. Because the first observations of the fibers were made at the fracture surface [2-61, it was viewed as a new fracture mechanism. However, fiber formation has also been observed at the deformed surface of unfailed specimens [7, 8, 14-161 which suggests its deformation origin. Although a number of possible explanations for the fiber formation have been proposed [2-161, the micro-mechanism for this phenomenon is still unclear. Meanwhile, its understanding may contribute to the further development of the theory of plasticity and serve as a basis for the development of new advanced materials and forming processes. In this paper, results of a micro-superplasticity study into four aluminum alloys are reported, and the physical mechanism for this phenomenon is analyzed. tPresent address: Concurrent 1450 Scalp Av., Johnstown,
Technologies Corporations, PA 15904, U.S.A.
Tensile specimens and punch bulged specimens have been studied. The chemical composition of the studied materials and test conditions are given in Table 1. The experimental details can be found elsewhere [ 17, 181. AA7475, AA2090 and AA51 82/ AA6090-25%SiC, laminated matrix composite (LMC) blanks were bulged with punches of hemispherical, cylindrical and conical shape. The blanks, as well as some of Al-S%Mg-2%Li and AA7475 tensile specimens were mechanically polished before deformation, and marker lines were inscribed at the polished surface by using a diamond paste. Deformed specimens were examined in a scanning electron microscope (SEM). The strain which the material underwent to form a fiber was estimated as t = In 8 - d& where d and do are the fiber diameter at the thinnest portion and at the base of the fiber, respectively. All measurements were performed on SEM micrographs with an accuracy of 0.1 pm. Fiber chemical composition was analyzed using EDS; the beam spot resolution was 1 pm. Fine fiber structure was studied in a TEM. The fibers for TEM observations were extracted from the failure tip of failed AA7475 tensile specimens.
3. EXPERIMENTAL
3.1. Fiber
RESULTS
morphology
In all studied materials, fiber appearance suggested a very high local strain (Figs 14). However, despite this similarity, the fiber morphology is distinct in each individual case. AA 7475. At the deformed surface of bulged domes, rows of fibers are observed between grains moving
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MICRO-SUPERPLASTICITY
Table 1. Materials Material
Comuosition
AA7475
Al-5.8Zn2.2Mg-1.5Cu
AA2090 Laminated
metal matrix
Al-5%Mg-2%Li
composite
AI-Cu-Mg AA5182/ 609OG25%SiC,
and test conditions
Specimen geometry (mm’) 13 x 3.4 x 2.5 50X50X 1.5 50 x 50 x 1.5 50X50X 1.5 13 x 3.4 x 2.5
apart [Figs l(a) and (b)]. Split fibers are seen at the triple junctions [Fig. l(b)]. The diameter of individual fibers is in the range from 0.05 to 1 pm, and their length is in the range from 3 to 10 pm. While the total strain of the bulk material is close to 0.5, the estimated strain value varies in the range from 1.5 to 4. For instance, the strain in fibers numbered 14 in the insert given in the upper right-hand corner in Fig. l(a) was 1.4, 1.8, 3.1 and 1.5, respectively. The existence of both failed and unfailed fibers at the same grain bounday facets indicates a significant variation in the fiber ductility. On the deformed surface and fracture surface of the tensile specimens, long individual fibers were observed along with rows of the fibers [Figs l(c) and (d)]. The length of the individual fibers reaches hundreds of microns, and their diameter can be up to
Deformation mode Umaxial
tension
Bulging Bulging Bulging Uniaxial
tension
Temperature (-0 516
2 x 10-4
415 550
2.5 x lo-” 10 ’
250
IO-”
3 pm. Droplet-like features were also observed at the fracture surface [arrowed in Fig. l(d)]. AA2090. Numerous individual fibers have a diameter from 0.1 to 2.5 pm and a length of up to more than 200 pm [Figs 2(a) and (b)]. Fibers of this type have been reported in aluminum metal matrix composites subjected to the high strain rate superplastic deformation [9%12]. Some fibers are normal to the dome surface [Fig. 2(a)]. Significant fiber lengths indicate a very high strain of the fiber material; however, even approximate local strain estimates were impossible because of the uncertainties at the base. The fiber surface in the fiber diameter appears to be severely oxidized, as well as the grain surface. Some steps at the fiber surface can be distinguished at high magnification [see the insert given in the right-hand upper corner in Fig. 2(b)].
Fig. I. Fibers formed at the deformed surface of AA7475 hemispherical dome (a, b) and the fracture surface of the tensile specimens (c, d). The insert given in the upper right-hand corner in (a) illustrates the region shown by crosses under a high magnification. Numerals 14 designate fibers in which strain was estimated. ; = 2 x 10S4 SK’, T= 516°C.
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Fig. 2. (a), (b) Fibers formed at the deformed surface of a conical AA2090 dome. the left-hand upper corner in (b 8) shows an individual fiber at high magnification. T = 470°C.
Al--5%Mg-2%Li. Both individual fibers and rows of fibers are seen in Figs 3(a)-(c), respectively. Individual fibers have a diameter of approximately 1 pm and can be as long as 15 pm. The estimated local strain in this case is approximately 1.4. Fibers forming rows have a diameter up to 0.3 pm and a length in the range of 1-5 pm. SAA5182/AA6090-25%SiC, composite. Fibers in AA5182 layers form rows [Fig. 4(a)], while fibers in AA6090-25%SiC, layers form bundles [Fig. 4(b)]. Similar bundles of fibers were also observed at the
Fig.
3. (a), (b) Isolated
fibers
and (c) rows Al-S%Mg-2%Li.
The insert given in 2 = 2.5 x 10mJs I,
fracture surface of IN-9021 [5] and Al-Fe-X alloys [6]. The diameter of both types of fibers is less than 1 pm, and their length can be up to 200 pm. An estimated local strain varies from 1.5 to 5. 3.2. Fiber location In all studied cases, fibers were observed only in the deformed specimen regions which indicates their deformation origin. The fibers were found both at the deformed surface and at the fracture surface, as well as inside internal cavities located close to the
of,fibers (arrowed) formed ( = lo-” SV’, T = 250’C.
at the deformed
surface
of
Fig. 4. Fibers formed at the surface cracks in (a) AA5086 layer and in (b) 6090-25%SiC, layer of a AA5182/609&25%SiC, laminated composite conical dome. ; = lo-* s-l, T = 550°C.
specimen edge [Fig. 5(a)]. However, the majority of the internal cavities which were not linked to the specimen surface did not show fiber formation [Fig.
the AA6090&25%SiC, layers, they are formed between and at SIC particles, which is typical for materials of this class [9].
WII. Detailed topographic study [l&18] showed fiber formation at the grain boundaries which experienced significant sliding. Figure 6 illustrates this fact using the example of an AA7475 hemispherical cup. Fibers are seen at grain boundaries surrounding grain groups which slid as an entity (for instance, labeled with letters A and B). Figure 7 illustrates such a cooperative grain boundary sliding (CGBS), i.e. sliding of grain groups, in the Al-S%Mg-2%Li alloy. The length of the segments in which marker lines were broken due to CGBS is up to 30 pm, i.e. approximately 30 grain diameters (for instance, the distance between points F and G). The offset of marker lines can be more than a grain size [it is approximately three grain sizes between points E and F in Fig. 7(b)]. In the AA5182/AA609&25%SiCP composite, fibers are observed at the fracture surface (Fig. 4). In
3.3. Fiber fine structure Figures 8(a) and (b) show TEM micrographs taken from a fiber colony and an individual fiber, respectively [193. Fibers have serrated edges that can be related to the presence of dispersoid particles. Blandin et al. [15] showed that these particles are enriched with Mg and Zn. Diffraction patterns taken from a fiber [see the insert given in Fig. 8(b)] are typical for a polycrystalline material. The fiber grain size varies from 10 to 50 nm. 3.4. Effect
qf strain
It was found [4, 51 that the length of fibers formed inside a crack increases with increase in the opening width. Figure 9 illustrates this fact in the case of the LMC composite showing fibers in the AA5182 surface layer. Note, that short failed fibers are seen
Fig. 5. SEM micrographs illustrating fibers formed inside the internal cavities located close specimen edge (a) and absence of fibers inside a typical internal close cavity (b) in AA7475 specimens. t = 2 x 10m4 SK’, T = 516°C.
to the tensile
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MICRO-SUPERPLASTICITY
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Fig. 6. A montage of SEM micrographs grain boundary
illustrating rows of fibers formed at the surfaces of cooperative in the top region of the hemispherical AA7475 dome. Letters designate groups of grains which slide as an entity. ; = 2 x IO-“ sm’, T= 516°C.
sliding (arrowed)
in addition to the long ones in the region of the wide crack opening. Studies performed in AA7475 domes [16, 171 and tensile specimens [ 141 indicate that formation of striated bands [20, 211 precedes fiber formation. For
instance, in an AA7475 conical cup, striation bands were observed in the base portion, while fibers were found in the top region. With increasing strain, striated bands evolve into the fibers because of continuing sliding apart of the grains.
Fig. 7. Distortion of marker lines due to grain boundary sliding in Al-S%Mg-2%Li tensile specimen. Letters E and F designate marker line displacement and F, G designate the undistorted marker line segment.
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Fig. 8. TEM micrographs showing groups of fibers (a) and fiber fine structure (b). The insert in (b) shows a typical polycrystalline material reflex pattern from the fiber. AA7475 spherical dome. t = 2 x 10m4SK’, T= 516°C.
3.5
Effect
of strain rate
7-he size of the sliding grain groups increases when the strain rate decreases (Fig. 10). Consequently,
fibers are seen at fewer grain boundaries (compare Figs 6 and 10). Evidence of bright lines, which appear to outline the interim positions of migrating grain boundaries (Fig. lo), indicates a high activity of
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MICRO-SUPERPLASTICITY
diffusion processes affecting the fiber morphology. Specifically, two fiber rows formed at either sides of the grain boundary are often observed (see the insert given in the lower left-hand corner of Fig. 10). The amount of grain boundary sliding reaches a maximum at the optimum strain rate [22]. Correspondingly, the longest fibers are observed in the dome formed at the optimum strain rate, ; = 2 x 1o-4 s-1. Note that in Al-9021, fiber formation was documented at slow strain, and it was not observed at the optimum superplastic strain rate [5]. At high strain rates, dislocation deformation becomes more active [22], and intergranular wavy slip lines related to fibers at grain boundaries are observed. Fibers in the material deformed at ; = 2 x 10-j s-’ look like folds. This is particularly clear when a stereographic technique is employed (Fig. 11). 3.4. Efect
of the environment
It has been demonstrated [4-61 that micro-superplasticity phenomenon is very sensitive to the atmospheric oxygen. Claeys et al. [6] observed fibers only at that portion of the fracture surface ,which was exposed to the air after a subsurface crack had emerged at the specimen surface. Fibers were much
Fig.
9. Fibers
formed
at the edges
3539
more pronounced when cracks were initiated at the specimen surface and there was more interaction of the freshly exposed crack surface with air. A high concentration of oxygen, zinc and silicon was found in AA7475 fibers; oxygen and iron were detected in the droplet-like features [see Figs l(c) and (d)]. Note, that there is some controversy in the reported results of fiber chemical analysis. Shaw [5] did not find any difference between the chemical composition of fibers and that of the bulk material. Varloteaux et al. [7] reported a higher concentration of the solute atoms in fibers observed in AA7475 tensile samples. A higher concentration of solute atoms in fibers formed in AA7475 domes was found only in approximately 60% of the studied cases (statistics was not large enough in the last case). It is possible that in AA7475 the difference in the results of chemical analysis can be caused by the presence of Zn-Mg precipitates in fibers. If the electron beam hits the particle, the measured concentration of solute atoms will be higher compared to that in the rest of the fiber [15]. It was found [7] that an annealing at 400°C for 48 h and even as small as for 6 h completely suppresses the fiber formation in AA7475. This indicates that it is a dynamic oxidation, i.e. an interaction of a freshly
of the surface crack in AA5086 composite hemispherical dome.
layer.
Laminated
metal
matrix
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Fig. 10. A montage of SEM micrographs illustrating fiber formation at the surface of an AA7475 conical dome at slow strain rate. An insert illustrates two fiber rows formed at either side of the grain boundary (arrowed). ; = 2 x 10mS SK’, T= 516°C.
exposed material with oxygen, not a static oxidation which took place during the annealing which facilitates the fiber formation.
(i) viscous flow [9915]; (ii) single crystalline plasticity [4, 5, 81; (iii) superplastic flow in micro-volumes [6]. The following is the analysis of these hypotheses on the basis of the experimental evidence discussed in the previous section.
4. MICRO-MECHANISM The proposed models of fiber formation following deformation mechanisms:
(i) Viscous Jlow invoke the
All metal matrix composites, in which fiber formation was reported, had been deformed under
Fig. 11. Fibers formed at the surface of an AA7475 conical dome stretched with the high strain rate. (a), (b) A stereo pair taken at 0 and 6” dome inclination with respect to the electron beam, respectively. ; = 2 x 10-3 s-‘, T = 516°C.
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MICRO-SUPERPLASTICITY
high strain rates close to the melting point temperatures or at slightly higher temperatures [9]. It has been suggested that the heat release caused by the high rate straining and segregation of solute atoms results in incipient melting, particularly at an interface between the matrix and ceramic particles. Formation of fibers in these materials is usually attributed to interface sliding at a liquid-like layer or to squeezing of a liquid material between hard particles [9]. Cao et al. [14] attributed fiber formation in AA7475 to the operation of the liquid-like grain boundary sliding. This explanation of the fiber formation is consistent with the morphology of fibers observed in these materials as well as with the droplet like shape of Fe-rich fibers in AA7475 [Fig. l(d)]. However, similar to the case of Al-S%Mg-2%Li alloy, fibers have been observed in materials deformed at between 200 and 300-C below the melting point [4]. Since incipient melting at such low temperatures appears improbable, the existence of a liquid material might not be the major requirement for fiber formation. Suery and Baudelet [8] considered a number of possible reasons for the micro-superplasticity phenomenon, including deformation of partially melted precipitates. However, the fact that precipitates are not observed in all fibers and they occupy only between 30 and 60% of the fiber volume indicates that fiber formation is not a direct consequence of precipitate melting. (ii) Single crystalline plasticity, Dislocation movement facilitated by oxygen [4, 51 or solute atoms [S] has been assumed to be a possible mechanism for fiber formation. Suppression of fiber formation by homogenization annealing of several hours before deformation can be interpreted [S] as a support for this explanation. Similarly, significant fiber ductility variation can also be related to the local chemical inhomogeneity. In dislocation slip, surface steps are formed when dislocations emerge at the surface. Surface features resembling dislocation steps were observed in some fibers (Fig. 2) which supports the dislocation nature of micro-superplasticity. However, Cao et al. [14] showed that the high density of mobile dislocations, which is needed to explain the observed rates of fiber formation, can hardly exist in the fibers. This is consistent with the TEM observations. (iii) Superplastic JIow in micro-volumes Clayes et al. [6] suggested that fibers originate due to the operation of the same processes which occur during regular micro-structural superplasticity. They assumed that in AI-Fe-X alloys a small subgrain size and constraint from the regions of high dispersoid density may promote superplastic deformation in the regions of low dispersoid density. Grains revealed by TEM in fibers are sufficiently small to participate in grain boundary sliding, which
3541
provides a dominant contribution to superplastic deformation [22]. These fine grains could have been formed in fibers as a result of a reaction between oxygen and the deformed metal. Shaw [5] referred to this reaction as a mechanical proximity reaction because the formed oxide can have a composition different from that of a stable AlzO,. Two other possible fiber formation mechanisms link grain boundary sliding with a pre-existing oxide film. If there are no strong bonds between the oxide film and the underlying grains, folding of the oxide film may occur between grains moving apart. Being formed, the folds can undergo tensile deformation while grains continue their motion. Fibers could also be formed because of delamination of the oxide film and its dissociation. Both these mechanisms can explain the observed fiber rows.
5. CONCLUSIONS (1) Micro-superplasticity phenomenon, i.e. formation of thin fibers, whose appearance suggests a significant strain, has been observed in a number of superplastically deformed materials, predominantly in aluminum alloys. The fibers can have different morphology and location. Rows of fibers and isolated fibers have been observed both at the fracture surface and at the deformed surface. (2) The length of the fibers increases with the total strain increase and is a function of the strain rate. The maximum fiber length, as well as the fiber number correspond to the optimum superplastic strain rate. (3) The fiber formation is sensitive to the environment: it is promoted by oxidation. A high concentration of oxygen and often alloy elements has been found in the fibers. Fibers have an ultra-fine grained structure. (4) A number of explanations for fiber formation, including viscous flow, single crystalline plasticity and operation of superplasticity mechanisms in micro-volumes, have been analyzed based on the existing experimental evidence. Observation of fibers after low temperature deformation suggests that the role of the viscous flow can be limited. The absence of dislocations in the fibers indicates the limited dislocation activity. Oxidation related processes can explain the observed results. AcknoH’ledgements--This work was supported by a grant (Number DMR-93-002171 from the U.S. National Science Foundation. The autho; wishes to thank Prof. A. K. Mukherjee for his valuable discussion and support, Dr R. Z. Valiev for providing the Al-S%Mg-2%Li alloy and Dr R. Grishaber for his assistance in conducting experiments on the AA5182/AA6090-25%SiC, composite.
REFERENCES 1. Hart, E. W., Acta metall., 1967, 15, 1545. 2. Bampton. C. C., Mahoney, M. W., Hamilton, C. H., Ghosh, A. K. and Raj, R.. Metall. Trans., 1983, 14A, 1583.
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3. Tighe, N. J., Wiedehorn, S. M., Chuang, T.-J. and McDaniel, C. L., in Deformation of Ceramic Materials II, ed. R. E. Tressler and R. C. Bradt. Plenum Press, NY, 1983, p. 587. 4. Shaw, W. J. D., Mater. Lett., 1985, 4, 1. 5. Shaw, W. J. D., J. Mater. Lett., 1989, 24, 4114. 6. Claeys, S. F., Jones, J. W. and Allison, J. E., in Dispersion Strengthened Aluminum Alloys, ed. Y.-W. Kim and W. M. Griffith. The Minerals, Metals & Materials Society, Warrendale, PA, 1988, p. 323. 7. Varloteaux, A., Blandin, J. J. and Suery, M., Mater. Sci. Technol., 1989, 5, 1109. in 8. Suery, M. and Baudelet, B., in Superplasticity Advanced Materials, ed. S. Hori, M. Tokizane and N. Furushiro. The Japan Society for Research on Superplasticity, Osaka, 1991. p. 105. 9. Imai, T., L’Esperance, G. and Hong, B. D., Scripra metall. mater., 1994, 31, 321. 10. Lim, S-W., Imai, T., Nishida, Y. and Choh, T., Scripta metall. mater., 1995, 32, 1713. 11. Lim, S.-W. and Nishida, Y., Scripta metall. mater., 1995, 32, 1821.
12. Lim, S.-W. and Nishida, Y., Scripta metall. mater., 1995, 32, 1911. 13. Satoh, T., Okimoto, K., Nishida, S. and Matsuki, K. Scripta metall. mater., 1995, 33, 819. 14. Cao, W. D., Lu, X. P. and Conrad, H., Acta mater., 1996, 44, 697. 15. Blandin, J. J., Hong, B., Varlotex, A., Suery, M. and L’Esperance, G., Acta mater., 1996, 44, 2317. 16. Zelin, M. G., Moore, W. L. and Chaudhary, P. K., in Superplasticity and Superplastic Forming, ed. A. K. Ghosh and T. R. Bieler. The Minerals, Metals & Materials Society, Warrendale, PA, 1995, p. 67. 17. Zelin, M. G., J. Mater. Sci., 1995, 31, 843. 18. Zelin, M. G., Metall. Trans., 1996, 27A, 1400. 19. Zelin, M. G., Guillard, S. and Chaudhury, P. K., To be published in Proc. of ICSMA-97. 20. Backofen, W. A., Murty G. S. and Zehr, S. W., Trans. Metall. Sot. AIME, 1968, 242, 329. 21. Novikov, I. I., Portnoy, V. K. and Levchenko, V. S., Acta metall., 1981, 29, 1077. 22. Pilling, J. and Ridley, N., Superplasticity in Crystalline Solids. The Institute of Metals, London, 1989, p. 240.
Pergamon
ON MICRO-SUPERPLASTICITY M. G. ZELINt Department
of Chemical
Engineering
(Received
and Materials Science, University U.S.A. 2 January
1997; accepted 28 January
of California,
Davis, CA 95616,
1997)
Abstract-Micro-superplasticity phenomenon, i.e. formation of thin fibers, whose appearance suggests extremely high strain in the local regions, has been documented in a number of plastically and superplastically deformed materials, predominantly in aluminum alloys. The fiber formation occurs both at the fracture surface and at the deformed surface. The fiber length increases with the total strain, and it is maximum under the optimum superplastic strain rate. An ultra-fine grain structure, a high concentration of oxygen and often high concentration of the alloy elements have been observed in the fibers. Suggested explanations for fiber formation have been reviewed; the important role of oxidation reactions has been emphasized. c 1997 Acta Metallurgica Inc.
2. EXPERIMENTAL
1. INTRODUCTION
Superplasticity is the ability of crystalline materials to undergo large deformations reaching 100 and 1000%. Since the early studies of superplastic (SP) flow, it has been realized that significant deformations in superplastic materials are achieved due to the high stability of SP flow [l]. Particularly, specimens superplastically deformed in tension do not demonstrate the formation of a fixed localized neck up to the final strains at which failure occurs. Recently [2-161, similar stable plastic flow in micro-volumes has been observed. Thin fibers with diameter from 0.1 to 1 pm have been reported in a number of materials, predominantly, aluminum alloys. Since the individual fibers look like micro-tensile specimens, which have undergone tremendous deformation, Shaw [S] suggested the term micro-superplasticity for this phenomenon of fiber formation. Because the first observations of the fibers were made at the fracture surface [2-61, it was viewed as a new fracture mechanism. However, fiber formation has also been observed at the deformed surface of unfailed specimens [7, 8, 14-161 which suggests its deformation origin. Although a number of possible explanations for the fiber formation have been proposed [2-161, the micro-mechanism for this phenomenon is still unclear. Meanwhile, its understanding may contribute to the further development of the theory of plasticity and serve as a basis for the development of new advanced materials and forming processes. In this paper, results of a micro-superplasticity study into four aluminum alloys are reported, and the physical mechanism for this phenomenon is analyzed. tPresent address: Concurrent 1450 Scalp Av., Johnstown,
Technologies Corporations, PA 15904, U.S.A.
Tensile specimens and punch bulged specimens have been studied. The chemical composition of the studied materials and test conditions are given in Table 1. The experimental details can be found elsewhere [ 17, 181. AA7475, AA2090 and AA51 82/ AA6090-25%SiC, laminated matrix composite (LMC) blanks were bulged with punches of hemispherical, cylindrical and conical shape. The blanks, as well as some of Al-S%Mg-2%Li and AA7475 tensile specimens were mechanically polished before deformation, and marker lines were inscribed at the polished surface by using a diamond paste. Deformed specimens were examined in a scanning electron microscope (SEM). The strain which the material underwent to form a fiber was estimated as t = In 8 - d& where d and do are the fiber diameter at the thinnest portion and at the base of the fiber, respectively. All measurements were performed on SEM micrographs with an accuracy of 0.1 pm. Fiber chemical composition was analyzed using EDS; the beam spot resolution was 1 pm. Fine fiber structure was studied in a TEM. The fibers for TEM observations were extracted from the failure tip of failed AA7475 tensile specimens.
3. EXPERIMENTAL
3.1. Fiber
RESULTS
morphology
In all studied materials, fiber appearance suggested a very high local strain (Figs 14). However, despite this similarity, the fiber morphology is distinct in each individual case. AA 7475. At the deformed surface of bulged domes, rows of fibers are observed between grains moving
ZELIN:
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MICRO-SUPERPLASTICITY
Table 1. Materials Material
Comuosition
AA7475
Al-5.8Zn2.2Mg-1.5Cu
AA2090 Laminated
metal matrix
Al-5%Mg-2%Li
composite
AI-Cu-Mg AA5182/ 609OG25%SiC,
and test conditions
Specimen geometry (mm’) 13 x 3.4 x 2.5 50X50X 1.5 50 x 50 x 1.5 50X50X 1.5 13 x 3.4 x 2.5
apart [Figs l(a) and (b)]. Split fibers are seen at the triple junctions [Fig. l(b)]. The diameter of individual fibers is in the range from 0.05 to 1 pm, and their length is in the range from 3 to 10 pm. While the total strain of the bulk material is close to 0.5, the estimated strain value varies in the range from 1.5 to 4. For instance, the strain in fibers numbered 14 in the insert given in the upper right-hand corner in Fig. l(a) was 1.4, 1.8, 3.1 and 1.5, respectively. The existence of both failed and unfailed fibers at the same grain bounday facets indicates a significant variation in the fiber ductility. On the deformed surface and fracture surface of the tensile specimens, long individual fibers were observed along with rows of the fibers [Figs l(c) and (d)]. The length of the individual fibers reaches hundreds of microns, and their diameter can be up to
Deformation mode Umaxial
tension
Bulging Bulging Bulging Uniaxial
tension
Temperature (-0 516
2 x 10-4
415 550
2.5 x lo-” 10 ’
250
IO-”
3 pm. Droplet-like features were also observed at the fracture surface [arrowed in Fig. l(d)]. AA2090. Numerous individual fibers have a diameter from 0.1 to 2.5 pm and a length of up to more than 200 pm [Figs 2(a) and (b)]. Fibers of this type have been reported in aluminum metal matrix composites subjected to the high strain rate superplastic deformation [9%12]. Some fibers are normal to the dome surface [Fig. 2(a)]. Significant fiber lengths indicate a very high strain of the fiber material; however, even approximate local strain estimates were impossible because of the uncertainties at the base. The fiber surface in the fiber diameter appears to be severely oxidized, as well as the grain surface. Some steps at the fiber surface can be distinguished at high magnification [see the insert given in the right-hand upper corner in Fig. 2(b)].
Fig. I. Fibers formed at the deformed surface of AA7475 hemispherical dome (a, b) and the fracture surface of the tensile specimens (c, d). The insert given in the upper right-hand corner in (a) illustrates the region shown by crosses under a high magnification. Numerals 14 designate fibers in which strain was estimated. ; = 2 x 10S4 SK’, T= 516°C.
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Fig. 2. (a), (b) Fibers formed at the deformed surface of a conical AA2090 dome. the left-hand upper corner in (b 8) shows an individual fiber at high magnification. T = 470°C.
Al--5%Mg-2%Li. Both individual fibers and rows of fibers are seen in Figs 3(a)-(c), respectively. Individual fibers have a diameter of approximately 1 pm and can be as long as 15 pm. The estimated local strain in this case is approximately 1.4. Fibers forming rows have a diameter up to 0.3 pm and a length in the range of 1-5 pm. SAA5182/AA6090-25%SiC, composite. Fibers in AA5182 layers form rows [Fig. 4(a)], while fibers in AA6090-25%SiC, layers form bundles [Fig. 4(b)]. Similar bundles of fibers were also observed at the
Fig.
3. (a), (b) Isolated
fibers
and (c) rows Al-S%Mg-2%Li.
The insert given in 2 = 2.5 x 10mJs I,
fracture surface of IN-9021 [5] and Al-Fe-X alloys [6]. The diameter of both types of fibers is less than 1 pm, and their length can be up to 200 pm. An estimated local strain varies from 1.5 to 5. 3.2. Fiber location In all studied cases, fibers were observed only in the deformed specimen regions which indicates their deformation origin. The fibers were found both at the deformed surface and at the fracture surface, as well as inside internal cavities located close to the
of,fibers (arrowed) formed ( = lo-” SV’, T = 250’C.
at the deformed
surface
of
Fig. 4. Fibers formed at the surface cracks in (a) AA5086 layer and in (b) 6090-25%SiC, layer of a AA5182/609&25%SiC, laminated composite conical dome. ; = lo-* s-l, T = 550°C.
specimen edge [Fig. 5(a)]. However, the majority of the internal cavities which were not linked to the specimen surface did not show fiber formation [Fig.
the AA6090&25%SiC, layers, they are formed between and at SIC particles, which is typical for materials of this class [9].
WII. Detailed topographic study [l&18] showed fiber formation at the grain boundaries which experienced significant sliding. Figure 6 illustrates this fact using the example of an AA7475 hemispherical cup. Fibers are seen at grain boundaries surrounding grain groups which slid as an entity (for instance, labeled with letters A and B). Figure 7 illustrates such a cooperative grain boundary sliding (CGBS), i.e. sliding of grain groups, in the Al-S%Mg-2%Li alloy. The length of the segments in which marker lines were broken due to CGBS is up to 30 pm, i.e. approximately 30 grain diameters (for instance, the distance between points F and G). The offset of marker lines can be more than a grain size [it is approximately three grain sizes between points E and F in Fig. 7(b)]. In the AA5182/AA609&25%SiCP composite, fibers are observed at the fracture surface (Fig. 4). In
3.3. Fiber fine structure Figures 8(a) and (b) show TEM micrographs taken from a fiber colony and an individual fiber, respectively [193. Fibers have serrated edges that can be related to the presence of dispersoid particles. Blandin et al. [15] showed that these particles are enriched with Mg and Zn. Diffraction patterns taken from a fiber [see the insert given in Fig. 8(b)] are typical for a polycrystalline material. The fiber grain size varies from 10 to 50 nm. 3.4. Effect
qf strain
It was found [4, 51 that the length of fibers formed inside a crack increases with increase in the opening width. Figure 9 illustrates this fact in the case of the LMC composite showing fibers in the AA5182 surface layer. Note, that short failed fibers are seen
Fig. 5. SEM micrographs illustrating fibers formed inside the internal cavities located close specimen edge (a) and absence of fibers inside a typical internal close cavity (b) in AA7475 specimens. t = 2 x 10m4 SK’, T = 516°C.
to the tensile
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Fig. 6. A montage of SEM micrographs grain boundary
illustrating rows of fibers formed at the surfaces of cooperative in the top region of the hemispherical AA7475 dome. Letters designate groups of grains which slide as an entity. ; = 2 x IO-“ sm’, T= 516°C.
sliding (arrowed)
in addition to the long ones in the region of the wide crack opening. Studies performed in AA7475 domes [16, 171 and tensile specimens [ 141 indicate that formation of striated bands [20, 211 precedes fiber formation. For
instance, in an AA7475 conical cup, striation bands were observed in the base portion, while fibers were found in the top region. With increasing strain, striated bands evolve into the fibers because of continuing sliding apart of the grains.
Fig. 7. Distortion of marker lines due to grain boundary sliding in Al-S%Mg-2%Li tensile specimen. Letters E and F designate marker line displacement and F, G designate the undistorted marker line segment.
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Fig. 8. TEM micrographs showing groups of fibers (a) and fiber fine structure (b). The insert in (b) shows a typical polycrystalline material reflex pattern from the fiber. AA7475 spherical dome. t = 2 x 10m4SK’, T= 516°C.
3.5
Effect
of strain rate
7-he size of the sliding grain groups increases when the strain rate decreases (Fig. 10). Consequently,
fibers are seen at fewer grain boundaries (compare Figs 6 and 10). Evidence of bright lines, which appear to outline the interim positions of migrating grain boundaries (Fig. lo), indicates a high activity of
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diffusion processes affecting the fiber morphology. Specifically, two fiber rows formed at either sides of the grain boundary are often observed (see the insert given in the lower left-hand corner of Fig. 10). The amount of grain boundary sliding reaches a maximum at the optimum strain rate [22]. Correspondingly, the longest fibers are observed in the dome formed at the optimum strain rate, ; = 2 x 1o-4 s-1. Note that in Al-9021, fiber formation was documented at slow strain, and it was not observed at the optimum superplastic strain rate [5]. At high strain rates, dislocation deformation becomes more active [22], and intergranular wavy slip lines related to fibers at grain boundaries are observed. Fibers in the material deformed at ; = 2 x 10-j s-’ look like folds. This is particularly clear when a stereographic technique is employed (Fig. 11). 3.4. Efect
of the environment
It has been demonstrated [4-61 that micro-superplasticity phenomenon is very sensitive to the atmospheric oxygen. Claeys et al. [6] observed fibers only at that portion of the fracture surface ,which was exposed to the air after a subsurface crack had emerged at the specimen surface. Fibers were much
Fig.
9. Fibers
formed
at the edges
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more pronounced when cracks were initiated at the specimen surface and there was more interaction of the freshly exposed crack surface with air. A high concentration of oxygen, zinc and silicon was found in AA7475 fibers; oxygen and iron were detected in the droplet-like features [see Figs l(c) and (d)]. Note, that there is some controversy in the reported results of fiber chemical analysis. Shaw [5] did not find any difference between the chemical composition of fibers and that of the bulk material. Varloteaux et al. [7] reported a higher concentration of the solute atoms in fibers observed in AA7475 tensile samples. A higher concentration of solute atoms in fibers formed in AA7475 domes was found only in approximately 60% of the studied cases (statistics was not large enough in the last case). It is possible that in AA7475 the difference in the results of chemical analysis can be caused by the presence of Zn-Mg precipitates in fibers. If the electron beam hits the particle, the measured concentration of solute atoms will be higher compared to that in the rest of the fiber [15]. It was found [7] that an annealing at 400°C for 48 h and even as small as for 6 h completely suppresses the fiber formation in AA7475. This indicates that it is a dynamic oxidation, i.e. an interaction of a freshly
of the surface crack in AA5086 composite hemispherical dome.
layer.
Laminated
metal
matrix
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Fig. 10. A montage of SEM micrographs illustrating fiber formation at the surface of an AA7475 conical dome at slow strain rate. An insert illustrates two fiber rows formed at either side of the grain boundary (arrowed). ; = 2 x 10mS SK’, T= 516°C.
exposed material with oxygen, not a static oxidation which took place during the annealing which facilitates the fiber formation.
(i) viscous flow [9915]; (ii) single crystalline plasticity [4, 5, 81; (iii) superplastic flow in micro-volumes [6]. The following is the analysis of these hypotheses on the basis of the experimental evidence discussed in the previous section.
4. MICRO-MECHANISM The proposed models of fiber formation following deformation mechanisms:
(i) Viscous Jlow invoke the
All metal matrix composites, in which fiber formation was reported, had been deformed under
Fig. 11. Fibers formed at the surface of an AA7475 conical dome stretched with the high strain rate. (a), (b) A stereo pair taken at 0 and 6” dome inclination with respect to the electron beam, respectively. ; = 2 x 10-3 s-‘, T = 516°C.
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high strain rates close to the melting point temperatures or at slightly higher temperatures [9]. It has been suggested that the heat release caused by the high rate straining and segregation of solute atoms results in incipient melting, particularly at an interface between the matrix and ceramic particles. Formation of fibers in these materials is usually attributed to interface sliding at a liquid-like layer or to squeezing of a liquid material between hard particles [9]. Cao et al. [14] attributed fiber formation in AA7475 to the operation of the liquid-like grain boundary sliding. This explanation of the fiber formation is consistent with the morphology of fibers observed in these materials as well as with the droplet like shape of Fe-rich fibers in AA7475 [Fig. l(d)]. However, similar to the case of Al-S%Mg-2%Li alloy, fibers have been observed in materials deformed at between 200 and 300-C below the melting point [4]. Since incipient melting at such low temperatures appears improbable, the existence of a liquid material might not be the major requirement for fiber formation. Suery and Baudelet [8] considered a number of possible reasons for the micro-superplasticity phenomenon, including deformation of partially melted precipitates. However, the fact that precipitates are not observed in all fibers and they occupy only between 30 and 60% of the fiber volume indicates that fiber formation is not a direct consequence of precipitate melting. (ii) Single crystalline plasticity, Dislocation movement facilitated by oxygen [4, 51 or solute atoms [S] has been assumed to be a possible mechanism for fiber formation. Suppression of fiber formation by homogenization annealing of several hours before deformation can be interpreted [S] as a support for this explanation. Similarly, significant fiber ductility variation can also be related to the local chemical inhomogeneity. In dislocation slip, surface steps are formed when dislocations emerge at the surface. Surface features resembling dislocation steps were observed in some fibers (Fig. 2) which supports the dislocation nature of micro-superplasticity. However, Cao et al. [14] showed that the high density of mobile dislocations, which is needed to explain the observed rates of fiber formation, can hardly exist in the fibers. This is consistent with the TEM observations. (iii) Superplastic JIow in micro-volumes Clayes et al. [6] suggested that fibers originate due to the operation of the same processes which occur during regular micro-structural superplasticity. They assumed that in AI-Fe-X alloys a small subgrain size and constraint from the regions of high dispersoid density may promote superplastic deformation in the regions of low dispersoid density. Grains revealed by TEM in fibers are sufficiently small to participate in grain boundary sliding, which
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provides a dominant contribution to superplastic deformation [22]. These fine grains could have been formed in fibers as a result of a reaction between oxygen and the deformed metal. Shaw [5] referred to this reaction as a mechanical proximity reaction because the formed oxide can have a composition different from that of a stable AlzO,. Two other possible fiber formation mechanisms link grain boundary sliding with a pre-existing oxide film. If there are no strong bonds between the oxide film and the underlying grains, folding of the oxide film may occur between grains moving apart. Being formed, the folds can undergo tensile deformation while grains continue their motion. Fibers could also be formed because of delamination of the oxide film and its dissociation. Both these mechanisms can explain the observed fiber rows.
5. CONCLUSIONS (1) Micro-superplasticity phenomenon, i.e. formation of thin fibers, whose appearance suggests a significant strain, has been observed in a number of superplastically deformed materials, predominantly in aluminum alloys. The fibers can have different morphology and location. Rows of fibers and isolated fibers have been observed both at the fracture surface and at the deformed surface. (2) The length of the fibers increases with the total strain increase and is a function of the strain rate. The maximum fiber length, as well as the fiber number correspond to the optimum superplastic strain rate. (3) The fiber formation is sensitive to the environment: it is promoted by oxidation. A high concentration of oxygen and often alloy elements has been found in the fibers. Fibers have an ultra-fine grained structure. (4) A number of explanations for fiber formation, including viscous flow, single crystalline plasticity and operation of superplasticity mechanisms in micro-volumes, have been analyzed based on the existing experimental evidence. Observation of fibers after low temperature deformation suggests that the role of the viscous flow can be limited. The absence of dislocations in the fibers indicates the limited dislocation activity. Oxidation related processes can explain the observed results. AcknoH’ledgements--This work was supported by a grant (Number DMR-93-002171 from the U.S. National Science Foundation. The autho; wishes to thank Prof. A. K. Mukherjee for his valuable discussion and support, Dr R. Z. Valiev for providing the Al-S%Mg-2%Li alloy and Dr R. Grishaber for his assistance in conducting experiments on the AA5182/AA6090-25%SiC, composite.
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3. Tighe, N. J., Wiedehorn, S. M., Chuang, T.-J. and McDaniel, C. L., in Deformation of Ceramic Materials II, ed. R. E. Tressler and R. C. Bradt. Plenum Press, NY, 1983, p. 587. 4. Shaw, W. J. D., Mater. Lett., 1985, 4, 1. 5. Shaw, W. J. D., J. Mater. Lett., 1989, 24, 4114. 6. Claeys, S. F., Jones, J. W. and Allison, J. E., in Dispersion Strengthened Aluminum Alloys, ed. Y.-W. Kim and W. M. Griffith. The Minerals, Metals & Materials Society, Warrendale, PA, 1988, p. 323. 7. Varloteaux, A., Blandin, J. J. and Suery, M., Mater. Sci. Technol., 1989, 5, 1109. in 8. Suery, M. and Baudelet, B., in Superplasticity Advanced Materials, ed. S. Hori, M. Tokizane and N. Furushiro. The Japan Society for Research on Superplasticity, Osaka, 1991. p. 105. 9. Imai, T., L’Esperance, G. and Hong, B. D., Scripra metall. mater., 1994, 31, 321. 10. Lim, S-W., Imai, T., Nishida, Y. and Choh, T., Scripta metall. mater., 1995, 32, 1713. 11. Lim, S.-W. and Nishida, Y., Scripta metall. mater., 1995, 32, 1821.
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