- Email: [email protected]
A comparative examination of superplastic flow and fracture in metals and ceramics
ELSEVIER
A comparative
Materials Science and Engineering A234-236
(1997) 986-990
examination of superplastic flow and fracture in metals and ceramics Atul H. Chokshi * Department
of Metallurgy,
Indian
Institute
of Science,
Bangalore
560012,
India
Received 21 February 1997
Abstract Although metals and ceramics can exhibit superplastic elongations to failure of > 500%,there are significant differences in their mechanicalbehavior and fracture characteristics.The transition from superplasticto non-superplasticbehavior at low stresses is sensitiveto impurity content. However, with an increasein purity, while the transition is retarded to lower stresses in metals,the transition occursat higher stressin ceramics.The cavitation behavior alsoappearsto be different. While metallic alloys typically exhibit rounded smallcavities suggestingquasi-equilibriumdiffusion growth, ceramicsdisplay crack-like cavity profiles indicating surfacediffusion controlled cavity growth. 0 1997Elsevier ScienceS.A. Keywords:
Superplasticity; Fracture; Metals; Ceramics
The ability of some fine-grained metals to exhibit large elongations to failure, termed superplasticity, has been established for over 6 decades and this phenomenon is being utilized commercially for forming components with complex shapes [l]. Recently, follow-
III, respectively and n < 3 in the superplastic region II at intermediate strain rates. The transition from the superplastic to the non-superplastic region at low stresses has been attributed variously to: (a) a threshold stress below which deformation by the superplastic mechanism cannot occur; (b) strain hardening due to grain growth; and (c) the operation of two sequential
ing the report of an elongation of > 100% in a 3 mol%
mechanism such that the slower one controls deforma-
yttria stabilized tetragonal zirconia (3YTZ) by Wakai et al. [2], there has been considerable research activity on superplasticity in ceramics. Although both metals and ceramics can exhibit elongations of > 500%, a close examination reveals significant differences in their mechanical behavior and fracture characteristics. The mechanical behavior of superplastic materials can be represented in the form 8~ 0” where B and 0 are the strain rate and stress, respectively and n is termed the stress exponent. It is now well established that superplastic metallic alloys generally exhibit a sigmoidal relationship between the flow stress and strain rate, so that n > 3 at low and high strain rates in regions I and
tion [l]. A fine and reasonably stable grain size is required for superplasticity in view of the importance of grain boundary sliding in this phenomenon. Flow localization in metallic materials can lead to early fracture. A low value of the stress exponent n retards flow localization and superplasticity is therefore associated with values of n < 3. The nucleation, growth and interlinkage of voids frequently lead to premature failure even under conditions where n < 3. In contrast to metals where superplasticity is observed only when grain sizes L c - 10 pm, ceramics exhibit superplastic-
1. Introduction
ity only when L < 1 w.
This brief report highlights differences in the mechanical * Tel.: + 91 80 3092684; fax: +91 80 3349472; e-mail: [email protected] 0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. PZI so921-5093(97)00355-9
some of the important behavior and fracture
characteristics of superplastic metals and ceramics, using examples from typical superplastic materials.
A.H. Chokshi /Materials
Science and Engineering A234-236
1.0
(1997) 986-990
987
IO T (HPal
t
I
I
E
ZrO2
t
Tension (as-received) Nleh and Wadsworth
T
IO’
0
c
IO
Wakai
et
=
1723
K
1
(1990)
al. (1986)
Compf ession Owen
and
Chokshi
-6
1 (b) Fig. 1. (a) Variation in shear strain rate with shear stress for a Zn-22% with stress for the 3 mol% yttria stabilized tetragonal zirconia (3YTZ)
2. Mechanical behavior
Fig. la illustrates, from the detailed studies by Mohamed and colleagues [3,4], the mechanical behavior of a typical superplastic Zn-22% Al eutectoid alloy in the
10 0
100
200
&Pa)
Al eutectoid from earlier
alloy with two different purities. (b) Variation studies and a more recent investigation.
in strain
rate
form of the variation in strain rate with stress at temperatures of 493 and 433 K. The data demonstrate clearly that a material with an impurity content of 180 ppm (filled datum points) exhibits the conventional behavior observed in superplastic materials of commer-
988
A.H.
Chokshi
/ Materials
Science
and Engineering
A2344236
(1997)
986-990
cial purity, with a transition from the superplastic region II with a stress exponent on 2.5 to a non-superplastic region at high and low stresses. Experiments conducted over a range of impurity contents revealed that the lower transition from the superplastic to the non-superplastic regime occurs at lower stresses with a decrease in the impurity content such that the transition is completely eliminated, under the experimental conditions utilized, for a material with an impurity content of only 6 ppm (open datum points in Fig. la). These observations are consistent with the presence of a threshold stress caused by the segregation of impurities to grain boundaries and their effect on the mobility of grain boundary dislocations [5]. Thus, the presence of impurities hinders grain boundary sliding at low stresses. The first two independent studies on the mechanical behavior of a nominally identical superplastic 3YTZ, obtained from the same source, yielded disparate data: Wakai et al. [2] reported a stress exponent of - 2 and Nieh et al. [6] observed a stress exponent of - 3. In addition, the flow stresses reported by Wakai et al. [2] were considerably lower than those noted by Nieh et al. [6]. The differences in mechanical behavior were assigned to variations in testing procedures and testing atmospheres. A more recent study on the mechanical behavior of 3YTZ revealed that there is a transition in stress exponent from a value of N 3 at low stresses to - 2 at high stresses and that the stress for the transition was dependent on the grain size. All three sets of data are shown in Fig. lb in the form of a logarithmic plot of strain rate versus stress. It is clear that the transition from IZ - 2 to n - 3 at low stresses is similar in form to that observed in metallic alloys (Fig. lb), however, the role of impurity in this transition appears to be quite different. An increase in grain size leads to a decrease in the transition stress in the superplastic 3YTZ [7,8], in contrast, the grain size has no effect on the transition in superplastic metallic alloys. Additional data reported in other investigations and the influence of grain size suggest that an increase in the impurity content facilitates grain
to grain growth [7-91. Therefore, the experimental data on a transition in stress exponents in ceramics can be interpreted in terms of either the operation of two sequential processes, with the slower process controlling deformation, or the presence of a threshold stress below which the superplastic deformation mechanism ceases to operate. Although a threshold stress can rationalize the experimental data in some ceramics [lo], a close inspection of detailed data suggests that all of the available results cannot be interpreted consistently on the basis of a threshold stress [7-9,111. Consequently, it is suggested that the data can be analyzed better in terms of grain boundary sliding and its accommodation as two sequential processes, although the details of the mechanisms are not yet clear. There is also the possibility of an amorphous or liquid phase along grain boundaries in ceramics, which can play an important role in their deformation and fracture.
boundary
sliding.
boundary
consistent
with
The
the
data
above
shown
in
suggestion.
Fig.
The
lb
are
earlier
3. Fracture characteristics In contrast to metallic alloys, where the elongations to failure attain a maximum value at intermediate strain rates in region II and decrease at both lower and higher strain rates corresponding to a change in the strain rate sensitivity, the experimental data on ceramics indicates that the ductility is insensitive to variations in strain rate [12]. Sakuma [13] and colleagues have examined experimentally the ductility of several ceramics and these have been analyzed in terms of the flow stress, strain hardening due to grain growth and differences in the crack growth rate. There is also a distinct difference in the cavity morphology in superplastic metals and ceramics. Fig. 2a illustrates cavitation in a superplastic metallic alloy [14]. The cavities generally have a rounded appearance suggesting a quasi-equilibrium diffusional growth process. In superplastic ceramics, the cavities appear to nucleate and grow along grain boundaries to develop grain facet
cracks
perpendicular
to
the
tensile
axis,
indicative of a surface diffusion controlled growth pro-
study of Wakai et al. [2] utilized material with a
cess,as shown in Fig. 2b [15]. It is the interaction and
somewhat higher impurity content so that the transition stress would be lower and a region with y1- 3 was not observed under the limited experimental conditions utilized in the study. In contrast, the report by Nieh et al. [6] involved material with a somewhat lower impurity content and a transition to a region with n - 2 was not observed under the limited range of strain rates employed in that study. Calculations reveal that the increase in stress exponent at low stressesin ceramics cannot be attributed
interlinkage of such cracks that governs the failure process in superplastic ceramics. A critical examination of fracture suggeststhat the dominant factors influencing ductility are the interface energies, flow stress and the grain size [16]. Thus, for example, it has been suggested that the introduction of a spine1 phase in alumina leads to the development of low energy spinelalumina interfaces [17] and a consequent increase in the ductility. In addition, it is suggestedthat the reduction in the critical grain size for the observation of super-
A.H.
Chokshi
/ Materials
Fig. 2. (a) Optical micrograph illustrating the development horizontal. (b) Scanning electron micrograph illustrating composite, the tensile stress is horizontal.
Science
and Engineering
(1997)
986-990
of rounded cavities in a superplastic commercial copper the development of crack-like cavities in a superplastic
plasticity, from < 10 urn in metals to < 1 urn in ceramics is related to the early development of facetsized cracks in ceramics.
4. Summary
A234-236
and conclusions
Although both ceramics and metals are capable of exhibiting superplastic elongations to failure of > 500%, a close inspection of the available data suggests there are significant differences in both their mechanical
989
alloy, the tensile axis is 3YTZ-20 wt.% alumina
behavior as well as their fracture characteristics. Both ceramics and metals exhibit a transition from a stress exponent of IZ - 2 to n - 3 at low stresses. However, under typical experimental conditions, while a reduction in the impurity content enhances the n - 2 region in metals it increases the IZ - 3 region in ceramics. In contrast to quasi-equilibrium type rounded cavities in metals, cavities in ceramics have a crack-like morphology indicative of surface diffusion control. The ductility of ceramics can be enhanced by decreasing the grain boundary/interphase energies, grain size and flow stress.
990
A.H.
Chokshi
/ Materials
Science
and Engineering
Acknowledgements This work was supported by the Aeronautical Research and Development Board. Additional support of this study by AFOSR is also gratefully acknowledged.
References [I] [2] [3] [4] [5] [6]
A.H. Chokshi, A.K. Mukherjee, T.G. Langdon, Mater. Sci. Eng. RlO (1993) 237. F. Wakai, S. Sakaguchi, Y. Matsuno, Adv. Ceram. Mater. 1 (1986) 259. P.K. Chaudhury, F.A. Mohamed, Acta Metall. 36 (1988) 1099. P.K. Chaudhury, V. Sivaramakrishnan, F.A. Mohamed, Metall. Trans. 19A (1988) 2741. F.A. Mohamed, J. Mater. Sci. Lett. 7 (1988) 215. T.G. Nieh. C.M. McNally, J. Wadsworth, Scripta Metall. 22 (1988) 1297.
A2344236
(1997)
986-990
[7] D.M. Owen, A.H. Chokshi, in: S.P.S. Badwal, M.J. Bannister, R.H.J. Hannink (Ed%), Science and Technology of Zirconia V, Technomic Press, Lancaster, 1993, p. 42. [S] A.H. Chokshi, Mater. Sci. Eng. Al66 (1993) 119. [9] D.M. Owen, A.H. Chokshi, in: R.C. Bradt, C.A. Brookes, J.L. Routbort (Eds.), Plastic Deformation of Ceramics, Plenum, New York, 1995, p. 507. [lo] M. Jimenez-Melendo, A. Bravo-Leon, A. Dominguez-Rodriguez, Mater. Sci. Forum 243-245 (1997) 363. [l l] M.Z. Berbon, T.G. Langdon, Mater. Sci. Forum 243-245 (1997) 357. [12] W.J. Kim, J. Wolfenstine, O.D. Sherby, Acta Metall. Mater. 39 (1991) 199. [13] T. Sakuma, Mater. Sci. Forum 2433245 (1997) 327. [14] A.H. Chokshi, T.G. Langdon, Acta Metall. 38 (1990) 867. [15] D.M. Owen, A.H. Chokshi, S.R. Nutt, J. Am. Ceram. Sot. (in press). [16] A.H. Chokshi, Mater. Sci. Forum 233-234 (1997) 89. [17] S.R. Shah, A.H. Chokshi, Mater. Sci. Forum 2433245 (1997) 381.
A comparative
Materials Science and Engineering A234-236
(1997) 986-990
examination of superplastic flow and fracture in metals and ceramics Atul H. Chokshi * Department
of Metallurgy,
Indian
Institute
of Science,
Bangalore
560012,
India
Received 21 February 1997
Abstract Although metals and ceramics can exhibit superplastic elongations to failure of > 500%,there are significant differences in their mechanicalbehavior and fracture characteristics.The transition from superplasticto non-superplasticbehavior at low stresses is sensitiveto impurity content. However, with an increasein purity, while the transition is retarded to lower stresses in metals,the transition occursat higher stressin ceramics.The cavitation behavior alsoappearsto be different. While metallic alloys typically exhibit rounded smallcavities suggestingquasi-equilibriumdiffusion growth, ceramicsdisplay crack-like cavity profiles indicating surfacediffusion controlled cavity growth. 0 1997Elsevier ScienceS.A. Keywords:
Superplasticity; Fracture; Metals; Ceramics
The ability of some fine-grained metals to exhibit large elongations to failure, termed superplasticity, has been established for over 6 decades and this phenomenon is being utilized commercially for forming components with complex shapes [l]. Recently, follow-
III, respectively and n < 3 in the superplastic region II at intermediate strain rates. The transition from the superplastic to the non-superplastic region at low stresses has been attributed variously to: (a) a threshold stress below which deformation by the superplastic mechanism cannot occur; (b) strain hardening due to grain growth; and (c) the operation of two sequential
ing the report of an elongation of > 100% in a 3 mol%
mechanism such that the slower one controls deforma-
yttria stabilized tetragonal zirconia (3YTZ) by Wakai et al. [2], there has been considerable research activity on superplasticity in ceramics. Although both metals and ceramics can exhibit elongations of > 500%, a close examination reveals significant differences in their mechanical behavior and fracture characteristics. The mechanical behavior of superplastic materials can be represented in the form 8~ 0” where B and 0 are the strain rate and stress, respectively and n is termed the stress exponent. It is now well established that superplastic metallic alloys generally exhibit a sigmoidal relationship between the flow stress and strain rate, so that n > 3 at low and high strain rates in regions I and
tion [l]. A fine and reasonably stable grain size is required for superplasticity in view of the importance of grain boundary sliding in this phenomenon. Flow localization in metallic materials can lead to early fracture. A low value of the stress exponent n retards flow localization and superplasticity is therefore associated with values of n < 3. The nucleation, growth and interlinkage of voids frequently lead to premature failure even under conditions where n < 3. In contrast to metals where superplasticity is observed only when grain sizes L c - 10 pm, ceramics exhibit superplastic-
1. Introduction
ity only when L < 1 w.
This brief report highlights differences in the mechanical * Tel.: + 91 80 3092684; fax: +91 80 3349472; e-mail: [email protected] 0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. PZI so921-5093(97)00355-9
some of the important behavior and fracture
characteristics of superplastic metals and ceramics, using examples from typical superplastic materials.
A.H. Chokshi /Materials
Science and Engineering A234-236
1.0
(1997) 986-990
987
IO T (HPal
t
I
I
E
ZrO2
t
Tension (as-received) Nleh and Wadsworth
T
IO’
0
c
IO
Wakai
et
=
1723
K
1
(1990)
al. (1986)
Compf ession Owen
and
Chokshi
-6
1 (b) Fig. 1. (a) Variation in shear strain rate with shear stress for a Zn-22% with stress for the 3 mol% yttria stabilized tetragonal zirconia (3YTZ)
2. Mechanical behavior
Fig. la illustrates, from the detailed studies by Mohamed and colleagues [3,4], the mechanical behavior of a typical superplastic Zn-22% Al eutectoid alloy in the
10 0
100
200
&Pa)
Al eutectoid from earlier
alloy with two different purities. (b) Variation studies and a more recent investigation.
in strain
rate
form of the variation in strain rate with stress at temperatures of 493 and 433 K. The data demonstrate clearly that a material with an impurity content of 180 ppm (filled datum points) exhibits the conventional behavior observed in superplastic materials of commer-
988
A.H.
Chokshi
/ Materials
Science
and Engineering
A2344236
(1997)
986-990
cial purity, with a transition from the superplastic region II with a stress exponent on 2.5 to a non-superplastic region at high and low stresses. Experiments conducted over a range of impurity contents revealed that the lower transition from the superplastic to the non-superplastic regime occurs at lower stresses with a decrease in the impurity content such that the transition is completely eliminated, under the experimental conditions utilized, for a material with an impurity content of only 6 ppm (open datum points in Fig. la). These observations are consistent with the presence of a threshold stress caused by the segregation of impurities to grain boundaries and their effect on the mobility of grain boundary dislocations [5]. Thus, the presence of impurities hinders grain boundary sliding at low stresses. The first two independent studies on the mechanical behavior of a nominally identical superplastic 3YTZ, obtained from the same source, yielded disparate data: Wakai et al. [2] reported a stress exponent of - 2 and Nieh et al. [6] observed a stress exponent of - 3. In addition, the flow stresses reported by Wakai et al. [2] were considerably lower than those noted by Nieh et al. [6]. The differences in mechanical behavior were assigned to variations in testing procedures and testing atmospheres. A more recent study on the mechanical behavior of 3YTZ revealed that there is a transition in stress exponent from a value of N 3 at low stresses to - 2 at high stresses and that the stress for the transition was dependent on the grain size. All three sets of data are shown in Fig. lb in the form of a logarithmic plot of strain rate versus stress. It is clear that the transition from IZ - 2 to n - 3 at low stresses is similar in form to that observed in metallic alloys (Fig. lb), however, the role of impurity in this transition appears to be quite different. An increase in grain size leads to a decrease in the transition stress in the superplastic 3YTZ [7,8], in contrast, the grain size has no effect on the transition in superplastic metallic alloys. Additional data reported in other investigations and the influence of grain size suggest that an increase in the impurity content facilitates grain
to grain growth [7-91. Therefore, the experimental data on a transition in stress exponents in ceramics can be interpreted in terms of either the operation of two sequential processes, with the slower process controlling deformation, or the presence of a threshold stress below which the superplastic deformation mechanism ceases to operate. Although a threshold stress can rationalize the experimental data in some ceramics [lo], a close inspection of detailed data suggests that all of the available results cannot be interpreted consistently on the basis of a threshold stress [7-9,111. Consequently, it is suggested that the data can be analyzed better in terms of grain boundary sliding and its accommodation as two sequential processes, although the details of the mechanisms are not yet clear. There is also the possibility of an amorphous or liquid phase along grain boundaries in ceramics, which can play an important role in their deformation and fracture.
boundary
sliding.
boundary
consistent
with
The
the
data
above
shown
in
suggestion.
Fig.
The
lb
are
earlier
3. Fracture characteristics In contrast to metallic alloys, where the elongations to failure attain a maximum value at intermediate strain rates in region II and decrease at both lower and higher strain rates corresponding to a change in the strain rate sensitivity, the experimental data on ceramics indicates that the ductility is insensitive to variations in strain rate [12]. Sakuma [13] and colleagues have examined experimentally the ductility of several ceramics and these have been analyzed in terms of the flow stress, strain hardening due to grain growth and differences in the crack growth rate. There is also a distinct difference in the cavity morphology in superplastic metals and ceramics. Fig. 2a illustrates cavitation in a superplastic metallic alloy [14]. The cavities generally have a rounded appearance suggesting a quasi-equilibrium diffusional growth process. In superplastic ceramics, the cavities appear to nucleate and grow along grain boundaries to develop grain facet
cracks
perpendicular
to
the
tensile
axis,
indicative of a surface diffusion controlled growth pro-
study of Wakai et al. [2] utilized material with a
cess,as shown in Fig. 2b [15]. It is the interaction and
somewhat higher impurity content so that the transition stress would be lower and a region with y1- 3 was not observed under the limited experimental conditions utilized in the study. In contrast, the report by Nieh et al. [6] involved material with a somewhat lower impurity content and a transition to a region with n - 2 was not observed under the limited range of strain rates employed in that study. Calculations reveal that the increase in stress exponent at low stressesin ceramics cannot be attributed
interlinkage of such cracks that governs the failure process in superplastic ceramics. A critical examination of fracture suggeststhat the dominant factors influencing ductility are the interface energies, flow stress and the grain size [16]. Thus, for example, it has been suggested that the introduction of a spine1 phase in alumina leads to the development of low energy spinelalumina interfaces [17] and a consequent increase in the ductility. In addition, it is suggestedthat the reduction in the critical grain size for the observation of super-
A.H.
Chokshi
/ Materials
Fig. 2. (a) Optical micrograph illustrating the development horizontal. (b) Scanning electron micrograph illustrating composite, the tensile stress is horizontal.
Science
and Engineering
(1997)
986-990
of rounded cavities in a superplastic commercial copper the development of crack-like cavities in a superplastic
plasticity, from < 10 urn in metals to < 1 urn in ceramics is related to the early development of facetsized cracks in ceramics.
4. Summary
A234-236
and conclusions
Although both ceramics and metals are capable of exhibiting superplastic elongations to failure of > 500%, a close inspection of the available data suggests there are significant differences in both their mechanical
989
alloy, the tensile axis is 3YTZ-20 wt.% alumina
behavior as well as their fracture characteristics. Both ceramics and metals exhibit a transition from a stress exponent of IZ - 2 to n - 3 at low stresses. However, under typical experimental conditions, while a reduction in the impurity content enhances the n - 2 region in metals it increases the IZ - 3 region in ceramics. In contrast to quasi-equilibrium type rounded cavities in metals, cavities in ceramics have a crack-like morphology indicative of surface diffusion control. The ductility of ceramics can be enhanced by decreasing the grain boundary/interphase energies, grain size and flow stress.
990
A.H.
Chokshi
/ Materials
Science
and Engineering
Acknowledgements This work was supported by the Aeronautical Research and Development Board. Additional support of this study by AFOSR is also gratefully acknowledged.
References [I] [2] [3] [4] [5] [6]
A.H. Chokshi, A.K. Mukherjee, T.G. Langdon, Mater. Sci. Eng. RlO (1993) 237. F. Wakai, S. Sakaguchi, Y. Matsuno, Adv. Ceram. Mater. 1 (1986) 259. P.K. Chaudhury, F.A. Mohamed, Acta Metall. 36 (1988) 1099. P.K. Chaudhury, V. Sivaramakrishnan, F.A. Mohamed, Metall. Trans. 19A (1988) 2741. F.A. Mohamed, J. Mater. Sci. Lett. 7 (1988) 215. T.G. Nieh. C.M. McNally, J. Wadsworth, Scripta Metall. 22 (1988) 1297.
A2344236
(1997)
986-990
[7] D.M. Owen, A.H. Chokshi, in: S.P.S. Badwal, M.J. Bannister, R.H.J. Hannink (Ed%), Science and Technology of Zirconia V, Technomic Press, Lancaster, 1993, p. 42. [S] A.H. Chokshi, Mater. Sci. Eng. Al66 (1993) 119. [9] D.M. Owen, A.H. Chokshi, in: R.C. Bradt, C.A. Brookes, J.L. Routbort (Eds.), Plastic Deformation of Ceramics, Plenum, New York, 1995, p. 507. [lo] M. Jimenez-Melendo, A. Bravo-Leon, A. Dominguez-Rodriguez, Mater. Sci. Forum 243-245 (1997) 363. [l l] M.Z. Berbon, T.G. Langdon, Mater. Sci. Forum 243-245 (1997) 357. [12] W.J. Kim, J. Wolfenstine, O.D. Sherby, Acta Metall. Mater. 39 (1991) 199. [13] T. Sakuma, Mater. Sci. Forum 2433245 (1997) 327. [14] A.H. Chokshi, T.G. Langdon, Acta Metall. 38 (1990) 867. [15] D.M. Owen, A.H. Chokshi, S.R. Nutt, J. Am. Ceram. Sot. (in press). [16] A.H. Chokshi, Mater. Sci. Forum 233-234 (1997) 89. [17] S.R. Shah, A.H. Chokshi, Mater. Sci. Forum 2433245 (1997) 381.