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Effect of stress on the phase redistribution during superplastic deformation of an alumina-zirconia composite
Materials Science and Engineering, A184 (1994) L21 -L23
L21
Letter
Effect of stress on the phase redistribution during superplastic deformation of an alumina-zirconia composite L. Clarisse, R. Baddi*, R. Duclos and J. Crampon Laboratoire de Structure et Propri~tOs de l'Etat Solide, URA CNRS 234, Bdtt. ('6, Universit~ de Lille I, F-59655 Villeneuve d'Ascq Cedex (France)
(Received December 20, 1993; in revised form January 24, 1994)
Abstract The phase redistribution during the superplastic deformation of an alumina-zirconia composite was analysed as a function of stress. It appears that intercalations between grains of the two phases are favoured by a high stress. In that case a trend to aggregate formation is observed.
The achievement of large superplastic strains is related to structural mechanisms such as grain boundary sliding and grain switching, that allow the structural stability to be maintained during deformation. In a single-phase material, the development of structure may be very weak and may require a topological approach to be revealed [1]. In a two-phase material, the problem is different because even if each phase can maintain equiaxed grains, the phase distribution can evolve with deformation and may consequently produce noticeable changes in structure which are the origin of the variation in physical and mechanical properties of the materials. Of the two-phase ceramic materials, the alumina-zirconia system has probably been investigated most thoroughly [2-6]. From the structural viewpoint, Wakai et al. [2] reported a tendency towards the formation of alumina aggregates in 73vol.%ZrO~27vol.%A1203 composites, while Martinez et al. [5] observed the formation of chains of zirconia grains around alumina grains in an 85vol.%A120~15vol.%ZrO 2 composite, the two phases being dis-
*Permanent address: Universit6 Mohamed ler, Facult6 des Sciences, Oujda, Morocco. 0921-5093/94/$7.00 SSDI 0921-5093(94)02700-Q
tributed more or less homogeneously in the as-sintered materials. In these two cases, the changes in structure were explained using the grain switching model due to Ashby and Verrall [7]. The purpose of this study was to investigate the structural changes that result from large strains (about 100%) in a 50vol.%A1203-50vol.%ZrO2 composite as a function of stress. Effectively, at stresses above about 140 MPa, alumina grains flowing in between zirconia grains, or vice versa, were observed by transmission electron microscopy while such observations were not made at low stress. This leads to finger-like bulges, located at boundary triple junctions, as reported in a previous paper [8]. They result from the normal stress distribution at grain boundaries in the vicinity of triple points and from the low solubility of aluminium and zirconium in zirconia and alumina respectively. In this paper we discuss whether these protuberances are the only way in which grain switching involving the two kinds of phase can occur, and what effect this has on the phase redistribution. The composite used was a mixture in a 50:50 volume ratio of a-alumina (HR8, Cricrram, France) and 3 mol% yttria partially stabilized zirconia (HSY-3, Daiichi Kigenso Kagaku Kogyo, Japan). The density was near the theoretical value (5.03 g cm-3), and the mean grain sizes were 1.1/~m and 0.8/~m for the alumina and zirconia phases respectively. The typical microstructure can be described schematically as an arrangement of small clusters containing four or five alumina grains embedded in a zirconia matrix (Fig. 1). The structure, and its development with deformation, were studied by scanning electron microscopy (SEM) on polished and thermally etched surfaces of unstrained and strained specimens using secondary and backscattered electrons. Strained specimens were crept at 1350°C under a constant stress of 200 or 100 MPa, corresponding respectively to the observation or not of grain boundary bulges. From SEM images, the mean grain surroundings (the distribution function of the number n of grain sides and the nature, i.e. alumina or zirconia, of first neighbours) was evaluated for the two phases as a function of deformation conditions. About 2000 grains were analysed per sample. Figure 2 presents the distribution of the number of grain sides of zirconia and alumina phases for an undeformed sample (labelled A) and for two samples crept to strains of 100% under stresses of 100 MPa (B) © 1994 - Elsevier Sequoia. All rights reserved
Letter
L22
TABLE 1. Mean grain size, mean number of grain sides and mean alumina fraction of neighbour grains for the three different test conditions (see text); the first number corresponds to the alumina phase, the second number to the zirconia phase
Fig. 1. Scanning electron micrograph showing the typical microstructure of as-sintered specimens. The dark phase is the alumina phase, scale bar 2/~m.
0,4 A e,. v O. co
,
0,3
I*C
J=l
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.=J_ c 0
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8
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(a)
8
10
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number n of grain sides
0,3 ~'
A w-
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.o
0,2
73 e~ t.
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._~ ~
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o 0,1 o
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(b)
6
8
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14
n u m b e r n of grain sides
Fig. 2. Distribution P(n) of the number n of grain sides in the zirconia phase (a) and in the alumina phase (b).
or 200 MPa (C). Analysis of these curves reveals two characteristics. (i) The average number of sides of alumina grains differs from that of zirconia grains (Table 1). It varies
Sample
Mean grain size (/~m)
Mean number of grain sides
Mean fraction (%) of alumina grains as first neighbours
A B C
1.08/0.8 ] . ] 1/0.82 1.13/0.81
6.26/5.52 6.45/5.49 6.21/5.39
35/37 34/32 49/31
from 6.2 (C) to 6.4 (B) for the alumina phase and from 5.4 (C) to 5.5 (A) for the zirconia grains. These values also differ from the theoretical mean number of grain sides in a planar section of a single-phase material, which is 6. The deviations from this value result from the following. (a) The alumina grain size is about 40% larger than the zirconia grain size. Thus, when an alumina grain is located at an alumina-zirconia interface, the number of first-neighbour grains is larger than when the alumina grain is surrounded only by other alumina grains. (b) There are numerous aluminazirconia interfaces. These features contribute to increase the average number of neighbours of alumina grains and inversely for the zirconia grains. (ii) For each phase, the three distribution curves are similar and do not reveal a strong influence of deformation on the average number of grain sides. By considering only these curves, no structural development is visible. Moreover, no grain coarsening was observed. Figure 3 presents the distribution function of the first-neighbour grains (represented by the fraction of alumina grains among these grains) of a given grain for the two phases. The three curves corresponding to the zirconia phase are very similar, indicating that the firstneighbour grains of a zirconia grain are relatively insensitive to deformation conditions. In contrast, for the alumina phase curve C differs significantly from curves A and B which are very alike. This means that on average, the fraction of alumina grains around a given alumina grain has increased in sample C relative to the initial value. Owing to the grain size stability, this new distribution of first-neighbour grains in sample C corresponds to changes in structure that can only be the result of grain intercalation processes implicating grains of the two phases. These intercalations are able to modify strongly the grain surroundings in a two-phase material as shown in Fig. 4. In this figure one can see that the structural development from (a) to (b) is responsible for an increase in the fraction of alumina neighbours for the involved alumina grains, the other neighbouring
Letter 0,6
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number of grain sides
Fig. 3. Distribution of the ratio of alumina grains as first neighbours around zirconia grains (a) and aluminagrains (b).
(a)
(b)
Fig. 4. Influence of grain intercalations on the nature of first-
neighbour grains.
grains being supposed unchanged. It is likely that grain switching also took place during deformation in sample B but the relatively similar grain surroundings in samples A and B, and consequently the stability of the structure, could mean that this mechanism only operated inside the alumina and zirconia phases and not on a large scale between these two phases. The only microstructural difference that could explain the different structural behaviours of samples B and C consists in the grain boundary bulges observed
L23
at the alumina-zirconia interfaces. These boundary singularities occur at the onset of the grain switching mechanism because when two alumina protuberances meet between two zirconia grains, changes in neighbours take place, as shown schematically in Fig. 4. As a result of these intercalations, the size of the alumina clusters increases. The relative stability of zirconia grain surroundings may result from several structural characteristics for example the following. (i) In contrast to alumina grain distribution, the observation of zirconia grains isolated inside alumina clusters was uncommon. Under these conditions the joining of two zirconia grains separated initially by alumina grains is less frequent than the reverse situation. Moreover, bulges of zirconia grains are probably less efficacious owing to the difference between the grain sizes of alumina and zirconia phases. (ii) Intercalations and resultant perturbations in the surroundings affect first the grains at the boundary of the two phases. If all alumina grains are more or less in this position, this is not true for the zirconia grains owing to their smaller size (there are about two zirconia grains for one alumina grain), and only one third to one half of the zirconia grains are in this position (in the assintered samples 63% of first-neighbour grains are zirconia grains whatever the phase). So, the effect of intercalations is less important in the case of the zirconia grains, and the changes in surroundings of this phase are probably too weak to be easily discernible. In the present case, one can then infer that intercalations involving grains of the two phases were favoured by high stresses. As a consequence the cluster size increased. The origin of these intercalations seems to be the formation of grain boundary protuberances. In actual fact, the absence of these protuberances corresponds, up to strains of 100%, to relatively stable structures in which no sharp structural interactions between the two phases were observed. References 1 R. Duclos, J. Crampon and B. Amana, Acta Metall., 37 (1989) 877-883. 2 F. Wakai, H. Kato, S. Sakaguchi and N. Muruyama, Yogyo Kyokai Shi, 94 (1986) 1017-1020. 3 F. Wakai and H. Kato, Adv. Ceram. Mater, 3 (1988) 71-76. 4 T. G. Nieh, C. M. McNally and J. Wadsworth, Scr. Metall., 23 (1989) 457-460. 5 R. Martinez, R. Duclos and J. Crampon, Scr. Metall. Mater., 24 (1990) 1979-1984. 6 T. G. Nieh and J. Wadsworth, Acta Metall. Mater., 39 (1991) 3037-3045. 7 M. F. Ashby and R. A. Verrall, Acta Metall., 21 (1973) 149-163. 8 R. Baddi, R. Duclos and J. Crampon, Mater. Sci. Eng., A165 (1993)L1-L3.
L21
Letter
Effect of stress on the phase redistribution during superplastic deformation of an alumina-zirconia composite L. Clarisse, R. Baddi*, R. Duclos and J. Crampon Laboratoire de Structure et Propri~tOs de l'Etat Solide, URA CNRS 234, Bdtt. ('6, Universit~ de Lille I, F-59655 Villeneuve d'Ascq Cedex (France)
(Received December 20, 1993; in revised form January 24, 1994)
Abstract The phase redistribution during the superplastic deformation of an alumina-zirconia composite was analysed as a function of stress. It appears that intercalations between grains of the two phases are favoured by a high stress. In that case a trend to aggregate formation is observed.
The achievement of large superplastic strains is related to structural mechanisms such as grain boundary sliding and grain switching, that allow the structural stability to be maintained during deformation. In a single-phase material, the development of structure may be very weak and may require a topological approach to be revealed [1]. In a two-phase material, the problem is different because even if each phase can maintain equiaxed grains, the phase distribution can evolve with deformation and may consequently produce noticeable changes in structure which are the origin of the variation in physical and mechanical properties of the materials. Of the two-phase ceramic materials, the alumina-zirconia system has probably been investigated most thoroughly [2-6]. From the structural viewpoint, Wakai et al. [2] reported a tendency towards the formation of alumina aggregates in 73vol.%ZrO~27vol.%A1203 composites, while Martinez et al. [5] observed the formation of chains of zirconia grains around alumina grains in an 85vol.%A120~15vol.%ZrO 2 composite, the two phases being dis-
*Permanent address: Universit6 Mohamed ler, Facult6 des Sciences, Oujda, Morocco. 0921-5093/94/$7.00 SSDI 0921-5093(94)02700-Q
tributed more or less homogeneously in the as-sintered materials. In these two cases, the changes in structure were explained using the grain switching model due to Ashby and Verrall [7]. The purpose of this study was to investigate the structural changes that result from large strains (about 100%) in a 50vol.%A1203-50vol.%ZrO2 composite as a function of stress. Effectively, at stresses above about 140 MPa, alumina grains flowing in between zirconia grains, or vice versa, were observed by transmission electron microscopy while such observations were not made at low stress. This leads to finger-like bulges, located at boundary triple junctions, as reported in a previous paper [8]. They result from the normal stress distribution at grain boundaries in the vicinity of triple points and from the low solubility of aluminium and zirconium in zirconia and alumina respectively. In this paper we discuss whether these protuberances are the only way in which grain switching involving the two kinds of phase can occur, and what effect this has on the phase redistribution. The composite used was a mixture in a 50:50 volume ratio of a-alumina (HR8, Cricrram, France) and 3 mol% yttria partially stabilized zirconia (HSY-3, Daiichi Kigenso Kagaku Kogyo, Japan). The density was near the theoretical value (5.03 g cm-3), and the mean grain sizes were 1.1/~m and 0.8/~m for the alumina and zirconia phases respectively. The typical microstructure can be described schematically as an arrangement of small clusters containing four or five alumina grains embedded in a zirconia matrix (Fig. 1). The structure, and its development with deformation, were studied by scanning electron microscopy (SEM) on polished and thermally etched surfaces of unstrained and strained specimens using secondary and backscattered electrons. Strained specimens were crept at 1350°C under a constant stress of 200 or 100 MPa, corresponding respectively to the observation or not of grain boundary bulges. From SEM images, the mean grain surroundings (the distribution function of the number n of grain sides and the nature, i.e. alumina or zirconia, of first neighbours) was evaluated for the two phases as a function of deformation conditions. About 2000 grains were analysed per sample. Figure 2 presents the distribution of the number of grain sides of zirconia and alumina phases for an undeformed sample (labelled A) and for two samples crept to strains of 100% under stresses of 100 MPa (B) © 1994 - Elsevier Sequoia. All rights reserved
Letter
L22
TABLE 1. Mean grain size, mean number of grain sides and mean alumina fraction of neighbour grains for the three different test conditions (see text); the first number corresponds to the alumina phase, the second number to the zirconia phase
Fig. 1. Scanning electron micrograph showing the typical microstructure of as-sintered specimens. The dark phase is the alumina phase, scale bar 2/~m.
0,4 A e,. v O. co
,
0,3
I*C
J=l
0,2 "0
÷
.=J_ c 0
0,1
9 o
8
0,0 6
(a)
8
10
12
14
6
number n of grain sides
0,3 ~'
A w-
e-
.o
0,2
73 e~ t.
o
._~ ~
B
o 0,1 o
0,0 4
(b)
6
8
10
12
14
n u m b e r n of grain sides
Fig. 2. Distribution P(n) of the number n of grain sides in the zirconia phase (a) and in the alumina phase (b).
or 200 MPa (C). Analysis of these curves reveals two characteristics. (i) The average number of sides of alumina grains differs from that of zirconia grains (Table 1). It varies
Sample
Mean grain size (/~m)
Mean number of grain sides
Mean fraction (%) of alumina grains as first neighbours
A B C
1.08/0.8 ] . ] 1/0.82 1.13/0.81
6.26/5.52 6.45/5.49 6.21/5.39
35/37 34/32 49/31
from 6.2 (C) to 6.4 (B) for the alumina phase and from 5.4 (C) to 5.5 (A) for the zirconia grains. These values also differ from the theoretical mean number of grain sides in a planar section of a single-phase material, which is 6. The deviations from this value result from the following. (a) The alumina grain size is about 40% larger than the zirconia grain size. Thus, when an alumina grain is located at an alumina-zirconia interface, the number of first-neighbour grains is larger than when the alumina grain is surrounded only by other alumina grains. (b) There are numerous aluminazirconia interfaces. These features contribute to increase the average number of neighbours of alumina grains and inversely for the zirconia grains. (ii) For each phase, the three distribution curves are similar and do not reveal a strong influence of deformation on the average number of grain sides. By considering only these curves, no structural development is visible. Moreover, no grain coarsening was observed. Figure 3 presents the distribution function of the first-neighbour grains (represented by the fraction of alumina grains among these grains) of a given grain for the two phases. The three curves corresponding to the zirconia phase are very similar, indicating that the firstneighbour grains of a zirconia grain are relatively insensitive to deformation conditions. In contrast, for the alumina phase curve C differs significantly from curves A and B which are very alike. This means that on average, the fraction of alumina grains around a given alumina grain has increased in sample C relative to the initial value. Owing to the grain size stability, this new distribution of first-neighbour grains in sample C corresponds to changes in structure that can only be the result of grain intercalation processes implicating grains of the two phases. These intercalations are able to modify strongly the grain surroundings in a two-phase material as shown in Fig. 4. In this figure one can see that the structural development from (a) to (b) is responsible for an increase in the fraction of alumina neighbours for the involved alumina grains, the other neighbouring
Letter 0,6
o J= ro~
-
0,5 • o
'3
$
er-
o +
0,4 '
o + •
0
o + •
+
o
o
+
•
o
o +
+
0,3
0
zlrconia 0,2 4
(a)
6
8
10
number of grain sides
0,7
-
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0,6 -
'~
0,5 -
EL_&
o o o
E
+
0,4 -
•
+
o •
o
0
0
0,3 +
alumina
"~ 0,2 -
4
(b)
6
8
10
2
number of grain sides
Fig. 3. Distribution of the ratio of alumina grains as first neighbours around zirconia grains (a) and aluminagrains (b).
(a)
(b)
Fig. 4. Influence of grain intercalations on the nature of first-
neighbour grains.
grains being supposed unchanged. It is likely that grain switching also took place during deformation in sample B but the relatively similar grain surroundings in samples A and B, and consequently the stability of the structure, could mean that this mechanism only operated inside the alumina and zirconia phases and not on a large scale between these two phases. The only microstructural difference that could explain the different structural behaviours of samples B and C consists in the grain boundary bulges observed
L23
at the alumina-zirconia interfaces. These boundary singularities occur at the onset of the grain switching mechanism because when two alumina protuberances meet between two zirconia grains, changes in neighbours take place, as shown schematically in Fig. 4. As a result of these intercalations, the size of the alumina clusters increases. The relative stability of zirconia grain surroundings may result from several structural characteristics for example the following. (i) In contrast to alumina grain distribution, the observation of zirconia grains isolated inside alumina clusters was uncommon. Under these conditions the joining of two zirconia grains separated initially by alumina grains is less frequent than the reverse situation. Moreover, bulges of zirconia grains are probably less efficacious owing to the difference between the grain sizes of alumina and zirconia phases. (ii) Intercalations and resultant perturbations in the surroundings affect first the grains at the boundary of the two phases. If all alumina grains are more or less in this position, this is not true for the zirconia grains owing to their smaller size (there are about two zirconia grains for one alumina grain), and only one third to one half of the zirconia grains are in this position (in the assintered samples 63% of first-neighbour grains are zirconia grains whatever the phase). So, the effect of intercalations is less important in the case of the zirconia grains, and the changes in surroundings of this phase are probably too weak to be easily discernible. In the present case, one can then infer that intercalations involving grains of the two phases were favoured by high stresses. As a consequence the cluster size increased. The origin of these intercalations seems to be the formation of grain boundary protuberances. In actual fact, the absence of these protuberances corresponds, up to strains of 100%, to relatively stable structures in which no sharp structural interactions between the two phases were observed. References 1 R. Duclos, J. Crampon and B. Amana, Acta Metall., 37 (1989) 877-883. 2 F. Wakai, H. Kato, S. Sakaguchi and N. Muruyama, Yogyo Kyokai Shi, 94 (1986) 1017-1020. 3 F. Wakai and H. Kato, Adv. Ceram. Mater, 3 (1988) 71-76. 4 T. G. Nieh, C. M. McNally and J. Wadsworth, Scr. Metall., 23 (1989) 457-460. 5 R. Martinez, R. Duclos and J. Crampon, Scr. Metall. Mater., 24 (1990) 1979-1984. 6 T. G. Nieh and J. Wadsworth, Acta Metall. Mater., 39 (1991) 3037-3045. 7 M. F. Ashby and R. A. Verrall, Acta Metall., 21 (1973) 149-163. 8 R. Baddi, R. Duclos and J. Crampon, Mater. Sci. Eng., A165 (1993)L1-L3.