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Combustion synthesis of Al2O3(-Cr2O3)-Cr cermets
Scripta mater. 42 (2000) 1167–1172 www.elsevier.com/locate/scriptamat
COMBUSTION SYNTHESIS OF Al2O3(-Cr2O3)-Cr CERMETS Xiaochun Zeng1, Guoxiong Sun1 and Shuge Zhang2 1
Department of Mechanical Engineering, Southeast University, Nanjing 210018, People’s Republic of China 2Nanjing Institute of Electric Light Source Materials, Nanjing 210015, People’s Republic of China (Received July 14, 1999) (Accepted in revised form February 3, 2000)
Keywords: Self-propagating high-temperature synthesis (SHS); Microstructure; Oxides; Composites 1. Introduction Combustion synthesis provides an attractive method of producing advanced materials, such as ceramics, intermetallic compounds, composites and cermets, since SHS offers advantages with respect to low cost and process simplicity [1,2]. Al2O3-Cr cermet is characterized by high oxidation resistance at high temperature, high corrosion resistance and high hardness, and it is commercially produced by conventional powder metallurgy [3]. The possibility of producing Al2O3-Cr cermets by thermite reaction has been demonstrated by Walton and Poules [4]. The objective of this study is to synthesize Al2O3(-Cr2O3)-Cr cermets from 2Al⫹Cr2O3 3 2Cr⫹Al2O3 reaction and investigate the influence of the reactant compact composition on product microstructures. 2. Experimental Procedure Powders of Al (75m), Cr2O3 (44m) and Al2O3 (55m) were used in this investigation. The purities of Al, Cr2O3 and Al2O3 particles were 96%, 99% and 99% respectively. The powders were well mixed in proportion of x Al2O3 ⫹ yCr2O3 ⫹ (1-y)(0.26 Al ⫹ 0.74 Cr2O3), where x and y were the mass fraction of Al2O3 and excess Cr2O3 in reaction compact respectively. The powder mixtures were then uniaxially pressed into cylindric green compacts of 20mm in diameter. The relative density of all the compacts was maintained at about 52%. Each compact weights 10g. A hole (0.4cm deep) was drilled on the top of the compact. The W-Re thermocouple junction was placed inside the hole for temperature measurement, and a recorder was connected to the thermocouple to record the temperature change of compact during heating and during reaction. The compact was placed in a graphite crucible in the experiment fixture shown in Fig. 1. A 15KW high-frequency induction furnace was used. All experiments were carried out in a quartz tube, placed within the copper-induction-coils. Experiments were run in flowing argon at 0.1Mpa. The compact was heated at a certain rate until the reaction was initiated. Combustion temperature Tc (the temperature at which the compact temperature reached its maximum during the reaction) was measured as a function of mass fraction of Al2O3 and excess Cr2O3 in reaction compacts. The composition range studied was x from 0 to 0.8 and y from 0 to 0.6 respectively. All polished samples were gold coated for examination by SEM (JSM-6300). The phase analysis of products was conducted in a Rikagu D/MAX III-A X-ray diffractometer. The composition analysis of the phases were conducted in KEVEX SIGMA X-ray energy spectrometer. 1359-6462/00/$–see front matter. © 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII S1359-6462(00)00353-5
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Figure 1. Schematic representation of combustion synthesis fixture. 1. graphite stopple 2. quartz tube 3. alumina tube 4. graphite crucible 5. green compact 6. copper induction coils 7. W-Re thermocouple.
3. Results and Discussion 3.1. Temperature Characteristics of Combustion Synthesis Fig. 2 shows the dependence of combustion temperature TC on the composition of reactant compacts. As the Al2O3 content increases, TC decreases gradually. One purpose of addition of Al2O3 to the compacts is to lower the maximum reaction temperature TC, that is, Al2O3 acts as diluent. Although diluent Al2O3 does not take part in the reaction, it will absorb the reaction heat released. The tendency of the change of TC with the increase of excess Cr2O3 is similar as the Al2O3 addition. 3.2. Phase Analysis and the Microstructure Observation of Reaction Products The X-ray phase analysis of reaction products have shown that for the case of Al2O3 addition, the product consists of chromium and ␣ - Al2O3. For the case of excess Cr2O3, only chromium and Al2O3-Cr2O3 solid solution exist in the products, which indicates that Al2O3 and Cr2O3 may form a complete solid solution [5]. As is shown in Fig. 2, the combustion temperature TC is higher than the melting point of chromium (2176K) when x is not larger than 0.2. The calculated value of chromium content in these products is not less than 40.6 weight percent. Because of the poor wetability between Cr and Al2O3, aggregation of liquid Cr occurs. Furthermore the density of Cr (7.19g/cm3) is larger than the density of Al2O3 (3.97g/cm3). The separation also occurs in the products, resulted in large chromium particles at the lower part of the products. For the same reason, the aggregation and separation of chromium was also
Figure 2. The dependence of combustion synthesis TC on the reaction compact compositions. (a) Al2O3 diluent, (b) excess Cr2O3.
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Figure 3. Distribution and morphology of chromium in samples with Al2O3 addition. (a) x ⫽ 0.1 (b) x ⫽ 0.2 (c) x ⫽ 0.3 (d) x ⫽ 0.4.
found in the product in which y is not larger then 0.2. In addition to the large chromium particles, there are large pores existed in these products. The distribution and morphology of chromium in samples in which x is 0.1, 0.2, 0.3, 0.4 were shown in Fig. 3. There are two kinds of morphology of chromium: large irregular and small spheric particles. The size of large and irregular chromium particles ranges from 4 m to 50 m. The small sphere chromium particles are about 1 m or less in size. With the increase of Al2O3 content in the compacts, the size of chromium particles decreases, and the variation of particle size decrease as well. It is suggested that large irregular chromium particles are due to the aggregation of small chromium particles. In the Al2O3-Cr system, a melting temperature of 1818°C, which is presumably the temperature of a binary eutectic has been reported [6]. However, the eutectic microstructure was not observed in that investigation. This is true for this study too. This fact may be due to a miscibility gap [7], relative slow diffusion of Al and O in Al2O3 compared with Cr and O in Cr2O3 [8], as well as the relative short time the products hold at high temperature. For samples in which y is 0.1, 0.2, 0.3 and 0.4, eutectic microstructures are observed, as shown in Fig. 4. The eutectic microstructure consists of rod-like chromium phase embedded in Al2O3-Cr2O3 solid solution. The Al2O3-Cr2O3-Cr eutectic microstructure was also observed by Schmitt for samples with an Al2O3:Cr2O3 weight ratio of 50:50 to 40:60 with 25wt%Cr addition [6]. In this study, the size of the chromium rod is less than 1 m in diameter. Furthermore, X-ray energy spectrum analysis shows that the Cr content in ceramic matrix of the eutectic is higher than that in non-eutectic area of the same sample with the content of excess Cr2O3 ranging from 0.1 to 0.4, while the Al content is in opposite way, as demonstrated by Fig. 5 for the excess Cr2O3 is 0.3. The numbers of eutectic regions in the samples with 0.2 and 0.3 excess Cr2O3 are larger than that in the samples with 0.1 and 0.4 excess Cr2O3. Other than rod-like chromiums in eutectic regions, spheric or irregular morphology of chromium particles were also found in non-eutectic regions, that is eutectic reaction does not occur all over the sample. For samples in which y is larger than 0.4, no eutectic microstructures were observed. Therefore,
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Figure 4. Eutectic formation in samples with excess Cr2O3. (a) y ⫽ 0.1 (b) y ⫽ 0.2 (c) y ⫽ 0.3 (d) y ⫽ 0.4.
it can be inferred that the occurrence of eutectic microstructure is related to the excess Cr2O3 and high temperature during combustion synthesis. In the Cr2O3-Cr system, two kinds of reaction exist in this system [5]: a monotectic reaction L1 3 Cr ⫹ L2 at 2wt.% Cr2O3 (1810°C) and a eutectic reaction L 3 Cr ⫹ Cr2O3 at 80wt.% Cr2O3 (1660°C). Compared with the interface between Al2O3 and Cr, the interface between Cr2O3 and Cr is more likely the place where the liquid solution is formed. Once the Cr2O3-Cr liquid exists, Al2O3 phase begins to dissolve into the liquid, forming the Cr2O3-Al2O3-Cr liquid solution. But this dissolution process may be limited by kinetic factors such as rapid cooling rate in high temperature range, resulting in low Al content in the Cr2O3-Al2O3-Cr liquid solution. During cooling, Cr phase will be first precipitated from the ternary liquid and then the eutectic reaction L 3 (Al,Cr)2O3 ⫹ Cr takes place. For the samples with excess Cr2O3 content up to 0.4, the calculated volume content of Cr2O3, Cr and Al2O3 ranges from 10% to 39.9%, 32.7% to 21.8%, and 57.3% to 38.3%, respectively. Some Cr2O3 particles may be located in the Cr-free regions and some Cr particles may be located in Cr2O3-free regions. In these regions, the dissolution may not occur, which means no eutectic microstructures will be formed. During cooling, Cr particles grow to large size by aggregation, and Cr2O3 particles solve into neighboring Al2O3 particles to form the solid solution. In the conventional production of Al2O3-Cr cermets by powder metallurgy, the chromium phase is dispersed in Al2O3 matrix in the particle form. Previous studies of the toughening of brittle materials by ductile reinforcement particles have suggested that the predominant contribution to toughness is the crack-bridging of ductile particles [9].Particles intercepted by the crack exhibit extensive plastic stretching, and contribute to the toughness by inhibiting crack opening. When such a bridging zone exists, residual stress present in the material, caused by thermal expansion mismatch, can also contribute to the toughness by means of its influence on the initial crack opening force. Simultaneously, plastic straining of particles in the bridging zone causes crack shielding. Therefore, thin and rod-like chromium in the eutectic regions should be more effective in toughening the ceramic matrix than
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Figure 5. The element linear distribution of Cr (a) and Al (b) for samples of 0.3 excess Cr2O3.
chromium in the particle form. The examination of mechanical properties of Al2O3(-Cr2O3)-Cr cermets produced by combustion synthesis and the toughening mechanism of different morphology of chromium are among the topics of further study. 4. Conclusions The Al2O3(-Cr2O3)-Cr cermets can be produced by combustion synthesis method. By addition Al2O3 in reaction compacts, Al2O3-Cr cermets consisted of chromium particles and Al2O3 matrix, and no eutectic microstructure was found. While excess Cr2O3 was introduced into reaction compacts, a eutectic microstructure containing rod-like chromium was observed. The cermets consisted of chromium particles, Al2O3-Cr2O3 solid solution matrix and (Al,Cr)2O3-Cr eutectic regions. The chromium content in the matrix of eutectic is higher than that in matrix of non-eutectic regions, while aluminum content is in the opposite way. The toughening effect of Cr in Al2O3(-Cr2O3)-Cr cermets produced by
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combustion synthesis is supposed to be better than that by conventional powder metallurgy due to eutectic formation in the former. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
A. G. Merzhanov, Int. J. SHS. 6(2), 119 (1997). Z. A. Munir, Ceram. Bull. 67, 342 (1988). W. Lingsen, Special Ceramics, pp. 1091–1095, Central South Polytechnic University Press, Changsha (1994) (in Chinese). J. D. Walton, Jr. and N. E. Poulos, J. Am. Ceram. Soc. 42, 40 (1959). E. M. Levin, C. R. Robbins, and H. F. McMurdie, ed, Phase Diagrams for Ceramists, pp. 38, 121, American Ceramic Society (1964). G. Banik, Th. Schmitt, W. Wruss, and B. Lux, High Temp. High Press. 12, 225 (1980). C. A. Handwerker, T. J. Foecke, J. S. Wallace, U. R. Kattner, and R. D. Jiggets, Mater. Sci. Eng. A195, 89 (1995). W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics, p. 240, John Wiley & Sons, New York (1976). L. S. Sigl, P. A. Mataga, B. J. Dalgleish, R. M. McMeeting, and A. G. Evans, Acta Metall. 36(4), 945 (1988).
COMBUSTION SYNTHESIS OF Al2O3(-Cr2O3)-Cr CERMETS Xiaochun Zeng1, Guoxiong Sun1 and Shuge Zhang2 1
Department of Mechanical Engineering, Southeast University, Nanjing 210018, People’s Republic of China 2Nanjing Institute of Electric Light Source Materials, Nanjing 210015, People’s Republic of China (Received July 14, 1999) (Accepted in revised form February 3, 2000)
Keywords: Self-propagating high-temperature synthesis (SHS); Microstructure; Oxides; Composites 1. Introduction Combustion synthesis provides an attractive method of producing advanced materials, such as ceramics, intermetallic compounds, composites and cermets, since SHS offers advantages with respect to low cost and process simplicity [1,2]. Al2O3-Cr cermet is characterized by high oxidation resistance at high temperature, high corrosion resistance and high hardness, and it is commercially produced by conventional powder metallurgy [3]. The possibility of producing Al2O3-Cr cermets by thermite reaction has been demonstrated by Walton and Poules [4]. The objective of this study is to synthesize Al2O3(-Cr2O3)-Cr cermets from 2Al⫹Cr2O3 3 2Cr⫹Al2O3 reaction and investigate the influence of the reactant compact composition on product microstructures. 2. Experimental Procedure Powders of Al (75m), Cr2O3 (44m) and Al2O3 (55m) were used in this investigation. The purities of Al, Cr2O3 and Al2O3 particles were 96%, 99% and 99% respectively. The powders were well mixed in proportion of x Al2O3 ⫹ yCr2O3 ⫹ (1-y)(0.26 Al ⫹ 0.74 Cr2O3), where x and y were the mass fraction of Al2O3 and excess Cr2O3 in reaction compact respectively. The powder mixtures were then uniaxially pressed into cylindric green compacts of 20mm in diameter. The relative density of all the compacts was maintained at about 52%. Each compact weights 10g. A hole (0.4cm deep) was drilled on the top of the compact. The W-Re thermocouple junction was placed inside the hole for temperature measurement, and a recorder was connected to the thermocouple to record the temperature change of compact during heating and during reaction. The compact was placed in a graphite crucible in the experiment fixture shown in Fig. 1. A 15KW high-frequency induction furnace was used. All experiments were carried out in a quartz tube, placed within the copper-induction-coils. Experiments were run in flowing argon at 0.1Mpa. The compact was heated at a certain rate until the reaction was initiated. Combustion temperature Tc (the temperature at which the compact temperature reached its maximum during the reaction) was measured as a function of mass fraction of Al2O3 and excess Cr2O3 in reaction compacts. The composition range studied was x from 0 to 0.8 and y from 0 to 0.6 respectively. All polished samples were gold coated for examination by SEM (JSM-6300). The phase analysis of products was conducted in a Rikagu D/MAX III-A X-ray diffractometer. The composition analysis of the phases were conducted in KEVEX SIGMA X-ray energy spectrometer. 1359-6462/00/$–see front matter. © 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII S1359-6462(00)00353-5
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Figure 1. Schematic representation of combustion synthesis fixture. 1. graphite stopple 2. quartz tube 3. alumina tube 4. graphite crucible 5. green compact 6. copper induction coils 7. W-Re thermocouple.
3. Results and Discussion 3.1. Temperature Characteristics of Combustion Synthesis Fig. 2 shows the dependence of combustion temperature TC on the composition of reactant compacts. As the Al2O3 content increases, TC decreases gradually. One purpose of addition of Al2O3 to the compacts is to lower the maximum reaction temperature TC, that is, Al2O3 acts as diluent. Although diluent Al2O3 does not take part in the reaction, it will absorb the reaction heat released. The tendency of the change of TC with the increase of excess Cr2O3 is similar as the Al2O3 addition. 3.2. Phase Analysis and the Microstructure Observation of Reaction Products The X-ray phase analysis of reaction products have shown that for the case of Al2O3 addition, the product consists of chromium and ␣ - Al2O3. For the case of excess Cr2O3, only chromium and Al2O3-Cr2O3 solid solution exist in the products, which indicates that Al2O3 and Cr2O3 may form a complete solid solution [5]. As is shown in Fig. 2, the combustion temperature TC is higher than the melting point of chromium (2176K) when x is not larger than 0.2. The calculated value of chromium content in these products is not less than 40.6 weight percent. Because of the poor wetability between Cr and Al2O3, aggregation of liquid Cr occurs. Furthermore the density of Cr (7.19g/cm3) is larger than the density of Al2O3 (3.97g/cm3). The separation also occurs in the products, resulted in large chromium particles at the lower part of the products. For the same reason, the aggregation and separation of chromium was also
Figure 2. The dependence of combustion synthesis TC on the reaction compact compositions. (a) Al2O3 diluent, (b) excess Cr2O3.
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Figure 3. Distribution and morphology of chromium in samples with Al2O3 addition. (a) x ⫽ 0.1 (b) x ⫽ 0.2 (c) x ⫽ 0.3 (d) x ⫽ 0.4.
found in the product in which y is not larger then 0.2. In addition to the large chromium particles, there are large pores existed in these products. The distribution and morphology of chromium in samples in which x is 0.1, 0.2, 0.3, 0.4 were shown in Fig. 3. There are two kinds of morphology of chromium: large irregular and small spheric particles. The size of large and irregular chromium particles ranges from 4 m to 50 m. The small sphere chromium particles are about 1 m or less in size. With the increase of Al2O3 content in the compacts, the size of chromium particles decreases, and the variation of particle size decrease as well. It is suggested that large irregular chromium particles are due to the aggregation of small chromium particles. In the Al2O3-Cr system, a melting temperature of 1818°C, which is presumably the temperature of a binary eutectic has been reported [6]. However, the eutectic microstructure was not observed in that investigation. This is true for this study too. This fact may be due to a miscibility gap [7], relative slow diffusion of Al and O in Al2O3 compared with Cr and O in Cr2O3 [8], as well as the relative short time the products hold at high temperature. For samples in which y is 0.1, 0.2, 0.3 and 0.4, eutectic microstructures are observed, as shown in Fig. 4. The eutectic microstructure consists of rod-like chromium phase embedded in Al2O3-Cr2O3 solid solution. The Al2O3-Cr2O3-Cr eutectic microstructure was also observed by Schmitt for samples with an Al2O3:Cr2O3 weight ratio of 50:50 to 40:60 with 25wt%Cr addition [6]. In this study, the size of the chromium rod is less than 1 m in diameter. Furthermore, X-ray energy spectrum analysis shows that the Cr content in ceramic matrix of the eutectic is higher than that in non-eutectic area of the same sample with the content of excess Cr2O3 ranging from 0.1 to 0.4, while the Al content is in opposite way, as demonstrated by Fig. 5 for the excess Cr2O3 is 0.3. The numbers of eutectic regions in the samples with 0.2 and 0.3 excess Cr2O3 are larger than that in the samples with 0.1 and 0.4 excess Cr2O3. Other than rod-like chromiums in eutectic regions, spheric or irregular morphology of chromium particles were also found in non-eutectic regions, that is eutectic reaction does not occur all over the sample. For samples in which y is larger than 0.4, no eutectic microstructures were observed. Therefore,
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Figure 4. Eutectic formation in samples with excess Cr2O3. (a) y ⫽ 0.1 (b) y ⫽ 0.2 (c) y ⫽ 0.3 (d) y ⫽ 0.4.
it can be inferred that the occurrence of eutectic microstructure is related to the excess Cr2O3 and high temperature during combustion synthesis. In the Cr2O3-Cr system, two kinds of reaction exist in this system [5]: a monotectic reaction L1 3 Cr ⫹ L2 at 2wt.% Cr2O3 (1810°C) and a eutectic reaction L 3 Cr ⫹ Cr2O3 at 80wt.% Cr2O3 (1660°C). Compared with the interface between Al2O3 and Cr, the interface between Cr2O3 and Cr is more likely the place where the liquid solution is formed. Once the Cr2O3-Cr liquid exists, Al2O3 phase begins to dissolve into the liquid, forming the Cr2O3-Al2O3-Cr liquid solution. But this dissolution process may be limited by kinetic factors such as rapid cooling rate in high temperature range, resulting in low Al content in the Cr2O3-Al2O3-Cr liquid solution. During cooling, Cr phase will be first precipitated from the ternary liquid and then the eutectic reaction L 3 (Al,Cr)2O3 ⫹ Cr takes place. For the samples with excess Cr2O3 content up to 0.4, the calculated volume content of Cr2O3, Cr and Al2O3 ranges from 10% to 39.9%, 32.7% to 21.8%, and 57.3% to 38.3%, respectively. Some Cr2O3 particles may be located in the Cr-free regions and some Cr particles may be located in Cr2O3-free regions. In these regions, the dissolution may not occur, which means no eutectic microstructures will be formed. During cooling, Cr particles grow to large size by aggregation, and Cr2O3 particles solve into neighboring Al2O3 particles to form the solid solution. In the conventional production of Al2O3-Cr cermets by powder metallurgy, the chromium phase is dispersed in Al2O3 matrix in the particle form. Previous studies of the toughening of brittle materials by ductile reinforcement particles have suggested that the predominant contribution to toughness is the crack-bridging of ductile particles [9].Particles intercepted by the crack exhibit extensive plastic stretching, and contribute to the toughness by inhibiting crack opening. When such a bridging zone exists, residual stress present in the material, caused by thermal expansion mismatch, can also contribute to the toughness by means of its influence on the initial crack opening force. Simultaneously, plastic straining of particles in the bridging zone causes crack shielding. Therefore, thin and rod-like chromium in the eutectic regions should be more effective in toughening the ceramic matrix than
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Figure 5. The element linear distribution of Cr (a) and Al (b) for samples of 0.3 excess Cr2O3.
chromium in the particle form. The examination of mechanical properties of Al2O3(-Cr2O3)-Cr cermets produced by combustion synthesis and the toughening mechanism of different morphology of chromium are among the topics of further study. 4. Conclusions The Al2O3(-Cr2O3)-Cr cermets can be produced by combustion synthesis method. By addition Al2O3 in reaction compacts, Al2O3-Cr cermets consisted of chromium particles and Al2O3 matrix, and no eutectic microstructure was found. While excess Cr2O3 was introduced into reaction compacts, a eutectic microstructure containing rod-like chromium was observed. The cermets consisted of chromium particles, Al2O3-Cr2O3 solid solution matrix and (Al,Cr)2O3-Cr eutectic regions. The chromium content in the matrix of eutectic is higher than that in matrix of non-eutectic regions, while aluminum content is in the opposite way. The toughening effect of Cr in Al2O3(-Cr2O3)-Cr cermets produced by
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combustion synthesis is supposed to be better than that by conventional powder metallurgy due to eutectic formation in the former. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
A. G. Merzhanov, Int. J. SHS. 6(2), 119 (1997). Z. A. Munir, Ceram. Bull. 67, 342 (1988). W. Lingsen, Special Ceramics, pp. 1091–1095, Central South Polytechnic University Press, Changsha (1994) (in Chinese). J. D. Walton, Jr. and N. E. Poulos, J. Am. Ceram. Soc. 42, 40 (1959). E. M. Levin, C. R. Robbins, and H. F. McMurdie, ed, Phase Diagrams for Ceramists, pp. 38, 121, American Ceramic Society (1964). G. Banik, Th. Schmitt, W. Wruss, and B. Lux, High Temp. High Press. 12, 225 (1980). C. A. Handwerker, T. J. Foecke, J. S. Wallace, U. R. Kattner, and R. D. Jiggets, Mater. Sci. Eng. A195, 89 (1995). W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics, p. 240, John Wiley & Sons, New York (1976). L. S. Sigl, P. A. Mataga, B. J. Dalgleish, R. M. McMeeting, and A. G. Evans, Acta Metall. 36(4), 945 (1988).