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Microstructures and mechanical characteristics of electromagnetic casting and direct-chill casting 2024 aluminum alloys
Materials Science and Engineering A327 (2002) 133– 137 www.elsevier.com/locate/msea
Microstructures and mechanical characteristics of electromagnetic casting and direct-chill casting 2024 aluminum alloys Cao Zhiqiang *, Jia Fei, Zhang Xingguo, Hao Hai, Jin Junze Research Center of Foundry Engineering, Dalian Uni6ersity of Technology, Dalian 116024, China Received 24 January 2001; received in revised form 7 June 2001
Abstract Electromagnetic casting (EMC) technology depends on the electromagnetic force to prevent the metal from touching the mold, leaving the ingot surface very smooth. As a result, the surface imperfections, such as oscillation and subsurface segregation etc. caused by a mold of direct-chill casting (DCC), are avoided. Moreover, the electromagnetic stirring makes the structure become more homogeneous over the entire cross-section. In this paper, 2024 aluminum alloys made by EMC and DCC were analyzed by optical microscope and scanning electron microscope, solution heat treatment and artificial aging was given to investigate the mechanical properties such as hardness, wear resistance and fatigue. Results showed EMC ingots had fine and uniform grain structure, which made the EMC ingots have high hardness and good fatigue. The hardness of EMC specimens increased one time than DCC ones and the fatigue was three times as high as DCC ones in as-cast state. The EMC specimens also expressed an excellent characterization in wear resistance; the weight loss was only a half of DCC specimens. © 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Electromagnetic casting; Microstructure; Hardness; Wear resistance; Fatigue
1. Introduction Electromagnetic casting (EMC) is a technology developed by combining the magnetohydrodynamics (MHD) and the casting engineering. The casting method employs the electromagnetic forces to pinch upon the liquid metal through an alternating electromagnetic field, which is induced by an inductor [1]. The electromagnetic forces are produced by the interaction of eddy currents induced in the metal with the produced magnetic field of the inductor. The main advantage of the EMC technology consists in the presence of stirring motion in the melt, which leads to a significant reduction of the grain size in the solidified product. Moreover surface quality and subsurface quality are improved due to the absence of ingot mould [2]. The surface finish of the ingots is usually smooth enough to be hot rolled without scalping operation that is required for direct-chill casting (DCC) [3].
* Corresponding author. Fax: + 86-411-4671110. E-mail address: [email protected] (C. Zhiqiang).
Aside from refining internal structures, electromagnetic stirring also has advantages of homogenizing alloy elements, reducing porosity and segregation, and minimizing internal cracks [4]. Because of these distinct merits of EMC technology, many scientists and engineers in different countries have engaged on the electromagnetic shaping and MHD phenomena in EMC of aluminum [5]; nevertheless, the research about the microstructure and mechanical characteristics has been comparatively few. This paper is an attempt to remedy this deficiency. The microstructure characteristics such as the grain structure is of great importance, since many mechanical properties are directly related to the size, shape and distribution of the grains [6]. As a result, the 2024 aluminum alloys were cast by DCC and EMC to investigate their microstructure and properties. Its nominal composition is Al –4.3Cu –l.6Mg –0.7Mn. As a kind of widely applied hardening alloys with high strength, it is often used to fabricate the plane wrappings and aeroengine vanes working under high temperature.
0921-5093/02/$ - see front matter © 2002 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 6 7 3 - 2
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C. Zhiqiang et al. / Materials Science and Engineering A327 (2002) 133–137
2. Experimental procedure
2.1. Principle and apparatus of electromagnetic casting The experimental campaigns were carried out on a pilot-scale caster, in which trial-and-error shaping experiments were performed and the optimized technical parameters have been obtained. The basic apparatus of EMC consists of a delivery system, a casting control system, a shaping and a cooling system, a melting furnace and a power supply etc. as shown in Fig. 1. A medium-frequency alternating current was used to generate the alternating magnetic field in the molten aluminum [7]. This magnetic field generated a heavy eddy current on the surface of the molten aluminum in opposite phase to the imposed current of the electromagnetic coil. This resulted in a force directing towards the center of ingots. The electromagnetic force located within the liquid column of ingots prevented the metal from touching the mold. The technique procedures are as follows: melt metal; adjust the position of inductor, screen, water jacket and bottom block; operate cooling system; turn on power supply; cast on in a withdrawal speed until the end of casting. The manufacturing conditions are: inductor current, 4800 A; height of liquid column, 40 mm; solidification front, 10 mm up to bottom of inductor; pouring temperature, 710 730 °C; flow rate of cooling water, 3 m3 h − 1; casting speed, 0.3 1.5 mm s − 1. Besides the as-cast state, some 2024 specimens were solution-treated at 495 °C in salt bath furnace for 1 h and then quenched in ice water. After this, they were immediately aged in the 190 °C silicon oil bath for various holding time to obtain the hardening effect. The metallographic specimens were prepared by cutting, grinding, polishing and etching. The etchant was Keller’s solution. The specimens from DCC and EMC ingots were examined and contrasted by optical mi-
Fig. 1. Schematic diagram of EMC equipment: (1) Furnace, (2) Liquid metal, (3) Trough, (4) Box, (5) Screen, (6) Inductor, (7) Transformer, (8) Power supply, (9) Cooling water, (10) Ingot, (11) Bottom block, (12) Motor, (13) Speed control.
crostructure (OM) and scanning electron microstructure (SEM).
2.2. Mechanical properties 2.2.1. Hardness test Hardness tests were performed before and after heattreatment by the load of 60 kg with the Rockwell hardness tester of F type. Take an average of seven times after omitting the highest and the lowest values. 2.2.2. Wear resistance test The PLINT TE88 multi-station friction and wear test machine was utilized to investigate the wear resistance of both DCC and EMC specimens. Cylindrical pin samples were machined to length of 15 mm and a diameter of 8 mm. The disc of counterpart was made of AISI 1055 steel quenched, the load was 60 N, sliding distance 200 m, sliding speed 1 m s − 1. The weight loss of various aluminum alloys was obtained by weighing the pins before and after each run. 2.2.3. Fatigue test The castings were cut into slices for the fatigue specimens basing on the ASTM 466-96. Both as-cast and T6 specimens were tested using an Instron 8516 hydraulic fatigue test machine. All fatigue tests were performed in a laboratory environment at ambient temperature. Constant-amplitude, fully reversed loading was applied (Sine wave, R=|max/|min = −l) at a frequency of 20 Hz to all specimens. Tests were conducted at a nominal stresses of 100 MPa.
3. Results and discussion
3.1. Microstructure Fig. 2 showed the transverse sectional grain structures of the 2024 DCC and EMC specimens in as-cast state. It was clear that the grains of EMC were small and equiaxed, no matter at edge, midthickness or center of ingots. The intensifying phases S and q distributed along the a-dendrite boundary off and on. In DCC condition, the grain size became coarse from the surface to the center of ingots with long and thick intensifying phase located among them. The dendrites grew and coarsened obviously in the center of DCC ingots due to Ostwald ripening [8]. The conventional DCC process is known to generate a non-uniform heat transfer due to air-gap formation. Therefore, the sub-surface often exists local big grains and segregation due to the slow loss of superheat. However, for the EMC, the grain was more homogeneous over the entire cross section due to the strong electromagnetic stirring in the liquid pool. This intense
C. Zhiqiang et al. / Materials Science and Engineering A327 (2002) 133–137
135
Fig. 2. Microstructures of 2024 alloy ingots (Ø174 mm) at as-cast state. (a), (c), (e), (g), DCC, from surface to center; (b), (d), (f), (h), EMC, from surface to center.
forced convection promoted a fast superheat evacuation and breaked the dendrite arms, which led to the grain multiplication. The suspended nuclei localized in the vicinity of the liquid solid interface were carried away and dispersed in a slightly super-heated melt. As a result, the crystallization took place simultaneously in most of the sump and caused the appearance of a fine and uniform equiaxed structure over the entire crosssection.
Fig. 3. The EMC specimens reached a peak at around 12 h, and the DCC ones got the peak after 36 h. That meaned the EMC ingots had much fast response to solution heat treatment and artificial aging. After the T6 treatment, the reinforcing phase S transformed into a transition phase S%% and G.P. zone, q became a q%% and G.P. zone. At this time, a stress field caused by the strain near the transitional phase could impede the movement of the dislocation, thus could largely improve the hardness and strength of alloys.
3.2. Mechanical characterization 3.2.1. Hardness properties As shown in Table 1, the hardness of EMC specimens was two times as high as DCC ones at as-cast state, owing to the much finer grain structure. The age-hardening curves for the 2024 alloy were plotted in
Table 1 Hardnesses of 2024 aluminum alloys Different alloys
DCC
EMC
DCCT6
EMCT6
Hardness
16.02
31.72
20.64
37.02
C. Zhiqiang et al. / Materials Science and Engineering A327 (2002) 133–137
136
Table 3 Fatigues of 2024 aluminum alloys
Fig. 3. Artificial aging curves of 2024 alloys: DCC, Rockwell;
EMC, Aging time (h). Table 2 Weight loss of 2024 aluminum alloys Different alloys
DCC
EMC
DCCT6
EMCT6
Weight loss (mg)
3.0
1.6
2.0
1.0
3.2.2. Wear resistance Table 2 showed that the weight loss of DCC was higher than the EMC’s no matter whether or not they were treated by T6 condition, the heat treatment obviously increased their abrasive resistance. Fig. 4 showed the characteristics of wear debris stripping from the worn surface. The wear debris of DCC specimen exhibited very obvious large block like particles. For the EMC ones, the wear debris existed an
Different alloys
DCC
EMC
DCCT6
EMCT6
Cycle number
37484
94674
612853
735567
aggregate of small pieces, no example of sheet like large particles found in the wear debris. The wear resistance of EMC specimens was improved remarkably due to their very fine, uniform and dense dendrite cells which could endure the friction with the wear surface. After the T6 treatment, the wear resistance increased in some degree. This coincides with the age hardening phenomenon.
3.2.3. Fatigue life Table 3 indicated that the fatigue cycle number of EMC specimens was almost three times as high as that of DCC ones in as-cast state. It is well known, the fracture initiates from the rough surface, sub-surface porosity, micropore and oxide inclusions [9,10]. The EMC ingots have smooth surface, free of porosity and little subsurface segregation, so there is an excellent fatigue life. Fig. 5 showed the striated fracture of DCC specimens and the dimple rupture of EMC ones. The former indicated the cleavage-like mechanism and the latter showed the ductile characteristics.
Fig. 4. SEM micrographs of worn debris of 2024 alloys: (a) DCC as-cast, (b) EMC as-cast, (c) DCC T6, (d) EMC T6.
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Fig. 5. SEM fractographs of 2024 aluminum alloys: (a) DCC as-cast, (b) EMC as-cast, (c) DCC T6, (d) EMC T6.
4. Conclusions (1) The EMC ingots owned much fine grain size and homogeneous structure and much effective response to aging for 2024 EMC ingots compared to the DCC ones, so that it have better mechanical properties not only in as-cast condition but also in T6 one. (2) The fatigue life of EMC ingots was about three times as high as that of DCC one in as-cast state due to the fine equiaxed grains and absence of microporosity of EMC ingots. (3) EMC specimens expressed excellent characteristics in hardness and wear resistance; the weight loss was only a half of DCC ones.
Acknowledgements The authors acknowledge the supports of the Na-
tional Natural Science Foundation of China (59901001) & (59995442) and National Major Fundamental Research Project of China (G1998061500).
References [1] Z.N. Getselev, J. Metals 23 (10) (1971) 38 – 39. [2] M. Garnier, International Symposium on Electromagnetic Processing of Materials, Nagoya, ISIJ, Japan, 1994, p. 1. [3] D.C. Prasso, J.W. Evans, I.J. Wilson, Metall. Trans. B 26B (1995) 1243 – 1251. [4] B.Q. Li, JOM-e 50 (2) (1998) 1 – 10. [5] S. Asai, ISIJ Internatl. 29 (12) (1989) 981 – 992. [6] C. Vives, J. Crystal Growth 94 (1989) 739 – 750. [7] M. Furui, ISIJ Internatl. 33 (3) (1993) 400 – 404. [8] N. Bryson, F. Weiberg (Eds.), International Symposium on Solidification Processing, Hamilton, Ont., Canada, 1990, p. 69. [9] B. Zhang, D.R. Poirier, W. Chen, Metall. Trans. A 30A (1999) 2659 – 2665. [10] H. Jiang, P. Brown, J.F. Knott, J. Mater. Sci. 34 (1999) 719 – 725.
Microstructures and mechanical characteristics of electromagnetic casting and direct-chill casting 2024 aluminum alloys Cao Zhiqiang *, Jia Fei, Zhang Xingguo, Hao Hai, Jin Junze Research Center of Foundry Engineering, Dalian Uni6ersity of Technology, Dalian 116024, China Received 24 January 2001; received in revised form 7 June 2001
Abstract Electromagnetic casting (EMC) technology depends on the electromagnetic force to prevent the metal from touching the mold, leaving the ingot surface very smooth. As a result, the surface imperfections, such as oscillation and subsurface segregation etc. caused by a mold of direct-chill casting (DCC), are avoided. Moreover, the electromagnetic stirring makes the structure become more homogeneous over the entire cross-section. In this paper, 2024 aluminum alloys made by EMC and DCC were analyzed by optical microscope and scanning electron microscope, solution heat treatment and artificial aging was given to investigate the mechanical properties such as hardness, wear resistance and fatigue. Results showed EMC ingots had fine and uniform grain structure, which made the EMC ingots have high hardness and good fatigue. The hardness of EMC specimens increased one time than DCC ones and the fatigue was three times as high as DCC ones in as-cast state. The EMC specimens also expressed an excellent characterization in wear resistance; the weight loss was only a half of DCC specimens. © 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Electromagnetic casting; Microstructure; Hardness; Wear resistance; Fatigue
1. Introduction Electromagnetic casting (EMC) is a technology developed by combining the magnetohydrodynamics (MHD) and the casting engineering. The casting method employs the electromagnetic forces to pinch upon the liquid metal through an alternating electromagnetic field, which is induced by an inductor [1]. The electromagnetic forces are produced by the interaction of eddy currents induced in the metal with the produced magnetic field of the inductor. The main advantage of the EMC technology consists in the presence of stirring motion in the melt, which leads to a significant reduction of the grain size in the solidified product. Moreover surface quality and subsurface quality are improved due to the absence of ingot mould [2]. The surface finish of the ingots is usually smooth enough to be hot rolled without scalping operation that is required for direct-chill casting (DCC) [3].
* Corresponding author. Fax: + 86-411-4671110. E-mail address: [email protected] (C. Zhiqiang).
Aside from refining internal structures, electromagnetic stirring also has advantages of homogenizing alloy elements, reducing porosity and segregation, and minimizing internal cracks [4]. Because of these distinct merits of EMC technology, many scientists and engineers in different countries have engaged on the electromagnetic shaping and MHD phenomena in EMC of aluminum [5]; nevertheless, the research about the microstructure and mechanical characteristics has been comparatively few. This paper is an attempt to remedy this deficiency. The microstructure characteristics such as the grain structure is of great importance, since many mechanical properties are directly related to the size, shape and distribution of the grains [6]. As a result, the 2024 aluminum alloys were cast by DCC and EMC to investigate their microstructure and properties. Its nominal composition is Al –4.3Cu –l.6Mg –0.7Mn. As a kind of widely applied hardening alloys with high strength, it is often used to fabricate the plane wrappings and aeroengine vanes working under high temperature.
0921-5093/02/$ - see front matter © 2002 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 6 7 3 - 2
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C. Zhiqiang et al. / Materials Science and Engineering A327 (2002) 133–137
2. Experimental procedure
2.1. Principle and apparatus of electromagnetic casting The experimental campaigns were carried out on a pilot-scale caster, in which trial-and-error shaping experiments were performed and the optimized technical parameters have been obtained. The basic apparatus of EMC consists of a delivery system, a casting control system, a shaping and a cooling system, a melting furnace and a power supply etc. as shown in Fig. 1. A medium-frequency alternating current was used to generate the alternating magnetic field in the molten aluminum [7]. This magnetic field generated a heavy eddy current on the surface of the molten aluminum in opposite phase to the imposed current of the electromagnetic coil. This resulted in a force directing towards the center of ingots. The electromagnetic force located within the liquid column of ingots prevented the metal from touching the mold. The technique procedures are as follows: melt metal; adjust the position of inductor, screen, water jacket and bottom block; operate cooling system; turn on power supply; cast on in a withdrawal speed until the end of casting. The manufacturing conditions are: inductor current, 4800 A; height of liquid column, 40 mm; solidification front, 10 mm up to bottom of inductor; pouring temperature, 710 730 °C; flow rate of cooling water, 3 m3 h − 1; casting speed, 0.3 1.5 mm s − 1. Besides the as-cast state, some 2024 specimens were solution-treated at 495 °C in salt bath furnace for 1 h and then quenched in ice water. After this, they were immediately aged in the 190 °C silicon oil bath for various holding time to obtain the hardening effect. The metallographic specimens were prepared by cutting, grinding, polishing and etching. The etchant was Keller’s solution. The specimens from DCC and EMC ingots were examined and contrasted by optical mi-
Fig. 1. Schematic diagram of EMC equipment: (1) Furnace, (2) Liquid metal, (3) Trough, (4) Box, (5) Screen, (6) Inductor, (7) Transformer, (8) Power supply, (9) Cooling water, (10) Ingot, (11) Bottom block, (12) Motor, (13) Speed control.
crostructure (OM) and scanning electron microstructure (SEM).
2.2. Mechanical properties 2.2.1. Hardness test Hardness tests were performed before and after heattreatment by the load of 60 kg with the Rockwell hardness tester of F type. Take an average of seven times after omitting the highest and the lowest values. 2.2.2. Wear resistance test The PLINT TE88 multi-station friction and wear test machine was utilized to investigate the wear resistance of both DCC and EMC specimens. Cylindrical pin samples were machined to length of 15 mm and a diameter of 8 mm. The disc of counterpart was made of AISI 1055 steel quenched, the load was 60 N, sliding distance 200 m, sliding speed 1 m s − 1. The weight loss of various aluminum alloys was obtained by weighing the pins before and after each run. 2.2.3. Fatigue test The castings were cut into slices for the fatigue specimens basing on the ASTM 466-96. Both as-cast and T6 specimens were tested using an Instron 8516 hydraulic fatigue test machine. All fatigue tests were performed in a laboratory environment at ambient temperature. Constant-amplitude, fully reversed loading was applied (Sine wave, R=|max/|min = −l) at a frequency of 20 Hz to all specimens. Tests were conducted at a nominal stresses of 100 MPa.
3. Results and discussion
3.1. Microstructure Fig. 2 showed the transverse sectional grain structures of the 2024 DCC and EMC specimens in as-cast state. It was clear that the grains of EMC were small and equiaxed, no matter at edge, midthickness or center of ingots. The intensifying phases S and q distributed along the a-dendrite boundary off and on. In DCC condition, the grain size became coarse from the surface to the center of ingots with long and thick intensifying phase located among them. The dendrites grew and coarsened obviously in the center of DCC ingots due to Ostwald ripening [8]. The conventional DCC process is known to generate a non-uniform heat transfer due to air-gap formation. Therefore, the sub-surface often exists local big grains and segregation due to the slow loss of superheat. However, for the EMC, the grain was more homogeneous over the entire cross section due to the strong electromagnetic stirring in the liquid pool. This intense
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135
Fig. 2. Microstructures of 2024 alloy ingots (Ø174 mm) at as-cast state. (a), (c), (e), (g), DCC, from surface to center; (b), (d), (f), (h), EMC, from surface to center.
forced convection promoted a fast superheat evacuation and breaked the dendrite arms, which led to the grain multiplication. The suspended nuclei localized in the vicinity of the liquid solid interface were carried away and dispersed in a slightly super-heated melt. As a result, the crystallization took place simultaneously in most of the sump and caused the appearance of a fine and uniform equiaxed structure over the entire crosssection.
Fig. 3. The EMC specimens reached a peak at around 12 h, and the DCC ones got the peak after 36 h. That meaned the EMC ingots had much fast response to solution heat treatment and artificial aging. After the T6 treatment, the reinforcing phase S transformed into a transition phase S%% and G.P. zone, q became a q%% and G.P. zone. At this time, a stress field caused by the strain near the transitional phase could impede the movement of the dislocation, thus could largely improve the hardness and strength of alloys.
3.2. Mechanical characterization 3.2.1. Hardness properties As shown in Table 1, the hardness of EMC specimens was two times as high as DCC ones at as-cast state, owing to the much finer grain structure. The age-hardening curves for the 2024 alloy were plotted in
Table 1 Hardnesses of 2024 aluminum alloys Different alloys
DCC
EMC
DCCT6
EMCT6
Hardness
16.02
31.72
20.64
37.02
C. Zhiqiang et al. / Materials Science and Engineering A327 (2002) 133–137
136
Table 3 Fatigues of 2024 aluminum alloys
Fig. 3. Artificial aging curves of 2024 alloys: DCC, Rockwell;
EMC, Aging time (h). Table 2 Weight loss of 2024 aluminum alloys Different alloys
DCC
EMC
DCCT6
EMCT6
Weight loss (mg)
3.0
1.6
2.0
1.0
3.2.2. Wear resistance Table 2 showed that the weight loss of DCC was higher than the EMC’s no matter whether or not they were treated by T6 condition, the heat treatment obviously increased their abrasive resistance. Fig. 4 showed the characteristics of wear debris stripping from the worn surface. The wear debris of DCC specimen exhibited very obvious large block like particles. For the EMC ones, the wear debris existed an
Different alloys
DCC
EMC
DCCT6
EMCT6
Cycle number
37484
94674
612853
735567
aggregate of small pieces, no example of sheet like large particles found in the wear debris. The wear resistance of EMC specimens was improved remarkably due to their very fine, uniform and dense dendrite cells which could endure the friction with the wear surface. After the T6 treatment, the wear resistance increased in some degree. This coincides with the age hardening phenomenon.
3.2.3. Fatigue life Table 3 indicated that the fatigue cycle number of EMC specimens was almost three times as high as that of DCC ones in as-cast state. It is well known, the fracture initiates from the rough surface, sub-surface porosity, micropore and oxide inclusions [9,10]. The EMC ingots have smooth surface, free of porosity and little subsurface segregation, so there is an excellent fatigue life. Fig. 5 showed the striated fracture of DCC specimens and the dimple rupture of EMC ones. The former indicated the cleavage-like mechanism and the latter showed the ductile characteristics.
Fig. 4. SEM micrographs of worn debris of 2024 alloys: (a) DCC as-cast, (b) EMC as-cast, (c) DCC T6, (d) EMC T6.
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137
Fig. 5. SEM fractographs of 2024 aluminum alloys: (a) DCC as-cast, (b) EMC as-cast, (c) DCC T6, (d) EMC T6.
4. Conclusions (1) The EMC ingots owned much fine grain size and homogeneous structure and much effective response to aging for 2024 EMC ingots compared to the DCC ones, so that it have better mechanical properties not only in as-cast condition but also in T6 one. (2) The fatigue life of EMC ingots was about three times as high as that of DCC one in as-cast state due to the fine equiaxed grains and absence of microporosity of EMC ingots. (3) EMC specimens expressed excellent characteristics in hardness and wear resistance; the weight loss was only a half of DCC ones.
Acknowledgements The authors acknowledge the supports of the Na-
tional Natural Science Foundation of China (59901001) & (59995442) and National Major Fundamental Research Project of China (G1998061500).
References [1] Z.N. Getselev, J. Metals 23 (10) (1971) 38 – 39. [2] M. Garnier, International Symposium on Electromagnetic Processing of Materials, Nagoya, ISIJ, Japan, 1994, p. 1. [3] D.C. Prasso, J.W. Evans, I.J. Wilson, Metall. Trans. B 26B (1995) 1243 – 1251. [4] B.Q. Li, JOM-e 50 (2) (1998) 1 – 10. [5] S. Asai, ISIJ Internatl. 29 (12) (1989) 981 – 992. [6] C. Vives, J. Crystal Growth 94 (1989) 739 – 750. [7] M. Furui, ISIJ Internatl. 33 (3) (1993) 400 – 404. [8] N. Bryson, F. Weiberg (Eds.), International Symposium on Solidification Processing, Hamilton, Ont., Canada, 1990, p. 69. [9] B. Zhang, D.R. Poirier, W. Chen, Metall. Trans. A 30A (1999) 2659 – 2665. [10] H. Jiang, P. Brown, J.F. Knott, J. Mater. Sci. 34 (1999) 719 – 725.