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Effects of chromium on the interface and bond strength of metal–ceramic joints
Materials Chemistry and Physics 75 (2002) 256–259
Effects of chromium on the interface and bond strength of metal–ceramic joints J.X. Zhang∗ , R.S. Chandel, H.P. Seow School of Materials Engineering, Nanyang Technological University, Singapore 639798, Singapore
Abstract In active brazing of alumina and stainless steel using copper-based filler, active elements such as chromium were introduced as additives. The effects of these additives on the microstructure of the interface and bond strength of the joints were investigated. The wetting characteristics of liquid Cu with Cr additives on the alumina substrate have also been studied by a sessile drop method, which indicates that the contact angle can be decreased even to less than 90◦ due to Cr additive. The segregation of Cr2 O3 at the interface between Cu and substrate was observed by SEM and EDS techniques. Through four point bending test, it is found that the additive of Cr in the filler materials can obviously improve the bend strength of the joints. These results demonstrate that the addition of Cr into liquid Cu can improve the wettability of the metal on alumina and facilitate the formation of a strong alumina–metal joint. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Chromium; Bond strength; Metal–ceramic joints
1. Introduction The joining of ceramics and metals has received much attention in recent years because of their compensative properties. Recently, a novel and promising method proposed for creating high temperature resistant interfaces between these two dissimilar materials is partial transient liquid phase (PTLP) method [1,2]. In this technique, multilayer metals composed of two lower melting and thin outer layers sandwiching a thick core of refractory metal are used. A film of liquid can be formed at the joining temperature either because the melting point of the outer layer has been exceeded or because a reaction between the core and the outer layer has produced a phase that is molten at that temperature. The liquid wets the solid surfaces and interdiffuses with the parent materials and core interlayer to transform into more refractory solid materials. Since the ionic or covalent structures of ceramics lack the delocalized bonding electrons present in metals and hence ceramic–metal interfaces are major electronic discontinuities, ceramics are difficult to be wetted by metals. Many technically important ceramics, including alumina, zirconia, are not wetted by silver, copper or gold, which are the basic constituents of most brazing alloys. Therefore, since non-wetting is associated with the basic chemistry characteristics of ceramic–metal ∗
Corresponding author. E-mail address: [email protected] (J.X. Zhang).
interfaces, it can be assumed that wetting might be promoted if the brazing alloy changes the surface chemistry properties of the ceramic. Some active elements such as titanium have been studied as active metal additions to improve the wettability of metal on ceramic, for example, Cu/Al2 O3 . However, the good wetting feature attained by Ti is accompanied by an undesirable intense reaction at the metal–ceramic interface such as the reduction of alumina and the formation of titanium oxides [3]. For certain applications, it is preferred to use additive elements which can improve wettability without serious chemical reaction. Some work about chromium has shown that this element increase the bonding strength of Al2 O3 without any significant chemical attack on the ceramic [4]. The mechanism and effects of improving the properties of metal–ceramic interface by chromium are still unclear. Therefore, in the present work, the effects of Cr additions on the wetting characteristic of liquid Cu on alumina were examined by sessile drop experiment. Stainless steel 304 and Al2 O3 were joined by PTLP methods and the bonding strength of the joints was also evaluated by four point bending test.
2. Experimental procedure Wetting experiments were carried out in an alumina tube furnace using the sessile drop technique. The tube is connected to a vacuum device consisting of a mechanical
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 0 7 2 - X
J.X. Zhang et al. / Materials Chemistry and Physics 75 (2002) 256–259
257
Fig. 1. The block/foil/block bonding assembly used in brazing.
pump and an oil diffusion pump, which can make a vacuum atmosphere of 10−7 mbar ␣-polycrystalline alumina substrates were cut to small pallets of 10.0 × 10.0 × 4.0 mm3 and polished. Before the experiment, the plates were cleaned in an ultrasonic bath with acetone, rinsed in distilled water and then annealed in air at 950 ◦ C to eliminate all work-hardening and any hydroxyl groups. High purity (purity >99.9%) powders of copper and chromium were ultrasonically mixed with ethanol to produce the Cu–Cr metal mixtures in the appropriate ratio. After dried, the metal mixtures with a total weight of about 300 mg were pressed into pellets of 10 mm diameter and the alloys were processed in situ by direct melting of copper and chromium on the substrate during the sessile drop experiments. For each experimental run, the temperature was increased at 4 ◦ C min−1 to 1150 ◦ C, and cooled at 5 ◦ C min−1 to room temperature after holding for 240 min at that temperature. PTLP brazing was also conducted in the vacuum furnace. In the bonding assemblies shown in Fig. 1, the sizes of stainless steel 304 and alumina are 20 × 12 × 4 mm3 respectively. Cu/Ni/Cu and Cu/80Ni20Cr/Cu were used as interlayers, where the thickness of Cu, Ni, 80Ni20Cr are 7, 125, 89 m, respectively. Just before bonding, the base materials were cleaned in an ultrasonic bath with isopropyl alcohol for 1 h. After drying in hot air, the block/foil/block assembly was prepared immediately, and bonding of the assembly was performed in an alumina fixture. The temperature was raised to brazing temperature at 4 ◦ C min−1 , maintained at the brazing temperature for 240 min, and then lowered to room temperature at 1 ◦ C min−1 . A range of temperatures and dwelling times were used in order to study their influence on the microstructure and bending strength of the joints. During this brazing cycle, the vacuum in the furnace was kept in a range of 8 × 10−6 to 2 × 10−5 mbar. After bonding, the joints were tested at room temperature using four point bending with an inner span of 10 mm, outer span of 30 mm and a displacement rate of 0.05 mm min−1 . Strengths were calculated from the load at failure using standard relationships derived for monolithic elastic materials. The samples consisted of the sessile drop specimen and metal–ceramic joints were sectioned for examination by optical microscopy, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) was also used to detect the new phases in the metal–ceramic interface.
Fig. 2. Change of contact angle of Cu on alumina substrate with the content of chromium.
3. Results and discussions 3.1. Sessile drop experiment Fig. 2 presents the relation between contact angle and chromium content of Cu–Cr alloy on Al2 O3 substrate after dwelling at 1150 ◦ C for 240 min. The contact angle of pure Cu on alumina was found to be around 125◦ , which is consistent with the results in literature [5,6,7] and indicates that Cu has poor wetting ability on alumina substrate. However, the contact angles formed by droplets of Cu with Cr additions decreased gradually with the content of chromium. Moreover, with the increasing of the content of additives, contact angles could be further reduced even by dwelling for the same time. 3.2. Bending test The four point bending test results of the ceramic-to-metal joints brazed at 1150 ◦ C are summarized in Fig. 3, where
Fig. 3. Four point bending test results of joints brazed at 1150 ◦ C for Ni and NiCr interlayer respectively.
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Fig. 5. Poor Al2 O3 /interlayer contact in the steel–Al2 O3 joints with Ni interlayer.
Fig. 4. Three different types of fracture in the bending tests of steel–ceramic joints.
the bending strengths reported are an average of three tests per processing condition. It has been shown that the bending strengths of metal–ceramic joints with chromium additions have been improved compared to those joints with no chromium additions. The Al2 O3 –Al2 O3 joints brazed at 1150 ◦ C also shows that the bonding strength including NiCr interlayer is 212 MPa, which is also higher than the bond strength with Ni interlayer of 151 MPa. Meanwhile, it was also found that the bond strength of Al2 O3 –Al2 O3 joints were much higher than that of the steel–Al2 O3 joints even with the same interlayer under the same brazing parameters. The fracture modes of the steel–Al2 O3 joints with NiCr interlayer also differs from those of the joints with Ni interlayer. Fig. 4 presents the three types of fracture modes existing in the ceramic-to-metal joints with Ni interlayer and NiCr interlayer. Type I is cracking along the interlayers between the two base materials, which suggests that this zone is relatively weak. In type II, the crack initiates in the edge of the interlayer and then changes its direction into the ceramic and propagates in the ceramic part near the interface. In type III, the crack exists only in the ceramic during initiating and propagating periods. Although failure happens along the ceramics in some ceramic-to-metal joints, their bond strengths cannot reach the same level of the ceramic itself. This phenomena can be explained by large mismatch in the coefficients of thermal expansion between Al2 O3 and stainless steel 304. Such thermal mismatch causes serious stress concentrations in the ceramic parts near the interfaces and leads to fracture of the ceramic parts under low external loads. Of course, there also exists considerable thermal residual stress in the interface of the ceramic-to-metal joint and this is the main reason for deterioration of the bond strength in the ceramic-to-metal joints. Bending tests show that the ceramic-to-ceramic bond strength is much higher than the ceramic-to-metal bond strength under the same brazing parameters, which can give support to the above explanation.
Through examining the bond fracture types, it can be found that the joints cracking along the interface have much weaker bending strength than the cracking in other types. After comparing the joints with Ni interlayers to those with Ni–Cr interlayers, it was found that the cracks in most of the Ni joints propagate along the interlayers between the ceramics and metals, while the cracks extend partly or completely along the ceramic parts in most of Ni–Cr joints. These fracture types observed from the joints with Ni or Ni–Cr interlayers correspond with the bending strength results shown in Fig. 3, where the joints with Ni–Cr interlayers generally have stronger bending strength than those joints with Ni interlayers. According to the bending tests, the bending strength of Al2 O3 was about 320 MPa, while the strength of steel– Al2 O3 joints bonded with Ni core interlayer were under 100 MPa. Through fractographic analysis, it could be concluded that low strengths in this system were associated with regions of poor Al2 O3 /interlayer or interlayer/interlayer contact, shown in Fig. 5. This suggested that the changes in interlayer chemistry could improve the wetting characteristics of the transient liquid layer and thus lead to the strength improvement. When Ni–Cr core layers were used, the average strengths of ceramic-to-metal joints were increased and failure seldom occurred along the alumina/interlayer interface, which suggests that Cr has “strengthened” the interface. As one active element, Cr additions from Ni–Cr alloys reduce the contact angle of Cu-rich liquids on Al2 O3 substrates and hence increase the driving force for liquid redistribution at bonding temperature. In order to identify the reaction phase in the metal–ceramic interface, the specimens achieved by sessile drop tests were dipped in HNO3 to expose the interface structure. XRD was performed on the interface area to identify the phases formed during wetting process. Fig. 6 is the XRD diffraction pattern of the sample with different content of chromium and all of them indicate the evidence of Cr2 O3 in the metal/ceramic interface. As Matsumoto et al. [8] reported, Cr does not react with Al2 O3 . Accordingly, it can be assumed that atmosphere is the source of oxygen for the creation of Cr2 O3 , which has a positive effects on the joints.
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conclusions were drawn: • Chromium addition can reduce the contact angle between copper and alumina and the extent of reduction depends on the content of chromium. • Four point bending tests show that the joints with Cr additive have stronger bonding strength than those joints with non-chromium additive. • It has been found that Cr2 O3 species formed at copper– alumina interface and such species obviously contribute to the improvement of wettability of copper on alumina.
Fig. 6. XRD patterns on the interface of Cu–Cr alloy and Al2 O3 . Symbol indicates the peaks of Cr2 O3 .
4. Conclusions The wetting characteristics of copper with chromium additives on alumina has been studied by sessile drop experiment. Stainless steel 304 and alumina were brazed by PTLP bonding method with NiCr core layers and Ni core layers to investigate the effects and mechanism of chromium on the metal–ceramic reactive brazing. The following
References [1] [2] [3] [4] [5] [6] [7] [8]
Y. Iino, J. Mater. Sci. Lett. 10 (1991) 104. J. Mei, P. Xiao, Script. Mater. 40 (1999) 587. R. Standing, M. Nicholas, J. Mater. Sci. 13 (1978) 1509. G. Ceccone, M.G. Nicholas, S.D. Peteves, A.P. Tomsia, B.J. Dalgleish, A.M. Glaeser, Acta Mater. 44 (1996) 657. Y. Naidich, Prog. Surf. Member. Sci. 14 (1981) 353. J. Li, L. Coudurier, I. Ansara, N. Eustathopoulos, Ann. Chim. Fr. 13 (1988) 145. P. Kritsala, L. Courdurier, N. Eustathopoulos, J. Mater. Sci. Lett. 9 (1990) 1332. H. Matsumoto, M.R. Locatelli, K. Nakashima, A.M. Glaeser, K. Mori, Mater. Trans., JIM V36 (1995) 555.
Effects of chromium on the interface and bond strength of metal–ceramic joints J.X. Zhang∗ , R.S. Chandel, H.P. Seow School of Materials Engineering, Nanyang Technological University, Singapore 639798, Singapore
Abstract In active brazing of alumina and stainless steel using copper-based filler, active elements such as chromium were introduced as additives. The effects of these additives on the microstructure of the interface and bond strength of the joints were investigated. The wetting characteristics of liquid Cu with Cr additives on the alumina substrate have also been studied by a sessile drop method, which indicates that the contact angle can be decreased even to less than 90◦ due to Cr additive. The segregation of Cr2 O3 at the interface between Cu and substrate was observed by SEM and EDS techniques. Through four point bending test, it is found that the additive of Cr in the filler materials can obviously improve the bend strength of the joints. These results demonstrate that the addition of Cr into liquid Cu can improve the wettability of the metal on alumina and facilitate the formation of a strong alumina–metal joint. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Chromium; Bond strength; Metal–ceramic joints
1. Introduction The joining of ceramics and metals has received much attention in recent years because of their compensative properties. Recently, a novel and promising method proposed for creating high temperature resistant interfaces between these two dissimilar materials is partial transient liquid phase (PTLP) method [1,2]. In this technique, multilayer metals composed of two lower melting and thin outer layers sandwiching a thick core of refractory metal are used. A film of liquid can be formed at the joining temperature either because the melting point of the outer layer has been exceeded or because a reaction between the core and the outer layer has produced a phase that is molten at that temperature. The liquid wets the solid surfaces and interdiffuses with the parent materials and core interlayer to transform into more refractory solid materials. Since the ionic or covalent structures of ceramics lack the delocalized bonding electrons present in metals and hence ceramic–metal interfaces are major electronic discontinuities, ceramics are difficult to be wetted by metals. Many technically important ceramics, including alumina, zirconia, are not wetted by silver, copper or gold, which are the basic constituents of most brazing alloys. Therefore, since non-wetting is associated with the basic chemistry characteristics of ceramic–metal ∗
Corresponding author. E-mail address: [email protected] (J.X. Zhang).
interfaces, it can be assumed that wetting might be promoted if the brazing alloy changes the surface chemistry properties of the ceramic. Some active elements such as titanium have been studied as active metal additions to improve the wettability of metal on ceramic, for example, Cu/Al2 O3 . However, the good wetting feature attained by Ti is accompanied by an undesirable intense reaction at the metal–ceramic interface such as the reduction of alumina and the formation of titanium oxides [3]. For certain applications, it is preferred to use additive elements which can improve wettability without serious chemical reaction. Some work about chromium has shown that this element increase the bonding strength of Al2 O3 without any significant chemical attack on the ceramic [4]. The mechanism and effects of improving the properties of metal–ceramic interface by chromium are still unclear. Therefore, in the present work, the effects of Cr additions on the wetting characteristic of liquid Cu on alumina were examined by sessile drop experiment. Stainless steel 304 and Al2 O3 were joined by PTLP methods and the bonding strength of the joints was also evaluated by four point bending test.
2. Experimental procedure Wetting experiments were carried out in an alumina tube furnace using the sessile drop technique. The tube is connected to a vacuum device consisting of a mechanical
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 0 7 2 - X
J.X. Zhang et al. / Materials Chemistry and Physics 75 (2002) 256–259
257
Fig. 1. The block/foil/block bonding assembly used in brazing.
pump and an oil diffusion pump, which can make a vacuum atmosphere of 10−7 mbar ␣-polycrystalline alumina substrates were cut to small pallets of 10.0 × 10.0 × 4.0 mm3 and polished. Before the experiment, the plates were cleaned in an ultrasonic bath with acetone, rinsed in distilled water and then annealed in air at 950 ◦ C to eliminate all work-hardening and any hydroxyl groups. High purity (purity >99.9%) powders of copper and chromium were ultrasonically mixed with ethanol to produce the Cu–Cr metal mixtures in the appropriate ratio. After dried, the metal mixtures with a total weight of about 300 mg were pressed into pellets of 10 mm diameter and the alloys were processed in situ by direct melting of copper and chromium on the substrate during the sessile drop experiments. For each experimental run, the temperature was increased at 4 ◦ C min−1 to 1150 ◦ C, and cooled at 5 ◦ C min−1 to room temperature after holding for 240 min at that temperature. PTLP brazing was also conducted in the vacuum furnace. In the bonding assemblies shown in Fig. 1, the sizes of stainless steel 304 and alumina are 20 × 12 × 4 mm3 respectively. Cu/Ni/Cu and Cu/80Ni20Cr/Cu were used as interlayers, where the thickness of Cu, Ni, 80Ni20Cr are 7, 125, 89 m, respectively. Just before bonding, the base materials were cleaned in an ultrasonic bath with isopropyl alcohol for 1 h. After drying in hot air, the block/foil/block assembly was prepared immediately, and bonding of the assembly was performed in an alumina fixture. The temperature was raised to brazing temperature at 4 ◦ C min−1 , maintained at the brazing temperature for 240 min, and then lowered to room temperature at 1 ◦ C min−1 . A range of temperatures and dwelling times were used in order to study their influence on the microstructure and bending strength of the joints. During this brazing cycle, the vacuum in the furnace was kept in a range of 8 × 10−6 to 2 × 10−5 mbar. After bonding, the joints were tested at room temperature using four point bending with an inner span of 10 mm, outer span of 30 mm and a displacement rate of 0.05 mm min−1 . Strengths were calculated from the load at failure using standard relationships derived for monolithic elastic materials. The samples consisted of the sessile drop specimen and metal–ceramic joints were sectioned for examination by optical microscopy, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) was also used to detect the new phases in the metal–ceramic interface.
Fig. 2. Change of contact angle of Cu on alumina substrate with the content of chromium.
3. Results and discussions 3.1. Sessile drop experiment Fig. 2 presents the relation between contact angle and chromium content of Cu–Cr alloy on Al2 O3 substrate after dwelling at 1150 ◦ C for 240 min. The contact angle of pure Cu on alumina was found to be around 125◦ , which is consistent with the results in literature [5,6,7] and indicates that Cu has poor wetting ability on alumina substrate. However, the contact angles formed by droplets of Cu with Cr additions decreased gradually with the content of chromium. Moreover, with the increasing of the content of additives, contact angles could be further reduced even by dwelling for the same time. 3.2. Bending test The four point bending test results of the ceramic-to-metal joints brazed at 1150 ◦ C are summarized in Fig. 3, where
Fig. 3. Four point bending test results of joints brazed at 1150 ◦ C for Ni and NiCr interlayer respectively.
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Fig. 5. Poor Al2 O3 /interlayer contact in the steel–Al2 O3 joints with Ni interlayer.
Fig. 4. Three different types of fracture in the bending tests of steel–ceramic joints.
the bending strengths reported are an average of three tests per processing condition. It has been shown that the bending strengths of metal–ceramic joints with chromium additions have been improved compared to those joints with no chromium additions. The Al2 O3 –Al2 O3 joints brazed at 1150 ◦ C also shows that the bonding strength including NiCr interlayer is 212 MPa, which is also higher than the bond strength with Ni interlayer of 151 MPa. Meanwhile, it was also found that the bond strength of Al2 O3 –Al2 O3 joints were much higher than that of the steel–Al2 O3 joints even with the same interlayer under the same brazing parameters. The fracture modes of the steel–Al2 O3 joints with NiCr interlayer also differs from those of the joints with Ni interlayer. Fig. 4 presents the three types of fracture modes existing in the ceramic-to-metal joints with Ni interlayer and NiCr interlayer. Type I is cracking along the interlayers between the two base materials, which suggests that this zone is relatively weak. In type II, the crack initiates in the edge of the interlayer and then changes its direction into the ceramic and propagates in the ceramic part near the interface. In type III, the crack exists only in the ceramic during initiating and propagating periods. Although failure happens along the ceramics in some ceramic-to-metal joints, their bond strengths cannot reach the same level of the ceramic itself. This phenomena can be explained by large mismatch in the coefficients of thermal expansion between Al2 O3 and stainless steel 304. Such thermal mismatch causes serious stress concentrations in the ceramic parts near the interfaces and leads to fracture of the ceramic parts under low external loads. Of course, there also exists considerable thermal residual stress in the interface of the ceramic-to-metal joint and this is the main reason for deterioration of the bond strength in the ceramic-to-metal joints. Bending tests show that the ceramic-to-ceramic bond strength is much higher than the ceramic-to-metal bond strength under the same brazing parameters, which can give support to the above explanation.
Through examining the bond fracture types, it can be found that the joints cracking along the interface have much weaker bending strength than the cracking in other types. After comparing the joints with Ni interlayers to those with Ni–Cr interlayers, it was found that the cracks in most of the Ni joints propagate along the interlayers between the ceramics and metals, while the cracks extend partly or completely along the ceramic parts in most of Ni–Cr joints. These fracture types observed from the joints with Ni or Ni–Cr interlayers correspond with the bending strength results shown in Fig. 3, where the joints with Ni–Cr interlayers generally have stronger bending strength than those joints with Ni interlayers. According to the bending tests, the bending strength of Al2 O3 was about 320 MPa, while the strength of steel– Al2 O3 joints bonded with Ni core interlayer were under 100 MPa. Through fractographic analysis, it could be concluded that low strengths in this system were associated with regions of poor Al2 O3 /interlayer or interlayer/interlayer contact, shown in Fig. 5. This suggested that the changes in interlayer chemistry could improve the wetting characteristics of the transient liquid layer and thus lead to the strength improvement. When Ni–Cr core layers were used, the average strengths of ceramic-to-metal joints were increased and failure seldom occurred along the alumina/interlayer interface, which suggests that Cr has “strengthened” the interface. As one active element, Cr additions from Ni–Cr alloys reduce the contact angle of Cu-rich liquids on Al2 O3 substrates and hence increase the driving force for liquid redistribution at bonding temperature. In order to identify the reaction phase in the metal–ceramic interface, the specimens achieved by sessile drop tests were dipped in HNO3 to expose the interface structure. XRD was performed on the interface area to identify the phases formed during wetting process. Fig. 6 is the XRD diffraction pattern of the sample with different content of chromium and all of them indicate the evidence of Cr2 O3 in the metal/ceramic interface. As Matsumoto et al. [8] reported, Cr does not react with Al2 O3 . Accordingly, it can be assumed that atmosphere is the source of oxygen for the creation of Cr2 O3 , which has a positive effects on the joints.
J.X. Zhang et al. / Materials Chemistry and Physics 75 (2002) 256–259
259
conclusions were drawn: • Chromium addition can reduce the contact angle between copper and alumina and the extent of reduction depends on the content of chromium. • Four point bending tests show that the joints with Cr additive have stronger bonding strength than those joints with non-chromium additive. • It has been found that Cr2 O3 species formed at copper– alumina interface and such species obviously contribute to the improvement of wettability of copper on alumina.
Fig. 6. XRD patterns on the interface of Cu–Cr alloy and Al2 O3 . Symbol indicates the peaks of Cr2 O3 .
4. Conclusions The wetting characteristics of copper with chromium additives on alumina has been studied by sessile drop experiment. Stainless steel 304 and alumina were brazed by PTLP bonding method with NiCr core layers and Ni core layers to investigate the effects and mechanism of chromium on the metal–ceramic reactive brazing. The following
References [1] [2] [3] [4] [5] [6] [7] [8]
Y. Iino, J. Mater. Sci. Lett. 10 (1991) 104. J. Mei, P. Xiao, Script. Mater. 40 (1999) 587. R. Standing, M. Nicholas, J. Mater. Sci. 13 (1978) 1509. G. Ceccone, M.G. Nicholas, S.D. Peteves, A.P. Tomsia, B.J. Dalgleish, A.M. Glaeser, Acta Mater. 44 (1996) 657. Y. Naidich, Prog. Surf. Member. Sci. 14 (1981) 353. J. Li, L. Coudurier, I. Ansara, N. Eustathopoulos, Ann. Chim. Fr. 13 (1988) 145. P. Kritsala, L. Courdurier, N. Eustathopoulos, J. Mater. Sci. Lett. 9 (1990) 1332. H. Matsumoto, M.R. Locatelli, K. Nakashima, A.M. Glaeser, K. Mori, Mater. Trans., JIM V36 (1995) 555.