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Cr(Mo) eutectic alloys
Intermetallics 14 (2006) 1326e1331 www.elsevier.com/locate/intermet
The effect of Ti-addition on plastic deformation and fracture behavior of directionally solidified NiAl/Cr(Mo) eutectic alloys K. Hagihara, Y. Sugino, Y. Umakoshi* Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan Received 2 September 2005; received in revised form 2 November 2005; accepted 28 November 2005 Available online 24 March 2006
Abstract To improve the high-temperature strength of NiAl/Cr(Mo) eutectic alloys, the effect of Ti-addition on microstructure and mechanical properties was examined. Three directionally solidified (DS) alloys with the composition of Nie(33 x)Ale31Cre3MoexTi (x ¼ 0, 3 and 5 at.%, respectively), denoted 0Ti-, 3Ti- and 5Ti-alloys hereafter, were prepared. Temperature dependence of the yield stress and the room temperature fracture toughness of these DS alloys was examined. The aligned lamellae with B2-NiAl and A2-Cr(Mo) were formed in 0Ti-alloy, but the formation of lamellar structure was hindered by the Ti-addition. Cellular microstructures containing short plate shapes of Cr(Mo) phases were obtained in 3Ti- and 5Ti-alloys. In 5Ti-alloy, the precipitation of the L21-Ni2AlTi was confirmed in NiAl matrix phase after the DS treatment. The Ti-addition induced a significant increase in high-temperature strength accompanied by a large deterioration of room temperature fracture toughness. The fracture toughness of 5Ti-alloy showed the low value of about 4 MPa m1/2 because of the disturbance of microstructure. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: A. Multiphase intermetallics; A. Nickel aluminides, based on NiAl; B. Fracture toughness; B. Mechanical properties at high temperatures
1. Introduction B2 intermetallic NiAl exhibits several good physical and thermal properties such as low density, high melting point, high thermal conductivity and excellent oxidation resistance at high temperatures [1]. NiAl is therefore considered a promising candidate material for replacing commercial Ni-based superalloys. However, poor fracture toughness at ambient temperature and low high-temperature strength restrict its use. To overcome the low fracture toughness (4e6 MPa m1/2 in polycrystalline NiAl [1]), many studies on incorporation of bcc phases such as Cr [2], Mo [3] and V [3] with NiAl have been reported. Since they show the eutectic reaction with NiAl, the directional solidification process has been especially focused on control of microstructure, where the longitudinal axes of the eutectic phases align themselves parallel to the growth direction.
* Corresponding author. Tel.: þ81 6 6879 7494; fax: þ81 6 6879 7495. E-mail address: [email protected] (Y. Umakoshi). 0966-9795/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2005.11.029
Among them, Raj et al. succeeded in superior improvement of fracture toughness to be about 12e17 MPa m1/2 in NiAl/ Cr(Mo) two-phase alloys [4]. However, greater improvement of high-temperature strength is still necessary. To improve the high-temperature strength, strengthening methods based on solution hardening and precipitation hardening mechanisms have been considered. Especially, the addition of some kinds of elements such as Ti, Ta, Zr and Hf induces an additional degree of order resulting in precipitation of Ni2AlXtype L21 Heusler phase. Many reports have shown that the formation of L21-Ni2AlTi phase greatly improves the hightemperature strength including the creep strength of NiAl [5]. Therefore, the addition of element which stabilizes the L21 structure in NiAl/Cr(Mo) is expected to obtain a good balance of high-temperature strength and room temperature toughness. The approach has been previously conducted by the addition of small amount of Zr [6] and Hf [7]. In this study, the addition of Ti, which has a wide solubility region in NiAl, was attempted into NiAl/Cr(Mo) eutectic alloys. Three directionally solidified (DS) alloys with different
K. Hagihara et al. / Intermetallics 14 (2006) 1326e1331
Ti contents were prepared, and the effect of the Ti-addition on microstructure, temperature dependence of yield stress and room temperature fracture toughness were examined.
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specimens used was 1 2 mm2 10 mm, and a notch 0.4 mm in depth was introduced by EDM. The loading direction (crack propagation direction) was selected to be perpendicular to the growth direction.
2. Experimental procedure 3. Results and discussion According to Raj et al. [4], the B2-NiAl/A2-Cr(Mo) twophase eutectic microstructure was obtained in alloy with composition of Nie33Ale31Cre3Mo (at.%). Three mother alloys of Nie(33 x)Ale31Cre3MoexTi (x ¼ 0, 3 and 5 at.%), containing different amounts of Ti were prepared by arc-melting a charge of appropriate weights of elemental Ni, Al, Cr, Mo and Ti under an Ar atmosphere. We denote the alloys hereafter as 0Ti-, 3Ti- and 5Ti-alloys. DS treatment was conducted for the alloys through a Bridgeman process in the high purity alumina crucible to control the microstructure. The growth rate of DS crystals was set to be 5 mm/h. For 0Ti- and 5Ti-alloys, DS rods at a growth rate of 100 mm/h were also prepared, in order to examine the effect of growth rate on microstructures and mechanical properties. The chemical composition of constituent phases was analyzed by the SEMeEDS and TEMe EDS method. Rectangular specimens with dimension of 2 2 mm2 5 mm were cut by an electro-discharge machine (EDM) and compression tests were conducted at a nominal strain rate of 1.7 104 s1. The loading direction was selected to be parallel to the growth direction. The tests were done at room temperature and between 800 and 1200 C in a vacuum in order to examine the temperature dependence of yield stress. Three-point bending tests were performed to evaluate the fracture toughness at room temperature. The size of
Fig. 1 shows the typical microstructures of three DS alloys grown at 5 mm/h, observed on the transverse and longitudinal sections. All alloys were macroscopically constructed by B2-NiAl and A2(bcc)-Cr(Mo) phases. Table 1 shows the chemical compositions of the constituent phases in the alloys analyzed by SEM(TEM)eEDS method. In Ti-bearing alloys, the Ti content in A2 phase was very low at about 1% and most Ti were distributed in the B2 phase. This induced the precipitation of L21-Ni2AlTi in 5Ti-alloy, as described later. Ti-addition strongly affects the microstructures in the alloys. As reported previously [4], the aligned fine lamellar structure with a thickness of about 4 mm was developed almost parallel to the growth direction in 0Ti-alloy (Fig. 1(a) and (b)). However, the addition of Ti disturbed the formation of lamellar microstructure. Fig. 1(c) and (d) shows the microstructure in 3Ti-alloy. The length of plate-like Cr phase in NiAl matrix became short and the morphology changed from a lamellar microstructure to a cellular eutectic microstructure. As seen in Fig. 1(d), the cell structure is developed along the growth direction. In the cells, however, the shortened Cr plates are poorly aligned and they were curled out toward the cell boundary. The increase in Ti-content further
Fig. 1. Typical microstructures of three DS alloys grown at 5 mm/h, observed on the (a,c,e) transverse and (b,d,f) longitudinal sections. (a,b) 0Ti- (c,d) 3Ti- and (e,f) 5Ti-alloys, respectively.
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Table 1 Chemical compositions of constituent phases in the three DS alloys analyzed by the SEM(TEM)eEDS method Alloy
Phase
Chemical composition (at.%) Ni
Al
Cr
Mo
Ti
Nie33Ale31Cre3Mo (0Ti-alloy)
A2 B2
4.84 50.71
5.34 44.81
83.07 4.19
6.75 0.29
Nie30Ale31Cre 3Moe3Ti (3Ti-alloy)
A2 B2
3.67 49.77
5.43 41.87
81.83 4.19
8.18 0.35
0.89 3.82
Nie28Ale31Cre3Moe A2 (SEM) 3.21 5Ti (5Ti-alloy) B2 (SEM) 50.02 B2 (TEM) 45.76 L21 (TEM) 47.00
6.36 39.70 45.42 30.78
83.15 4.00 4.05 3.20
6.39 0.14 0.22 0.09
0.89 6.14 4.55 18.93
induced the shortening of Cr-phase plates and the eutectic microstructure containing nearly rod-like Cr phases was developed in 5Ti-alloy. The effect of Ti-addition on the change in microstructure was more remarkably observed in specimens grown at higher growth rate. Fig. 2 shows the longitudinal microstructure of 0Ti- and 5Ti-alloys grown at 100 mm/h. In 0Ti-alloy, although some NiAl phases dendritically precipitated, the lamellar microstructure still exists between them. The lamellar spacing in them was about 1 mm, which was much smaller than that at 5 mm/h. In contrast, in 5Ti-alloy, the development of
microstructure along the growth direction was very weak. Cr-precipitates were observed to be homogeneously distributed in NiAl matrix. The results clearly suggest that the Ti-addition strongly hinders the development of aligned lamellar structure in NiAl/Cr(Mo) alloys. Fig. 3 shows TEM images of NiAl phase in the three DS alloys grown at 5 mm/h. Precipitates with spherical shape are seen in all the alloys. They were identified to be the Cr(Mo) phases by EDS analysis, which may be precipitated from the over-saturated NiAl matrix during the DS process. Although no other precipitates were confirmed in 0Ti- and 3Ti-alloys, many plate-like precipitates were also observed in 5Ti-alloy. They were confirmed to be the L21-Ni2AlTi phase; in the diffraction pattern analysis the 1/2[111] extra spots were clearly seen when viewed along ½110 direction in the NiAl matrix phase. Fig. 3(c) shows the bright field image viewed along [111] direction. On observation from this direction, the L21-Ni2AlTi plates align parallel to three h110i direction. The result indicates that the Ni2AlTi phases precipitate on {100} in NiAl with the cube-on-cube crystal orientation relationship, which is in accordance with the previous report in NiAl/Ni2AlTi two-phase alloy [8]. The thermal stability and growth kinetics of Ni2AlTi in this 5Ti-alloy are not yet clarified. According to the phase diagram reported by Ishikawa et al. [9], Cr is known to reduce the stability of Ni2AlTi phase.
Fig. 2. Microstructures on the longitudinal section in directionally solidified (a,b) 0Ti- and (c,d) 5Ti-alloys grown at 100 mm/h, (b) and (d) are higher magnification images in (a) and (c), respectively.
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Fig. 3. Bright field TEM images of NiAl phases in the three DS alloys grown at 5 mm/h. (a) 0Ti-alloy, (b) 3Ti-alloy and (c) 5Ti-alloy.
Table 2 shows the yield stress of alloys examined by compression tests at room temperature. The yield stress intensely increased with increase in the amount of Ti. This increase in yield stress was accompanied by a decrease in fracture strain, however. Approximately 5% of compressive plastic strain was obtained in 0Ti-alloy, but decreased to a few percent in Ti-bearing alloys. Concerning the effect of growth rate on the yield stress, the yield stress tends to show higher value in alloys grown at 100 mm/h than that grown at 5 mm/h. This may be due to the finer microstructure in alloys grown at 100 mm/h. A similar tendency was also reported in the previous paper [10]. Fig. 4 shows the temperature dependence of yield stress at high temperatures above 800 C. The Ti-bearing alloy shows higher yield stress at 800 C as well as at room temperature, while the temperature dependence is different depending on the alloy. The difference in yield stress between 3Ti-alloy and 0Ti-alloy decreased rapidly with increasing temperature above 800 C, and showed almost the same value above 900 C. Since no specific precipitation was observed in 3Ti-alloy, the increase in yield stress of 3Ti-alloy must be caused by solution hardening. The result indicates that the effect of solution hardening by Ti-addition is remarkably moderated above 900 C. In contrast, the yield stress of 5Ti-alloys maintains much higher value than others up to 1200 C, demonstrating the effectiveness of Ni2AlTi phase in improving the high-temperature strength. Fig. 5 shows the room temperature fracture toughness of alloys examined by three-point bending tests. As previously reported [2e4], the incorporation of bcc-Cr phase into the NiAl phase was very effective to improve the low fracture
toughness of NiAl. This was also confirmed in our results. The fracture toughness of 0Ti-alloy shows relatively high value of over 8 MPa m1/2 when the stress is loaded perpendicular to the growth direction. This value is higher than that of NiAl polycrystals (about 4e6 MPa m1/2 [1]). The toughness of 0Ti-alloy specimens grown at 100 mm/h also shows a high value, although it contains large scatters because of the inhomogeneous microstructure containing the dendritic NiAl phases. However, the Ti-addition causes a sharp decrease in fracture toughness. Even in the 3Ti-alloy specimens, the fracture toughness rapidly decreases to about 4 MPa m1/2. This value is almost the same as that of 0Tialloy when loaded parallel to the growth direction, in which the crack propagates along the two-phase lamellar interfaces. One explanation for the deterioration of fracture toughness is the decrease in toughness of constituent NiAl and Cr phases themselves by Ti-addition. The fracture toughness of Ni2AlTi may be much lower than that of NiAl because of its lower crystal symmetry. However, we assume that the main reason is related to the disturbance of microstructure in Ti-bearing
Table 2 Yield stress of alloys examined by compression tests at room temperature Alloy
0Ti (5 mm/h)
0Ti (100 mm/h)
3Ti (5 mm/h)
5Ti (5 mm/h)
5Ti (100 mm/h)
Yield stress (MPa)
619
699
1091
1269
1479
Fig. 4. Temperature dependence of yield stress of DS alloys between 800 and 1200 C. The loading direction is parallel to the growth direction.
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propagates almost straight and parallel to the loading direction. The cracks showed the tendency to propagate along the two-phase interface. However, the crack deflection did not occur macroscopically since the Cr plates were not well aligned along the growth direction and their length was not enough. Therefore, the toughening mechanism by Cr phase did not perform effectively and hence the fracture toughness showed low values comparable to NiAl polycrystals. The result indicates that another strategy or treatment to control the microstructure including the structure of twophase interface is strongly required to improve the fracture toughness of Ti-bearing alloys.
4. Conclusion Fig. 5. Room temperature fracture toughness of alloys examined by three-point bending tests.
alloys, in which the toughening mechanism induced by the incorporation of Cr phase does not act effectively. Fig. 6 shows the side surface of fractured specimens after threepoint bending tests. The features of crack propagation are different in each alloy. In 0Ti-alloy the main crack was frequently deflected from the loading direction and propagated in a zigzag manner. Since the NiAl/Cr(Mo) two-phase interfaces act as dislocation source, the significant plasticity was obtained in NiAl phases in the two-phase composite alloy [11]. This may lead to the increase in intrinsic toughening. Moreover the suppression of rapid propagation of cracks induces the frequent occurrence of debonding of the lamellar interface, called delamination, as shown in the high magnification image in Fig. 6(b). This induces the deflection of crack perpendicular to the loading direction, resulting in effective increase in fracture toughness. On the other hand, a main crack initiated from the notch in Ti-bearing alloys
(1) Ti-addition to the NiAl/Cr(Mo) eutectic alloy disturbs the formation of lamellar microstructure. Cellular microstructures containing the short plates shape of the Cr(Mo) phase were obtained in Ti-bearing alloys. In 5Ti-alloy, L21Ni2AlTi phase precipitated in NiAl matrix phase after the DS treatment. (2) The Ti-addition induces an increase of yield stress of the alloys. Especially, high-temperature strength shows significantly higher value in 5Ti-alloy. However, it accompanied the large deterioration of room temperature fracture toughness because of the disturbance of microstructure.
Acknowledgements This work was supported by ‘‘Priority Assistance for the Formation of Worldwide Renowned Centers of Research e The 21st Century COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design)’’ from the Japanese Ministry of Education, Culture, Sports,
Fig. 6. The side surface of fractured specimens after the three-point bending tests. (a,b) 0Ti-alloy, (c,d) 3Ti-alloy and (e,f) 5Ti-alloy, (b), (d) and (f) are higher magnification images in (a), (c) and (e), respectively.
K. Hagihara et al. / Intermetallics 14 (2006) 1326e1331
Science and Technology. A part of this work was carried out at the Strategic Research Base ‘‘Handai Frontier Research Center’’ supported by the Japanese Government’s Special Coordination Fund for Promoting Science and Technology. References [1] Miracle DB. Acta Metall Mater 1993;41:649. [2] Johnson DR, Chen XF, Oliver BF, Noebe RD, Whittenberger JD. Intermetallics 1995;3:99.
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[3] Joslin SM, Chen XF, Oliver BF, Noebe RD. Mater Sci Eng A 1995;196:9. [4] Raj SV, Locci IE, Salem JA, Pawlik RJ. Metall Mater Trans A 2002;33A:597. [5] Strutt PR, Polvani RS, Ingram JC. Metall Trans A 1976;7:23. [6] Qi YH, Guo JT, Cui CY. Mater Sci Technol 2003;19:399. [7] Cui CY, Guo JT, Qi YH, Ye HQ. Intermetallics 2002;10:1001. [8] Oh-ishi K, Horita Z, Nemoto M. Mater Trans JIM 1997;2:99. [9] Ishikawa K, Ohnuma I, Kainuma R, Aoki K, Ishida K. J Alloys Compd 2004;367:2. [10] Guo JT, Xu CM, Du XH, Fu HZ. Mater Lett 2004;58:3233. [11] Misra A, Wu ZL, Kush MT, Gibala R. Philos Mag A 1998;78:533.
The effect of Ti-addition on plastic deformation and fracture behavior of directionally solidified NiAl/Cr(Mo) eutectic alloys K. Hagihara, Y. Sugino, Y. Umakoshi* Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan Received 2 September 2005; received in revised form 2 November 2005; accepted 28 November 2005 Available online 24 March 2006
Abstract To improve the high-temperature strength of NiAl/Cr(Mo) eutectic alloys, the effect of Ti-addition on microstructure and mechanical properties was examined. Three directionally solidified (DS) alloys with the composition of Nie(33 x)Ale31Cre3MoexTi (x ¼ 0, 3 and 5 at.%, respectively), denoted 0Ti-, 3Ti- and 5Ti-alloys hereafter, were prepared. Temperature dependence of the yield stress and the room temperature fracture toughness of these DS alloys was examined. The aligned lamellae with B2-NiAl and A2-Cr(Mo) were formed in 0Ti-alloy, but the formation of lamellar structure was hindered by the Ti-addition. Cellular microstructures containing short plate shapes of Cr(Mo) phases were obtained in 3Ti- and 5Ti-alloys. In 5Ti-alloy, the precipitation of the L21-Ni2AlTi was confirmed in NiAl matrix phase after the DS treatment. The Ti-addition induced a significant increase in high-temperature strength accompanied by a large deterioration of room temperature fracture toughness. The fracture toughness of 5Ti-alloy showed the low value of about 4 MPa m1/2 because of the disturbance of microstructure. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: A. Multiphase intermetallics; A. Nickel aluminides, based on NiAl; B. Fracture toughness; B. Mechanical properties at high temperatures
1. Introduction B2 intermetallic NiAl exhibits several good physical and thermal properties such as low density, high melting point, high thermal conductivity and excellent oxidation resistance at high temperatures [1]. NiAl is therefore considered a promising candidate material for replacing commercial Ni-based superalloys. However, poor fracture toughness at ambient temperature and low high-temperature strength restrict its use. To overcome the low fracture toughness (4e6 MPa m1/2 in polycrystalline NiAl [1]), many studies on incorporation of bcc phases such as Cr [2], Mo [3] and V [3] with NiAl have been reported. Since they show the eutectic reaction with NiAl, the directional solidification process has been especially focused on control of microstructure, where the longitudinal axes of the eutectic phases align themselves parallel to the growth direction.
* Corresponding author. Tel.: þ81 6 6879 7494; fax: þ81 6 6879 7495. E-mail address: [email protected] (Y. Umakoshi). 0966-9795/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2005.11.029
Among them, Raj et al. succeeded in superior improvement of fracture toughness to be about 12e17 MPa m1/2 in NiAl/ Cr(Mo) two-phase alloys [4]. However, greater improvement of high-temperature strength is still necessary. To improve the high-temperature strength, strengthening methods based on solution hardening and precipitation hardening mechanisms have been considered. Especially, the addition of some kinds of elements such as Ti, Ta, Zr and Hf induces an additional degree of order resulting in precipitation of Ni2AlXtype L21 Heusler phase. Many reports have shown that the formation of L21-Ni2AlTi phase greatly improves the hightemperature strength including the creep strength of NiAl [5]. Therefore, the addition of element which stabilizes the L21 structure in NiAl/Cr(Mo) is expected to obtain a good balance of high-temperature strength and room temperature toughness. The approach has been previously conducted by the addition of small amount of Zr [6] and Hf [7]. In this study, the addition of Ti, which has a wide solubility region in NiAl, was attempted into NiAl/Cr(Mo) eutectic alloys. Three directionally solidified (DS) alloys with different
K. Hagihara et al. / Intermetallics 14 (2006) 1326e1331
Ti contents were prepared, and the effect of the Ti-addition on microstructure, temperature dependence of yield stress and room temperature fracture toughness were examined.
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specimens used was 1 2 mm2 10 mm, and a notch 0.4 mm in depth was introduced by EDM. The loading direction (crack propagation direction) was selected to be perpendicular to the growth direction.
2. Experimental procedure 3. Results and discussion According to Raj et al. [4], the B2-NiAl/A2-Cr(Mo) twophase eutectic microstructure was obtained in alloy with composition of Nie33Ale31Cre3Mo (at.%). Three mother alloys of Nie(33 x)Ale31Cre3MoexTi (x ¼ 0, 3 and 5 at.%), containing different amounts of Ti were prepared by arc-melting a charge of appropriate weights of elemental Ni, Al, Cr, Mo and Ti under an Ar atmosphere. We denote the alloys hereafter as 0Ti-, 3Ti- and 5Ti-alloys. DS treatment was conducted for the alloys through a Bridgeman process in the high purity alumina crucible to control the microstructure. The growth rate of DS crystals was set to be 5 mm/h. For 0Ti- and 5Ti-alloys, DS rods at a growth rate of 100 mm/h were also prepared, in order to examine the effect of growth rate on microstructures and mechanical properties. The chemical composition of constituent phases was analyzed by the SEMeEDS and TEMe EDS method. Rectangular specimens with dimension of 2 2 mm2 5 mm were cut by an electro-discharge machine (EDM) and compression tests were conducted at a nominal strain rate of 1.7 104 s1. The loading direction was selected to be parallel to the growth direction. The tests were done at room temperature and between 800 and 1200 C in a vacuum in order to examine the temperature dependence of yield stress. Three-point bending tests were performed to evaluate the fracture toughness at room temperature. The size of
Fig. 1 shows the typical microstructures of three DS alloys grown at 5 mm/h, observed on the transverse and longitudinal sections. All alloys were macroscopically constructed by B2-NiAl and A2(bcc)-Cr(Mo) phases. Table 1 shows the chemical compositions of the constituent phases in the alloys analyzed by SEM(TEM)eEDS method. In Ti-bearing alloys, the Ti content in A2 phase was very low at about 1% and most Ti were distributed in the B2 phase. This induced the precipitation of L21-Ni2AlTi in 5Ti-alloy, as described later. Ti-addition strongly affects the microstructures in the alloys. As reported previously [4], the aligned fine lamellar structure with a thickness of about 4 mm was developed almost parallel to the growth direction in 0Ti-alloy (Fig. 1(a) and (b)). However, the addition of Ti disturbed the formation of lamellar microstructure. Fig. 1(c) and (d) shows the microstructure in 3Ti-alloy. The length of plate-like Cr phase in NiAl matrix became short and the morphology changed from a lamellar microstructure to a cellular eutectic microstructure. As seen in Fig. 1(d), the cell structure is developed along the growth direction. In the cells, however, the shortened Cr plates are poorly aligned and they were curled out toward the cell boundary. The increase in Ti-content further
Fig. 1. Typical microstructures of three DS alloys grown at 5 mm/h, observed on the (a,c,e) transverse and (b,d,f) longitudinal sections. (a,b) 0Ti- (c,d) 3Ti- and (e,f) 5Ti-alloys, respectively.
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Table 1 Chemical compositions of constituent phases in the three DS alloys analyzed by the SEM(TEM)eEDS method Alloy
Phase
Chemical composition (at.%) Ni
Al
Cr
Mo
Ti
Nie33Ale31Cre3Mo (0Ti-alloy)
A2 B2
4.84 50.71
5.34 44.81
83.07 4.19
6.75 0.29
Nie30Ale31Cre 3Moe3Ti (3Ti-alloy)
A2 B2
3.67 49.77
5.43 41.87
81.83 4.19
8.18 0.35
0.89 3.82
Nie28Ale31Cre3Moe A2 (SEM) 3.21 5Ti (5Ti-alloy) B2 (SEM) 50.02 B2 (TEM) 45.76 L21 (TEM) 47.00
6.36 39.70 45.42 30.78
83.15 4.00 4.05 3.20
6.39 0.14 0.22 0.09
0.89 6.14 4.55 18.93
induced the shortening of Cr-phase plates and the eutectic microstructure containing nearly rod-like Cr phases was developed in 5Ti-alloy. The effect of Ti-addition on the change in microstructure was more remarkably observed in specimens grown at higher growth rate. Fig. 2 shows the longitudinal microstructure of 0Ti- and 5Ti-alloys grown at 100 mm/h. In 0Ti-alloy, although some NiAl phases dendritically precipitated, the lamellar microstructure still exists between them. The lamellar spacing in them was about 1 mm, which was much smaller than that at 5 mm/h. In contrast, in 5Ti-alloy, the development of
microstructure along the growth direction was very weak. Cr-precipitates were observed to be homogeneously distributed in NiAl matrix. The results clearly suggest that the Ti-addition strongly hinders the development of aligned lamellar structure in NiAl/Cr(Mo) alloys. Fig. 3 shows TEM images of NiAl phase in the three DS alloys grown at 5 mm/h. Precipitates with spherical shape are seen in all the alloys. They were identified to be the Cr(Mo) phases by EDS analysis, which may be precipitated from the over-saturated NiAl matrix during the DS process. Although no other precipitates were confirmed in 0Ti- and 3Ti-alloys, many plate-like precipitates were also observed in 5Ti-alloy. They were confirmed to be the L21-Ni2AlTi phase; in the diffraction pattern analysis the 1/2[111] extra spots were clearly seen when viewed along ½110 direction in the NiAl matrix phase. Fig. 3(c) shows the bright field image viewed along [111] direction. On observation from this direction, the L21-Ni2AlTi plates align parallel to three h110i direction. The result indicates that the Ni2AlTi phases precipitate on {100} in NiAl with the cube-on-cube crystal orientation relationship, which is in accordance with the previous report in NiAl/Ni2AlTi two-phase alloy [8]. The thermal stability and growth kinetics of Ni2AlTi in this 5Ti-alloy are not yet clarified. According to the phase diagram reported by Ishikawa et al. [9], Cr is known to reduce the stability of Ni2AlTi phase.
Fig. 2. Microstructures on the longitudinal section in directionally solidified (a,b) 0Ti- and (c,d) 5Ti-alloys grown at 100 mm/h, (b) and (d) are higher magnification images in (a) and (c), respectively.
K. Hagihara et al. / Intermetallics 14 (2006) 1326e1331
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Fig. 3. Bright field TEM images of NiAl phases in the three DS alloys grown at 5 mm/h. (a) 0Ti-alloy, (b) 3Ti-alloy and (c) 5Ti-alloy.
Table 2 shows the yield stress of alloys examined by compression tests at room temperature. The yield stress intensely increased with increase in the amount of Ti. This increase in yield stress was accompanied by a decrease in fracture strain, however. Approximately 5% of compressive plastic strain was obtained in 0Ti-alloy, but decreased to a few percent in Ti-bearing alloys. Concerning the effect of growth rate on the yield stress, the yield stress tends to show higher value in alloys grown at 100 mm/h than that grown at 5 mm/h. This may be due to the finer microstructure in alloys grown at 100 mm/h. A similar tendency was also reported in the previous paper [10]. Fig. 4 shows the temperature dependence of yield stress at high temperatures above 800 C. The Ti-bearing alloy shows higher yield stress at 800 C as well as at room temperature, while the temperature dependence is different depending on the alloy. The difference in yield stress between 3Ti-alloy and 0Ti-alloy decreased rapidly with increasing temperature above 800 C, and showed almost the same value above 900 C. Since no specific precipitation was observed in 3Ti-alloy, the increase in yield stress of 3Ti-alloy must be caused by solution hardening. The result indicates that the effect of solution hardening by Ti-addition is remarkably moderated above 900 C. In contrast, the yield stress of 5Ti-alloys maintains much higher value than others up to 1200 C, demonstrating the effectiveness of Ni2AlTi phase in improving the high-temperature strength. Fig. 5 shows the room temperature fracture toughness of alloys examined by three-point bending tests. As previously reported [2e4], the incorporation of bcc-Cr phase into the NiAl phase was very effective to improve the low fracture
toughness of NiAl. This was also confirmed in our results. The fracture toughness of 0Ti-alloy shows relatively high value of over 8 MPa m1/2 when the stress is loaded perpendicular to the growth direction. This value is higher than that of NiAl polycrystals (about 4e6 MPa m1/2 [1]). The toughness of 0Ti-alloy specimens grown at 100 mm/h also shows a high value, although it contains large scatters because of the inhomogeneous microstructure containing the dendritic NiAl phases. However, the Ti-addition causes a sharp decrease in fracture toughness. Even in the 3Ti-alloy specimens, the fracture toughness rapidly decreases to about 4 MPa m1/2. This value is almost the same as that of 0Tialloy when loaded parallel to the growth direction, in which the crack propagates along the two-phase lamellar interfaces. One explanation for the deterioration of fracture toughness is the decrease in toughness of constituent NiAl and Cr phases themselves by Ti-addition. The fracture toughness of Ni2AlTi may be much lower than that of NiAl because of its lower crystal symmetry. However, we assume that the main reason is related to the disturbance of microstructure in Ti-bearing
Table 2 Yield stress of alloys examined by compression tests at room temperature Alloy
0Ti (5 mm/h)
0Ti (100 mm/h)
3Ti (5 mm/h)
5Ti (5 mm/h)
5Ti (100 mm/h)
Yield stress (MPa)
619
699
1091
1269
1479
Fig. 4. Temperature dependence of yield stress of DS alloys between 800 and 1200 C. The loading direction is parallel to the growth direction.
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propagates almost straight and parallel to the loading direction. The cracks showed the tendency to propagate along the two-phase interface. However, the crack deflection did not occur macroscopically since the Cr plates were not well aligned along the growth direction and their length was not enough. Therefore, the toughening mechanism by Cr phase did not perform effectively and hence the fracture toughness showed low values comparable to NiAl polycrystals. The result indicates that another strategy or treatment to control the microstructure including the structure of twophase interface is strongly required to improve the fracture toughness of Ti-bearing alloys.
4. Conclusion Fig. 5. Room temperature fracture toughness of alloys examined by three-point bending tests.
alloys, in which the toughening mechanism induced by the incorporation of Cr phase does not act effectively. Fig. 6 shows the side surface of fractured specimens after threepoint bending tests. The features of crack propagation are different in each alloy. In 0Ti-alloy the main crack was frequently deflected from the loading direction and propagated in a zigzag manner. Since the NiAl/Cr(Mo) two-phase interfaces act as dislocation source, the significant plasticity was obtained in NiAl phases in the two-phase composite alloy [11]. This may lead to the increase in intrinsic toughening. Moreover the suppression of rapid propagation of cracks induces the frequent occurrence of debonding of the lamellar interface, called delamination, as shown in the high magnification image in Fig. 6(b). This induces the deflection of crack perpendicular to the loading direction, resulting in effective increase in fracture toughness. On the other hand, a main crack initiated from the notch in Ti-bearing alloys
(1) Ti-addition to the NiAl/Cr(Mo) eutectic alloy disturbs the formation of lamellar microstructure. Cellular microstructures containing the short plates shape of the Cr(Mo) phase were obtained in Ti-bearing alloys. In 5Ti-alloy, L21Ni2AlTi phase precipitated in NiAl matrix phase after the DS treatment. (2) The Ti-addition induces an increase of yield stress of the alloys. Especially, high-temperature strength shows significantly higher value in 5Ti-alloy. However, it accompanied the large deterioration of room temperature fracture toughness because of the disturbance of microstructure.
Acknowledgements This work was supported by ‘‘Priority Assistance for the Formation of Worldwide Renowned Centers of Research e The 21st Century COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design)’’ from the Japanese Ministry of Education, Culture, Sports,
Fig. 6. The side surface of fractured specimens after the three-point bending tests. (a,b) 0Ti-alloy, (c,d) 3Ti-alloy and (e,f) 5Ti-alloy, (b), (d) and (f) are higher magnification images in (a), (c) and (e), respectively.
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