- Email: [email protected]
High temperature strength of Nb3Al-base alloys
Intermetallics 6 (1998) 735-739 C 1998 Elsevier Science Limited PII:
SO966-9795(98)00052-l
Printed in Great Britain. All rights reserved 0966-9795/98/%see front matter
ELSEVIER
High temperature strength of NbsAl-base alloys T. Tabaru & S. Hanada” Institute for Materials Research, Tohoku University, 2-l-l Katahira. Aoba-ku, Sendai 980-8577, Japan
(Received 13 April 1998; revised 6 May 1998; accepted 7 May 1998)
High temperature strength was investigated as a function of volume percent of NbsAl using ternary alloys with controlled microstructures of equiaxed NbsAl and Nb,, (Nb solid solution) grains. Creep strength was examined in MO-added NbsAl-base alloy with two types of different microstructures, equiaxed grains and directionally elongated grains. MO addition increases high temperature strength at all the volume percents of NbaAl, while Ta addition is effective only at high volume percents of NbsAl. Ti addition decreases high temperature strength at all the volume percents of NbsAl. MO-added NbsAl-base alloy consisting of directionally elongated grains has high creep strength compared to other refractory intermetallic alloys such as MoSiz alloy and (Cr,Mo)$i/(Cr,Mo)sSis alloy. Creep strength is decreased under a low applied stress in MO-added NbsAl-base alloy with equiaxed grains probably because of easy grain boundary sliding. The obtained results are discussed in terms of solid solution strengthening of the constituent phases. 0 1998 Elsevier Science Limited. All rights reserved Key words: A niobium aluminides, B creep, mechanical properties at high temperatures, ternary alloying, D microstructure.
1 INTRODUCTION
2 EXPERIMENTAL
Refractory intermetallic Nb3Al alloys with high melting temperature around 2300 K are expected as structural materials operated at much higher temperatures compared to conventional Ni superalloys. It is well known, however, that monolithic NbsAl with the ordered Al5type crystal structure is very brittle and has poor fracture toughness at ambient temperature.’ The incorporation of a ductile Nb,, in the brittle NbsAl phase is believed to be a promising method to increase fracture toughness to an acceptable value. Since the incorporation of a ductile phase leads to a decrease in high temperature strength,2 solid solution strengthening of a ductile phase and NbsAl as well as the incorporation of a ductile phase is needed to develop high temperature NbsAl-base alloys. The objective of this work is to increase high temperature strength of ductile phase toughened Nb3Al-base alloys by ternary additions. High temperature strength and creep strength were investigated using microstructure controlled ternary NbsAl alloys.
NbsAl-base binary and ternary Nb-Al-Ta, NbAI-MO and Nb-Al-Ti alloys with various NbsAl volume percents ranging from 0 to 100% were arcmelted in an Ar atmosphere using a non-consumable tungsten electrode and solution treated above 1973 K for more than 1 h in a vacuum of less than 1 x 10e3 Pa. The solution treated ingots were isothermally forged at an initial strain rate of 3x lop5 s-l to 75% reduction in thickness at a temperature from 1473 to 1873 K, and then annealed in vacuum at 1473 K for 170 h. Compression tests were carried out at 1473 K under a constant crosshead speed. In order to investigate the effect of microstructure on creep strength, NbsAl-base alloy Nb18.1 mol%Al-31.4mol%Mo (alloy will be abbreviated hereafter as Nb-18Al-31Mo) was selected. The alloy was arc-melted in an Ar atmosphere. Two types of heat treatments were given to Nb 18Al-31Mo. The arc-melted ingots were solution treated at 2173 K for 3 h, and then isothermally forged at 1873K to 75% reduction in thickness. After forging, the sample A was annealed at 1773 K for 100 h, and the sample B was solution treated at 2173 K for 3 h and annealed at 1773 K for 100 h. Compressive creep tests were performed under a constant load.
*To whom correspondence 022-215-2116
should be addressed. Fax: 0081
735
PROCEDURE
736
T. Tabaru, S. Hanada
Microstructures were observed, by optical microscopy, scanning electron microscopy and transmission electron microscopy. Existing phases were identified by electron probe microanalysis, transmission electron microscopy and X-ray diffraction. Chemical compositions were determined by inductively coupled plasma atomic emission spectroscopy.
seen in Ta-added Nb3Al alloys, although yield stress at high ~01% of NbsAl is very high conipared to NbsAl binary alloys. Yield stress of Tiadded NbsAl alloys is lower than that of NbsAl binary alloys at all the volume percents tested. It should be noted that yield stress of Ta- or Ti-added NbsAl alloys is similar to that of NbsAl binary
3 RESULTS AND DISCUSSION 3.1 High temperature strength of Nb&base ternary alloys Microstructures of NbsAl ternary cast alloys consisting of Nb3A1 and Nb,, were found to be very sensitive to alloy composition. When alloys were solution treated and annealed, directionally elongated Nb3A1 or irregular-shaped, blocky NbsAl particles evolved depending on alloy composition even though the same heat treatment was employed. These differences in microstructure may obscure an intrinsic effect of ternary addition on high temperature strength. In order to investigate the effect of ternary addition on high temperature strength under a similar microstructure, equiaxed microstructure was produced in all the alloys by isothermal forging and heat treatment. Figure 1 shows typical scanning electron micrographs of (a) Nb-16A1, (b) Nb-16Al-1OTi and (c) Nb-17Al10Mo. These microstructures are composed of equiaxed NbsAl and Nb,, grains with similar grain sizes, and volume percents of NbsAl are 77% for (a), 69% for (b) and 78% for (c). The equiaxed microstructures appear to be produced when the decomposition of supersaturated Nb,, into NbsAl and Nb,, takes place during isothermal forging. This is probably because the preferential growth of NbsAl is suppressed in strained Nb,,. Using Nb3A1 ternary alloys with two phase equiaxed microstructures, yield stress was examined at 1473 K under 1.7x lop4 s-l. Since grain boundary sliding does not occur under this deformation condition as discussed later, and an extrinsic effect resulting from different microstructures is excluded, yield stress in Fig. 2 as a function of NbsAl ~01% will represent intrinsic strength due to dislocation glide influenced by ternary additions. Data of NbsAl binary alloys and Nb-18Al-3 1Mo (sample A) are included in Fig. 2 for comparison. Yield stress of binary alloys is very high at 100 ~01% NbsAl, whereas it is remarkably decreased by incorporating Nb,,. A similar tendency can be
Fig. 1. Scanning electron micrographs of (a) Nb-16A1, (b) Nbl6Al-1OTi and (c) Nbl7Al-1OMo.
High temperature strength of NbjAl-base alloys 1000 1473
K
_ initial strain rate ; 1.7X 10-4s-1 *O” _ -O-Nb-Al alloys . + Nb-Al-1OTi alloys tNb-Al-1OMo alloys ‘, 600 - + Nb-18Al-31Mo (A) a” . + Nb-AI- IOTa alloys D 2 2 400 u z._ *
0
“‘~“~“~~~‘~~~‘~~~’ 0 20 40 60 80 Volume percent of Nb,Al
100
737
bit high strength at 1473 K. Furthermore, it was pointed out that high temperature strength would be increased with increasing MO content. In this section creep strength of Nb-18Al-31Mo will be discussed. Nb-18Al-3 1MO alloy contained 28 ~01% A2 phase, and the compositions of the NbsAl and Nb,, phases were found to be Nb-22Al-29Mo and Nb-9Al-37M0, respectively, after equilibration at 1773 IL Evolved microstructures are shown in Fig. 3. As clearly seen, the sample A has an equiaxed microstructure consisting of NbsAl and Nb,, (Fig. 3(a)) and average grain sizes are 16 pm in Nb3A1 and 12 pm in Nb,,. By contrast, most of Nb,, particles are directionally elongated to a few hundred pm at a maximum in the sample B (Fig. 3(b)). Stress dependence of minimum creep rate at 1573 K is shown in Fig. 4, where data of MoS& alloy,
Fig. 2. Yield stress at 1473K of MO-, Ta- and Ti-added Nb3Al-base ternary alloys as a function of ~01% of Nb3AI.
alloys at volume percents iess than 60 ~01% NbsAl. On the other hand, yield stress of MO-added NbjAl alloys is much higher than that of NbsAl binary alloys at all the volume percents tested. These results can be explained by considering the effect of ternary additions on yield stresses of the constituent phases, Nb3Al and Nb,,. Monolithic NbJAl has been found to be strengthened at elevated temperatures by alloying with refractory metals such as Ta, MO or W with melting temperature higher than Nb, which was explained by sluggish diffusion of these elements in Nb3A1.3 Also, yield stress of Nb,, at elevated temperatures was reported to be increased by alloying with MO and not by alloying with Ta due to differences in size misfit between solvent and solute atoms and in diffusion rate of solute atoms in Nb.“-6 Thus, high volume percent of Nb3Al is required in Ta-added Nb3A1 alloys to obtain high strength at elevated temperature, while high strength can be attained by increasing MO content in MO-added Nb3Al alloys even at relatively low ~01% of NbsAl. MO-added Nb3A1 alloys may be more advantageous to attain a good balance between strength at elevated temperature and fracture toughness at ambient temperature compared to Ta- or Ti-added Nb3Al alloys, because fracture toughness at ambient temperature will be increased by decreasing volume percent of Nb3Al. 3.2 Creep strength of Mo- and Ti-added w
alloys Fig. 3. Scanning
In the preceding section MO-added NbsAl alloys consisting of Nb3Al and Nb,, were shown to exhi-
electron micrographs of Nb-18Al-3 1Mo annealed at 1773K for 100 h after (a) isothermal forging and (b) solution treated at 2173 K for 3 h and annealed at 1773 K for 100 h after isothermal forging.
T. Tabaru, S. Hanada
738
1573 K
Nb,Al/Nb, (41~01% A15)’ /
NbsAl/Nb,, (87/vol% A15)
I I
I”-” 6*‘Nb-18AL31Mo/
.w Nh-18Al-31Mo
102 Applied stress, a/ MPa Fig. 4. Stress dependence of minimum creep rate at 1573 K in NbJAl-base alloys.
(Cr,Mo)sSi/(Cr,Mo)& alloy and NbsAl/Nb,, alloy are included for comparison.7-g Evidently, MO-added NbsAl alloy, especially the sample B, exhibits good creep strength compared to the other refractory intermetallic alloys. Also, Fig. 4 reveals that MO alloying is very effective to increase creep strength of binary NbsAl/Nb,, alloys. It should be noted in Fig. 4 that a significant difference in the stress dependence of minimum creep rate for the sample A takes place at stresses lower than 200MPa. Stress exponent values are calculated to be n = 5 throughout the stress range tested for the sample B and to be n = 5 at high stresses and n = 1 at low stresses for the sample A. Since the same heat treatment at 1773 K and for 100 h was finally given to both the samples, the difference would result from the difference in microstructure. Figure 5 shows transmission electron micrographs of the sample A crept at 1573 K under 80 MPa to a strain 0.04 (Fig. 5(a)) and to a strain 0.20 (Fig. 5(b)). At 0.04 strain dislocations can be hardly observed in NbJAl (A15) grains, although a few dislocations are present in Nb,, (A2) grains. This feature is not significantly changed on further straining. In Fig. 5(b), one can see a few dislocations in NbsAl grains as well as in Nb,, grains, but dislocation densities in both the phases are still low. In addition, no apparent difference was observed in the two-phase SEM microstructures before and after creep testing. These results suggest that creep in the sample A having n = 1 is associated with grain boundary sliding. In contrast to the sample A, a considerable density of dislocations are observed in a NbJAl grain of the sample B crept at 1573 K under
Fig. 5. Transmission electron micrographs of Nb-1 SAl-3 1MO (sample A) crept at 1573 K under 80 MPa to a strain (a) 0.04 and (b) 0.20.
98 MPa to a strain 0.02, as shown in Fig. 6. Creep deformation of the sample B, therefore, may be controlled by dislocation glide in NbsAl grains. The obtained value n = 5 seems to support this explanation. It is concluded that creep strength of Nb3Albase alloys can be increased by suppressing grain boundary sliding through microstructure control. Solution treated Nb-Al-Ti ingots were annealed at 1473K for 170 h to obtain equilibrated microstructures and compositions. Chemical compositions of constituent phases were determined by EPMA. The present result was in fairly good agreement with Kattner and Boettinger’s result,‘O but it was found that two phase NbJAl/Nb,, region is slightly broadened. This expansion has been recently pointed out by Semboshi et al. in NbsAl/ Nb,, binary alloys. l1 Directionally elongated microstructure similar to Fig. 3(b) was obtained in Nb-18Al-20Ti (Nb-17.8 mol%Al-20.4 mol%Ti). Equiaxed or blocky NbsAl particles were produced in the NbAl-Ti alloys except for Nb-18Al-20Ti. The reason for the difficulty of producing elongated microstructure in Ti-added NbsAl alloys is not clear at present. Creep rate of Nb-18Al-20Ti was examined at 1473 K, because it was so high at 1573 K. The obtained result is included in Fig. 4.
High temperature strength of NbjAl-base alloys
739
melted and microstructure was controlled by heat treatments and isothermal forging. High temperature strength was investigated as a function of volume percent of NbsAl using ternary alloys with controlled microstructures composed of equiaxed grains. Creep strength was examined in MO-added NbsAl-base alloy with two types of different microstructures, equiaxed grains and directionally elongated grains. MO addition increases high temperature strength at all the volume percents of NbsAl, while Ta addition is effective only at ~01% of NbsAl. Ti addition decreases high temperature strength at all the volume percents of NbsAl. These results can be explained in terms of solid solution strengthening of constituent phases. MO-added NbsAl-base alloy consisting of directionally elongated grains has high creep strength compared to other refractory intermetallic alloys such as MoSi2 alloy and (Cr,Mo)sSi/(Cr,Mo)$33 alloy. Fig. 6. Transmission electron micrographs of Nb-18Al-31Mo (sample B) crept at 1573 K under 98 MPa to a strain 0.02.
Compared to binary NbsAl/Nb,,, it is suggested that the addition of Ti decreases creep strength. Recently, it has been found that solid solution strengthening of monolithic NbsAl alloys at high temperature becomes more effective in the order of W > MO > Ta additions.‘* In addition, the present result reveals that the addition of Ti to binary monolithic NbsAl decreases yield strength. Thus, the order of the effective solid solution strengthening in NbsAl is concluded to be W > MO > Ta > Ti. Although there are no available diffusion data for ternary elements in NbsAl, it has been reported that diffusion rate of solute atoms in Nb,, increases in the order W < MO >Ta
4 SUMMARY
REFERENCES 1. Murugesh, L., Rao, K. T. V. and Ritchie, R. O., Scripta Metall., 1993, 29, 1107. 2. Hanada, S., Murayama, Y. and Abe, Y., Intermetallics, 1994, 2, 155. R., 3. Hanada, S., Tabaru, T., Miura, E., Gnanamoorthy, and Murayama, Y., in THERMEC’97, ed. T. Chandra and T. Sakai. TMS, Warrendale, PA, 1997, p. 1583. 4. English, C., in Niobium, ed. H. Stuart. AIME, Warrendale, PA, 1984, p. 239. 5. Begley, R. T., in Evolution of Refractory Metals and Alloys, ed. E. N. C. Dalder, T. Grobstein and C. S. Olsen. TMS, Warrendale, PA, 1994, p. 29. 6. Chang, W. H., Refractory Metals and Alloys, AIME Metallurgical Society Conferences, Vol. 11, 1961, p. 83. 7. Sadananda, K., Feng, C. R., Jones, H. K. and Petrovic, J. J., in Structural Intermetallics, ed. R. Darolia et al. TMS, Warrendale, PA, 1993, p. 809. 8. Nazamy, M., Noseda, C., Sauthoff, G., Zeumer, B. and Anton, D., in High-Temperature Ordered Intermetallic Alloy VZ, ed. J. A. Horton et al. MRS Symp. Proc., Vol. 364, 1995, p. 1333. 9. Nomura, N., Yoshimi, K. and Hanada, S., in Structural Intermetallics 1997, ed. M. V. Nathal et al. TMS, Warrendale, PA, 1997, p. 923. 10. Kattner, U. R. and Boettinger, W. J., Mat. Sci. Eng., 1992, A152, 9.
11 Semboshi, S., Tabaru,
Ternary NbsAl-base alloy ingots consisting of a NbsAl phase and a Nb solid solution were arc-
T., Hosoda, H. and Hanada,
S.,
Intermetallics, 1998, 6, 61.
12. Murayama, 949.
Y. and Hanada, S., Scripta Mater., 1997, 37,
SO966-9795(98)00052-l
Printed in Great Britain. All rights reserved 0966-9795/98/%see front matter
ELSEVIER
High temperature strength of NbsAl-base alloys T. Tabaru & S. Hanada” Institute for Materials Research, Tohoku University, 2-l-l Katahira. Aoba-ku, Sendai 980-8577, Japan
(Received 13 April 1998; revised 6 May 1998; accepted 7 May 1998)
High temperature strength was investigated as a function of volume percent of NbsAl using ternary alloys with controlled microstructures of equiaxed NbsAl and Nb,, (Nb solid solution) grains. Creep strength was examined in MO-added NbsAl-base alloy with two types of different microstructures, equiaxed grains and directionally elongated grains. MO addition increases high temperature strength at all the volume percents of NbaAl, while Ta addition is effective only at high volume percents of NbsAl. Ti addition decreases high temperature strength at all the volume percents of NbsAl. MO-added NbsAl-base alloy consisting of directionally elongated grains has high creep strength compared to other refractory intermetallic alloys such as MoSiz alloy and (Cr,Mo)$i/(Cr,Mo)sSis alloy. Creep strength is decreased under a low applied stress in MO-added NbsAl-base alloy with equiaxed grains probably because of easy grain boundary sliding. The obtained results are discussed in terms of solid solution strengthening of the constituent phases. 0 1998 Elsevier Science Limited. All rights reserved Key words: A niobium aluminides, B creep, mechanical properties at high temperatures, ternary alloying, D microstructure.
1 INTRODUCTION
2 EXPERIMENTAL
Refractory intermetallic Nb3Al alloys with high melting temperature around 2300 K are expected as structural materials operated at much higher temperatures compared to conventional Ni superalloys. It is well known, however, that monolithic NbsAl with the ordered Al5type crystal structure is very brittle and has poor fracture toughness at ambient temperature.’ The incorporation of a ductile Nb,, in the brittle NbsAl phase is believed to be a promising method to increase fracture toughness to an acceptable value. Since the incorporation of a ductile phase leads to a decrease in high temperature strength,2 solid solution strengthening of a ductile phase and NbsAl as well as the incorporation of a ductile phase is needed to develop high temperature NbsAl-base alloys. The objective of this work is to increase high temperature strength of ductile phase toughened Nb3Al-base alloys by ternary additions. High temperature strength and creep strength were investigated using microstructure controlled ternary NbsAl alloys.
NbsAl-base binary and ternary Nb-Al-Ta, NbAI-MO and Nb-Al-Ti alloys with various NbsAl volume percents ranging from 0 to 100% were arcmelted in an Ar atmosphere using a non-consumable tungsten electrode and solution treated above 1973 K for more than 1 h in a vacuum of less than 1 x 10e3 Pa. The solution treated ingots were isothermally forged at an initial strain rate of 3x lop5 s-l to 75% reduction in thickness at a temperature from 1473 to 1873 K, and then annealed in vacuum at 1473 K for 170 h. Compression tests were carried out at 1473 K under a constant crosshead speed. In order to investigate the effect of microstructure on creep strength, NbsAl-base alloy Nb18.1 mol%Al-31.4mol%Mo (alloy will be abbreviated hereafter as Nb-18Al-31Mo) was selected. The alloy was arc-melted in an Ar atmosphere. Two types of heat treatments were given to Nb 18Al-31Mo. The arc-melted ingots were solution treated at 2173 K for 3 h, and then isothermally forged at 1873K to 75% reduction in thickness. After forging, the sample A was annealed at 1773 K for 100 h, and the sample B was solution treated at 2173 K for 3 h and annealed at 1773 K for 100 h. Compressive creep tests were performed under a constant load.
*To whom correspondence 022-215-2116
should be addressed. Fax: 0081
735
PROCEDURE
736
T. Tabaru, S. Hanada
Microstructures were observed, by optical microscopy, scanning electron microscopy and transmission electron microscopy. Existing phases were identified by electron probe microanalysis, transmission electron microscopy and X-ray diffraction. Chemical compositions were determined by inductively coupled plasma atomic emission spectroscopy.
seen in Ta-added Nb3Al alloys, although yield stress at high ~01% of NbsAl is very high conipared to NbsAl binary alloys. Yield stress of Tiadded NbsAl alloys is lower than that of NbsAl binary alloys at all the volume percents tested. It should be noted that yield stress of Ta- or Ti-added NbsAl alloys is similar to that of NbsAl binary
3 RESULTS AND DISCUSSION 3.1 High temperature strength of Nb&base ternary alloys Microstructures of NbsAl ternary cast alloys consisting of Nb3A1 and Nb,, were found to be very sensitive to alloy composition. When alloys were solution treated and annealed, directionally elongated Nb3A1 or irregular-shaped, blocky NbsAl particles evolved depending on alloy composition even though the same heat treatment was employed. These differences in microstructure may obscure an intrinsic effect of ternary addition on high temperature strength. In order to investigate the effect of ternary addition on high temperature strength under a similar microstructure, equiaxed microstructure was produced in all the alloys by isothermal forging and heat treatment. Figure 1 shows typical scanning electron micrographs of (a) Nb-16A1, (b) Nb-16Al-1OTi and (c) Nb-17Al10Mo. These microstructures are composed of equiaxed NbsAl and Nb,, grains with similar grain sizes, and volume percents of NbsAl are 77% for (a), 69% for (b) and 78% for (c). The equiaxed microstructures appear to be produced when the decomposition of supersaturated Nb,, into NbsAl and Nb,, takes place during isothermal forging. This is probably because the preferential growth of NbsAl is suppressed in strained Nb,,. Using Nb3A1 ternary alloys with two phase equiaxed microstructures, yield stress was examined at 1473 K under 1.7x lop4 s-l. Since grain boundary sliding does not occur under this deformation condition as discussed later, and an extrinsic effect resulting from different microstructures is excluded, yield stress in Fig. 2 as a function of NbsAl ~01% will represent intrinsic strength due to dislocation glide influenced by ternary additions. Data of NbsAl binary alloys and Nb-18Al-3 1Mo (sample A) are included in Fig. 2 for comparison. Yield stress of binary alloys is very high at 100 ~01% NbsAl, whereas it is remarkably decreased by incorporating Nb,,. A similar tendency can be
Fig. 1. Scanning electron micrographs of (a) Nb-16A1, (b) Nbl6Al-1OTi and (c) Nbl7Al-1OMo.
High temperature strength of NbjAl-base alloys 1000 1473
K
_ initial strain rate ; 1.7X 10-4s-1 *O” _ -O-Nb-Al alloys . + Nb-Al-1OTi alloys tNb-Al-1OMo alloys ‘, 600 - + Nb-18Al-31Mo (A) a” . + Nb-AI- IOTa alloys D 2 2 400 u z._ *
0
“‘~“~“~~~‘~~~‘~~~’ 0 20 40 60 80 Volume percent of Nb,Al
100
737
bit high strength at 1473 K. Furthermore, it was pointed out that high temperature strength would be increased with increasing MO content. In this section creep strength of Nb-18Al-31Mo will be discussed. Nb-18Al-3 1MO alloy contained 28 ~01% A2 phase, and the compositions of the NbsAl and Nb,, phases were found to be Nb-22Al-29Mo and Nb-9Al-37M0, respectively, after equilibration at 1773 IL Evolved microstructures are shown in Fig. 3. As clearly seen, the sample A has an equiaxed microstructure consisting of NbsAl and Nb,, (Fig. 3(a)) and average grain sizes are 16 pm in Nb3A1 and 12 pm in Nb,,. By contrast, most of Nb,, particles are directionally elongated to a few hundred pm at a maximum in the sample B (Fig. 3(b)). Stress dependence of minimum creep rate at 1573 K is shown in Fig. 4, where data of MoS& alloy,
Fig. 2. Yield stress at 1473K of MO-, Ta- and Ti-added Nb3Al-base ternary alloys as a function of ~01% of Nb3AI.
alloys at volume percents iess than 60 ~01% NbsAl. On the other hand, yield stress of MO-added NbjAl alloys is much higher than that of NbsAl binary alloys at all the volume percents tested. These results can be explained by considering the effect of ternary additions on yield stresses of the constituent phases, Nb3Al and Nb,,. Monolithic NbJAl has been found to be strengthened at elevated temperatures by alloying with refractory metals such as Ta, MO or W with melting temperature higher than Nb, which was explained by sluggish diffusion of these elements in Nb3A1.3 Also, yield stress of Nb,, at elevated temperatures was reported to be increased by alloying with MO and not by alloying with Ta due to differences in size misfit between solvent and solute atoms and in diffusion rate of solute atoms in Nb.“-6 Thus, high volume percent of Nb3Al is required in Ta-added Nb3A1 alloys to obtain high strength at elevated temperature, while high strength can be attained by increasing MO content in MO-added Nb3Al alloys even at relatively low ~01% of NbsAl. MO-added Nb3A1 alloys may be more advantageous to attain a good balance between strength at elevated temperature and fracture toughness at ambient temperature compared to Ta- or Ti-added Nb3Al alloys, because fracture toughness at ambient temperature will be increased by decreasing volume percent of Nb3Al. 3.2 Creep strength of Mo- and Ti-added w
alloys Fig. 3. Scanning
In the preceding section MO-added NbsAl alloys consisting of Nb3Al and Nb,, were shown to exhi-
electron micrographs of Nb-18Al-3 1Mo annealed at 1773K for 100 h after (a) isothermal forging and (b) solution treated at 2173 K for 3 h and annealed at 1773 K for 100 h after isothermal forging.
T. Tabaru, S. Hanada
738
1573 K
Nb,Al/Nb, (41~01% A15)’ /
NbsAl/Nb,, (87/vol% A15)
I I
I”-” 6*‘Nb-18AL31Mo/
.w Nh-18Al-31Mo
102 Applied stress, a/ MPa Fig. 4. Stress dependence of minimum creep rate at 1573 K in NbJAl-base alloys.
(Cr,Mo)sSi/(Cr,Mo)& alloy and NbsAl/Nb,, alloy are included for comparison.7-g Evidently, MO-added NbsAl alloy, especially the sample B, exhibits good creep strength compared to the other refractory intermetallic alloys. Also, Fig. 4 reveals that MO alloying is very effective to increase creep strength of binary NbsAl/Nb,, alloys. It should be noted in Fig. 4 that a significant difference in the stress dependence of minimum creep rate for the sample A takes place at stresses lower than 200MPa. Stress exponent values are calculated to be n = 5 throughout the stress range tested for the sample B and to be n = 5 at high stresses and n = 1 at low stresses for the sample A. Since the same heat treatment at 1773 K and for 100 h was finally given to both the samples, the difference would result from the difference in microstructure. Figure 5 shows transmission electron micrographs of the sample A crept at 1573 K under 80 MPa to a strain 0.04 (Fig. 5(a)) and to a strain 0.20 (Fig. 5(b)). At 0.04 strain dislocations can be hardly observed in NbJAl (A15) grains, although a few dislocations are present in Nb,, (A2) grains. This feature is not significantly changed on further straining. In Fig. 5(b), one can see a few dislocations in NbsAl grains as well as in Nb,, grains, but dislocation densities in both the phases are still low. In addition, no apparent difference was observed in the two-phase SEM microstructures before and after creep testing. These results suggest that creep in the sample A having n = 1 is associated with grain boundary sliding. In contrast to the sample A, a considerable density of dislocations are observed in a NbJAl grain of the sample B crept at 1573 K under
Fig. 5. Transmission electron micrographs of Nb-1 SAl-3 1MO (sample A) crept at 1573 K under 80 MPa to a strain (a) 0.04 and (b) 0.20.
98 MPa to a strain 0.02, as shown in Fig. 6. Creep deformation of the sample B, therefore, may be controlled by dislocation glide in NbsAl grains. The obtained value n = 5 seems to support this explanation. It is concluded that creep strength of Nb3Albase alloys can be increased by suppressing grain boundary sliding through microstructure control. Solution treated Nb-Al-Ti ingots were annealed at 1473K for 170 h to obtain equilibrated microstructures and compositions. Chemical compositions of constituent phases were determined by EPMA. The present result was in fairly good agreement with Kattner and Boettinger’s result,‘O but it was found that two phase NbJAl/Nb,, region is slightly broadened. This expansion has been recently pointed out by Semboshi et al. in NbsAl/ Nb,, binary alloys. l1 Directionally elongated microstructure similar to Fig. 3(b) was obtained in Nb-18Al-20Ti (Nb-17.8 mol%Al-20.4 mol%Ti). Equiaxed or blocky NbsAl particles were produced in the NbAl-Ti alloys except for Nb-18Al-20Ti. The reason for the difficulty of producing elongated microstructure in Ti-added NbsAl alloys is not clear at present. Creep rate of Nb-18Al-20Ti was examined at 1473 K, because it was so high at 1573 K. The obtained result is included in Fig. 4.
High temperature strength of NbjAl-base alloys
739
melted and microstructure was controlled by heat treatments and isothermal forging. High temperature strength was investigated as a function of volume percent of NbsAl using ternary alloys with controlled microstructures composed of equiaxed grains. Creep strength was examined in MO-added NbsAl-base alloy with two types of different microstructures, equiaxed grains and directionally elongated grains. MO addition increases high temperature strength at all the volume percents of NbsAl, while Ta addition is effective only at ~01% of NbsAl. Ti addition decreases high temperature strength at all the volume percents of NbsAl. These results can be explained in terms of solid solution strengthening of constituent phases. MO-added NbsAl-base alloy consisting of directionally elongated grains has high creep strength compared to other refractory intermetallic alloys such as MoSi2 alloy and (Cr,Mo)sSi/(Cr,Mo)$33 alloy. Fig. 6. Transmission electron micrographs of Nb-18Al-31Mo (sample B) crept at 1573 K under 98 MPa to a strain 0.02.
Compared to binary NbsAl/Nb,,, it is suggested that the addition of Ti decreases creep strength. Recently, it has been found that solid solution strengthening of monolithic NbsAl alloys at high temperature becomes more effective in the order of W > MO > Ta additions.‘* In addition, the present result reveals that the addition of Ti to binary monolithic NbsAl decreases yield strength. Thus, the order of the effective solid solution strengthening in NbsAl is concluded to be W > MO > Ta > Ti. Although there are no available diffusion data for ternary elements in NbsAl, it has been reported that diffusion rate of solute atoms in Nb,, increases in the order W < MO >Ta
4 SUMMARY
REFERENCES 1. Murugesh, L., Rao, K. T. V. and Ritchie, R. O., Scripta Metall., 1993, 29, 1107. 2. Hanada, S., Murayama, Y. and Abe, Y., Intermetallics, 1994, 2, 155. R., 3. Hanada, S., Tabaru, T., Miura, E., Gnanamoorthy, and Murayama, Y., in THERMEC’97, ed. T. Chandra and T. Sakai. TMS, Warrendale, PA, 1997, p. 1583. 4. English, C., in Niobium, ed. H. Stuart. AIME, Warrendale, PA, 1984, p. 239. 5. Begley, R. T., in Evolution of Refractory Metals and Alloys, ed. E. N. C. Dalder, T. Grobstein and C. S. Olsen. TMS, Warrendale, PA, 1994, p. 29. 6. Chang, W. H., Refractory Metals and Alloys, AIME Metallurgical Society Conferences, Vol. 11, 1961, p. 83. 7. Sadananda, K., Feng, C. R., Jones, H. K. and Petrovic, J. J., in Structural Intermetallics, ed. R. Darolia et al. TMS, Warrendale, PA, 1993, p. 809. 8. Nazamy, M., Noseda, C., Sauthoff, G., Zeumer, B. and Anton, D., in High-Temperature Ordered Intermetallic Alloy VZ, ed. J. A. Horton et al. MRS Symp. Proc., Vol. 364, 1995, p. 1333. 9. Nomura, N., Yoshimi, K. and Hanada, S., in Structural Intermetallics 1997, ed. M. V. Nathal et al. TMS, Warrendale, PA, 1997, p. 923. 10. Kattner, U. R. and Boettinger, W. J., Mat. Sci. Eng., 1992, A152, 9.
11 Semboshi, S., Tabaru,
Ternary NbsAl-base alloy ingots consisting of a NbsAl phase and a Nb solid solution were arc-
T., Hosoda, H. and Hanada,
S.,
Intermetallics, 1998, 6, 61.
12. Murayama, 949.
Y. and Hanada, S., Scripta Mater., 1997, 37,