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Effect of Al content and Nb addition on the strength and fault energy of TiAl alloys
Materials Science and Engineering A329– 331 (2002) 649– 652 www.elsevier.com/locate/msea
Effect of Al content and Nb addition on the strength and fault energy of TiAl alloys W.J. Zhang a,b,*, F. Appel a b
a Institute for Materials Research, GKSS Research Center, 21502 Geesthacht, Germany State Key Laboratory for Ad6anced Metals and Materials, Uni6ersity of Science and Technology Beijing, Beijing 100083, People’s Republic of China
Abstract The yield strengths of Ti–45Al– xNb and Ti–49Al–xNb (x = 0, 10) with a nearly-equiaxed gamma microstructure were measured at 900 °C. Nb addition was found to largely increase the yield strengths but the variation of Al content has little influence on the yield strength of binary TiAl alloys. To understand the strengthening effect of Nb, the stacking fault energies in these alloys were measured using weak-beam transmission electron microscope (TEM) techniques. The SISF energy decreases significantly with the decreasing of Al content of k phase in binary alloys but is practically independent of the Al content in ternary alloys. The implications of the dissociation behavior on the mechanical properties of the material will be discussed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Intermetallic compounds; Dislocations; TEM; Yield strength; Fault energy
1. Introduction The Al-lean TiAl-base alloys have received a great deal of attentions chiefly owing to their attractive combination of low density, good oxidation resistance and high temperature strength retention [1]. It has been recently recognized that Nb addition of 5 – 10 at.% can significantly improve the strength of TiAl-base alloys [2–6]. The origin of this strengthening effect is not altogether clear. It may arise from solid solution as has been suggested by Zhang et al. [2]. Alloying with Nb often leads to a refinement of the microstructure, as has been recognized in [5]. Thus, the strengthening effect may in terms of a Hall –Petch mechanism be attributed to dislocation interactions with internal boundaries [5]. Another hypothesis is that additions of Nb lead to a decrease of the stacking fault energy. This hypothesis is
* Corresponding author. Present address: The Research Center, Chrysalis Technologies Inc., 7801 Whitepine Road, Richmond, VA 23237, USA. E-mail address: [email protected] (W.J. Zhang).
supported by the observation that deformed high Nb containing alloys exhibit an abundant activation of twinning [4–6]. This would increase the dissociation width of the dislocations, which in turn has significant implications on their resistance against glide and climb. To examine this hypothesis the stacking fault energies in Ti–(45, 49)Al alloys with and without Nb additions were measured in the present study using weak-beam TEM techniques. To the author’s knowledge, the stacking fault energy in Ti-rich TiAl alloys has not been measured experimentally up to date.
2. Experimental details Four kinds of TiAl alloys were investigated in this study with the nominal compositions of Ti –45Al, Ti – 49Al, Ti –45Al –10Nb, and Ti –49Al –10Nb (at.%). The oxygen content of the materials was less than 900 ppm. All the alloys (that is cast Ti –49Al –xNb and forged Ti –45Al – xNb) were finally annealed at 1100 °C for 30 h. After annealing, all the studied materials exhibited
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 6 6 3 - X
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W.J. Zhang, F. Appel / Materials Science and Engineering A329–331 (2002) 649–652
3. Results and discussions
3.1. Effect of Al and Nb contents on the yield strength of TiAl
Fig. 1. Comparison of the yield strengths of binary and ternary alloys at 900 °C. The grain sizes are indicated in the legend.
a nearly-equiaxed gamma microstructure with the volume fraction of h2 grains ranging from 2 to 20 vol.% in different alloys. The microstructures in detail were described elsewhere [3]. Specimens of dimensions 5 mm diameter by 10 mm length were cut from the annealed materials by spark erosion for compression tests. Compression tests were carried out in an MTS-type testing machine at 900 °C in air at a strain rate of 5×10 − 4 s − 1 to a plastic strain of about 2%. Thin foils for TEM observation were cut perpendicular to the loading axis from the center of the specimens. Electron transparent foils were prepared by grinding and standard twin jet polishing method. Examination was performed on Philips CM 200 microscope operated at 200 kV. Weak-beam images were taken using g/3 –5 g contrast conditions.
Fig. 1 compared the yield strengths of the four studied alloys at 900 °C. The k grain sizes of the studied materials were indicated in the legend. The yield strengths of the ternary alloys are significantly higher than that of binary alloys, which are almost consistent with the results at room temperature [3]. Addition of Nb leads to a slight refinement of the microstructure as indicated by the grain sizes give in Fig. 1. With this data about 12–26% of the observed yield strength difference between the binary and the ternary alloys of the same Al content can be explained, provided that a Hall–Petch constant of 1 MPa1/2 is utilized. The remaining difference in yield strengths might be associated with precipitation hardening due to oxides, carbides or nitrides. However, in none of the alloys have precipitates been observed within the resolution power of the utilized weak beam techniques, although the existence of very small precipitates cannot be ruled out. Thus, the strengthening effect might be associated with solid solution hardening. Clearly, the problem needs further investigation.
3.2. Effect of Al and Nb contents on the stacking fault energy The intrinsic stacking fault (ISF) energies were estimated in the studied alloys by analyzing the dissociation configurations of 1/2[1 1 2] superdislocations.
Fig. 2. Weak-beam images of the dissociation configurations of 1/6[1( 1 2] superdislocations in Ti – 45Al (a, b) and Ti – 45Al – 10Nb (c, d) after deformation at 900 °C. (a) g= 0 0 2, B near [1 1( 0]; (b) g = 2( 0 2, B near [01( 0]; (c) g =2( 02( , B near [01( 0]; (d) g =2 0 2, B near [0 1( 0].
W.J. Zhang, F. Appel / Materials Science and Engineering A329–331 (2002) 649–652
651
Fig. 2 and Fig. 3 shows the examples of weak-beam images of dislocation dissociation in the four studied alloys. The three-fold dissociation can be clearly seen. The analyses in detail were reported elsewhere [7]. Using the extinction criterion, the dissociation reactions were determined as the scheme (1) in Ti–45Al and Ti– 45Al– 10Nb alloys and the scheme (2) in Ti–49Al and Ti – 49Al–10Nb alloys, respectively. 1/2[1( 1 2]= 1/6[1( 1 2]+ SESF +1/6[1( 1 2]+ SISF +1/6[1( 1 2],
(1)
1/2[1( 1 2]=1/6[1( 1 2]+SISF+ 1/6[2( 1( 1]+ APB +1/2[0 1 1].
(2)
For a three fold dissociation: S= b1 + SF1 +b2 + SF2 + b3, the energies of the faults SF1 and SF2 are given by the radial forces acting on the left-hand partial b1 and on the right-hand partial b3, respectively [8] kSF1 = F12 + F13,
(3)
kSF2 = F32 + F31.
(4)
Assuming isotropic elasticity, the radial interaction force Fij between two parallel dislocations bi and bj can be written as [9] Fij = (v/2yR)[(bi ·¨)(bj ·¨)+ [(bi × ¨)·(bj × ¨)]/(1 −w)], (5)
Fig. 3. Weak-beam images of the dissociation configurations of 1/6[1( 1 2] superdislocations in Ti –49Al (a, b) and Ti –49Al –10Nb (c, d) after deformation at 900 °C. (a) g= 0 0 2, B near [1 1( 0]; (b) g =2( 0 2, B near [0 1( 0]; (c) g =2( 0 2( , B near [0 1( 0]; (d) g= 2 0 2, B near [0 1( 0].
Fig. 4. The measured SISF energies at 900 °C as a function of the Al content in the k phase of binary and ternary alloys. The calculated energies at room temperature using LKKR theory [8] are also presented for reference.
where R is the separation distance, ¨ is a unit vector in the line direction of dislocations, and bi and bj are the Burgers vectors of the dislocations. Using the dissociation separations being measured, the SISF energies were estimated [7]. The data are plotted in Fig. 4 as a function of the Al content in the k phase (rather than the nominal Al content of the alloys). As can be seen, in the binary alloys the SISF energies decrease significantly with the decreasing of Al content, from 97 mJ m − 2 for 49.6% Al to 67 mJ m − 2 for 48% Al. This trend is in good agreement with the theoretical prediction by LKKR method [10]. In the ternary alloys, the SISF energies are relatively low (68 mJ m − 2) and almost independent of the Al concentration. By comparing the dependence of yield strengths and stacking fault energies on Al and Nb contents (Figs. 1 and 4), we concluded that the decreased faults energy in Nb-containing alloys is not the major cause of its good yield strength at 900 °C. The binary Ti –45Al alloy has the fault energy comparable to that of ternary alloys but its yield strength is much lower. Thus, in view of the present data it is tempting to speculate that there are more restrictions upon dislocation climb in both, Al-lean binary alloys and Nb containing alloys. In Nb containing alloys climb might further be impeded, since these materials exhibit a relatively low diffusivity when compared with their binary counterparts [5,6]. The other plausible explanation of Nb strengthening is the increased friction stress due to Nb addition, as confirmed by estimation of the curvature of dislocations in [2–4].
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W.J. Zhang, F. Appel / Materials Science and Engineering A329–331 (2002) 649–652
4. Conclusions The yield strengths and stacking fault energies were measured at 900 °C in binary Ti– (45, 49)Al and ternary Ti– (45, 49)Al– 10Nb alloys in the form of nearly-equiaxed gamma microstructure. The yield strength of TiAl largely increases with Nb addition and is insensitive to the variation of Al content. On the other hand, the stacking fault energy decreases significantly with the decreasing of Al content in binary alloys and is practically independent of Al content in ternary alloys.
Acknowledgements The authors would like to thank Professor G.L. Chen (University of Science and Technology Beijing, People’s Republic of China) and Dr S.C. Deevi (Chrysalis Technology Inc., Richmond) for helpful discussions. One of the authors (WJZ) greatly acknowl-
edges the Alexander von Humboldt Foundation and Chrysalis Technology Inc., for financial support.
References [1] Y.W. Kim, J. Metals 7 (1994) 30. [2] W.J. Zhang, Z.C. Liu, G.L. Chen, Y.W. Kim, Mater. Sci. Eng. A 271 (1999) 416. [3] G.L. Chen, W.J. Zhang, Z.C. Liu, S.J. Li, Y.W. Kim, in: Y.-W. Kim, et al. (Eds.), Gamma Titanium Aluminides, TMS, PA, 1999, p. 371. [4] W.J. Zhang, Z.C. Liu, G.L. Chen, Y.W. Kim, Phil. Mag. A 5 (1999) 1073. [5] J.D.H. Paul, F. Appel, R. Wagner, Acta Mater. 46 (1998) 1075. [6] F. Appel, U. Lorenz, J.D.H. Paul, M. Oehring, in: Y.-W. Kim, et al. (Eds.), Gamma Titanium Aluminides, TMS, PA, 1999, p. 381. [7] W.J. Zhang, F. Appel, submitted to Mater. Sci. Eng. A (2000). [8] V. Paidar, H. Inui, K. Kishida, M. Yamaguchi, Mater. Sci. Eng. A 233 (1997) 111. [9] J.P. Hirth, J. Lothe, Theory of Dislocations, 2nd ed., Krieger Publishing Company, Florida, 1992, p. 117. [10] C. Woodward, J.M. MacLaren, Phil. Mag. A 74 (1996) 337.
Effect of Al content and Nb addition on the strength and fault energy of TiAl alloys W.J. Zhang a,b,*, F. Appel a b
a Institute for Materials Research, GKSS Research Center, 21502 Geesthacht, Germany State Key Laboratory for Ad6anced Metals and Materials, Uni6ersity of Science and Technology Beijing, Beijing 100083, People’s Republic of China
Abstract The yield strengths of Ti–45Al– xNb and Ti–49Al–xNb (x = 0, 10) with a nearly-equiaxed gamma microstructure were measured at 900 °C. Nb addition was found to largely increase the yield strengths but the variation of Al content has little influence on the yield strength of binary TiAl alloys. To understand the strengthening effect of Nb, the stacking fault energies in these alloys were measured using weak-beam transmission electron microscope (TEM) techniques. The SISF energy decreases significantly with the decreasing of Al content of k phase in binary alloys but is practically independent of the Al content in ternary alloys. The implications of the dissociation behavior on the mechanical properties of the material will be discussed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Intermetallic compounds; Dislocations; TEM; Yield strength; Fault energy
1. Introduction The Al-lean TiAl-base alloys have received a great deal of attentions chiefly owing to their attractive combination of low density, good oxidation resistance and high temperature strength retention [1]. It has been recently recognized that Nb addition of 5 – 10 at.% can significantly improve the strength of TiAl-base alloys [2–6]. The origin of this strengthening effect is not altogether clear. It may arise from solid solution as has been suggested by Zhang et al. [2]. Alloying with Nb often leads to a refinement of the microstructure, as has been recognized in [5]. Thus, the strengthening effect may in terms of a Hall –Petch mechanism be attributed to dislocation interactions with internal boundaries [5]. Another hypothesis is that additions of Nb lead to a decrease of the stacking fault energy. This hypothesis is
* Corresponding author. Present address: The Research Center, Chrysalis Technologies Inc., 7801 Whitepine Road, Richmond, VA 23237, USA. E-mail address: [email protected] (W.J. Zhang).
supported by the observation that deformed high Nb containing alloys exhibit an abundant activation of twinning [4–6]. This would increase the dissociation width of the dislocations, which in turn has significant implications on their resistance against glide and climb. To examine this hypothesis the stacking fault energies in Ti–(45, 49)Al alloys with and without Nb additions were measured in the present study using weak-beam TEM techniques. To the author’s knowledge, the stacking fault energy in Ti-rich TiAl alloys has not been measured experimentally up to date.
2. Experimental details Four kinds of TiAl alloys were investigated in this study with the nominal compositions of Ti –45Al, Ti – 49Al, Ti –45Al –10Nb, and Ti –49Al –10Nb (at.%). The oxygen content of the materials was less than 900 ppm. All the alloys (that is cast Ti –49Al –xNb and forged Ti –45Al – xNb) were finally annealed at 1100 °C for 30 h. After annealing, all the studied materials exhibited
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 6 6 3 - X
650
W.J. Zhang, F. Appel / Materials Science and Engineering A329–331 (2002) 649–652
3. Results and discussions
3.1. Effect of Al and Nb contents on the yield strength of TiAl
Fig. 1. Comparison of the yield strengths of binary and ternary alloys at 900 °C. The grain sizes are indicated in the legend.
a nearly-equiaxed gamma microstructure with the volume fraction of h2 grains ranging from 2 to 20 vol.% in different alloys. The microstructures in detail were described elsewhere [3]. Specimens of dimensions 5 mm diameter by 10 mm length were cut from the annealed materials by spark erosion for compression tests. Compression tests were carried out in an MTS-type testing machine at 900 °C in air at a strain rate of 5×10 − 4 s − 1 to a plastic strain of about 2%. Thin foils for TEM observation were cut perpendicular to the loading axis from the center of the specimens. Electron transparent foils were prepared by grinding and standard twin jet polishing method. Examination was performed on Philips CM 200 microscope operated at 200 kV. Weak-beam images were taken using g/3 –5 g contrast conditions.
Fig. 1 compared the yield strengths of the four studied alloys at 900 °C. The k grain sizes of the studied materials were indicated in the legend. The yield strengths of the ternary alloys are significantly higher than that of binary alloys, which are almost consistent with the results at room temperature [3]. Addition of Nb leads to a slight refinement of the microstructure as indicated by the grain sizes give in Fig. 1. With this data about 12–26% of the observed yield strength difference between the binary and the ternary alloys of the same Al content can be explained, provided that a Hall–Petch constant of 1 MPa1/2 is utilized. The remaining difference in yield strengths might be associated with precipitation hardening due to oxides, carbides or nitrides. However, in none of the alloys have precipitates been observed within the resolution power of the utilized weak beam techniques, although the existence of very small precipitates cannot be ruled out. Thus, the strengthening effect might be associated with solid solution hardening. Clearly, the problem needs further investigation.
3.2. Effect of Al and Nb contents on the stacking fault energy The intrinsic stacking fault (ISF) energies were estimated in the studied alloys by analyzing the dissociation configurations of 1/2[1 1 2] superdislocations.
Fig. 2. Weak-beam images of the dissociation configurations of 1/6[1( 1 2] superdislocations in Ti – 45Al (a, b) and Ti – 45Al – 10Nb (c, d) after deformation at 900 °C. (a) g= 0 0 2, B near [1 1( 0]; (b) g = 2( 0 2, B near [01( 0]; (c) g =2( 02( , B near [01( 0]; (d) g =2 0 2, B near [0 1( 0].
W.J. Zhang, F. Appel / Materials Science and Engineering A329–331 (2002) 649–652
651
Fig. 2 and Fig. 3 shows the examples of weak-beam images of dislocation dissociation in the four studied alloys. The three-fold dissociation can be clearly seen. The analyses in detail were reported elsewhere [7]. Using the extinction criterion, the dissociation reactions were determined as the scheme (1) in Ti–45Al and Ti– 45Al– 10Nb alloys and the scheme (2) in Ti–49Al and Ti – 49Al–10Nb alloys, respectively. 1/2[1( 1 2]= 1/6[1( 1 2]+ SESF +1/6[1( 1 2]+ SISF +1/6[1( 1 2],
(1)
1/2[1( 1 2]=1/6[1( 1 2]+SISF+ 1/6[2( 1( 1]+ APB +1/2[0 1 1].
(2)
For a three fold dissociation: S= b1 + SF1 +b2 + SF2 + b3, the energies of the faults SF1 and SF2 are given by the radial forces acting on the left-hand partial b1 and on the right-hand partial b3, respectively [8] kSF1 = F12 + F13,
(3)
kSF2 = F32 + F31.
(4)
Assuming isotropic elasticity, the radial interaction force Fij between two parallel dislocations bi and bj can be written as [9] Fij = (v/2yR)[(bi ·¨)(bj ·¨)+ [(bi × ¨)·(bj × ¨)]/(1 −w)], (5)
Fig. 3. Weak-beam images of the dissociation configurations of 1/6[1( 1 2] superdislocations in Ti –49Al (a, b) and Ti –49Al –10Nb (c, d) after deformation at 900 °C. (a) g= 0 0 2, B near [1 1( 0]; (b) g =2( 0 2, B near [0 1( 0]; (c) g =2( 0 2( , B near [0 1( 0]; (d) g= 2 0 2, B near [0 1( 0].
Fig. 4. The measured SISF energies at 900 °C as a function of the Al content in the k phase of binary and ternary alloys. The calculated energies at room temperature using LKKR theory [8] are also presented for reference.
where R is the separation distance, ¨ is a unit vector in the line direction of dislocations, and bi and bj are the Burgers vectors of the dislocations. Using the dissociation separations being measured, the SISF energies were estimated [7]. The data are plotted in Fig. 4 as a function of the Al content in the k phase (rather than the nominal Al content of the alloys). As can be seen, in the binary alloys the SISF energies decrease significantly with the decreasing of Al content, from 97 mJ m − 2 for 49.6% Al to 67 mJ m − 2 for 48% Al. This trend is in good agreement with the theoretical prediction by LKKR method [10]. In the ternary alloys, the SISF energies are relatively low (68 mJ m − 2) and almost independent of the Al concentration. By comparing the dependence of yield strengths and stacking fault energies on Al and Nb contents (Figs. 1 and 4), we concluded that the decreased faults energy in Nb-containing alloys is not the major cause of its good yield strength at 900 °C. The binary Ti –45Al alloy has the fault energy comparable to that of ternary alloys but its yield strength is much lower. Thus, in view of the present data it is tempting to speculate that there are more restrictions upon dislocation climb in both, Al-lean binary alloys and Nb containing alloys. In Nb containing alloys climb might further be impeded, since these materials exhibit a relatively low diffusivity when compared with their binary counterparts [5,6]. The other plausible explanation of Nb strengthening is the increased friction stress due to Nb addition, as confirmed by estimation of the curvature of dislocations in [2–4].
652
W.J. Zhang, F. Appel / Materials Science and Engineering A329–331 (2002) 649–652
4. Conclusions The yield strengths and stacking fault energies were measured at 900 °C in binary Ti– (45, 49)Al and ternary Ti– (45, 49)Al– 10Nb alloys in the form of nearly-equiaxed gamma microstructure. The yield strength of TiAl largely increases with Nb addition and is insensitive to the variation of Al content. On the other hand, the stacking fault energy decreases significantly with the decreasing of Al content in binary alloys and is practically independent of Al content in ternary alloys.
Acknowledgements The authors would like to thank Professor G.L. Chen (University of Science and Technology Beijing, People’s Republic of China) and Dr S.C. Deevi (Chrysalis Technology Inc., Richmond) for helpful discussions. One of the authors (WJZ) greatly acknowl-
edges the Alexander von Humboldt Foundation and Chrysalis Technology Inc., for financial support.
References [1] Y.W. Kim, J. Metals 7 (1994) 30. [2] W.J. Zhang, Z.C. Liu, G.L. Chen, Y.W. Kim, Mater. Sci. Eng. A 271 (1999) 416. [3] G.L. Chen, W.J. Zhang, Z.C. Liu, S.J. Li, Y.W. Kim, in: Y.-W. Kim, et al. (Eds.), Gamma Titanium Aluminides, TMS, PA, 1999, p. 371. [4] W.J. Zhang, Z.C. Liu, G.L. Chen, Y.W. Kim, Phil. Mag. A 5 (1999) 1073. [5] J.D.H. Paul, F. Appel, R. Wagner, Acta Mater. 46 (1998) 1075. [6] F. Appel, U. Lorenz, J.D.H. Paul, M. Oehring, in: Y.-W. Kim, et al. (Eds.), Gamma Titanium Aluminides, TMS, PA, 1999, p. 381. [7] W.J. Zhang, F. Appel, submitted to Mater. Sci. Eng. A (2000). [8] V. Paidar, H. Inui, K. Kishida, M. Yamaguchi, Mater. Sci. Eng. A 233 (1997) 111. [9] J.P. Hirth, J. Lothe, Theory of Dislocations, 2nd ed., Krieger Publishing Company, Florida, 1992, p. 117. [10] C. Woodward, J.M. MacLaren, Phil. Mag. A 74 (1996) 337.