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The effect of B addition on charge distribution in Co3Ti
Intermetallics 9 (2001) 705–709 www.elsevier.com/locate/intermet
The effect of B addition on charge distribution in Co3Ti M.-Y. Wua,b, Jing Zhua,b,*, X.-J. Wanc a
Electron Microscopy Laboratory, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China b Central Iron and Steel Research Institute, Beijing 100081, PR China c Shanghai University, Shanghai 200072, PR China Received 26 May 2000; accepted 29 May 2001
Abstract In this paper, the chemistries at grain boundaries in Co3Ti intermetallics with and without boron doping were examined. The charge density distributions of the two kinds of alloys were obtained by their experimentally determined structure factors. The differences between them were analyzed and compared with the effect of boron on charge density distribution in Ni3Al. It is found that B has quite different effects on the charge distribution, and segregation behavior as well as mechanical properties in Co3Ti and Ni3Al. It is concluded that boron has no effect on suppressing the environmental embrittlement in Co3Ti because of the weakened Co–B–Co bonding. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Intermetallics, miscellaneous; B. Brittleness and ductility; B. Bonding; B. Crystallography; F. Electron microscopy, transmission
1. Introduction Many intermetallic compounds with Li2 structure, such as Co3Ti [1,2], Ni-based compounds [3], exhibit attractive properties as they show an increased strength with increasing temperature. But they are susceptible to environmental embrittlement. Therefore, to solve this problem will be a benefit to the extensive use of those intermetallic compounds. It has been well known that addition of a small amount of boron in Ni3Al can dramatically increase tensile ductility and change the fracture mode from intergranular fracture to transgranular fracture [4,5]. Moreover, with optimum boron addition the Ni3Al alloy is not susceptible to the testing atmosphere [5,6]. As in Ni3Al, boron is also quite effective on reducing the environmental embrittlement of other L12 compounds like Ni3Si [7] and Ni3 (Si, Ti) [8] alloys but Co3Ti [1,9–11]. By X-ray diffraction experiments and by comparing the geometrical size of the interstitial site with boron atom, Takasugi [9] confirmed that boron occupies the body centered position in Co3Ti, which is the same
* Corresponding author. Tel.: +86-10-62773998; fax: +86-1062772507. E-mail address: [email protected] (J. Zhu).
position as boron in Ni3Al [12]. Why boron has different effects on environmental embrittlement in intermetallic compounds with the same crystal structure, even though it occupies the same position in the crystals, is still not known at present. Understanding the different roles that boron plays in these two alloys will give a better insight into the effect of boron on the other alloys with L12 structure. Theoretical calculations by Lu et al. [13] have tried to analyze the effect of boron on the electronic structure of the grain boundary in Co3Ti. By using the first principles discrete variational method (DVM) and the polyhedral model of the grain boundary [13], a conclusion of the anti-bonding Co–B may provide the reason why B has little effect on improving the ductility of Co3Ti alloy. To test the model constituted in the theoretical calculation, the experimental confirmation of the charge density redistribution by alloying is needed. Using the quantitative convergent beam electron diffraction pattern method (QCBED), charge density distributions in the real materials can be obtained accurately, so that the alloying effect on macro-property of the materials can be explained. This is an effective experimental means which can demonstrate the macro-property of the alloys, either macro or micro alloyed with the third element. In fact, it has been used to explain the mechanical properties of many intermetallics. The Cr, V and Mn additions to the
0966-9795/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(01)00055-3
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M.-Y. Wu et al. / Intermetallics 9 (2001) 705–709
charge density in TiAl have been analyzed using this method [14–17]. Zhu et al. [16] investigated the charge density distribution in both Ni-rich Ni3Al single crystals, with and without B-addition, and they attribute the beneficial effect of B in Ni3Al to the strengthened bonding between Ni–B–Ni. In this paper, to elucidate why boron plays a different role in Co3Ti alloy, firstly we examine the chemistry at grain boundaries in Co3Ti, with and without boron doping. Secondly, we get the difference charge distribution of the two kinds of alloys and analyze the different effect of boron on the charge distribution, and compare the results with that in Ni3Al.
2. Experimental 2.1. Specimen preparation The material Co3Ti (the nominal compositions Co–23 at.% Ti doped with 0, 0.02 wt.% boron) were melted in argon atmosphere using high purity cobalt and titanium, with boron added through a Co–10 wt.% B master alloy. To ensure the chemical homogeneity, the buttons were melted three times and finally cast into 20 mm diameter bars. After an anneal in vacuum at 1050 C for 24 h, the bars were sliced into sheets with a thickness of about 4 mm and each sheet was repeatedly rolled to about 1 mm thickness at 400 C in air, with an intermediate vacuum recrystallization anneal at 1000 C for 2 h. The sheets were finally given a vacuum anneal of 1000 C for 5 h, followed by furnace cooling. The average grain diameter was about 20 mm. The thin foils (3 mm in diameter) of Co3Ti with and without boron doping for TEM were prepared by standard twin-jet electro-polishing using a 10% H2SO4– 90% CH3OH electrolyte at 30 C.
3. Chemistry at grain boundaries in Co3Ti with and without boron doping Before analyzing the charge density of the polycrystalline Co3Ti with and without boron doping, the chemistry at the grain boundaries of the two kinds of alloys was measured. Although much work has been done on analyzing the segregation of Ni atoms to grain boundaries in Ni3Al, with and without boron doping [5,18–20], the documentation of segregation of Co atoms to grain boundary in Co3Ti has not been done. In this work, we detected the segregation behavior of Co and B near grain boundaries. As EDS is sensitive to heavy elements, it was used to study the segregation of Co to grain boundaries. Fig. 1 shows the concentration of Co near grain boundaries in the two kinds of alloys. It can be seen from Fig. 1 that cobalt segregates to grain boundaries in both alloys, but the amounts are somewhat different. It is found that Co3Ti with boron doping has much greater cobalt segregation ( 3 at.%) than Co3Ti without boron doping
2.2. EELS and EDS acquiring The TEM observation, EELS and EDS studies were carried out on a field emission gun Jeol-2010F transmission electron microscope attached with a Gatan image filter system (GIF, model 678) and a LINK EDX system. Liquid-nitrogen cooling trap of the specimen was used to minimize thermal diffuse scattering and to reduce contamination. To ensure the information from EELS and EDS was obtained from the grain boundaries, the probe size was controlled at 0.5 nm. The grain boundaries analyzed were tilted to be parallel to the incident beam direction. To enhance the signal–noise ratio, the microscope was adjusted to the diffraction mode using a camera length of 30 mm, while EELS was acquired. At the operating voltage of 200 kV, an energy resolution of about 1.0 eV is obtained for EELS.
Fig. 1. The concentration of Co near grain boundaries detected by EDX in Co3Ti doped with 200 wppm B (a) and without B (b).
M.-Y. Wu et al. / Intermetallics 9 (2001) 705–709
( 1 at.%). From Fig. 1, the Co-rich region is seen to be 4 nm wide Co3Ti with boron doping and 5 nm wide Co3Ti without boron doping. It should be mentioned that the measured concentrations of Co in both kinds of alloys are likely to be only a lower limit, since the effect of beam broadening and the width of the boundary due to tilt must be taken into account. It is likely that the Co-rich region will be thinner than those limits. Similar results as shown in Fig. 1 were obtained near different kinds of large angle grain boundaries including twin grain boundaries in the two kinds of alloys. To examine the compositional ordering of atoms near the grain boundaries, high resolution electron images (HREM) near the grain boundaries were acquired. These HREMs indicate a range of disordered area near grain boundaries confirming the EDX results above. Fig. 2 shows the HREM of a [110] 180 twin in Co3Ti with boron doping. In the interior of the upper grain, a rectangle array corresponding to the (100) and (110) planes is present about 3–4 nm away from the grain boundary. When approaching the boundary plane, the rectangular pattern gradually disappears and diamond shaped {111} fringes characteristic of compositional disorder are seen. Since Co3Ti alloys with and without boron doping were prepared through the same heat treatment, their different chemistry near the grain boundaries is attributed to the boron doping. It means that the boron must make the charge density redistribute in Co3Ti. To analyze this effect, it should be known whether boron segregates to grain boundaries in Co3Ti. Although a beryllium sample stage was used and the TEM was equipped with a windowless energy dispersive detector, the B–K edge was too weak to be detected
Fig. 2. HREM of [110] 180 twin in Co3Ti with boron doping.
707
with EDX. EELS is much more sensitive than EDX to light elements such as B. Therefore, EELS was used to detect whether boron segregates to the grain boundary. No segregation of B to grain boundary has been detected in this work. This is the same as the result from the Augur experiment [8]. Because boron does not segregate to the grain boundary, its effect will be in the matrix: that effect is reflected in the charge density in Co3Ti.
4. Charge density distribution 4.1. Method The QCBED method was used to get the charge density in the two kinds of alloys. Since this method has been entirely described elsewhere [21–23], here we just introduce the major procedure of this method. The intensity distribution in the CBED pattern is a function of several parameters such as sample thickness, incident beam direction and structure factors, etc.
Fig. 3. The charge density difference map on the full Co atomic plane between Co3Ti (23 at % Ti) doped with 0.02 wt.% boron and undoped with boron.
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M.-Y. Wu et al. / Intermetallics 9 (2001) 705–709
Comparing and refining the experimental intensity distribution with the theoretical calculated intensity distribution, accurate structure factors will be obtained. With the knowledge of structure factors in the material, the Fourier coefficient of the Coulomb potential, the charge density in materials is directly obtained through Poisson’s equation. The low order structure factors can be refined with the accuracy required to get information on bonding in the crystal by the QCBED method; this bonding information will be reflected in the charge density. 4.2. Results and analysis The low order structure factors of Co3Ti with and without boron doping which most reflect bonding information have been measured using the QCBED method and given in another paper [24]. In order to analyze the boron effect on the charge density in the two kinds of alloys, we used the charge density difference map described by Zhu et al. [16]. Consequently, B ðrÞ is defined as the charge density in Co3Ti with boron doping, ðrÞ is the charge density in Co3Ti without boron
Fig. 4. The charge density difference map on the 50%Co–50%Ti atomic plane between Co3Ti (23 at.% Ti) doped with 0.02 wt.% boron and undoped with boron.
doping, and ðrÞ ¼ B ðrÞ ðrÞ, the charge density difference between the two doped and undoped boron alloys were given to analyze the boron effect. To explain how boron affects various kinds of nearest and second nearest internal atom bonding, three projected charge density difference maps are shown in Figs. 3–5. When ðrÞ has a negative sign, it indicates a gain of electrons, while a positive sign indicates a loss of electrons. The solid lines centered at the indicated atom in the figures show its metal atom radius or covalent radius. Here the metal atom radius RCo(M)=0.125 nm, RTi(M)=0.145 nm, and the covalent radius RCo(C)= 0.116 nm, RTi(C)=0.132 nm [25]. According to the results of Takasugi and Izumi [9], B atoms occupy the body centered positions, the center of an octahedron with 6 nearest neighboring Co atoms in the unit cell shown in Figs. 3–5. This position is called site 1, hereafter, and the center of another octahedron with nearest neighboring 4 Co and 2 Ti is called site 2. From Fig. 2–4, it can be seen that: (1) near the Co metal atomic radius, electrons decrease along the Co–site 1 direction; (2) electrons decrease along the nearest Co–Ti direction; (3) electrons increase along Ti– site 2 direction; and (4) electrons around site 2 increase. It indicates clearly that the strength of Co–site 1–Co bonding is a little reduced by boron while the strength of Ti–site 2–Ti bonding is enhanced. These results confirm the segregation behaviors that were detected above. Since boron weakens the strength
Fig. 5. The charge density difference map on the (110) atomic plane between Co3Ti (23 at.% Ti) doped with 0.02 wt.% boron and undoped with boron.
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5. Conclusion With the information of chemistry near grain boundaries in Co3Ti with and without boron doping and charge density difference map of the two, we conclude as follows: 1. due to boron doping, Co segregates more to large angle grain boundaries; 2. boron gives the opposite effect on the charge density in Ni3Al and Co3Ti with the same Li2 structure. The enhanced nearest metal–B–metal bonding in intermetallic compounds will suppress the environmental embrittlement.
Acknowledgements This work was supported by the National Nature Science Foundation of China and National Advanced Materials Committee of China. References
Fig. 6. The charge density difference map on the full Ni atomic plane from 75.66 Ni–23.82 Al–0.52 B (at.%) and 77.1 Ni–22.9 Al (at.%) single crystal alloys.
of Co–site 1–Co bonding, there is no surprise on more Co segregation to grain boundaries in Co3Ti doped with boron than that in Co3Ti undoped with boron. Also due to no strengthened Co–B–Co bonding on grain boundary, any suppressing environmental embrittlement of Co3Ti doped with boron will not be expected. This experimental result that boron weakens Co–site 1– Co agrees with the result by theoretical calculation, which says Co–B is anti-bonding. It is interesting to compare the results in this paper with those in Ni3Al. The charge density difference map of Ni3Al and Co3Ti display in nearly opposite tendency. Fig. 6 shows the charge density difference map on the full Ni atomic plane for 75.66 Ni–23.82 Al–0.52 B (at.%) and 77.1 Ni–22.9 Al (at.%) for comparing. It is clearly seen that boron strengthens Ni–site 1–Ni bonding. Therefore, boron has the opposite effect on bonding between nearest neighboring atoms around it in Ni3Al and Co3Ti. As suggested by Zhu et al. [16], we believe that boron cannot suppress the environmental embrittlement for Co3Ti is due to Co– site 1–Co weakened by boron, and there is no boron segregation at grain boundaries.
[1] Takasugi T, Izumi O. Acta Metall 1985;33:39. [2] Liu Y, Takasugi T, Izumi O, Yamada T. Acta Metall 1989; 37:507–17. [3] Suzuki T. Materials Transactions, JIM 1996;37(1):1–11. [4] Aoki K, Izumi O. J Jpn Inst Met 1979;43:1190. [5] Liu CT, White CL, Horton JA. Acta Metall 1985;33:213. [6] George EP, Liu CT. Mater Res Soc Proc 1995;s64:1131. [7] Taub AI, Brian CL. Met Trans 1989;20A:2025. [8] Takasugi T, Hono K, Suzuki S, Hanada S, Sakurai T. Scripta Metall 1993;29:1587. [9] Takasugi T, Takazawa M, Izumi O. J Mater Sci 1990;25:4226, 4239 (1990). [10] Takasugi T, Izumi O. Acta Metall 1986;34:607. [11] Cheng XY, Wan XJ, Chen YX. Scripta Materialia 1997;7:1065– 9. [12] Shinohara T, Takasugi T. J Mater Res 1996;8(6):1285. [13] Lu G, Guo JT, Chen KY, Hu ZQ. Acta Mater 1996;44(10):4019– 26. [14] Holmestad R, Zuo JM, Spence JCH, Hoier R, Horita Z. Phil Mag A 1995;72:579. [15] Holmestad R, Birkeland CR. Phil Mag A 1998;77:1231–54. [16] Zhu J, Miao Y, Guo JT. Acta Mater 1997;45(5):1989. [17] Miao Y, Zhu J, Lin XW, Jiang WJ. J Mater Res 1995;10:1913. [18] Sieloff DD, Brenner SS, Hua MJ. High-temperature ordered intermetallic alloy III. In: Liu CT, Taub AI, Stoloff NS, Koch CC, editors. Proc Mat Res Soc Symp. Pittsburgh, PA: Mater Res Soc, 1989. p. 155. [19] Baker I, Schluson EM, Michael JR. Phil Mag B 1989;57:379. [20] George EP, Liu CT, Padgett RA. Scr Metall 1989;23:979. [21] Saunders M, Bird DM, Zaluzec NJ, Burgess WG, Preston AR, Humphreys CJ. Ultramicroscopy 1995;60:311–23. [22] Zuo JM, Spence JCH. Ultramicroscopy 1991;35:185. [23] Wu MY, Li SY, Zhu J, Du ZH, Li SL. Acta Cryst 1999;A56:189– 92. [24] Wu MY, Zhu J, Li SY, Du ZH, Li SL. J Appl Crystallog 2000;33:1119–21. [25] Vainshtein BK, Fridkin VM, Indenbom VI. In: Sovremennaya Kristallografiya, 2, Struktura Kristallor. Moscow: Nayka, 1979. (Chinese translation, by Wu ZQ, editor. Publishing house of Chinese Science and Technology University, 1990).
The effect of B addition on charge distribution in Co3Ti M.-Y. Wua,b, Jing Zhua,b,*, X.-J. Wanc a
Electron Microscopy Laboratory, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China b Central Iron and Steel Research Institute, Beijing 100081, PR China c Shanghai University, Shanghai 200072, PR China Received 26 May 2000; accepted 29 May 2001
Abstract In this paper, the chemistries at grain boundaries in Co3Ti intermetallics with and without boron doping were examined. The charge density distributions of the two kinds of alloys were obtained by their experimentally determined structure factors. The differences between them were analyzed and compared with the effect of boron on charge density distribution in Ni3Al. It is found that B has quite different effects on the charge distribution, and segregation behavior as well as mechanical properties in Co3Ti and Ni3Al. It is concluded that boron has no effect on suppressing the environmental embrittlement in Co3Ti because of the weakened Co–B–Co bonding. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Intermetallics, miscellaneous; B. Brittleness and ductility; B. Bonding; B. Crystallography; F. Electron microscopy, transmission
1. Introduction Many intermetallic compounds with Li2 structure, such as Co3Ti [1,2], Ni-based compounds [3], exhibit attractive properties as they show an increased strength with increasing temperature. But they are susceptible to environmental embrittlement. Therefore, to solve this problem will be a benefit to the extensive use of those intermetallic compounds. It has been well known that addition of a small amount of boron in Ni3Al can dramatically increase tensile ductility and change the fracture mode from intergranular fracture to transgranular fracture [4,5]. Moreover, with optimum boron addition the Ni3Al alloy is not susceptible to the testing atmosphere [5,6]. As in Ni3Al, boron is also quite effective on reducing the environmental embrittlement of other L12 compounds like Ni3Si [7] and Ni3 (Si, Ti) [8] alloys but Co3Ti [1,9–11]. By X-ray diffraction experiments and by comparing the geometrical size of the interstitial site with boron atom, Takasugi [9] confirmed that boron occupies the body centered position in Co3Ti, which is the same
* Corresponding author. Tel.: +86-10-62773998; fax: +86-1062772507. E-mail address: [email protected] (J. Zhu).
position as boron in Ni3Al [12]. Why boron has different effects on environmental embrittlement in intermetallic compounds with the same crystal structure, even though it occupies the same position in the crystals, is still not known at present. Understanding the different roles that boron plays in these two alloys will give a better insight into the effect of boron on the other alloys with L12 structure. Theoretical calculations by Lu et al. [13] have tried to analyze the effect of boron on the electronic structure of the grain boundary in Co3Ti. By using the first principles discrete variational method (DVM) and the polyhedral model of the grain boundary [13], a conclusion of the anti-bonding Co–B may provide the reason why B has little effect on improving the ductility of Co3Ti alloy. To test the model constituted in the theoretical calculation, the experimental confirmation of the charge density redistribution by alloying is needed. Using the quantitative convergent beam electron diffraction pattern method (QCBED), charge density distributions in the real materials can be obtained accurately, so that the alloying effect on macro-property of the materials can be explained. This is an effective experimental means which can demonstrate the macro-property of the alloys, either macro or micro alloyed with the third element. In fact, it has been used to explain the mechanical properties of many intermetallics. The Cr, V and Mn additions to the
0966-9795/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(01)00055-3
706
M.-Y. Wu et al. / Intermetallics 9 (2001) 705–709
charge density in TiAl have been analyzed using this method [14–17]. Zhu et al. [16] investigated the charge density distribution in both Ni-rich Ni3Al single crystals, with and without B-addition, and they attribute the beneficial effect of B in Ni3Al to the strengthened bonding between Ni–B–Ni. In this paper, to elucidate why boron plays a different role in Co3Ti alloy, firstly we examine the chemistry at grain boundaries in Co3Ti, with and without boron doping. Secondly, we get the difference charge distribution of the two kinds of alloys and analyze the different effect of boron on the charge distribution, and compare the results with that in Ni3Al.
2. Experimental 2.1. Specimen preparation The material Co3Ti (the nominal compositions Co–23 at.% Ti doped with 0, 0.02 wt.% boron) were melted in argon atmosphere using high purity cobalt and titanium, with boron added through a Co–10 wt.% B master alloy. To ensure the chemical homogeneity, the buttons were melted three times and finally cast into 20 mm diameter bars. After an anneal in vacuum at 1050 C for 24 h, the bars were sliced into sheets with a thickness of about 4 mm and each sheet was repeatedly rolled to about 1 mm thickness at 400 C in air, with an intermediate vacuum recrystallization anneal at 1000 C for 2 h. The sheets were finally given a vacuum anneal of 1000 C for 5 h, followed by furnace cooling. The average grain diameter was about 20 mm. The thin foils (3 mm in diameter) of Co3Ti with and without boron doping for TEM were prepared by standard twin-jet electro-polishing using a 10% H2SO4– 90% CH3OH electrolyte at 30 C.
3. Chemistry at grain boundaries in Co3Ti with and without boron doping Before analyzing the charge density of the polycrystalline Co3Ti with and without boron doping, the chemistry at the grain boundaries of the two kinds of alloys was measured. Although much work has been done on analyzing the segregation of Ni atoms to grain boundaries in Ni3Al, with and without boron doping [5,18–20], the documentation of segregation of Co atoms to grain boundary in Co3Ti has not been done. In this work, we detected the segregation behavior of Co and B near grain boundaries. As EDS is sensitive to heavy elements, it was used to study the segregation of Co to grain boundaries. Fig. 1 shows the concentration of Co near grain boundaries in the two kinds of alloys. It can be seen from Fig. 1 that cobalt segregates to grain boundaries in both alloys, but the amounts are somewhat different. It is found that Co3Ti with boron doping has much greater cobalt segregation ( 3 at.%) than Co3Ti without boron doping
2.2. EELS and EDS acquiring The TEM observation, EELS and EDS studies were carried out on a field emission gun Jeol-2010F transmission electron microscope attached with a Gatan image filter system (GIF, model 678) and a LINK EDX system. Liquid-nitrogen cooling trap of the specimen was used to minimize thermal diffuse scattering and to reduce contamination. To ensure the information from EELS and EDS was obtained from the grain boundaries, the probe size was controlled at 0.5 nm. The grain boundaries analyzed were tilted to be parallel to the incident beam direction. To enhance the signal–noise ratio, the microscope was adjusted to the diffraction mode using a camera length of 30 mm, while EELS was acquired. At the operating voltage of 200 kV, an energy resolution of about 1.0 eV is obtained for EELS.
Fig. 1. The concentration of Co near grain boundaries detected by EDX in Co3Ti doped with 200 wppm B (a) and without B (b).
M.-Y. Wu et al. / Intermetallics 9 (2001) 705–709
( 1 at.%). From Fig. 1, the Co-rich region is seen to be 4 nm wide Co3Ti with boron doping and 5 nm wide Co3Ti without boron doping. It should be mentioned that the measured concentrations of Co in both kinds of alloys are likely to be only a lower limit, since the effect of beam broadening and the width of the boundary due to tilt must be taken into account. It is likely that the Co-rich region will be thinner than those limits. Similar results as shown in Fig. 1 were obtained near different kinds of large angle grain boundaries including twin grain boundaries in the two kinds of alloys. To examine the compositional ordering of atoms near the grain boundaries, high resolution electron images (HREM) near the grain boundaries were acquired. These HREMs indicate a range of disordered area near grain boundaries confirming the EDX results above. Fig. 2 shows the HREM of a [110] 180 twin in Co3Ti with boron doping. In the interior of the upper grain, a rectangle array corresponding to the (100) and (110) planes is present about 3–4 nm away from the grain boundary. When approaching the boundary plane, the rectangular pattern gradually disappears and diamond shaped {111} fringes characteristic of compositional disorder are seen. Since Co3Ti alloys with and without boron doping were prepared through the same heat treatment, their different chemistry near the grain boundaries is attributed to the boron doping. It means that the boron must make the charge density redistribute in Co3Ti. To analyze this effect, it should be known whether boron segregates to grain boundaries in Co3Ti. Although a beryllium sample stage was used and the TEM was equipped with a windowless energy dispersive detector, the B–K edge was too weak to be detected
Fig. 2. HREM of [110] 180 twin in Co3Ti with boron doping.
707
with EDX. EELS is much more sensitive than EDX to light elements such as B. Therefore, EELS was used to detect whether boron segregates to the grain boundary. No segregation of B to grain boundary has been detected in this work. This is the same as the result from the Augur experiment [8]. Because boron does not segregate to the grain boundary, its effect will be in the matrix: that effect is reflected in the charge density in Co3Ti.
4. Charge density distribution 4.1. Method The QCBED method was used to get the charge density in the two kinds of alloys. Since this method has been entirely described elsewhere [21–23], here we just introduce the major procedure of this method. The intensity distribution in the CBED pattern is a function of several parameters such as sample thickness, incident beam direction and structure factors, etc.
Fig. 3. The charge density difference map on the full Co atomic plane between Co3Ti (23 at % Ti) doped with 0.02 wt.% boron and undoped with boron.
708
M.-Y. Wu et al. / Intermetallics 9 (2001) 705–709
Comparing and refining the experimental intensity distribution with the theoretical calculated intensity distribution, accurate structure factors will be obtained. With the knowledge of structure factors in the material, the Fourier coefficient of the Coulomb potential, the charge density in materials is directly obtained through Poisson’s equation. The low order structure factors can be refined with the accuracy required to get information on bonding in the crystal by the QCBED method; this bonding information will be reflected in the charge density. 4.2. Results and analysis The low order structure factors of Co3Ti with and without boron doping which most reflect bonding information have been measured using the QCBED method and given in another paper [24]. In order to analyze the boron effect on the charge density in the two kinds of alloys, we used the charge density difference map described by Zhu et al. [16]. Consequently, B ðrÞ is defined as the charge density in Co3Ti with boron doping, ðrÞ is the charge density in Co3Ti without boron
Fig. 4. The charge density difference map on the 50%Co–50%Ti atomic plane between Co3Ti (23 at.% Ti) doped with 0.02 wt.% boron and undoped with boron.
doping, and ðrÞ ¼ B ðrÞ ðrÞ, the charge density difference between the two doped and undoped boron alloys were given to analyze the boron effect. To explain how boron affects various kinds of nearest and second nearest internal atom bonding, three projected charge density difference maps are shown in Figs. 3–5. When ðrÞ has a negative sign, it indicates a gain of electrons, while a positive sign indicates a loss of electrons. The solid lines centered at the indicated atom in the figures show its metal atom radius or covalent radius. Here the metal atom radius RCo(M)=0.125 nm, RTi(M)=0.145 nm, and the covalent radius RCo(C)= 0.116 nm, RTi(C)=0.132 nm [25]. According to the results of Takasugi and Izumi [9], B atoms occupy the body centered positions, the center of an octahedron with 6 nearest neighboring Co atoms in the unit cell shown in Figs. 3–5. This position is called site 1, hereafter, and the center of another octahedron with nearest neighboring 4 Co and 2 Ti is called site 2. From Fig. 2–4, it can be seen that: (1) near the Co metal atomic radius, electrons decrease along the Co–site 1 direction; (2) electrons decrease along the nearest Co–Ti direction; (3) electrons increase along Ti– site 2 direction; and (4) electrons around site 2 increase. It indicates clearly that the strength of Co–site 1–Co bonding is a little reduced by boron while the strength of Ti–site 2–Ti bonding is enhanced. These results confirm the segregation behaviors that were detected above. Since boron weakens the strength
Fig. 5. The charge density difference map on the (110) atomic plane between Co3Ti (23 at.% Ti) doped with 0.02 wt.% boron and undoped with boron.
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5. Conclusion With the information of chemistry near grain boundaries in Co3Ti with and without boron doping and charge density difference map of the two, we conclude as follows: 1. due to boron doping, Co segregates more to large angle grain boundaries; 2. boron gives the opposite effect on the charge density in Ni3Al and Co3Ti with the same Li2 structure. The enhanced nearest metal–B–metal bonding in intermetallic compounds will suppress the environmental embrittlement.
Acknowledgements This work was supported by the National Nature Science Foundation of China and National Advanced Materials Committee of China. References
Fig. 6. The charge density difference map on the full Ni atomic plane from 75.66 Ni–23.82 Al–0.52 B (at.%) and 77.1 Ni–22.9 Al (at.%) single crystal alloys.
of Co–site 1–Co bonding, there is no surprise on more Co segregation to grain boundaries in Co3Ti doped with boron than that in Co3Ti undoped with boron. Also due to no strengthened Co–B–Co bonding on grain boundary, any suppressing environmental embrittlement of Co3Ti doped with boron will not be expected. This experimental result that boron weakens Co–site 1– Co agrees with the result by theoretical calculation, which says Co–B is anti-bonding. It is interesting to compare the results in this paper with those in Ni3Al. The charge density difference map of Ni3Al and Co3Ti display in nearly opposite tendency. Fig. 6 shows the charge density difference map on the full Ni atomic plane for 75.66 Ni–23.82 Al–0.52 B (at.%) and 77.1 Ni–22.9 Al (at.%) for comparing. It is clearly seen that boron strengthens Ni–site 1–Ni bonding. Therefore, boron has the opposite effect on bonding between nearest neighboring atoms around it in Ni3Al and Co3Ti. As suggested by Zhu et al. [16], we believe that boron cannot suppress the environmental embrittlement for Co3Ti is due to Co– site 1–Co weakened by boron, and there is no boron segregation at grain boundaries.
[1] Takasugi T, Izumi O. Acta Metall 1985;33:39. [2] Liu Y, Takasugi T, Izumi O, Yamada T. Acta Metall 1989; 37:507–17. [3] Suzuki T. Materials Transactions, JIM 1996;37(1):1–11. [4] Aoki K, Izumi O. J Jpn Inst Met 1979;43:1190. [5] Liu CT, White CL, Horton JA. Acta Metall 1985;33:213. [6] George EP, Liu CT. Mater Res Soc Proc 1995;s64:1131. [7] Taub AI, Brian CL. Met Trans 1989;20A:2025. [8] Takasugi T, Hono K, Suzuki S, Hanada S, Sakurai T. Scripta Metall 1993;29:1587. [9] Takasugi T, Takazawa M, Izumi O. J Mater Sci 1990;25:4226, 4239 (1990). [10] Takasugi T, Izumi O. Acta Metall 1986;34:607. [11] Cheng XY, Wan XJ, Chen YX. Scripta Materialia 1997;7:1065– 9. [12] Shinohara T, Takasugi T. J Mater Res 1996;8(6):1285. [13] Lu G, Guo JT, Chen KY, Hu ZQ. Acta Mater 1996;44(10):4019– 26. [14] Holmestad R, Zuo JM, Spence JCH, Hoier R, Horita Z. Phil Mag A 1995;72:579. [15] Holmestad R, Birkeland CR. Phil Mag A 1998;77:1231–54. [16] Zhu J, Miao Y, Guo JT. Acta Mater 1997;45(5):1989. [17] Miao Y, Zhu J, Lin XW, Jiang WJ. J Mater Res 1995;10:1913. [18] Sieloff DD, Brenner SS, Hua MJ. High-temperature ordered intermetallic alloy III. In: Liu CT, Taub AI, Stoloff NS, Koch CC, editors. Proc Mat Res Soc Symp. Pittsburgh, PA: Mater Res Soc, 1989. p. 155. [19] Baker I, Schluson EM, Michael JR. Phil Mag B 1989;57:379. [20] George EP, Liu CT, Padgett RA. Scr Metall 1989;23:979. [21] Saunders M, Bird DM, Zaluzec NJ, Burgess WG, Preston AR, Humphreys CJ. Ultramicroscopy 1995;60:311–23. [22] Zuo JM, Spence JCH. Ultramicroscopy 1991;35:185. [23] Wu MY, Li SY, Zhu J, Du ZH, Li SL. Acta Cryst 1999;A56:189– 92. [24] Wu MY, Zhu J, Li SY, Du ZH, Li SL. J Appl Crystallog 2000;33:1119–21. [25] Vainshtein BK, Fridkin VM, Indenbom VI. In: Sovremennaya Kristallografiya, 2, Struktura Kristallor. Moscow: Nayka, 1979. (Chinese translation, by Wu ZQ, editor. Publishing house of Chinese Science and Technology University, 1990).