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Microstructure of Ni3Al ingots containing boron
207
METALLOGRAPHY 21:207-215 (1988)
Microstructure of NlaAI Ingots Containing Boron
V. ZINOVIEV,* E. M. SCHULSON, AND I. BAKER
Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire
The microstructures of nominally stoichiometric ingots of NiaA1 with boron contents up to 1.12 at.% were examined in the as-received and in the homogenized states. A second phase was observed in the ingot containing 1.12 at.% boron. The effect of homogenization on this phase is described. On a examine les microstructures des lingots stoichiometriques de Ni3AI dont la teneur en bore allait jusqu'a 1,12 at.%. Les etudes furent faites dans l'etat initial des lingots et apres homogeneisation. On a observe une seconde phase dans le lingot contenant 1,12 at.% bore. L'effet de l'homogeneisation sur cette phase est decrit. Die Feinstruktur nominaler, boronhaltiger, stOchiometrischer Ni3AI Gussproben, die bis zu 1,12 At.% Boron enthielten, wurden nach der Produktion und nach einer Homogenisierung untersucht. Eine zweite Phase wurde in der Gussprobe beobachtet, die 1,12 At.% Boron enthielt. Die Auswirkung der Homogenisierung auf diese Phase wird beschrieben.
Introduction Previous success with the extrusion of rapidly solidified powders [I] of Ni3AI doped with boron led to an attempt to extrude ingots directly. Thus, nominally stoichiometric ingots with varying boron contents were cast, given a homogenization anneal, and extruded. This note presents the microstructure of the ingots. A separate note [2] will present the microstructures of the extruded product. This work is part of a larger effort focused on the structure and properties of the potentially useful structural aluminide, Ni3A1.
* Present address: Burndy Corporation, Norwalk, Connecticut 06856.
© Elsevier Science Publishing Co., Inc., 1988 52 Vanderbilt Ave., New York, NY 10017
0026-0800/88/$3.50
208
V. Zinoviev, E. M. Schulson, and I. Baker
Experimental Procedure Nominally stoichiometric ingots (50 mm in diameter) of Ni3AI were prepared with boron contents of 0.05, 0.09, 0.19, 0.37, 0.75, and 1.12 at.%. The boron was added as a nickel boride. The ingots were homogenized in air at 1473 K for 24 hr. Specimens were cut from the ingots, metallographically prepared and then examined using optical microscopy and scanning electron microscopy. Polished as-cast and homogenized specimens from the ingot containing 1.12 at.% boron were further examined using wavelength dispersive spectroscopy (WDS). In addition, scrapings from the homogenized ingot (1.12 at.% boron) were examined using WDS and Debye-Scherrer x-ray analysis.
(a) FIo. 1. Optical micrograph of an as-received ingot (1.12 at.% boron) showing dendritic segregation (coring) with fine plate-like precipitates, A, and additional phase at the grain boundaries and as islands in the matrix, B. The etchant preferably attacked the cored regions " A , " thus they appear black. At higher magnification the globular nodules are seen in (b) arrowed. Both (a) and (b) etched with Marble's Reagent.
209
Ni3AI Ingots Containing B
i li~ ii iiii,,Liiiiii!!!!!Aiiii ¸
(b) FIG. 1. (continued)
Results
THE AS-RECEIVED INGOTS All of the as-cast ingots exhibited dendritic segregation or coring but little porosity. The coring structures found in this study, including fine plate-like regions within the secondary structure (see Fig. 1[a], " A " ) , are very much like the structures found by Grala [3]. X-ray diffraction studies performed by Grala [3] on as-cast Ni3A1 (undoped) suggest that some NiAI was retained as the secondary structure. Coring is, therefore, more or less a result of segregation in the Ni-A1 system, and the only effect of boron seems to be to increase its extent. Two unusual features were found in the highest boron containing ingot. In addition to coring, another phase was observed, Figure 1[a] " B " , which had precipitated at grain boundaries and as islands in the matrix. Within these islands were globular nodules budding off the parent alloy (shown
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V. Zinoviev, E. M. Schulson, and I. Baker
with an arrow in Fig. l[b]). These nodules appear to be a third phase. With the exception of the nodules, these features are similar to those exhibited by castings of nickel-based alloys Hastelloy B and C, which c o n t a i n M 6 C precipitates at grain boundaries and within the gamma matrix [4]. In this c a s e M 6 B may be the precipitate. THE H O M O G E N I Z E D INGOTS After homogenization, the microstructure of all ingots changed. The cored structure was removed, and the "islands" in the 1.12 at.% boron ingot developed into a few isolated intergranular pathways about 8 p,m
(a) FIG. 2. Optical micrograph of the homogenized ingot (1.12 at.% boron) showing coring removed but additional phase represented as a few isolated pathways. Note the minimal porosity (black spots) in (a). The globular nodules of the as-received ingot persist and are shown with arrow in (b) at a higher magnification. Photo (a) unetched. Photo (b) etched with Marble's Reagent.
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(b) FIG. 2.
(continued)
in width (Fig. 2). The globular nodules, however, persisted as seen in Figure 2[b]. The overall outward appearance of the homogenized ingots is noteworthy. The ingots with the highest boron concentrations (0.75 and 1.12 at.%) showed unusual oxidation. A blue-colored scale and green-colored "pimples" were distributed randomly over the surface with a higher number density around the pipe (Fig. 3[a]). In the 1.12 at.% boron ingot the "pimples" were larger in diameter (1-2 mm). The ingot containing 0.37 at.% boron showed some "pimpling", whereas the ingots containing less boron showed little or none. This effect resembled "tin sweat," which occurs in phosphorus bronzes of high phosphorus and tin concentrations
[5]. A WDS analysis performed on the "pimples" scraped from the surface of the ingot (1.12 at.% boron) showed a high concentration of boron ( ~ 2 6 at.%) as well as ~49 at.% oxygen. There were scattered concentrations
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of nickel (~3-48 at.%) while the aluminium concentrations averaged around ~25 at.%. Although the results of the WDS were only semi-quantative, the relatively high levels of boron suggest that this element may have "sweated" toward the free surface. A Debye-Scherrer x-ray analysis on the scrapings produced diffraction patterns consistent with the compounds of NiO, NiA1204, and 9A1203"2B203. These nickel oxides have been seen on the surfaces of dilute Ni-A1 alloys [6] oxidized in air at 1473 K. These compounds are also in agreement with the elemental contents determined by the WDS analyses. Scanning electron microscopy of the pimples revealed a faceted structure (Fig. 3[b]), suggesting that boron may be part of a complex compound as suggested by the Debye-Scherrer results.
(a) FIG.3. (a) Homogenizedingots(50-mmdiameter, 114mmlong)showingpimpling,boron content increases to the right, 0.19, 0.75, and 1.12 at.%; (b) scanningelectron micrograph of pimples from ingot with 1.12 at.% boron.
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(b) Flo. 3. (continued)
Discussion In comparing the as-cast and the homogenized ingots, we found that both have a matrix that is more or less stoichiometric Ni3AI. Inside both the islands and the pathways, however, the aluminium concentration decreased to ~ 10 at.%, the nickel concentration increased to ~84 at.%, and the boron concentration increased to ~6 at.%. Additionally, the closeness of the elemental concentrations in both the islands and the pathways suggest they are the same phase. The globular nodules had intermediate compositional values, i.e., compositions ranging between the matrix and the island/pathway phase. The high boron and the high nickel levels may have resulted either from poor mixing of the nickel boride (used in the preparation of the alloys) or from microsegregation during freezing. The latter possibility is the more likely, because the Ni3A1 +B system, like the Ni-B system, probably contains a low melting point eutectic. Furthermore, the globular appearance of the nodules suggests precipitation out of a liquid rather than a
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solid solution, and implies that the island/pathway feature is a low melting phase. The solubility limit of boron in Ni-rich Ni3A1 (24 at.% AI) is estimated to be between 0.93-1.87 at.% [7]. In melt-spun stoichiometric ribbons [8] up to 1.5 at.% boron can be in solution. The island/pathway phase, therefore, could be a low melting phase rich in nickel and in boron and may only be present in the ingot with 1.12 at.% boron because of its proximity to the solubility limit. During homogenization, therefore, the grain boundary and island-like phase (Fig. l[a] " B " ) most likely melted, wetted the grain boundaries and, upon freezing, formed the pathways (Fig. 2[a]). The similarity in compostions of the pathways and the "pimples" suggests that near and on the surfaces, the liquidified phase, sweated out and formed oxides ("pimples"). The grain boundary-island phase and/or pathways may not have formed in other ingots owing to their lower boron concentrations. Conclusion Investigation of the microstructure of Ni3AI ingots containing boron has revealed a phase, rich in both nickel and boron, which is present only in the ingot with the highest boron content (1.12 at.%). Homogenization changed the morphology of this phase, i.e., numerous island features changed into a few isolated intergranular pathways.
The authors acknowledge Dr. F. D. Lemkey of the United Technologies Research Center, East Hartford, Ct., for providing the ingots. Also appreciated was the help of Mr. V. A. Surprenant and Louisa Howard. The use of the Dartmouth College electron microscope facility is gratefully acknowledged. This work was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy, Grant No. DE-FG02-84 ER 45148. References 1. I. Baker, J. A. Horton, and E. M. Schulson, The structure of consolidatedrapidly solidified powders of Ni3A1,J. Mat. Sci., 21:3297-3301 (1986). 2. V. Zinovievet al., in press. 3. E. M. Grala, Investigationsof NiA1and Ni3A1,in MechanicalPropertiesoflntermetallic Compounds (J. H. Westbrook,ed.), John Wileyand Sons, Inc., New York (1960), Chap. 17, pp. 358-404. 4. ASM Committee,Microstructureof nickel-base and cobalt-base heat-resistancecasting alloys, in Metals Handbook, Vol 7., 8th ed., (T. Lyman, ed.), American Society for Metals, Metals Park, Ohio (1972), pp. 187-196.
Ni3Al Ingots Containing B
215
5. J. L. Francis, Copper and heavy non-ferrous casting alloys, in Applied Science in the Casting of Metals (K. Strauss, ed.), Pergamon Press, Oxford (1970), Chap. 6, p. 219. 6. H. Hindam and D. P. Whittle, High temperature internal oxidation behavior of dilute Ni-A1 alloys, J. Mat. Sci., 18:1389-1404 (1983). 7. C. T. Liu, C. L. White, and J. A. Horton, Effect of boron on grain-boundaries in Ni3AI, Acta Metall., 33:213-229 (1985). 8. S. C. Huang, A. I. Taub, and K. M. Chang, Boron extended solubility and strengthening potency in rapidly solidified Ni3AI, Acta Metall., 32:1703-1707 (1984).
Received June 1987; accepted October 1987.
METALLOGRAPHY 21:207-215 (1988)
Microstructure of NlaAI Ingots Containing Boron
V. ZINOVIEV,* E. M. SCHULSON, AND I. BAKER
Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire
The microstructures of nominally stoichiometric ingots of NiaA1 with boron contents up to 1.12 at.% were examined in the as-received and in the homogenized states. A second phase was observed in the ingot containing 1.12 at.% boron. The effect of homogenization on this phase is described. On a examine les microstructures des lingots stoichiometriques de Ni3AI dont la teneur en bore allait jusqu'a 1,12 at.%. Les etudes furent faites dans l'etat initial des lingots et apres homogeneisation. On a observe une seconde phase dans le lingot contenant 1,12 at.% bore. L'effet de l'homogeneisation sur cette phase est decrit. Die Feinstruktur nominaler, boronhaltiger, stOchiometrischer Ni3AI Gussproben, die bis zu 1,12 At.% Boron enthielten, wurden nach der Produktion und nach einer Homogenisierung untersucht. Eine zweite Phase wurde in der Gussprobe beobachtet, die 1,12 At.% Boron enthielt. Die Auswirkung der Homogenisierung auf diese Phase wird beschrieben.
Introduction Previous success with the extrusion of rapidly solidified powders [I] of Ni3AI doped with boron led to an attempt to extrude ingots directly. Thus, nominally stoichiometric ingots with varying boron contents were cast, given a homogenization anneal, and extruded. This note presents the microstructure of the ingots. A separate note [2] will present the microstructures of the extruded product. This work is part of a larger effort focused on the structure and properties of the potentially useful structural aluminide, Ni3A1.
* Present address: Burndy Corporation, Norwalk, Connecticut 06856.
© Elsevier Science Publishing Co., Inc., 1988 52 Vanderbilt Ave., New York, NY 10017
0026-0800/88/$3.50
208
V. Zinoviev, E. M. Schulson, and I. Baker
Experimental Procedure Nominally stoichiometric ingots (50 mm in diameter) of Ni3AI were prepared with boron contents of 0.05, 0.09, 0.19, 0.37, 0.75, and 1.12 at.%. The boron was added as a nickel boride. The ingots were homogenized in air at 1473 K for 24 hr. Specimens were cut from the ingots, metallographically prepared and then examined using optical microscopy and scanning electron microscopy. Polished as-cast and homogenized specimens from the ingot containing 1.12 at.% boron were further examined using wavelength dispersive spectroscopy (WDS). In addition, scrapings from the homogenized ingot (1.12 at.% boron) were examined using WDS and Debye-Scherrer x-ray analysis.
(a) FIo. 1. Optical micrograph of an as-received ingot (1.12 at.% boron) showing dendritic segregation (coring) with fine plate-like precipitates, A, and additional phase at the grain boundaries and as islands in the matrix, B. The etchant preferably attacked the cored regions " A , " thus they appear black. At higher magnification the globular nodules are seen in (b) arrowed. Both (a) and (b) etched with Marble's Reagent.
209
Ni3AI Ingots Containing B
i li~ ii iiii,,Liiiiii!!!!!Aiiii ¸
(b) FIG. 1. (continued)
Results
THE AS-RECEIVED INGOTS All of the as-cast ingots exhibited dendritic segregation or coring but little porosity. The coring structures found in this study, including fine plate-like regions within the secondary structure (see Fig. 1[a], " A " ) , are very much like the structures found by Grala [3]. X-ray diffraction studies performed by Grala [3] on as-cast Ni3A1 (undoped) suggest that some NiAI was retained as the secondary structure. Coring is, therefore, more or less a result of segregation in the Ni-A1 system, and the only effect of boron seems to be to increase its extent. Two unusual features were found in the highest boron containing ingot. In addition to coring, another phase was observed, Figure 1[a] " B " , which had precipitated at grain boundaries and as islands in the matrix. Within these islands were globular nodules budding off the parent alloy (shown
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V. Zinoviev, E. M. Schulson, and I. Baker
with an arrow in Fig. l[b]). These nodules appear to be a third phase. With the exception of the nodules, these features are similar to those exhibited by castings of nickel-based alloys Hastelloy B and C, which c o n t a i n M 6 C precipitates at grain boundaries and within the gamma matrix [4]. In this c a s e M 6 B may be the precipitate. THE H O M O G E N I Z E D INGOTS After homogenization, the microstructure of all ingots changed. The cored structure was removed, and the "islands" in the 1.12 at.% boron ingot developed into a few isolated intergranular pathways about 8 p,m
(a) FIG. 2. Optical micrograph of the homogenized ingot (1.12 at.% boron) showing coring removed but additional phase represented as a few isolated pathways. Note the minimal porosity (black spots) in (a). The globular nodules of the as-received ingot persist and are shown with arrow in (b) at a higher magnification. Photo (a) unetched. Photo (b) etched with Marble's Reagent.
Ni3AI Ingots Containing B
211
(b) FIG. 2.
(continued)
in width (Fig. 2). The globular nodules, however, persisted as seen in Figure 2[b]. The overall outward appearance of the homogenized ingots is noteworthy. The ingots with the highest boron concentrations (0.75 and 1.12 at.%) showed unusual oxidation. A blue-colored scale and green-colored "pimples" were distributed randomly over the surface with a higher number density around the pipe (Fig. 3[a]). In the 1.12 at.% boron ingot the "pimples" were larger in diameter (1-2 mm). The ingot containing 0.37 at.% boron showed some "pimpling", whereas the ingots containing less boron showed little or none. This effect resembled "tin sweat," which occurs in phosphorus bronzes of high phosphorus and tin concentrations
[5]. A WDS analysis performed on the "pimples" scraped from the surface of the ingot (1.12 at.% boron) showed a high concentration of boron ( ~ 2 6 at.%) as well as ~49 at.% oxygen. There were scattered concentrations
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V. Zinoviev, E. M. Schulson, and I. Baker
of nickel (~3-48 at.%) while the aluminium concentrations averaged around ~25 at.%. Although the results of the WDS were only semi-quantative, the relatively high levels of boron suggest that this element may have "sweated" toward the free surface. A Debye-Scherrer x-ray analysis on the scrapings produced diffraction patterns consistent with the compounds of NiO, NiA1204, and 9A1203"2B203. These nickel oxides have been seen on the surfaces of dilute Ni-A1 alloys [6] oxidized in air at 1473 K. These compounds are also in agreement with the elemental contents determined by the WDS analyses. Scanning electron microscopy of the pimples revealed a faceted structure (Fig. 3[b]), suggesting that boron may be part of a complex compound as suggested by the Debye-Scherrer results.
(a) FIG.3. (a) Homogenizedingots(50-mmdiameter, 114mmlong)showingpimpling,boron content increases to the right, 0.19, 0.75, and 1.12 at.%; (b) scanningelectron micrograph of pimples from ingot with 1.12 at.% boron.
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(b) Flo. 3. (continued)
Discussion In comparing the as-cast and the homogenized ingots, we found that both have a matrix that is more or less stoichiometric Ni3AI. Inside both the islands and the pathways, however, the aluminium concentration decreased to ~ 10 at.%, the nickel concentration increased to ~84 at.%, and the boron concentration increased to ~6 at.%. Additionally, the closeness of the elemental concentrations in both the islands and the pathways suggest they are the same phase. The globular nodules had intermediate compositional values, i.e., compositions ranging between the matrix and the island/pathway phase. The high boron and the high nickel levels may have resulted either from poor mixing of the nickel boride (used in the preparation of the alloys) or from microsegregation during freezing. The latter possibility is the more likely, because the Ni3A1 +B system, like the Ni-B system, probably contains a low melting point eutectic. Furthermore, the globular appearance of the nodules suggests precipitation out of a liquid rather than a
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V. Zinoviev, E. M. Schulson, and I. Baker
solid solution, and implies that the island/pathway feature is a low melting phase. The solubility limit of boron in Ni-rich Ni3A1 (24 at.% AI) is estimated to be between 0.93-1.87 at.% [7]. In melt-spun stoichiometric ribbons [8] up to 1.5 at.% boron can be in solution. The island/pathway phase, therefore, could be a low melting phase rich in nickel and in boron and may only be present in the ingot with 1.12 at.% boron because of its proximity to the solubility limit. During homogenization, therefore, the grain boundary and island-like phase (Fig. l[a] " B " ) most likely melted, wetted the grain boundaries and, upon freezing, formed the pathways (Fig. 2[a]). The similarity in compostions of the pathways and the "pimples" suggests that near and on the surfaces, the liquidified phase, sweated out and formed oxides ("pimples"). The grain boundary-island phase and/or pathways may not have formed in other ingots owing to their lower boron concentrations. Conclusion Investigation of the microstructure of Ni3AI ingots containing boron has revealed a phase, rich in both nickel and boron, which is present only in the ingot with the highest boron content (1.12 at.%). Homogenization changed the morphology of this phase, i.e., numerous island features changed into a few isolated intergranular pathways.
The authors acknowledge Dr. F. D. Lemkey of the United Technologies Research Center, East Hartford, Ct., for providing the ingots. Also appreciated was the help of Mr. V. A. Surprenant and Louisa Howard. The use of the Dartmouth College electron microscope facility is gratefully acknowledged. This work was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy, Grant No. DE-FG02-84 ER 45148. References 1. I. Baker, J. A. Horton, and E. M. Schulson, The structure of consolidatedrapidly solidified powders of Ni3A1,J. Mat. Sci., 21:3297-3301 (1986). 2. V. Zinovievet al., in press. 3. E. M. Grala, Investigationsof NiA1and Ni3A1,in MechanicalPropertiesoflntermetallic Compounds (J. H. Westbrook,ed.), John Wileyand Sons, Inc., New York (1960), Chap. 17, pp. 358-404. 4. ASM Committee,Microstructureof nickel-base and cobalt-base heat-resistancecasting alloys, in Metals Handbook, Vol 7., 8th ed., (T. Lyman, ed.), American Society for Metals, Metals Park, Ohio (1972), pp. 187-196.
Ni3Al Ingots Containing B
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5. J. L. Francis, Copper and heavy non-ferrous casting alloys, in Applied Science in the Casting of Metals (K. Strauss, ed.), Pergamon Press, Oxford (1970), Chap. 6, p. 219. 6. H. Hindam and D. P. Whittle, High temperature internal oxidation behavior of dilute Ni-A1 alloys, J. Mat. Sci., 18:1389-1404 (1983). 7. C. T. Liu, C. L. White, and J. A. Horton, Effect of boron on grain-boundaries in Ni3AI, Acta Metall., 33:213-229 (1985). 8. S. C. Huang, A. I. Taub, and K. M. Chang, Boron extended solubility and strengthening potency in rapidly solidified Ni3AI, Acta Metall., 32:1703-1707 (1984).
Received June 1987; accepted October 1987.