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Precipitate crystallography and morphology in undercooled, rapidly solidified titanium rare earth alloys
Scripta Materialia, Vol. 36, No. 2, pp. 157-163, 1997 Elsevier Science Ltd Comiaht 0 1996 Acta Metallwaica Inc. 6&d in the USA. All rights &served 13594462197 $17.00 + .OO
Pergamon
PI1 S1359-6462(96)00364-S
PRECIPllTATE CRYSTALLOGRAPHY AND MORPHOLOGY UNDERCOOLED, RAPIDLY SOLIDIFIED TITANIUM RARE EARTH ALLOYS
IN
M.V. k-al*, W.H. Hofmeister and J.E. Wittig Department of Applied and Engineering Sciences, Vanderbilt University, Nashville, TN 37240 *Currently at Physical Metallurgy Branch, Naval Research Laboratory, Washington, DC 20375-5000 (Received April 15, 1996) (Accepted June 11, 1996) Introduction Earlier researclh showed that the addition of rare earth elements to titanium alloys can produce dispersions of $80 mm diameter rare earth-rich precipitates in an a titanium matrix. [l] Subsequent work used rapid solidification processing techniques such as melt spinning, laser surface melting and splat quenching to optimize the distribution of rare earth particles. [2,3] Rapidly solidified titanium-rare earth alloys generally form supersaturated martensitic a’ Ti with annealing required to precipitate the rare earth-rich particles. Characterization by x-ray and electron diffraction usually indicates that these particles are rare earth sesquioxides (REZ03) [l-6], although there have been reports of elemental rare earths [7, 81, non-stoichiometric oxides and carbides [ 1,5]. In the present experiments on three titanium-rare earth alloys (Ti- 1.4 at. % Ce, Ti- 1.7 Er and Ti-1.5 La), electromagnetic levitation techniques allowed large bulk liquid undercoolings to be induced prior to rapid solidification via splat quenching. The resulting microstructnres are quite different from those found in earlier rapid solidification experiments. As-solidified Ti-Ce and Ti-Er alloys exhibited a distribution of elemental rare earth precipitates within regularly-spaced planar or curved sheets throughout an equiaxed aTi matrix. The Ti-La alloy contained randomly distributed metallic lanthanum precipitates within equiaxed 01Ti grains. As in previous studies, annealing treatments resulted in oxidation of the particles. However, annealed precipitates showed different crystal structures than those identified in prior research. This paper describes the unique precipitate morphologies, structures and orientation relationships present in undercooled and rapidly solidified titanium-rare earth alloys. Experimental Procedure The present alloys were prepared by arc-melting with nominal compositions of Ti-1.4 Ce, Ti-1.7 Er and Ti-1.5 La (at. %) and an initial nominal oxygen content of 0.1 at. %. Samples of the arc-melted materials (0.3 * O.Olg) were electromagnetically levitated, melted, undercooled approximately 2OO’K and splat quenched between copper anvils. Splat quenching of undercooled alloys resulted in discs
157
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150-500 urn thick and l-3 cm across. This experiment has been described in detail by Bertero. [9] Selected samples were annealed at 973°K for 10 hours in quartz tubes backfilled with argon. Transmission electron microscopy (TEM) samples were mechanically punched from the rapidly quenched and annealed materials and ground so that after electropolishing regions approximately 50 pm from the contact surface could be examined. Thin foils were produced by twin-jet electropolishing in an electrolyte composed of 59% methanol, 35% n-butyl alcohol and 6% perchloric acid at 248”K, 21 V, 23 mA. Transmission electron microscopy was performed using a Philips CM20 TEIWSTEM. Multiple selected area diffraction patterns (SAD) were obtained from several zone axes for each sample in the as-quenched and annealed condition. Diffraction spot spacings were measured using a Nikon Profile Projector, Model 6c. The known lattice spacings of the a Ti matrix provided an internal calibration for each diffraction pattern, which allowed for precipitate lattice parameter determination with an error of approximately f 0.01 nm. Lattice spacings in the patterns were compared to published values for titanium, elemental rare earths and rare earth oxides. [lo] Diffract software version 1.3 [ 1 l] was used for diffraction pattern simulation to confirm the determined structures and orientation relationships. Solidification Microstructure Microstructures in the as-rapidly solidified materials are consistent with non-equilibrium solidification of hypoeutectic (Ti-Er) and hypomonotectic (Ti-Ce and Ti-La) alloys, as predicted by the equilibrium phase diagrams. [ 121 Scanning electron microscopy revealed the presence of large, 0. l-l pm diameter rare earth-rich precipitates at prior p Ti grain boundaries. This microstructure is a result of primary solidification of the p Ti phase accompanied by the rejection of rare earth-rich liquid, followed by a eutectic or monotectic reaction. Transmission electron microscopy showed that samples with small prior undercoolings were composed of martensitic a’ Ti, similar to microstructures typically found in rapidly solidified titanium alloys. [2-4] However, samples solidified with large prior undercoolings (lOO-300°K beneath the liquidus) contained equiaxed a. Ti grains. Solid state cooling rates have been shown to decrease with increasing undercooling due to an increase in the specimens’ cross-sectional thickness. [ 131 In undercooled samples, the solid state cooling rate is less than the critical rate for martensite formation. Precipitate Morphology Figures l-3 (a and b) are transmission electron micrographs of representative areas within equiaxed a Ti grains for each alloy. In the Ti-Ce and Ti-Er alloys, 3-20 nm diameter spherical precipitates are distributed in planar or curved sheets. As shown in figures la and 2a, precipitate sheets become evident in these alloys when the sample is tilted appropriately. Previous investigations of titanium-rare earth alloys have identified precipitate sheet morphologies in a Ti-Al-Y alloy [3] and a Ti- 1.4 at. % Ce alloy [ 141. These microstructures are consistent with eutectoid-like decomposition of the p Ti phase, despite the current understanding that Ti-Ce and Ti-Er alloys transform by peritectic and peritectoid reactions, respectively. [12] It should be noted that similar precipitate morphologies have been extensively documented in alloy steels and attributed to precipitation at interphase boundaries during the austentite+ferrite transformation. [ 151 The diverse particle morphologies in the Ti-Er alloy and the mechanisms for their formation will be described in detail in subsequent work. [16] In contrast with the other titanium-rare earth alloys, the Ti-La alloy contained 20 nm diameter spherical particles randomly dispersed within the olTi matrix (Figure 3a). Annealing treatments result in precipitate
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Figure 1. (a,b) TEM micrographs of typical as-quenched microstructures of an undercooled Ti-1.4Ce ahoy. Bands of particles are characteristic of interphase boundary precipitation. (c,d) Experimental and simulated a Ti matrix selected area diffraction (SAD) patterns from as-quenched Ti-1.4Ce indicate that the particles are Ce (Fm3 m). (e,t) Experimental and simulated a Ti matrix SAD patterns from an annealed Ti-1.4Ce alloy indicate that both Ce and CeOr (Fmj m) particles are present. Orientation relationships are given in Table 2.
coarsening and\ diminished evidence of the as-quenched morphologies. Table 1 summarizes the average particle siz,e and standard deviations for each alloy in the as-quenched and annealed states.
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. l * .*.
Figure 2. (a,b) TEM micrographs of a typical as-quenched microstructure of an undercooled Ti-1.7Er alloy. Interphase boundary precipitates are approximately 5 nm in diameter within sheets spaced approximately 20 mn apart. (c,d) Experimental and simulated a Ti matrix SAD patterns from an as-quenched sample show that the particles are Er (P63/mmc). (e,f) SAD patterns from an annealed specimen indicate that the particles are composed of Er203 (Fmjm). Orientation relationships are given in Table 2.
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Figure 3. (a,b) TEIM micrograpbs of typical as-quenched microstructures of an undercooled Ti-I.SLa alloy. The random arrangement of precipitates indicates that they formed by decomposition of supersaturated a Ti. (c,d) Experimental and simulated a Ti matrix SAD patterns from as-quenched samples show that the particles are La (Fm?.m) (e,I) Experimental and simulated a Ti matrix SAD patterns from an annealed sample indicate that both elemental La and LqO, (13 ) particles are present. Orientation relationships are given in Table 2.
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TABLE 1 Average Precipitate Sizes Allov (at. %) Ti- I .4 Cc Ti-1.7 Er Ti-1.5 La
As-quenched Diameter (nm) 17*4 5f2 17f4
Annealed Diameter (nm) 2:::2 22f6
Precipitate Crystallograph;ll Figures l-3 (c and d) show typical experimental and simulated (0001) SAD zone axis patterns for asquenched Ti-Ce, Er and La alloys. Precipitate spots are consistent with the structure and lattice spacings of elemental rare earths, i.e., Ce (Fmgm), Er (P6Jmmc) and La (Fm?m). Although simulated elemental erbium and erbium sesquioxide diffraction patterns are similar, low index zone axis patterns consistently indicated that as-quenched Ti-Er alloys contained elemental erbium precipitates. Reflections are present that cannot be attributed to any erbium oxide, carbide or nitride. The large negative heat of formation of rare earth oxides and the unavoidable presence of oxygen result in the oxidation of elemental rare earth particles during annealing treatments. Figures l-3 (e and f) show experimental and simulated a Ti matrix zone axis patterns for annealed Ti-Ce, Er and La alloys. In annealed Ti-Ce and La alloys, precipitate spots are consistent with the presence of both elemental rare earth and rare earth oxide precipitates. Comparing annealed sample patterns with asquenched SAD patterns shows that the intensity of the elemental rare earth spots has decreased and that rare earth oxide spots have appeared. Precipitate spots in the annealed Ti-Ce alloy compare well with the structure and lattice spacings of elemental Ce (FmJm) and Ce02 (Fmh). In annealed Ti-La specimens, precipitates have been identified as elemental La (Fmjm) and La20) (Iaj). Simulated SAD patterns for a Ti with Er203 precipitates having either the Frnh or the Ia space group were compared with experimental patterns from the annealed Ti-Er alloy. Precipitate planar spacings are similar for both precipitate allotropes in a Ti (0001) zone axis patterns. Although the simulated Id allotrope [4] shows slightly better correlation to experimental spot spacings, precipitate spots in (1210) and (1213) zone axis patterns clearly show the symmetry that would be expected if the precipitates were Er203 (Fm-$n). No evidence of elemental Er precipitates was found in the annealed specimens. Experimentally determined d-spacings were generally within 1% of calculated values, except in the case of the elemental erbium and Erz03 (Fmjm), which showed maximum differences of 3.75 and 2.5%, respectively. Coherency strain and the presence of a non-stoichiometric proportion of oxygen (or other interstitials) in erbium-rich particles may account for these discrepancies. Table 2 summarizes the particles’ structures and matrix orientation relationships that were determined in the present work.
Precipitate
Structures
TABLE 2 and OrientationRelationships
Vol. 36, No. 2
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Conclusions Previous investigations of rapidly solidified titanium-rare earth alloys show supersaturated martensitic a’ Ti microstructures that are essentially devoid of precipitates in the as-quenched condition. In contrast, as-quenched Ti-Ce, Ti-Er and T&La alloys with large bulk liquid undercoolings just prior to rapid solidification contain dispersions of 3-20 nm diameter elemental rare earth particles within equiaxed a Ti grains. Electron diffraction experiments indicate that the precipitates are semi-coherent metallic rare earths. In the Ti-Ce and Ti-Er alloys, particles are arranged in planar and curved sheets. These are the first observations of this precipitate morphology in a Ti-Er alloy. Annealed materials contain both semi-coherent elemental rare earth particles and semi-coherent rare earth oxide particles with structures and orientation relationships that are unique to this undercooling/rapid solidification study. Acknowledgments This research was supported by the National Science Foundation, Grant No. 92-02308 and the NASA Microgravity Science and Applications Division. Alloys were prepared by General Electric Corporate Research and Development, Schenectady, NY. Discussions with D.R. Kegley were greatly appreciated. MVK gratefully acknowledges the support of the American Society for Engineering Education during preparation of this manuscript. References 1. B.B. Rath, R.J. Lederich and J.E. O’Neal, in “Grain Boundaries in Engineering Materials”, Ed. by J.L. Walter, J.H. Westbrook and D.A. Woodford, p. 39, Claitor’s Publishing Division, Baton Rouge, La. (1974). 2. S.M.L. Sastry, P. J. Meschter and J. E. ONeal, Metall. Trans. A 15, 1451 (1984). 3. D.G. Konitzer, R.C. Muddle and H.L. Fraser, Scripta Metal]. 17,963 (1983). 4. D.G. Konitzer, J.T. Stanley, M.H. Loretto and H.L. Fraser, Acta. Metal]. 34, 1269 (1986). 5. S. Naka, H. Octor, E. Bouchud and T. Khan, Scripta Metal]. 23,501 (1989) 6. J. P. A. Lovander, S. A. Court, R. Wheeler, J. W. Sears, D. A. Watson and H. L. Fraser, in “Titanium Rapid Solidification Technology”, Ed. by F.H. Froes and D. Eylon, p. 77, Metallurgical Society of AIME (1986). 7. D.R. Kegley, .J.E. Wittig, W.H. Hofmeister, R.J. Bayuzick and R.G. Rowe, Trans. Tech. Pub. 50, 129 (1989). 8. S.A. Court, J.W. Sears, M.H. Loretto and H.L. Fraser, Mat. Sci. Eng. 98,243 (1988). 9. G.A. Bertero, W.H. Hofmeister, M.B. Robinson and R.J. Bayuzick, Metall. Trans. A 22,2723 (1991). 10. P. Villars and L.D. Calvert, Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, American Society for Metals, Metats Park, OH. (1991). 11. “Diffract” software version 1.3 was used under license from microdev Company, Evergreen, Co. 12. J.L. Murray, Phase Diagrams of Binary Titanium Alloy, American Society for Metals, Metals Park, OH. (1987). 13. M.V. Kral, Ph.D. thesis, Vanderbilt University (1996). 14. D.R. Kegley, private communication, Vanderbilt University, (1989). 15. R.W.K. Honeycombe, Metall. Trans. A 7,915 (1976). 16. M.V. Kral, W H. Hofmeister and J.E. Wittig, submitted to Metall. Trans. A, (1996).
Pergamon
PI1 S1359-6462(96)00364-S
PRECIPllTATE CRYSTALLOGRAPHY AND MORPHOLOGY UNDERCOOLED, RAPIDLY SOLIDIFIED TITANIUM RARE EARTH ALLOYS
IN
M.V. k-al*, W.H. Hofmeister and J.E. Wittig Department of Applied and Engineering Sciences, Vanderbilt University, Nashville, TN 37240 *Currently at Physical Metallurgy Branch, Naval Research Laboratory, Washington, DC 20375-5000 (Received April 15, 1996) (Accepted June 11, 1996) Introduction Earlier researclh showed that the addition of rare earth elements to titanium alloys can produce dispersions of $80 mm diameter rare earth-rich precipitates in an a titanium matrix. [l] Subsequent work used rapid solidification processing techniques such as melt spinning, laser surface melting and splat quenching to optimize the distribution of rare earth particles. [2,3] Rapidly solidified titanium-rare earth alloys generally form supersaturated martensitic a’ Ti with annealing required to precipitate the rare earth-rich particles. Characterization by x-ray and electron diffraction usually indicates that these particles are rare earth sesquioxides (REZ03) [l-6], although there have been reports of elemental rare earths [7, 81, non-stoichiometric oxides and carbides [ 1,5]. In the present experiments on three titanium-rare earth alloys (Ti- 1.4 at. % Ce, Ti- 1.7 Er and Ti-1.5 La), electromagnetic levitation techniques allowed large bulk liquid undercoolings to be induced prior to rapid solidification via splat quenching. The resulting microstructnres are quite different from those found in earlier rapid solidification experiments. As-solidified Ti-Ce and Ti-Er alloys exhibited a distribution of elemental rare earth precipitates within regularly-spaced planar or curved sheets throughout an equiaxed aTi matrix. The Ti-La alloy contained randomly distributed metallic lanthanum precipitates within equiaxed 01Ti grains. As in previous studies, annealing treatments resulted in oxidation of the particles. However, annealed precipitates showed different crystal structures than those identified in prior research. This paper describes the unique precipitate morphologies, structures and orientation relationships present in undercooled and rapidly solidified titanium-rare earth alloys. Experimental Procedure The present alloys were prepared by arc-melting with nominal compositions of Ti-1.4 Ce, Ti-1.7 Er and Ti-1.5 La (at. %) and an initial nominal oxygen content of 0.1 at. %. Samples of the arc-melted materials (0.3 * O.Olg) were electromagnetically levitated, melted, undercooled approximately 2OO’K and splat quenched between copper anvils. Splat quenching of undercooled alloys resulted in discs
157
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150-500 urn thick and l-3 cm across. This experiment has been described in detail by Bertero. [9] Selected samples were annealed at 973°K for 10 hours in quartz tubes backfilled with argon. Transmission electron microscopy (TEM) samples were mechanically punched from the rapidly quenched and annealed materials and ground so that after electropolishing regions approximately 50 pm from the contact surface could be examined. Thin foils were produced by twin-jet electropolishing in an electrolyte composed of 59% methanol, 35% n-butyl alcohol and 6% perchloric acid at 248”K, 21 V, 23 mA. Transmission electron microscopy was performed using a Philips CM20 TEIWSTEM. Multiple selected area diffraction patterns (SAD) were obtained from several zone axes for each sample in the as-quenched and annealed condition. Diffraction spot spacings were measured using a Nikon Profile Projector, Model 6c. The known lattice spacings of the a Ti matrix provided an internal calibration for each diffraction pattern, which allowed for precipitate lattice parameter determination with an error of approximately f 0.01 nm. Lattice spacings in the patterns were compared to published values for titanium, elemental rare earths and rare earth oxides. [lo] Diffract software version 1.3 [ 1 l] was used for diffraction pattern simulation to confirm the determined structures and orientation relationships. Solidification Microstructure Microstructures in the as-rapidly solidified materials are consistent with non-equilibrium solidification of hypoeutectic (Ti-Er) and hypomonotectic (Ti-Ce and Ti-La) alloys, as predicted by the equilibrium phase diagrams. [ 121 Scanning electron microscopy revealed the presence of large, 0. l-l pm diameter rare earth-rich precipitates at prior p Ti grain boundaries. This microstructure is a result of primary solidification of the p Ti phase accompanied by the rejection of rare earth-rich liquid, followed by a eutectic or monotectic reaction. Transmission electron microscopy showed that samples with small prior undercoolings were composed of martensitic a’ Ti, similar to microstructures typically found in rapidly solidified titanium alloys. [2-4] However, samples solidified with large prior undercoolings (lOO-300°K beneath the liquidus) contained equiaxed a. Ti grains. Solid state cooling rates have been shown to decrease with increasing undercooling due to an increase in the specimens’ cross-sectional thickness. [ 131 In undercooled samples, the solid state cooling rate is less than the critical rate for martensite formation. Precipitate Morphology Figures l-3 (a and b) are transmission electron micrographs of representative areas within equiaxed a Ti grains for each alloy. In the Ti-Ce and Ti-Er alloys, 3-20 nm diameter spherical precipitates are distributed in planar or curved sheets. As shown in figures la and 2a, precipitate sheets become evident in these alloys when the sample is tilted appropriately. Previous investigations of titanium-rare earth alloys have identified precipitate sheet morphologies in a Ti-Al-Y alloy [3] and a Ti- 1.4 at. % Ce alloy [ 141. These microstructures are consistent with eutectoid-like decomposition of the p Ti phase, despite the current understanding that Ti-Ce and Ti-Er alloys transform by peritectic and peritectoid reactions, respectively. [12] It should be noted that similar precipitate morphologies have been extensively documented in alloy steels and attributed to precipitation at interphase boundaries during the austentite+ferrite transformation. [ 151 The diverse particle morphologies in the Ti-Er alloy and the mechanisms for their formation will be described in detail in subsequent work. [16] In contrast with the other titanium-rare earth alloys, the Ti-La alloy contained 20 nm diameter spherical particles randomly dispersed within the olTi matrix (Figure 3a). Annealing treatments result in precipitate
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Figure 1. (a,b) TEM micrographs of typical as-quenched microstructures of an undercooled Ti-1.4Ce ahoy. Bands of particles are characteristic of interphase boundary precipitation. (c,d) Experimental and simulated a Ti matrix selected area diffraction (SAD) patterns from as-quenched Ti-1.4Ce indicate that the particles are Ce (Fm3 m). (e,t) Experimental and simulated a Ti matrix SAD patterns from an annealed Ti-1.4Ce alloy indicate that both Ce and CeOr (Fmj m) particles are present. Orientation relationships are given in Table 2.
coarsening and\ diminished evidence of the as-quenched morphologies. Table 1 summarizes the average particle siz,e and standard deviations for each alloy in the as-quenched and annealed states.
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Figure 2. (a,b) TEM micrographs of a typical as-quenched microstructure of an undercooled Ti-1.7Er alloy. Interphase boundary precipitates are approximately 5 nm in diameter within sheets spaced approximately 20 mn apart. (c,d) Experimental and simulated a Ti matrix SAD patterns from an as-quenched sample show that the particles are Er (P63/mmc). (e,f) SAD patterns from an annealed specimen indicate that the particles are composed of Er203 (Fmjm). Orientation relationships are given in Table 2.
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Figure 3. (a,b) TEIM micrograpbs of typical as-quenched microstructures of an undercooled Ti-I.SLa alloy. The random arrangement of precipitates indicates that they formed by decomposition of supersaturated a Ti. (c,d) Experimental and simulated a Ti matrix SAD patterns from as-quenched samples show that the particles are La (Fm?.m) (e,I) Experimental and simulated a Ti matrix SAD patterns from an annealed sample indicate that both elemental La and LqO, (13 ) particles are present. Orientation relationships are given in Table 2.
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TABLE 1 Average Precipitate Sizes Allov (at. %) Ti- I .4 Cc Ti-1.7 Er Ti-1.5 La
As-quenched Diameter (nm) 17*4 5f2 17f4
Annealed Diameter (nm) 2:::2 22f6
Precipitate Crystallograph;ll Figures l-3 (c and d) show typical experimental and simulated (0001) SAD zone axis patterns for asquenched Ti-Ce, Er and La alloys. Precipitate spots are consistent with the structure and lattice spacings of elemental rare earths, i.e., Ce (Fmgm), Er (P6Jmmc) and La (Fm?m). Although simulated elemental erbium and erbium sesquioxide diffraction patterns are similar, low index zone axis patterns consistently indicated that as-quenched Ti-Er alloys contained elemental erbium precipitates. Reflections are present that cannot be attributed to any erbium oxide, carbide or nitride. The large negative heat of formation of rare earth oxides and the unavoidable presence of oxygen result in the oxidation of elemental rare earth particles during annealing treatments. Figures l-3 (e and f) show experimental and simulated a Ti matrix zone axis patterns for annealed Ti-Ce, Er and La alloys. In annealed Ti-Ce and La alloys, precipitate spots are consistent with the presence of both elemental rare earth and rare earth oxide precipitates. Comparing annealed sample patterns with asquenched SAD patterns shows that the intensity of the elemental rare earth spots has decreased and that rare earth oxide spots have appeared. Precipitate spots in the annealed Ti-Ce alloy compare well with the structure and lattice spacings of elemental Ce (FmJm) and Ce02 (Fmh). In annealed Ti-La specimens, precipitates have been identified as elemental La (Fmjm) and La20) (Iaj). Simulated SAD patterns for a Ti with Er203 precipitates having either the Frnh or the Ia space group were compared with experimental patterns from the annealed Ti-Er alloy. Precipitate planar spacings are similar for both precipitate allotropes in a Ti (0001) zone axis patterns. Although the simulated Id allotrope [4] shows slightly better correlation to experimental spot spacings, precipitate spots in (1210) and (1213) zone axis patterns clearly show the symmetry that would be expected if the precipitates were Er203 (Fm-$n). No evidence of elemental Er precipitates was found in the annealed specimens. Experimentally determined d-spacings were generally within 1% of calculated values, except in the case of the elemental erbium and Erz03 (Fmjm), which showed maximum differences of 3.75 and 2.5%, respectively. Coherency strain and the presence of a non-stoichiometric proportion of oxygen (or other interstitials) in erbium-rich particles may account for these discrepancies. Table 2 summarizes the particles’ structures and matrix orientation relationships that were determined in the present work.
Precipitate
Structures
TABLE 2 and OrientationRelationships
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Conclusions Previous investigations of rapidly solidified titanium-rare earth alloys show supersaturated martensitic a’ Ti microstructures that are essentially devoid of precipitates in the as-quenched condition. In contrast, as-quenched Ti-Ce, Ti-Er and T&La alloys with large bulk liquid undercoolings just prior to rapid solidification contain dispersions of 3-20 nm diameter elemental rare earth particles within equiaxed a Ti grains. Electron diffraction experiments indicate that the precipitates are semi-coherent metallic rare earths. In the Ti-Ce and Ti-Er alloys, particles are arranged in planar and curved sheets. These are the first observations of this precipitate morphology in a Ti-Er alloy. Annealed materials contain both semi-coherent elemental rare earth particles and semi-coherent rare earth oxide particles with structures and orientation relationships that are unique to this undercooling/rapid solidification study. Acknowledgments This research was supported by the National Science Foundation, Grant No. 92-02308 and the NASA Microgravity Science and Applications Division. Alloys were prepared by General Electric Corporate Research and Development, Schenectady, NY. Discussions with D.R. Kegley were greatly appreciated. MVK gratefully acknowledges the support of the American Society for Engineering Education during preparation of this manuscript. References 1. B.B. Rath, R.J. Lederich and J.E. O’Neal, in “Grain Boundaries in Engineering Materials”, Ed. by J.L. Walter, J.H. Westbrook and D.A. Woodford, p. 39, Claitor’s Publishing Division, Baton Rouge, La. (1974). 2. S.M.L. Sastry, P. J. Meschter and J. E. ONeal, Metall. Trans. A 15, 1451 (1984). 3. D.G. Konitzer, R.C. Muddle and H.L. Fraser, Scripta Metal]. 17,963 (1983). 4. D.G. Konitzer, J.T. Stanley, M.H. Loretto and H.L. Fraser, Acta. Metal]. 34, 1269 (1986). 5. S. Naka, H. Octor, E. Bouchud and T. Khan, Scripta Metal]. 23,501 (1989) 6. J. P. A. Lovander, S. A. Court, R. Wheeler, J. W. Sears, D. A. Watson and H. L. Fraser, in “Titanium Rapid Solidification Technology”, Ed. by F.H. Froes and D. Eylon, p. 77, Metallurgical Society of AIME (1986). 7. D.R. Kegley, .J.E. Wittig, W.H. Hofmeister, R.J. Bayuzick and R.G. Rowe, Trans. Tech. Pub. 50, 129 (1989). 8. S.A. Court, J.W. Sears, M.H. Loretto and H.L. Fraser, Mat. Sci. Eng. 98,243 (1988). 9. G.A. Bertero, W.H. Hofmeister, M.B. Robinson and R.J. Bayuzick, Metall. Trans. A 22,2723 (1991). 10. P. Villars and L.D. Calvert, Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, American Society for Metals, Metats Park, OH. (1991). 11. “Diffract” software version 1.3 was used under license from microdev Company, Evergreen, Co. 12. J.L. Murray, Phase Diagrams of Binary Titanium Alloy, American Society for Metals, Metals Park, OH. (1987). 13. M.V. Kral, Ph.D. thesis, Vanderbilt University (1996). 14. D.R. Kegley, private communication, Vanderbilt University, (1989). 15. R.W.K. Honeycombe, Metall. Trans. A 7,915 (1976). 16. M.V. Kral, W H. Hofmeister and J.E. Wittig, submitted to Metall. Trans. A, (1996).