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A phase separation reaction in a binary titanium-chromium alloy
A Phase Separation Reaction in a Binary T i t a n i u m - C h r o m i u m Alloy
G. HARI NARAYANAN, T. S. L U H M A N , T. F. ARCHBOLD, R. T A G G A R T AND D. H. P O L O N I S
College of Engineering, University of Washington, Seattle, Washington
Experimental evidence is presented to show that a phase separation reaction occurs in the beta phase of T i - C r binary alloys. The reaction has been detected prior to the precipitation of both the omega and the alpha phases, and it becomes a prominent mode of decomposition when omega precipitation is inhibited either by alloying additions or through appropriate heat-treatments. It is suggested that the f12 particles in T i - C r alloys are leaner in solute than the matrix. The precipitation of the alpha phase during prolonged aging has been observed to occur on the f12 particles formed during prior phase separation.
Eine Entmischungsreaktion in einer bindren Titan-Chrom-Legierung Es wird tiber Experimente berichtet, die zeigen, dab in der fl-Phase bin/irer T i - C r Legierungen eine Entmischungsreaktion stattfindet. Diese Reaktion wurde nachgewiesen noch bevor sich die Omega- oder Alpha-Phase ausgeschieden hatte. Sie wird die wesent liche Zerfallsreaktion wenn die Omega-Ausscheidung entweder durch Legierungszus~itze oder durch eine geeignete Wiirmebehandlung unterdrtickt wird. Es wird angenommen, dab die /32-Teilchen in Ti-Cr-Legierungen ~irmer am Legierungselement sind als die Matrix. Die Ausscheidung der a-Phase bei langem Anlassen finder an den fl2-Teilchen statt, die sich bei der vorhergehenden Entmischung gebildet haben.
Rgaction pour la s@aration d'une phase dans un alliage binaire titane-chrome Une preuve exp~rimentale est pr~sentfie pour montrer qu'une r6action de s~paration de phase se produit dans la phase b~ta des alliages Ti-Cr. La r~action fur d~tect6e avant la precipitation des phases omega et alpha et devient un mode pro~minent de d~composition lorsque la pr6cipitation omdga est entrav6e soit par les dl~ments d'addition ou soit par des traitements thermiques approprifis. On sugg6re que les particules f12 dans les alliages T i - C r contiennent moins de solut~ que la matrice. On a observ~ que la prdcipitation de la phase alpha pendant des recuits prolong6s se produit sur les particules 82 form6es pendant la s~paration de phase qui a pr6c~d~.
Introduction T h e c o n c e p t of c l u s t e r i n g of s o l u t e a t o m s in t h e solid s o l u t i o n s of T i w i t h t h e t r a n s i t i o n m e t a l s was o r i g i n a l l y p r o p o s e d b y M c Q u i l l a n 1 to explain c e r t a i n
Metallography, 4 (1971) 343-358 Copyright © 1971 by American Elsevier Publishing Company, Inc.
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discrepancies in the results of earlier investigations of Ti-rich Ti-Cr alloys. Subsequently, it was shown by Harmon and Troiano z that segregation occurs within the beta phase of Ti-20% V* and Ti-16% V-2.5% A1 alloys during isothermal aging at temperatures below 300°C. More recently, it has been shown by means of superconductivity measurements that compositional fluctuations exist in the beta phase of Ti-Cr binary alloys.3 In another study 4 transmission electron microscopy techniques have revealed that the metastable beta phase in Ti-13 V-11 Cr and Ti-13 V-11 Cr-3 A1 alloys can decompose into two b.c.c, phases having different compositions. The present paper is concerned with experimental evidence for the occurrence of a phase separation reaction in the beta phase of a binary Ti-15 wt.% Cr alloy.
Experimental Procedure Samples of the Ti-15 wt.% Cr alloy were prepared from iodide titanium and 99.99% pure chromium by levitation melting in a purified helium atmosphere. The alloys were homogenized at 1000°C for 36 hours in vacuum and subsequently rolled into the form of strip 0.4 mm thick. Specimens 15 x 10 mm in size were cut from the strips and solution-treated at 850°C for 1½ hours in an atmosphere of purified helium using titanium as a "getter." One set of specimens was water-quenched to room temperature, after which they were aged for various times at 300°C or at 450°C. A second set of specimens was step-quenched into a salt bath maintained at 450°C, held isothermally for times ranging from 5 to 150 minutes, and then quenched to room temperature. The thin foils for transmission electron microscopy were prepared from the heattreated samples by employing the conventional electropolishing techniques. 5
Results In the solution-treated and quenched condition, the Ti-15% Cr alloy exhibits a single-phase beta structure. Aging the water-quenched specimens for 21 minutes at 300°C results in the formation of fine-scale precipitates in the beta matrix. Selected area diffraction patterns obtained from these specimens exhibited only beta-phase reflections with no additional spots that could be attributed to the transitional omega phase. A typical microstructure is shown in Fig. la, and the corresponding selected area diffraction patterns for the (111)~ and (311)~ zones are shown in Figs. lb and lc for an alloy that has been aged for 5½ minutes at 300°C. The size of the unidentified particles was found to increase slightly with an increase in the aging time. Omega-phase reflections were detected in the electron diffraction patterns (Fig. 2a) after aging the alloy for 18 minutes *A]l compositions referred to in the paper are weight percent.
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at 300°C. Figure 2b illustrates the fine-scale dispersion of the omega phase; such a fine dispersion of omega in beta makes it impossible to obtain a direct correlation between the nucleation sites of the omega particles and those of the unidentified particles observed in the beta matrix prior to omega formation.
Fro. la. Electron micrograph illustrating the fine-scale precipitates in the beta matrix of a Ti-15% Cr alloy following aging for 5½ minutes at 300°C. Magnification 100,000 ×. The selected area diffraction patterns shown in Figs. lb and lc were obtained from the specimens containing the unidentified precipitates and show splitting of the higher-order reflections from the beta phase. The Kikuchi lines obtained from these specimens were paired as shown in Fig. 3, a phenomenon which has been identified with diffraction from regions having different lattice spacing within a single crystal. G Both of these observations provide strong evidence that the unidentified precipitates have a b.c.c, structure and a composition differing from that of the matrix. Prior to the formation of the omega phase the diffraction patterns of the aged alloys exhibited diffuse streaks, the configurations of which were characteristic of the zone being examined. Similar diffuse streaking appears on the electron diffraction patterns from the metastable b.c.c, solid solutions of a number of binary and complex alloy systems based on either Ti or Zr.
FIO. lb. Selected area electron diffraction p a t t e r n taken f r o m the region s h o w n in Fig. l a . (111)¢ zone normal.
FIG. lc. Selected area electron diffraction p a t t e r n taken f r o m a s p e c i m e n following the same h e a t - t r e a t m e n t as in Fig. l a , illustrating the diffuse streak effects in reciprocal space. (311)/~ zone normal.
FIG. 2a. Selected area diffraction pattern obtained from a T i - 1 5 % Cr alloy, quenched from 850'~C and aged for 18 minutes at 300°C, showing reflections from two variants of the "oJ" phase. <110>~ zone normal.
FIe;. 2b. Dark-field micrograph taken from a (1011)co reflection showing the fine dispersion of "co" phase particles in the beta matrix. Foil orientation is {ll0)p zone normal. Magnification 78.000 × .
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Recently, de Fontaine, Williams and Paton (unpublished research 1970) have investigated this phenomenon in some detail and have proposed that streaking is due to a mechanical instability characteristic of the b.c.c, lattice of these alloys. De Fontaine 7 has also illustrated how such mechanical instabilities can lead to the formation of the omega phase by subjecting the b.c.c, lattice to a transverse sinusoidal displacement wave of wave vector ½ (112)~ which is polarized in the (111)B direction. The Ti-Cr alloy specimens were step-quenched to 450°C to obtain particle sizes large enough for the examination of precipitate morphology without obscuring the phase separation reaction by the onset of omega precipitation. The specimens were held at 450°C for times ranging from 5 to 150 minutes prior to a final quench to room temperature. Omega precipitation does not occur, because this holding temperature is above the upper limit for the formation of this phase. The precipitation of the alpha phase requires a holding time in excess of 90 minutes after direct quenching to 450°C, whereas this phase appears within 15 minutes of aging a specimen that has been water-quenched and reheated to 450°C. Discrete precipitate particles were observed after quenching to 450°C and holding for 5 minutes as shown in Fig. 4. After continued aging for 30 minutes, sufficient precipitate growth occurred to reveal a disk-shaped morphology. Trace analysis showed that the precipitate disks lie parallel to the {100}p planes of the matrix; three variants of this habit relationship can be observed in Fig. 5. The precipitate particles continued to increase in size for holding times up to 90 minutes. Selected area diffraction patterns obtained from the specimens containing the disk-shaped precipitates show matrix spot splitting similar to that in Figs. lb and lc, but not reflections associated with either the alpha or omega phases. Reciprocal lattice streaking of the beta-phase reflections and paired Kikuchi lines were observed on the electron diffraction patterns of the step-quenched alloys. The precipitate particles could be brought into contrast against the matrix as shown in Fig. 6 by imaging the matrix reflections in a two-beam situation by means of high-resolution dark-field techniques. The dark-field conditions were varied to obtain maximum resolution; they confirmed that the precipitate and the matrix have the same structure but differ only in composition. These observations in the binary Ti-Cr alloys are similar to those in the Ti-13 V-11 Cr-3AI and Ti-13 V-11 Cr alloys for which the product b.c.c, phase was designated as a/32 precip itate.4 For holding times in excess of 90 minutes after step-quenching to 450°C, alpha-phase precipitation was found to occur on the disk-shaped/32 precipitates. Using dark-field analysis, it was possible to identify plate-like alpha precipitates co-existing with and comparable in size to the/32 precipitates. Figures 7a and 7b suggest that the alpha phase forms by either (1) the direct transformation of the disk-shaped /33 precipitates, or (2) by nucleating the alpha phase at the
FIG. 3. Electron diffraction pattern taken from a Ti-15°i~ Cr specimen, quenched from 850°C and aged for 12 minutes at 300°C, showing the paired Kikuchi lines. Close to (111)/~ zone normal.
Fic. 4. Electron micrograph showing the presence of discrete particles in the beta matrix, in a Ti-15~o Cr alloy after quenching directly to 450°C from 850°C and holding for 5 minutes. (111)# zone normal. Magnification 52,000 ×.
FIC. 5.--Electron micrograph illustrating the disk-shaped morphology of the precipitates in specimens quenched directly to 450°C and held at this temperature for 30 minutes. (100)fl zone; three variants are present. Magnification 52,000 ×.
FIG. 6. Dark-field electron micrograph obtained by imaging a (ll0)fl reflection from the general area corresponding to Fig. 5. (110)8 zone normal. Magnification 52,000 x .
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interface separating the/32 particles and the matrix, followed by the growth of the alpha particles to consume the/32 particles. The latter mechanism is favoured when consideration is given to the fact that aggregates of very fine alpha particles form with a cluster morphology similar to that of the original ]~2 zones; each cluster is parallel to the {100}# planes. An observation supporting this idea has been reported by Williams et al. s in the Ti-V-A1 and T i - V - Z r ternary systems.
FIG. 7a. Bright-field micrograph showing the precipitation of the alpha phase associated with the 132 particles in a Ti-15% alloy, quenched directly to 450°C from 850C and held for 90 minutes. (120)/t zone normal. Magnification 52,000 x. On the basis of superconducting transition temperature measurements, it has been shown that remanent compositional gradients persist in the beta phase of a Ti-15°'o Cr alloy aged at 300°C to produce a large volume fraction of the omega phase and subsequently up-quenched to 450°C to cause reversion of this phase. 9 The stability of the remanent compositional gradients in the beta phase of an up-quenched alloy was examined by electron microscopy, in order to make a comparison between the products of the reversion treatment and the phase separation reaction that occurs during step-quenching treatments to 450°C. Figure 8a is an electron micrograph of the Ti-15% Cr alloy that was initially aged at 300°C to produce a large volume fraction of the omega phase, and then
FIG. 7b. Dark-field micrograph taken from (i011)~ reflection showing aggregates of fine platelets of the alpha phase formed within the disk-shaped fl~ precipitates. <120)/~ zone normal. Magnification 52,000 × .
FiG. 7c. Selected area electron diffraction pattern taken from the region shown in Fig. 7a. <120)~ zone normal.
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up-quenched to 450°C and held at this temperature for 2½ minutes to cause omega reversion. The selected area diffraction pattern shown in Fig. 8b confirmed the complete reversion of the isothermal omega phase. Although the up-quenching and aging treatments eliminated the structure of the omega phase, electron images of the former locations of the omega particles revealed that gradients in composition still persist, as shown in Fig. 8a. Similar remanent particles were also reported when the omega structure was destroyed by a spontaneous transformation during the thin-foil preparation of a Ti-11.6% Mo alloy. TM
FIG. 8a. Electron micrograph of the Ti-15% Cr alloy aged at 300°C for 35 hours to produce a large volume fraction of omega, subsequently up-quenched to 450°C and held for 2~ minutes to cause omega reversion. (ll0)fl zone normal. Magnification 78,000 ×. Figure 8c is a dark-field micrograph obtained after imaging a matrix reflection from an alloy that had been quenched to room temperature from the beta field and then up-quenched to 450°C for aging. The micrograph reveals discrete particles in contrast against the beta matrix, and these particles grow with increasing holding time at 450°C after up-quenching. The increase in particle size is illustrated by comparing Fig. 8c with Fig. 9a and Fig. 9b. The features obsecved in the electron diffraction patterns corresponding to the up-quenching
FIc. 8b. Selected area electron diffraction pattern obtained from a specimen given the same heat-treatment as in Fig. 8a (110)# zone pattern. Pronounced diffuse streak effects can be noticed.
Fta. 8c. Dark-field electron micrograph corresponding to Fig. 8a obtained by imaging a (110)# reflection showing the precipitate particles in contrast; (110)~ zone normal. Magnification 78,000 x .
FIG. 9a. Dark-field micrograph illustrating the growth of the fl~ precipitate particles Aged at 300°C for 35 hours, up-quenched to 450°C and held for 18 minutes. (111)/~ zone normal. Magnification 78,000 X.
FIG. 9b. Dark-field micrograph illustrating the growth of the f12 precipitate~particles due to increasing the holding times at 450°C to 30 minutes.
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and aging treatment are the same as those seen in the patterns from a stepquenched alloy. In comparison with an alloy step-quenched directly to 450°C as shown in Figs. 4 and 5, the precipitates observed in the same alloy up-quenched to 450°C exhibited a smaller particle size and higher particle density after corresponding isothermal holding times.
Discussion The results of the present investigation show conclusively that a phase separation reaction occurs in the metastable beta phase of a Ti-15% Cr alloy. This reaction is consistent with the available thermodynamic data from the Ti-Cr system11 which shows positive heats of mixing for the beta solid solutions of Ti and Cr. From a careful examination of the influence of the heat-treatment procedure on the phase separation reaction in the Ti-15% Cr alloy and a comparison of these results with those obtained from Ti-V-Cr and Ti-13 V11 Cr-3 A1 alloys,4 it is possible to deduce some basic features of this reaction. 1. In the metastable beta phase of T i - C r alloys the phase separation process is a transition reaction which has been detected prior to the precipitation of both the omega phase and the alpha phase. Depending on the alloy additions, the ~l-t-flz structure is metastable with respect to either the to-i-fl or the ~-kfl phase mixtures. 2. The phase separation reaction may become a prominent mode of decomposition in alloys of titanium if the beta phase is stabilized at room temperature, and when the precipitation of the omega phase is inhibited either through alloying additions or through appropriate heat-treatments. Recently, Williams et al. s showed that when ternary elements such as AI, Zr, or Sn are added to the binary Ti-V and T i - M o alloys, the formation of the omega phase is completely suppressed, and beta decomposition occurs by means of a phase separation reaction. In the present investigation, it has been shown that in a Ti-15% Cr alloy in which the omega phase forms only during isothermal aging, the phase separation process can occur as a competitive reaction during aging at temperatures below approximately 300°C. However, when omega phase formation is completely inhibited by appropriate heat-treatments, phase separation becomes a significant mode of beta decomposition. 3. The product phase fl~ resulting from the phase separation reaction is leaner in solute than the matrix, contrary to the conclusion reported earlier.4 This conclusion is based on the fact that the precipitation of the alpha phase occurs on the fl~ particles at these aging temperatures. Such a mode of nucleation is favored, since only small compositional readjustments are required to form the alpha phase from the solute lean beta. This hypothesis for a binary alloy is in agreement with the work of Williams et al. on ternary alloys Ti-V-AI and Ti-V-Zr. s
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4. The stabilization of the composition gradients, produced by the reversion of the omega phase on up-quenching, can be explained by the inherent tendency for phase separation to occur in the beta phase. In view of the fact that the precipitation of the alpha phase occurs on the f12 particles, the up-quenching of specimens aged at lower temperatures provides a means of controlling the scale of dispersion of the alpha phase; this could provide the basis for a desirable strengthening mechanism for Ti AUoys. In alloys where the omega phase forms, the duplex aging treatment is of potential practical interest for reducing the embrittling effects of the omega phase while simultaneously enhancing the strength of the alloy due to a uniform dispersion of the alpha precipitates.
Summary The occurrence of a phase separation reaction in the metastable beta phase of a Ti-15°,o Cr alloy has been confirmed by means of transmission electron microscopy techniques. The phase separation process has been detected prior to the precipitation of both the omega phase and the alpha phase; it becomes a prominent mode of decomposition when the formation of the omega phase is inhibited. During aging below 300°C the phase separation reaction occurs in competition with the formation of the omega phase. The association of alphaphase nucleation with the fi2 particles formed during prior phase separation has been interpreted as an indication that the/32 regions have a lower solute content than the surrounding/3 matrix. Up-quenching experiments have shown that the omega particles transform to the beta structure, but composition gradients remain, which is consistent with the inherent tendency for phase separation in the beta phase.
Acknowledgment We acknowledge support for this work provided by the U.S. Atomic Energy Commission Contract AT(45-1)-2225-T13. (RLO AT(45-1)-2225-R13-8.)
References 1. 2. 3. 4. 5. 6. 7.
M. K. McQuillan, J. Int. Metals., 82, (1953) 433. E. L. Harmon and A. R. Troiano, Trans. ASM, 53 (1961) 43. T. S. Luhman, R. Taggart and D. H. Polonis, Scripta Met., 3, (1969) 777. G. Hari Narayanan and T. F. Archbold, Met. Trans., I, (1970), 2281. M. J. Blackburn and J. C. Williams, Trans. TMS-.4IME, 239 (1967) 287. G. Thomas, Trans. TMS-AIME, 233 (1965) 1608. D. de Fontaine, .dcta Met., 18 (1970) 275.
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358 8. 9. 10. 11.
J. C. Williams, B. S. Hiekman and D. H. Leslie, Met. Trans., in press. T. S. Luhman, R. Taggart and D. H. Polonis, Scripta Met., 5 (1971) 81. M. J. Blackburn and J. C. Williams, Trans. TMS-AIME, 242 (1968) 2461. M. J. Pool, R. Speiser and G. R. St. Pierre, Trans. TMS-AIME, 239 (1967) 1180.
Accepted February 8, 1971
G. HARI NARAYANAN, T. S. L U H M A N , T. F. ARCHBOLD, R. T A G G A R T AND D. H. P O L O N I S
College of Engineering, University of Washington, Seattle, Washington
Experimental evidence is presented to show that a phase separation reaction occurs in the beta phase of T i - C r binary alloys. The reaction has been detected prior to the precipitation of both the omega and the alpha phases, and it becomes a prominent mode of decomposition when omega precipitation is inhibited either by alloying additions or through appropriate heat-treatments. It is suggested that the f12 particles in T i - C r alloys are leaner in solute than the matrix. The precipitation of the alpha phase during prolonged aging has been observed to occur on the f12 particles formed during prior phase separation.
Eine Entmischungsreaktion in einer bindren Titan-Chrom-Legierung Es wird tiber Experimente berichtet, die zeigen, dab in der fl-Phase bin/irer T i - C r Legierungen eine Entmischungsreaktion stattfindet. Diese Reaktion wurde nachgewiesen noch bevor sich die Omega- oder Alpha-Phase ausgeschieden hatte. Sie wird die wesent liche Zerfallsreaktion wenn die Omega-Ausscheidung entweder durch Legierungszus~itze oder durch eine geeignete Wiirmebehandlung unterdrtickt wird. Es wird angenommen, dab die /32-Teilchen in Ti-Cr-Legierungen ~irmer am Legierungselement sind als die Matrix. Die Ausscheidung der a-Phase bei langem Anlassen finder an den fl2-Teilchen statt, die sich bei der vorhergehenden Entmischung gebildet haben.
Rgaction pour la s@aration d'une phase dans un alliage binaire titane-chrome Une preuve exp~rimentale est pr~sentfie pour montrer qu'une r6action de s~paration de phase se produit dans la phase b~ta des alliages Ti-Cr. La r~action fur d~tect6e avant la precipitation des phases omega et alpha et devient un mode pro~minent de d~composition lorsque la pr6cipitation omdga est entrav6e soit par les dl~ments d'addition ou soit par des traitements thermiques approprifis. On sugg6re que les particules f12 dans les alliages T i - C r contiennent moins de solut~ que la matrice. On a observ~ que la prdcipitation de la phase alpha pendant des recuits prolong6s se produit sur les particules 82 form6es pendant la s~paration de phase qui a pr6c~d~.
Introduction T h e c o n c e p t of c l u s t e r i n g of s o l u t e a t o m s in t h e solid s o l u t i o n s of T i w i t h t h e t r a n s i t i o n m e t a l s was o r i g i n a l l y p r o p o s e d b y M c Q u i l l a n 1 to explain c e r t a i n
Metallography, 4 (1971) 343-358 Copyright © 1971 by American Elsevier Publishing Company, Inc.
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discrepancies in the results of earlier investigations of Ti-rich Ti-Cr alloys. Subsequently, it was shown by Harmon and Troiano z that segregation occurs within the beta phase of Ti-20% V* and Ti-16% V-2.5% A1 alloys during isothermal aging at temperatures below 300°C. More recently, it has been shown by means of superconductivity measurements that compositional fluctuations exist in the beta phase of Ti-Cr binary alloys.3 In another study 4 transmission electron microscopy techniques have revealed that the metastable beta phase in Ti-13 V-11 Cr and Ti-13 V-11 Cr-3 A1 alloys can decompose into two b.c.c, phases having different compositions. The present paper is concerned with experimental evidence for the occurrence of a phase separation reaction in the beta phase of a binary Ti-15 wt.% Cr alloy.
Experimental Procedure Samples of the Ti-15 wt.% Cr alloy were prepared from iodide titanium and 99.99% pure chromium by levitation melting in a purified helium atmosphere. The alloys were homogenized at 1000°C for 36 hours in vacuum and subsequently rolled into the form of strip 0.4 mm thick. Specimens 15 x 10 mm in size were cut from the strips and solution-treated at 850°C for 1½ hours in an atmosphere of purified helium using titanium as a "getter." One set of specimens was water-quenched to room temperature, after which they were aged for various times at 300°C or at 450°C. A second set of specimens was step-quenched into a salt bath maintained at 450°C, held isothermally for times ranging from 5 to 150 minutes, and then quenched to room temperature. The thin foils for transmission electron microscopy were prepared from the heattreated samples by employing the conventional electropolishing techniques. 5
Results In the solution-treated and quenched condition, the Ti-15% Cr alloy exhibits a single-phase beta structure. Aging the water-quenched specimens for 21 minutes at 300°C results in the formation of fine-scale precipitates in the beta matrix. Selected area diffraction patterns obtained from these specimens exhibited only beta-phase reflections with no additional spots that could be attributed to the transitional omega phase. A typical microstructure is shown in Fig. la, and the corresponding selected area diffraction patterns for the (111)~ and (311)~ zones are shown in Figs. lb and lc for an alloy that has been aged for 5½ minutes at 300°C. The size of the unidentified particles was found to increase slightly with an increase in the aging time. Omega-phase reflections were detected in the electron diffraction patterns (Fig. 2a) after aging the alloy for 18 minutes *A]l compositions referred to in the paper are weight percent.
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at 300°C. Figure 2b illustrates the fine-scale dispersion of the omega phase; such a fine dispersion of omega in beta makes it impossible to obtain a direct correlation between the nucleation sites of the omega particles and those of the unidentified particles observed in the beta matrix prior to omega formation.
Fro. la. Electron micrograph illustrating the fine-scale precipitates in the beta matrix of a Ti-15% Cr alloy following aging for 5½ minutes at 300°C. Magnification 100,000 ×. The selected area diffraction patterns shown in Figs. lb and lc were obtained from the specimens containing the unidentified precipitates and show splitting of the higher-order reflections from the beta phase. The Kikuchi lines obtained from these specimens were paired as shown in Fig. 3, a phenomenon which has been identified with diffraction from regions having different lattice spacing within a single crystal. G Both of these observations provide strong evidence that the unidentified precipitates have a b.c.c, structure and a composition differing from that of the matrix. Prior to the formation of the omega phase the diffraction patterns of the aged alloys exhibited diffuse streaks, the configurations of which were characteristic of the zone being examined. Similar diffuse streaking appears on the electron diffraction patterns from the metastable b.c.c, solid solutions of a number of binary and complex alloy systems based on either Ti or Zr.
FIO. lb. Selected area electron diffraction p a t t e r n taken f r o m the region s h o w n in Fig. l a . (111)¢ zone normal.
FIG. lc. Selected area electron diffraction p a t t e r n taken f r o m a s p e c i m e n following the same h e a t - t r e a t m e n t as in Fig. l a , illustrating the diffuse streak effects in reciprocal space. (311)/~ zone normal.
FIG. 2a. Selected area diffraction pattern obtained from a T i - 1 5 % Cr alloy, quenched from 850'~C and aged for 18 minutes at 300°C, showing reflections from two variants of the "oJ" phase. <110>~ zone normal.
FIe;. 2b. Dark-field micrograph taken from a (1011)co reflection showing the fine dispersion of "co" phase particles in the beta matrix. Foil orientation is {ll0)p zone normal. Magnification 78.000 × .
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Recently, de Fontaine, Williams and Paton (unpublished research 1970) have investigated this phenomenon in some detail and have proposed that streaking is due to a mechanical instability characteristic of the b.c.c, lattice of these alloys. De Fontaine 7 has also illustrated how such mechanical instabilities can lead to the formation of the omega phase by subjecting the b.c.c, lattice to a transverse sinusoidal displacement wave of wave vector ½ (112)~ which is polarized in the (111)B direction. The Ti-Cr alloy specimens were step-quenched to 450°C to obtain particle sizes large enough for the examination of precipitate morphology without obscuring the phase separation reaction by the onset of omega precipitation. The specimens were held at 450°C for times ranging from 5 to 150 minutes prior to a final quench to room temperature. Omega precipitation does not occur, because this holding temperature is above the upper limit for the formation of this phase. The precipitation of the alpha phase requires a holding time in excess of 90 minutes after direct quenching to 450°C, whereas this phase appears within 15 minutes of aging a specimen that has been water-quenched and reheated to 450°C. Discrete precipitate particles were observed after quenching to 450°C and holding for 5 minutes as shown in Fig. 4. After continued aging for 30 minutes, sufficient precipitate growth occurred to reveal a disk-shaped morphology. Trace analysis showed that the precipitate disks lie parallel to the {100}p planes of the matrix; three variants of this habit relationship can be observed in Fig. 5. The precipitate particles continued to increase in size for holding times up to 90 minutes. Selected area diffraction patterns obtained from the specimens containing the disk-shaped precipitates show matrix spot splitting similar to that in Figs. lb and lc, but not reflections associated with either the alpha or omega phases. Reciprocal lattice streaking of the beta-phase reflections and paired Kikuchi lines were observed on the electron diffraction patterns of the step-quenched alloys. The precipitate particles could be brought into contrast against the matrix as shown in Fig. 6 by imaging the matrix reflections in a two-beam situation by means of high-resolution dark-field techniques. The dark-field conditions were varied to obtain maximum resolution; they confirmed that the precipitate and the matrix have the same structure but differ only in composition. These observations in the binary Ti-Cr alloys are similar to those in the Ti-13 V-11 Cr-3AI and Ti-13 V-11 Cr alloys for which the product b.c.c, phase was designated as a/32 precip itate.4 For holding times in excess of 90 minutes after step-quenching to 450°C, alpha-phase precipitation was found to occur on the disk-shaped/32 precipitates. Using dark-field analysis, it was possible to identify plate-like alpha precipitates co-existing with and comparable in size to the/32 precipitates. Figures 7a and 7b suggest that the alpha phase forms by either (1) the direct transformation of the disk-shaped /33 precipitates, or (2) by nucleating the alpha phase at the
FIG. 3. Electron diffraction pattern taken from a Ti-15°i~ Cr specimen, quenched from 850°C and aged for 12 minutes at 300°C, showing the paired Kikuchi lines. Close to (111)/~ zone normal.
Fic. 4. Electron micrograph showing the presence of discrete particles in the beta matrix, in a Ti-15~o Cr alloy after quenching directly to 450°C from 850°C and holding for 5 minutes. (111)# zone normal. Magnification 52,000 ×.
FIC. 5.--Electron micrograph illustrating the disk-shaped morphology of the precipitates in specimens quenched directly to 450°C and held at this temperature for 30 minutes. (100)fl zone; three variants are present. Magnification 52,000 ×.
FIG. 6. Dark-field electron micrograph obtained by imaging a (ll0)fl reflection from the general area corresponding to Fig. 5. (110)8 zone normal. Magnification 52,000 x .
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interface separating the/32 particles and the matrix, followed by the growth of the alpha particles to consume the/32 particles. The latter mechanism is favoured when consideration is given to the fact that aggregates of very fine alpha particles form with a cluster morphology similar to that of the original ]~2 zones; each cluster is parallel to the {100}# planes. An observation supporting this idea has been reported by Williams et al. s in the Ti-V-A1 and T i - V - Z r ternary systems.
FIG. 7a. Bright-field micrograph showing the precipitation of the alpha phase associated with the 132 particles in a Ti-15% alloy, quenched directly to 450°C from 850C and held for 90 minutes. (120)/t zone normal. Magnification 52,000 x. On the basis of superconducting transition temperature measurements, it has been shown that remanent compositional gradients persist in the beta phase of a Ti-15°'o Cr alloy aged at 300°C to produce a large volume fraction of the omega phase and subsequently up-quenched to 450°C to cause reversion of this phase. 9 The stability of the remanent compositional gradients in the beta phase of an up-quenched alloy was examined by electron microscopy, in order to make a comparison between the products of the reversion treatment and the phase separation reaction that occurs during step-quenching treatments to 450°C. Figure 8a is an electron micrograph of the Ti-15% Cr alloy that was initially aged at 300°C to produce a large volume fraction of the omega phase, and then
FIG. 7b. Dark-field micrograph taken from (i011)~ reflection showing aggregates of fine platelets of the alpha phase formed within the disk-shaped fl~ precipitates. <120)/~ zone normal. Magnification 52,000 × .
FiG. 7c. Selected area electron diffraction pattern taken from the region shown in Fig. 7a. <120)~ zone normal.
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up-quenched to 450°C and held at this temperature for 2½ minutes to cause omega reversion. The selected area diffraction pattern shown in Fig. 8b confirmed the complete reversion of the isothermal omega phase. Although the up-quenching and aging treatments eliminated the structure of the omega phase, electron images of the former locations of the omega particles revealed that gradients in composition still persist, as shown in Fig. 8a. Similar remanent particles were also reported when the omega structure was destroyed by a spontaneous transformation during the thin-foil preparation of a Ti-11.6% Mo alloy. TM
FIG. 8a. Electron micrograph of the Ti-15% Cr alloy aged at 300°C for 35 hours to produce a large volume fraction of omega, subsequently up-quenched to 450°C and held for 2~ minutes to cause omega reversion. (ll0)fl zone normal. Magnification 78,000 ×. Figure 8c is a dark-field micrograph obtained after imaging a matrix reflection from an alloy that had been quenched to room temperature from the beta field and then up-quenched to 450°C for aging. The micrograph reveals discrete particles in contrast against the beta matrix, and these particles grow with increasing holding time at 450°C after up-quenching. The increase in particle size is illustrated by comparing Fig. 8c with Fig. 9a and Fig. 9b. The features obsecved in the electron diffraction patterns corresponding to the up-quenching
FIc. 8b. Selected area electron diffraction pattern obtained from a specimen given the same heat-treatment as in Fig. 8a (110)# zone pattern. Pronounced diffuse streak effects can be noticed.
Fta. 8c. Dark-field electron micrograph corresponding to Fig. 8a obtained by imaging a (110)# reflection showing the precipitate particles in contrast; (110)~ zone normal. Magnification 78,000 x .
FIG. 9a. Dark-field micrograph illustrating the growth of the fl~ precipitate particles Aged at 300°C for 35 hours, up-quenched to 450°C and held for 18 minutes. (111)/~ zone normal. Magnification 78,000 X.
FIG. 9b. Dark-field micrograph illustrating the growth of the f12 precipitate~particles due to increasing the holding times at 450°C to 30 minutes.
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G. Hari Narayanan, et aL
and aging treatment are the same as those seen in the patterns from a stepquenched alloy. In comparison with an alloy step-quenched directly to 450°C as shown in Figs. 4 and 5, the precipitates observed in the same alloy up-quenched to 450°C exhibited a smaller particle size and higher particle density after corresponding isothermal holding times.
Discussion The results of the present investigation show conclusively that a phase separation reaction occurs in the metastable beta phase of a Ti-15% Cr alloy. This reaction is consistent with the available thermodynamic data from the Ti-Cr system11 which shows positive heats of mixing for the beta solid solutions of Ti and Cr. From a careful examination of the influence of the heat-treatment procedure on the phase separation reaction in the Ti-15% Cr alloy and a comparison of these results with those obtained from Ti-V-Cr and Ti-13 V11 Cr-3 A1 alloys,4 it is possible to deduce some basic features of this reaction. 1. In the metastable beta phase of T i - C r alloys the phase separation process is a transition reaction which has been detected prior to the precipitation of both the omega phase and the alpha phase. Depending on the alloy additions, the ~l-t-flz structure is metastable with respect to either the to-i-fl or the ~-kfl phase mixtures. 2. The phase separation reaction may become a prominent mode of decomposition in alloys of titanium if the beta phase is stabilized at room temperature, and when the precipitation of the omega phase is inhibited either through alloying additions or through appropriate heat-treatments. Recently, Williams et al. s showed that when ternary elements such as AI, Zr, or Sn are added to the binary Ti-V and T i - M o alloys, the formation of the omega phase is completely suppressed, and beta decomposition occurs by means of a phase separation reaction. In the present investigation, it has been shown that in a Ti-15% Cr alloy in which the omega phase forms only during isothermal aging, the phase separation process can occur as a competitive reaction during aging at temperatures below approximately 300°C. However, when omega phase formation is completely inhibited by appropriate heat-treatments, phase separation becomes a significant mode of beta decomposition. 3. The product phase fl~ resulting from the phase separation reaction is leaner in solute than the matrix, contrary to the conclusion reported earlier.4 This conclusion is based on the fact that the precipitation of the alpha phase occurs on the fl~ particles at these aging temperatures. Such a mode of nucleation is favored, since only small compositional readjustments are required to form the alpha phase from the solute lean beta. This hypothesis for a binary alloy is in agreement with the work of Williams et al. on ternary alloys Ti-V-AI and Ti-V-Zr. s
Binary Ti-Cr Alloy
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4. The stabilization of the composition gradients, produced by the reversion of the omega phase on up-quenching, can be explained by the inherent tendency for phase separation to occur in the beta phase. In view of the fact that the precipitation of the alpha phase occurs on the f12 particles, the up-quenching of specimens aged at lower temperatures provides a means of controlling the scale of dispersion of the alpha phase; this could provide the basis for a desirable strengthening mechanism for Ti AUoys. In alloys where the omega phase forms, the duplex aging treatment is of potential practical interest for reducing the embrittling effects of the omega phase while simultaneously enhancing the strength of the alloy due to a uniform dispersion of the alpha precipitates.
Summary The occurrence of a phase separation reaction in the metastable beta phase of a Ti-15°,o Cr alloy has been confirmed by means of transmission electron microscopy techniques. The phase separation process has been detected prior to the precipitation of both the omega phase and the alpha phase; it becomes a prominent mode of decomposition when the formation of the omega phase is inhibited. During aging below 300°C the phase separation reaction occurs in competition with the formation of the omega phase. The association of alphaphase nucleation with the fi2 particles formed during prior phase separation has been interpreted as an indication that the/32 regions have a lower solute content than the surrounding/3 matrix. Up-quenching experiments have shown that the omega particles transform to the beta structure, but composition gradients remain, which is consistent with the inherent tendency for phase separation in the beta phase.
Acknowledgment We acknowledge support for this work provided by the U.S. Atomic Energy Commission Contract AT(45-1)-2225-T13. (RLO AT(45-1)-2225-R13-8.)
References 1. 2. 3. 4. 5. 6. 7.
M. K. McQuillan, J. Int. Metals., 82, (1953) 433. E. L. Harmon and A. R. Troiano, Trans. ASM, 53 (1961) 43. T. S. Luhman, R. Taggart and D. H. Polonis, Scripta Met., 3, (1969) 777. G. Hari Narayanan and T. F. Archbold, Met. Trans., I, (1970), 2281. M. J. Blackburn and J. C. Williams, Trans. TMS-.4IME, 239 (1967) 287. G. Thomas, Trans. TMS-AIME, 233 (1965) 1608. D. de Fontaine, .dcta Met., 18 (1970) 275.
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358 8. 9. 10. 11.
J. C. Williams, B. S. Hiekman and D. H. Leslie, Met. Trans., in press. T. S. Luhman, R. Taggart and D. H. Polonis, Scripta Met., 5 (1971) 81. M. J. Blackburn and J. C. Williams, Trans. TMS-AIME, 242 (1968) 2461. M. J. Pool, R. Speiser and G. R. St. Pierre, Trans. TMS-AIME, 239 (1967) 1180.
Accepted February 8, 1971