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Effects of B and W alloying additions on the formation and stability of lamellar structures in two-phase γ-TiAl
1997
PII:
(1997) 83-95
lntermerallics 5 © Elsevier Science Limited Printed in Great Britain. All rights reserved 0966-9795/97/$17.00
S0966-9795(96)00070-2
Effects of Band W alloying additions on the formation and stability of lamellar structures in two-phase y-TiAI P. J. Maziasz, R.
v. Ramanujan,* C. T. Liu &
J. L. Wright
Metals and Ceramics Division. Oak Ridge National Laboratory. PO Box 2008. Oak Ridge. Tennessee 37831. USA (Received 14 May 1996; accepted 25 June 1996)
The effects of additions of 140 appm Band 0·5 at.% W on the formation and stability of fully-lamellar structures in a Ti-47AI base alloy were studied. The fully-lamellar structure was formed by heat-treatment at 1400°C for I h (in vacuum) followed by furnace cooling. Stability was studied by aging these alloys for 168 h at 800, 1000 and 1200°C and making quantitative measurements of lamellar structure parameters (i.e. average lamellar spacing, £l!2-£l!2 spacing) using transmission electron microscopy (TEM) and hardness measurements. The B addition alone refines the lamellar structure relative to the binary TiAI alloy, but causes fragmented and discontinuous £l!2 lamellae. By contrast, W+B addition refines the lamellar structure and produces more uniform and continuous £l!, lamellae. The as-heat-treated Ti-47AI+W+B alloy is harder than the other t~o alloys. Aging for 168 h at 800°C does not change the fine initial lamellar microstructure of any of these alloys. Aging at 1000°C significantly coarsens the Ti-47AI alloy, causes some coarsening in the Ti-47AI+B alloy, but has little effect on the lamellar structure of the Ti-47AI+W+B alloy. Aging at 1200°C causes discontinuous coarsening and complete loss of the initial, fine, lamellar microstructure in all three alloys. Aging in vacuum produces a subsurface 'damage region' consisting of an £l!(Ti3AI) surface layer with another layer of coarsened y microstructure underneath. At 1000°C, this layer is greatest in the binary alloy and least in the Ti-47AI+W+B alloy. Specimens were 5·5% cold-forged and then aged at 1000°C to evaluate the effects of subsurface deformation caused during cutting of the specimens. Prior cold-deformation caused considerable recrystallization in the binary alloy, but almost none in the Ti-47AI+W+B alloy. The additions of Wand B to a Ti-47AI alloy refine the fully-lamellar structure obtained by heat-treatment, and make it more resistant to lamellar coarsening (and recrystallization if cold deformed) during aging at 800-1000°C. © 1997 Elsevier Science Limited. All rights reserved.
Key words: A. titanium aluminides (based on TiAI), B. microalloying, D. microstructure.
alloys provides much better high-temperature strength and creep-resistance than other structures (i.e. duplex), but the room-temperature ductility of fully-lamellar TiAl alloys has usually been low.4-9 Recent work, however, has shown that controlled processing which gives a refined colony size and very fine lamellar spacings in P/M materials can produce good ductility (1·4%) and high strength (YS = 971 MPa) at room temperature, as well as very good strength and creep-resistance at 760-S00°c. IO,I' The fully-lamellar microstructure consists of parallel laths of the (X2 (DO I9 structure) and y (Llo structure) phases forming colonies, with little or
INTRODUCTION
y-based titanium aluminides have been developed and are being tested for high temperature structural applications because they possess very low density (4 g/cm 3), good oxidation resistance,. and good creep resistance and high temperature strength. ' -3 The balance of room-temperature ductility and high-temperature strength is very sensitive to the microstructure. 1-6 Generally, the fully-lamellar structure of Ti-(47-4S)Al (at.%) *Permanent address: Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India. 83
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no y grains at the boundaries of those colonies. Lamellar spacings can range from > 0.5 mm to < 0.1 J.Lm. IO•12 Lamellar microstructures can be formed by heating the alloy into the a phase region, and then cooling through the a + y phase region to form the y lamellae. The residual a trapped between these growing y lamellae then transforms to the ordered 0'2 phase below the eutectoid temperature. Ti-47AI alloys are more resistant to discontinuous coarsening of their lamellar structures during aging than alloys with less Al. 13 Tungsten additions have been shown to be very beneficial for enhancing creep-resistance of simple and more complex Ti-(47-48)AI alloys.l4-l6 Recent studies of lamellar coarsening mechanisms have found that B+W additions to Ti-47AI alloys enhance their resistance to lamellar degredation at 800-1000°CY This was found not only in the bulk material, but also at surfaces that had experienced mechanical damage during cutting. One purpose of this paper is to present more detailed data showing the effects of Band W additions on the formation of the initial, fine lamellar structures in Ti-47 Al alloys, and to show the stability of such fine lamellar structures during subsequent aging. In particular, the behavior of the 0'2 component of the microstructure is emphasized. Another purpose is to study the effects of 5.5% cold-forging prior to aging to understand better the benefits of B+ W additions on subsurface damage resistance. Such information is vital for improving the 'damage tolerance' of machined TiAI components.
EXPERIMENTAL PROCEDURE Alloy ingots (2·5 cm in diameter and 5 cm long, weighing 175 gm) of nominal composition Ti-47AI, Ti-47AI-0·014B (at.%) (Ti-47AI+B) and Ti-47AI-0·5W-0·014B (Ti-47AI+W+B) alloys were prepared by arc melting and drop casting, and the difference in weight after melting was less than O· 5%. These ingots were then cut into slices, and specimens for aging or microstructural examination were made with a slow-speed saw. These specimens were then heat-treated in a vacuum furnace at 1400°C for 1 h in the a-phase region of the phase diagram6 and furnace cooled. This heattreatment homogenized the materials and established a fully-lamellar microstructure in all three alloys. Specimens were then aged at either 800, 1000 or 1200°C for 168 h and furnace cooled. To study better the effects of surface preparation on
sub-surface damage, some as-cast specimens of each alloy were gently ground with 180, 240, 400, 600 and 0 grit emery paper, and then heat-treated in vacuum at 1400°C for I h. They were then cold-forged at room temperature to a compression strain of 5-5
RESULTS
Bulk microstructure - as heat-treated The as-cast material of all the alloys had lamellar colonies (100-300 J.Lm) with about 5-15% of equiaxed y grains located mainly along colony boundaries (Fig. l(a». Heat-treatment of these
Band Walloying additions
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Table 1. Effects of Band W on average lamellar spacing in TiAI alloys Quantitative lamellar microstructure data (nm) Alloy
Condition
Average spacing
ara2 spacing
y-width
a2 width
Ti--47AI Ti--47AI+B Ti--47AI+W+B Ti--47AI+B Ti--47AI Ti--47AI+B Ti--47AI+W+B Ti--47AI+B
I I I I I I I I
400 170 190 144 800-1950 350--400 190 >1900
520 330-500 420-560 340--400 >1300 1420 660 3700
200-650 70--400 140-720 120-630 520-2200 200-1200 150--440 2000--4000
60-120 50-100 50-130 35-150 220-1030 50-155 60-130 >1000
h at h at h at h at h in hat hat hat
1400°C" 1400°C" 1400°C" 1400°C" 1400°C" 1400°C" 1400°C" 1400°C"
+ + + + +
aged aged aged aged aged
168 168 168 168 168
h/800°C hll OOO°C hIlOOO°C hIlOOO°C h1l200°C
"Furnace cooled (fc).
Electron microscopy analysis of these initial lamellar structures does show relatively fine lamellar microstructures, with some differences related to alloy compositional effects that are not seen optically. The Ti-47AI binary alloy has an average lamellar spacing of about 400 nm, with about 520 nm spacing between the thinner U2 laths (Table 1). The Ti-47 AI+ B alloys have a finer total interlamellar spacing of about 170 nm and a smaller distance between U2 laths. However, the U2 lamellae in the B-modified alloy are not continuous as they are in the binary alloy, but many are short fragments that look like chopper fibers, as shown in Fig. 2 by both bright-field (BF) and dark-field (DF) photomicrographs with the U2 lamellae tilted about 10-12° from their edge-on orientation. The y (TiAI) and U2 (Ti3Al) phase lamellae are always observed to have the Sastry-Lipsitt crystallonamely graphic habit relationship, 17 {111}yll(0001)u2 and < 11 0> yll<1120>u2' as shown in Fig. 2(c). The Ti-47AI+B alloy has patches of very fine y+u2 lamellae and more yly twins instead of the wider y lamellae found in the binary alloy (Table 1). The Ti-47AI+W+B alloy also has a similar refined average lamellar spacing (190 nm) and U2 spacing compared to the binary alloy. However, the U2 lamellae are more uniform and continuous, and the average lamellar structure is more uniform (no fine lamellar patches and less yly twins in wide lamellae) relative to the Ti-47AI+B alloy (Fig. 3(a». Fig. 1. Optical microstructures of (a) Ti--47AI alloy, as-cast, showing lamellar colonies and equiaxed y grains, and (b) Ti--47AI+W+B, heat-treated for I hat 1400°C to produce a fully-lamellar microstructure.
as-cast alloys for 1 h at 1400°C in vacuum (furnace cooled) produced a fully-lamellar structure with much coarser colony (average size - 700 11m) (Fig. l(b».
Hardness - as-heat-treated Hardness measurements were also made on the as-heat-treated alloys to assess relative differences in strength (Table 2). The Ti-47AI+B alloy was about as strong as or slightly weaker (297 compared· to 309 dph, respectively) than the binary alloy, despite its refined lamellar spacing.
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zone axis: <101 >"{ II <1120> ~
Fig. 2. TEM of the fully-lamellar microstructure of as-heat-treated Ti-47AI-140 appm B alloy, showing (a) bright-field of the lamellae, with the £1'2 lamellae in dark contrast, and (b) dark-field of the £1'2 lamellae, and (c) selected area diffraction showing superimposed zone axis patterns from < 10 I >y//< 1120>£1'2. (c) was taken with the lamellae edge-on and parallel to the electron beam, whereas (a) and (b) were taken with a 10-12 0 tilt to show better the fragmented nature of the £1'2 lamellae.
However, the Ti-47AI+W+B alloy was clearly stronger than the other two alloys (331 dph). While W would be expected to contribute some solid-solution strengthening, the more continuous refinement of the a2 component of the lamellar microstructure is also probably a factor in the strength difference between the W + B- and B-modified alloys. Recent work has emphasized that the ara2 spacing is a major strength-determining factor in such fully-lamellar TiAI alloys.lo.16
Bulk microstructure - aging at 800-1200 o e Aging the fully-lamellar alloys at 800 0 e for 168 h produced little change in the microstructures. The
lamellar structure para~eters for the Ti-47AI+B alloy were measured after aging at 800, 1000, and 1200°C, whereas all three alloys were compared to determine alloying effects at 1000°C. The Ti-47Al+B alloy aged at 800°C showed similar lamellar structure parameters (average lamellar and ara2 spacing) to the initial structure in the as-heat-treated material (Table I and Fig. 4(a)). However, subtle changes did indicate the onset of coarsening mechanisms even at 800°C. For example, the finer a2 lamellar fragments present initially in this alloy appear to have dissolved, and groups of finer a2 lamellae are merging together, which is reflected in the wider range of a2 lath sizes (some thinner and some thicker) after aging.
87
Band Walloying additions Table 2. Hardness of heat-treated and aged TiAI alloys Alloy
Condition
Ti-47AI Ti-47Al+B Ti-47Al+W+B Ti-47AI Ti-47AI+B Ti-47Al+W+B Ti-47AI Ti-47Al+B Ti-47AI+W+B Ti-47AI Ti-47AI+B Ti-47AI+W+B
Ih at Ih at Ih at Ih at Ih at Ih at Ih at Ih at Ih at Ih at Ih at Ih at
1400°Ch 1400°Ch 1400°Ch 1400°Cb + aged 1400°Ch + aged 1400°Cb + aged 1400°Cb + aged 1400°Cb + aged 1400°Cb + aged 1400°Ch + aged 1400°Cb + aged 1400°Cb + aged
Hardness (dph," I kg)
168 168 168 168 168 168 168 168 168
hl800°C hl800°C hl800°C hllOOO°C hllOOO°C hllOOO°C hll200°C hl1200°C hl1200°C
307 297 331 316 305 349 324 309 344 267 293 291
"Average of four-five measurements on metallographically polished specimens. "Furnace cooled (fc).
Fig. 4. TEM of Ti-47Al+B alloy showing the changes in microstructure after aging for 168 h at (a) 800°C, (b) lOOO°C and (c) l200°e. Fig. 3. TEM of fully-lamellar Ti-47AI+W+B alloy showing (a) the initial lamellar structure and (b) the structure after aging at lOOO°C for 168 h.
These doublet and triplet clusters of finer a2 lamellae that are coarsening cause the average lamellae spacing number to decrease (Table 1), even though the overall microstructure shows
sIgns of early coarsening. The early stages of coarsening can be recognized better by noticing the 'Y lamellae size range, which significantly increases during aging at 800°e. Aging for 168 h at lOOO°C produces a large amount of continuous lamellar coarsening in the binary TiAI alloy, less in the B-modified alloy, and little or no coarsening in the W+B-modified
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Fig. 5. SEM of the electro-polished surfaces of TEM discs of (a) as-heat-treated Ti-47AI+B, (b) as-heat-treated Ti-47 AI+W+B, (c) Ti-47AI+B aged at lOOO°C, and (d) Ti-47 AI+W+B aged at lOOO°C. The a2 phase lamellae are visible in the secondary electron images because they protrude (polish slightly less than y) and are bright (higher Z than y).
alloy. As reported previously, the Ti-47AI alloy aged at 1000°C has a much more non-uniform microstructure, because it exhibits both continuous and discontinuous lamellar coarsening modes of instabilityY To the best of our knowledge, lamellar parameter measurements were made in colonies exhibiting continuous coarsening behavior. Both the average overall lamellar spacing and the 'Y lamellar size range show increases by a factor of 2-5 due to coarsening during aging, with the (X2 lamellae showing similar or larger changes (much thicker and spaced farther apart) (Table I). The Ti-47Al+B alloy shows much better resistance of its lamellar structure to continuous coarsening during aging at 1000°C than the binary alloy; however, the coarsening is much more evident at 1000°C than at 800°C (Figs 4(a) and 5(b), and Table 1). The average overall lamellar spacing of the Ti-47 AI+ B alloy nearly doubles during aging at 1000°C, but the (XZ-(X2 spacing almost triples. This can be seen more clearly in Fig. 5, showing SEM of the electropolished
surfaces of TEM disks in the as-heat-treated and aged conditions. These secondary electron images easily show the (X2 lamellae due to topographical (the (X2 lamellae polish a little slower than the adjacent 'Y) and Z (the (X2 is bright because it is the higher Z (atomic number) phase) contrast effects. Wider 'Y lamellae in this alloy have smaller, thinner 'Y fragments within them with heavy dislocation networks emanating from the ends. This suggests that once the (X2 lamellae dissolve, the 'Y/'Y boundaries are less stable and lamellar coarsening proceeds more rapidly. Consistently, the Ti-47AI+W+B alloy shows the best resistance to lamellar coarsening of these three alloys, and the lamellar structure parameters of this W + Bmodified alloy are almost unchanged during aging at 1000°C compared to the initial structure (Figs 3(a) and 3(b), and Table 1). In particular, the (X2-(X2 spacing increases only slightly and has only a few (X2 fragments, as shown in Fig. 5 via SEM, after aging relative to the initial as-heat-treated material. This demonstrates that W stabilizes the (X2 lamellae component of the microstructure against dissolution during aging, and that such (X2 lamellae dissolution is a critical first step in the processes that lead to continuous lamellar coarsening of these fine structures during aging. Finally, Fig. 4(c) shows the almost 10fold coarsening (with Ti-47Al+B as the example) of the lamellar structure that occurs during aging at 1200°C. Previously, optical microscopy of these same alloys showed similar behavior of both Ti-47AI+B and Ti-47Al+W+B at this temperature. 15 Hardness - aging at 800-1200°C
Hardness measurements were also made on metallographic specimens prepared from these same aged materials (Table '2). There is a slight increase in hardness of all three alloys after aging at 800 and 1000°C, but the relative strengths of the different alloys after aging remain the same as they were initially (Ti-47AI+W+B being the hardest). All of the alloys soften after aging at 1200°C, which completely coarsens the initially fine lamellar structures. After aging at 1200°C, the binary alloy is significantly softer than both the Ti-47AI+B and Ti-47AI+W+B alloys, which have similar hardnesses. The relative hardness values of the as-heat-treated alloys, and the changes in hardness observed in the different alloys after aging correlate consistently with the changes observed in their lamellar microstructures.
89
Band Walloying additions
Fig. 6. Optical microstructures of Ti--47AI alloy, showing the subsurface microstructure after (a) heat-treatment for I hat 1400°C, (b) aging for 168 h at 800°C, (c) aging for 168 h at 1000°C, and (d) aging for 168 h at l200°e.
Surface microstructure 800-1000 D C
unaged, and aged at
Studies of subsurface damage that developed in these alloys after aging were made previously,15 and that damage was suspected to be due to plastic deformation beneath the surfaces that were cut to prepare these specimens. Metallographic observations will be summarized and used as a comparison baseline for a more careful evaluation of cold-work effects on the microstructure during aging of these three alloys in the next section. The as-heat-treated specimens were sectioned prior to aging, with little or no detectable damage below most of the surfaces (Fig. 6(a)). After aging, these same surfaces showed a 'damage region' that penetrated into the bulk microstructure below the surface, with the depth of penetration varying with both aging temperature and alloy composition. The 'damage region' consisted of mainly small equiaxed 'Y grains at 800 D e, an a (Ti3Al) surface layer plus larger 'Y grains at lOOODe, and a thicker a layer with varying degrees of porosity at the external surface at 1200 D e (Figs 6(b)-(d) and Table 3). The a surface layer formed during aging on the Ti-47AI binary alloy is shown using backscattered electron (BSE) imaging in an electron-
Table 3. Sub-surface damage layer depth of aged TiAI alloysa Alloy
Condition
Ti--47AI Ti--47Al+B Ti--47AI+W+B Ti--47AI Ti--47AI+B Ti--47AI+W+B Ti--47AI Ti--47AI+B Ti--47AI+W+B
168 h at 168 h at 168 h at 168 hat 168 hat 168 h at 168 h at 168 h at 168 h at
800°C 800°C 800°C 1000°C 1000°C 1000°C l200°C l200°C l200°C
Layer thickness (ILm) 20-25 20-25 20-25 70-80 20-50 20-25 70-75 65-70 85-100
aAll heat-treated for 1 h at 1400°C in vacuum and furnace cooled (fc), then aged in vacuum.
microprobe in Fig. 7, and quantitative XEDS analysis confirms that this outer layer is Ti-25at.%AI, while the material deeper beneath the surface is Ti-47at.% AI. All of the alloys have a similar sub-surface damage layer that is about 25 JLm deep after aging at 800 D e. All the alloys have similar a surface layers that are 20-25 JLm deep after aging at lOOODe, but the depth of the equiaxed 'Y region beneath the a layer varies significantly with alloy composition. The binary alloy has the largest equiaxed 'Y region beneath the a layer (25 JLm), while the Ti-47AI+W+B alloy has none (Table 3 and Fig. 8). After aging at
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Fig. 9. Optical microstructures of 5·5% cold-pressed material aged for 168 h at 1000°C of (a) Ti-47AI alloy, and (b) Ti-47AI+W+B alloy. Note the lack of recrystallized grains in the Ti-47AI+W+B alloy. Fig. 7. Back-scattered image of the subsurface microstructure of Ti-4 7AI aged for 168 h at 1200°C taken in a scanning electron microprobe. The a (Ti3AI) phase appears bright in the picture and was identified using quantitative XEDS analysis (courtesy of D. L. Joslin, ORNL).
Fig. 8. Optical subsurface microstructures after aging for 168 h at 1000°C of (a) Ti-47AI+B and (b) Ti-47AI+W+B. Note only the a layer in the Ti-47AI+W+B alloy.
1200DC, all of the alloys have a thick (75-100 JLm) layer with a very coarse lamellar microstructure beneath it. The thicknesses of the (X layers are about the same on all three alloys aged at 1200°C, as one would expect. (X
Bulk microstructure - 5·5% cold-worked, and aged at lOOO°C The hypothesis was suggested previously that the reduced size of the sub-surface damage layer in the Ti-47Al+W+B alloy aged at 1000°C was due to the alloying elements Wand B making the TiAI alloys more resistant to recrystallization caused by
subsurface deformation and/or residual stresses due to cutting. To test this hypothesis, specimens of all three alloys were cold-forged about 5·5% and then aged for 48 and 168 h at 1000°C. Although all of the alloys had cracks, metallographic examination of the as-deformed specimens indicated that all had undergone significant plastic strain. All three alloys showed some development of fine equiaxed 'Y grains along intercolony boundaries after aging for 48 h. After 168 h, there were very significant differences between the three alloys, with the binary alloy showing the most recrystallization and equiaxed grain growth, and the Ti-47Al+W+B alloy showing little to none (Fig. 9). TEM examination of the Ti-47Al+B alloy showed very coarse, broken (X2 lamellae (Fig. 10) and considerably more continuous and discontinuous lamellar coarsening than was found anywhere in the same alloy aged without any prior deformation (cp. Figs 4(b) and 10).
Surface microstructure - 5·5% cold-worked, and aged at lOOO°C The carefully polished surfaces of these 5·5% coldpressed specimens exposed to aging in vacuum for 48 and 168 h at 1000°C were also examined for comparison with the previous subsurface 'damage regions' developed on cut and heat-treated specimens. The binary Ti-47 Al alloy developed an (X surface layer that was about 3-6 JLm thick right at the surface, with another layer of 'Yand (X2 grains that was about 10-12 JLm thick, for a total average damage region of 15 JLm after aging for 168 h (Fig. II(a)). The Ti-47Al+W+B alloy shows a 2-4 JLm thick (X layer after 48 h that grows to about 6 JLm after 168 h, but does not develop the discon-
Band Walloying additions
91
Fig. 10. TEM of the lamellar structure of the Ti-47AI+B aHoy, 5·5'Yo cold-pressed and aged for 168 h at IOOO°C.
tinuously coarsened Y+CI'2 region found in the binary alloy (Figs 11(b) and (c)). The surface behavior of the Ti-47AI+B alloy is close to that found in the binary, with a slightly thinner total damage layer thickness. Finally, it can be seen that there is some discontinuous coarsening adjacent to the crack seen in the Ti-47AI+W+B alloy in Fig. 11 (c). Most likely this reflects the effect of enhanced lamellar coarsening in what was the plastic zone initially associated with the crack. Degradation of the lamellar structure in that region is more severe than at the free surface because the careful polishing at that surface removed all such damaged material prior to aging. DISCUSSION
The previous paper on part of this work by Ramanujan et al. 15 categorized the various modes of continuous and discontinuous coarsening by which the lamellar microstructures in these alloys change during aging, and emphasized the changes in modes of lamellar instability as the aging temperature increased from 800 to 1200°e. These TiAI alloys exhibited primarily continuous coarsening of the initial fine lamellar structures at 800 and 1000°C, and discontinuous coarsening to form new and very coarse lamellar structures at 1200°e. The binary and Ti-47AI+B alloys also formed very large non-lamellar y grains which contain spherodized Cl'2 particles, whereas the Ti-47AI+W+B alloy did not. Ramanujan et al. noted that the additions of Wand B retarded the coarsening of the bulk lamellar structures in this Ti-47 Al series of alloys, and minimized the development of subsurface 'damage regions' beneath the cut surfaces
Fig. II. Optical subsurface microstructures of 5·5% coldpressed specimens aged at IOOO°C of (a) Ti-47AI aged for 168 h, (b) Ti-47AI+W+B aged for 48 h, and (c) Ti-47AI+W+B aged for 168 h.
of these specimens during subsequent aging, but those solute effects were not the main focus of that paper. In this paper, the discussion will focus in more detail on the effects that Band W have on the formation of the initial fully-lamellar structures in these Ti-47 Al alloys, and on the detailed behavior of the y and Cl'2 phase components of the overall lamellar structures during aging. It will also emphasize the role of cold-work and the effects of Band W additions on the subsurface damage that develops during aging.
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Relative to other studies of TiAI alloys with 40-50 at. (Yo AI, the initial lamellar structures formed in these Ti--47 Al alloys are relatively fine,18-24 although not as fine as the structures observed recently in P/M processed Ti--47 AI-2Cr-2Nb alloys.1O Following the classification of TiAI microstructures by Kim,20 the Ti--4 7Al alloys in this work that were heat-treated at 1400°C have a type I fully-lamellar (FL) structure, in which the y lamellae form in the c¥ phase grains as the material is cooled. Other work indicates that increased cooling rates can refine the lamellar structure of Ti--47AI and Ti--48AI alloys up to a point, but when the cooling rate becomes too high, massive transformation of the c¥ into y occurs.20.2I.24 In binary alloys, slow furnace cooling tends to produce lamellar structures with lamellar spacing of the order of 1 JLm or more. 21 ,24 Polysynthetically twinned TiAI has a lamellar structure with spacings of 1-2 JLm or slightly less, but that structure is dominated by y/y boundaries and the C¥2 lamellar are quite thick and spaced 5-10 /Lm apart. 6,19 Kim shows a lamellar spacing of 0·03-0·2 /Lm in a fully-lamellar Ti--47AI-I Cr-1V-2·5Mo alloy.20 However, to our knowledge, there are no systematic data on the effects of alloying on lamellar refinement in the same base TiAI alloy for the same heat-treatment/cooling conditions. 15 The effects that both Band B+ W additions have on refining the lamellar microstructure for the same heat-treatment/cooling rate conditions indicate that these elements are affecting how the y lamellae nucleate and grow to form this structure. Boron appears to increase the nucleation rate of y lamellae relative to the binary alloy, but it does not seem to have much effect on the way those individual y lamellae grow. The fact that there are so many fragmented C¥2 lamellae suggests that the y lamellae grow together rapidly during cooling to pinch-off the c¥ phase as the lamellar structure forms. Consistently then, adding W together with the B appears to retard the growth rate of the y lamellae after they nucleate, resulting in the more continuous and uniformly dispersed C¥2 lamellae that we observe in this work. Recent work on TiAI alloys containing W does not include detailed observations of C¥2 phase behavior within the general lamellar structure. 7,9,25 The differences in hardness of the as heattreated alloys are somewhat surprising in light of the differences observed in the initial lamellar microstructures of these alloys. The B modified alloy has about the same hardness or is slightly
softer than the binary Ti--47 Al alloy, despite its refined lamellar structure. Even though the C¥2 lamellae are fragmented, it is not clear why a finer dispersion of y/y interfaces would not increase the hardness, but softening effects associated with B additions to TiAI alloys have also been observed by others. 26 Compared to the amounts of B added by others (up to 1 at.%), the amount of B added in this study (0·014 at.%) is quite low. 3 By contrast, the Ti--47AI+W+B alloy is stronger than the binary alloy (331 dph compared to 307 dph), consistent with the refined lamellar structure that includes more continuous and uniformly dispersed C¥2 lamellae, and the solid-solution strengthening effects of W. 16 Such details of the lamellar structure are important to note, because recent work has indicated that both ultrafine lamellar structure and defect-free C¥2 lamellae within that structure contribute significantly to the high-temperature strength and creep-resistance of the FL structures in more complex TiAI alloys.IO,1l,16 The improved aging/coarsening resistance of the B- and W + B-modified alloys is particularly important because without such alloying effects enhancing stability, finer lamellar structures would be expected to be less stable rather than more stable during aging. One of the driving forces for discontinuous coarsening in the Livingston-Cahn treatment of general coarsening of lamellar structures (their examples are eutectics) for discontinuous coarsening, in which much coarser lamellar structures replaced finer structures, is a reduction of surface area to reduce the free energy of the system.27 Boron is well known to segregate to boundaries and interfaces in other intermetallic systems,28,29 but such segregation was not measured in this study. A comparison of the aging behavior of the Ti--4 7Al + B alloy to the binary definitely shows retarded coarsening kinetics at 1000°C, although there is considerable continuous coarsening due to dissolution of the C¥2 lamellae and instability of the y/y boundary structure. Relative to the other two alloys, the Ti--47AI+W+B alloy shows very little continuous coarsening of the initial fine lamellar structure during aging at 800 and 1000°e. The more continuous C¥2 lamellae of the initial as-heat-treated microstructure of the W+B modified alloy would be a positive factor in aging resistance of the microstructure, because broken or fragmented lamellae are unstable. Bartholomeusz and Wert 29 ,30 studied much coarser lamellar structures in Ti--44AI alloys, but they do show that terminal C¥2 lamellae dissolve as adjacent lamellae thicken due to the highly curved surface (short
Band Walloying additions
radius of curvature) at the terminal end affecting the solubility of Ti and Al in the adjacent TiAI matrix. Therefore, structures without such lamellar defects would be more stable because parallel lamellae without terminal defects have an infinite radius of curvature. However, this one factor alone is not sufficient to explain the beneficial effect of W on aging resistance, because the Ti--47 Al binary also has relatively defect-free (l'2 lamellae and yet it coarsens tremendously during aging. The stabilizing effects of W most likely also come from direct solid-solution effects that W has when it is present in both the (l'2 and I' lamellar phases. The microstructural data presented here show that the (l'2 lamellae in the Ti--47Al+W+B alloy are more resistant to dissolution, and show that the general lamellar structure is more resistant to the various modes of continuous 1'/1' lamellar coarsening. IS ,31 Tungsten is present in both the (l'2 and I' phase lamellae in the initial structure, with slightly more W in the (l'2 phase (0·9 at. % compared to 0·6% in the 1'), as shown previously by measurements of the compositions of individual lamellae using high-spacial resolution XEDS in this alloy. IS Similar measurements of Wand Al in individual lamellae of the Ti--4 7Al + W + B alloy aged for 168 h at 1000°C indicate that W is repartitioning to achieve equal concentrations (0· 7 at.%) in both phases while the Al difference between the (l'2 and I' phase lamellae is increasing (aging lowers the Al content of the (l'2 phase from 39 to 35 at.%, but has almost no effect on the I' phase composition). These compositional effects during aging are consistent with the (l'2 phase compositional differences above and below the eutectoid temperature, and would suggest that the microstructure forms with a metastable, nonequilibrium composition above the eutectoid temperature forced by the continuous cooling. During aging at lower temperatures, the microstructure appears to be approaching the equilibrium composition of the (l'2 and I' phases as the lamellar structure coarsens. If W diffusion is necessary before the (l'2 lamellae can dissolve and coarsen, and W is a slower diffusing species than Ti or AI, then that would help to explain how W retards coarsening of the lamellar structure. Tungsten also appears to generally retard the various modes of lamellar coarsening and microstructural instability that both result from the migration/propagation of I' boundaries and interfaces during aging at 800-1000°C. Such an effect could be related to W affecting the kinetics
93
of Ti and Al diffusion in the I' phase, but more work would be necessary to verify such a mechanism. However, the suggestion that 1'/1' interfacial movement is more sluggish in the W-modified alloy during aging than it is in the alloys without W fits together with the argument made earlier that W retards the growth of I' lamellae during heat-treatment and cooling at higher temperatures to form the initial lamellar structure. The same structure-stabilizing effect of W is further supported by the lack of discontinuous coarsening or equiaxed I' grain formation beneath the surface of the Ti--47Al+W+B alloy specimens relative to the other alloys, and the recrystallization resistance found in the cold-worked and aged W + B modified alloy. There is little effect of W evident after the initial lamellar microstructure is completely removed during aging at 1200°C, because the microstructures of the Band W + B modified alloys look similar and have the same hardness values. This would suggest that W is most effective in TiAI processed/heat-treated to produce a fullylamellar structure, as suggested by creep studies of others in W -modified TiAI alloys.7,9 It is technologically significant that Band W+B additions can refine the lamellar microstructure even at relatively slow furnace cooling rates. In TiAI components with greater section thicknesses, cooling rate alone cannot be used to control and refine the structure. Since the mechanical properties of TiAI alloys are so sensitive to microstructural changes, alloying elements that minimize gradients in microstructure should also minimize properties gradients too. The addition of W+B together appear to be very beneficial for producing a refined FL structure that is less sensitive to cooling rate effects. Another technological benefit of the W + B addition is its minimization or elimination of subsurface damage layer evolution. This may be very important for resistance to mechanical failures in which cracks initiate at the surface, such as tensile straining, creep, or creep-fatigue. Analysis of the various damage layers formed on the aged specimens suggests that the (l'-case formation is more of a generic factor in these alloys that is affected by a loss of Al from the surface to the vacuum at a particular aging temperature. Indeed, in the 5·5% cold-pressed specimens, the careful surface preparation after heat-treatment at 1400°C and after the cold-deformation greatly reduced the (l'case formation in all of the alloys aged for 168 h at 1000°C. This demonstrates that the initial surface of the first group of aged specimens (cut and
94
P. J. Ma::ias:: et al.
vacuum-heat-treated) lost some AI in the vacuum, even though it was not enough to form a visible a-case. The growth of the a-case during subsequent vacuum-aging then begins sooner and proceeds more rapidly at 1000°C compared to the cold-pressed specimens with all such surface affects from previous treatments removed. Recent work has shown that an a-phase surface layer significantly decreases room temperature ductility of a FL Ti-47AI-2Cr-2Nb alloy.32 The 'damage layer' of coarser equiaxed y grains with a2 particles is definitely affected by any deformation that occurs below the surface during cutting and by the alloy composition. The addition of W+B makes the Ti-47 AI alloys very resistant to such damage evolution with or without subsurface deformation. The binary alloy shows some discontinuous coarsening beneath the surface of the 5·5% cold-pressed specimen, despite all efforts to remove subsurface damage beforehand, but less than the as-heattreated material aged at 1000°C with its initial subsurface deformation intact. Ideally, it would appear to be important to remove any adverse surface effects of vacuum heat-treating or machining by polishing, and to add W + B to make the TiAI alloy more resistant to in-service damage layer evolution associated with such machined and/or heat-treated surfaces. Tungsten and boron additions together have a significant benefit of producing finer and more uniform lamellar structures initially in Ti-47 AI alloys heat-treated at 1400°C. These combined additions furthermore make the fine lamellar structure robust and resistant to coarsening during aging at 800 and 1000°C for 168 h.
CONCLUSIONS (1) The addition of B (140 appm) to Ti-47AI
refines the fully-lamellar structure produced by heat-treatment for I h at I 400°C, but with many a2 lamellae within that structure, A combination of W (0·5 at.%) and B additions produces a similarly refined lamellar structure, but with more uniformly continuous and defect-free lamellae. The Ti-47 AI +W + B was harder than the other two alloys. (2) Aging of these alloys for 168 h produced little effect at 800°C, but caused significant changes at 1000°C that depend strongly on alloy composition, and complete degradation of the microstructure by discontinuous coarsening
at 1200°C. All of the alloys aged at 800 and 1000°C were slightly stronger than the initial as-heat-treated material, with the Ti-47 AI+W+B alloy always being harder than the other two. However, all the alloys softened after aging at 1200°C, with the binary being much softer than the other two alloys. (3) After aging at 1000°C the binary alloy had a significantly coarser lamellar structure (average spacing - I-211m). The Ti-47AI+B alloy showed much less coarsening of the overall lamellar structure (average spacing - 0·35-0A 11m), but the a2-a2 spacing increased from O· 3-0· 5 11m to IA 11m due to dissolution and coarsening of this phase. The Ti-47AI+W+B alloy showed almost no change in the average lamellar spacing or the a2-a2 spacing after aging at 1000°C. (4) Aging of initially heat-treated (in vacuum) and cut specimens produced subsurface damage in all alloys and aging (in vacuum) conditions. All alloys showed a similar a-case that was about 20-25 11m thick at 800 and 1000°C, and 65-100 JLm thick at 1200°C. The binary Ti-47AI alloy had a coarse y grain layer below the a-case that was 50-60 JLm thick. That 'damage layer' was 25 JLm or less in the Ti-47AI+B alloy, and was not found at all III the Ti-47AI+W+B alloy. (5) Cold-pressing (5·5%) increased the lamellar degradation in all of the alloys aged at 1000°C for 168 h. The binary alloy showed extensive recrystallization into fine equiaxed "y grains, while the B-modified alloy showed less and W + B-modified alloy showed almost none. (6) The surfaces of the 5·5% cold-pressed and aged specimens were carefully polished to remove all damage from prior heat-treatment, deformation and specimen cutting. Consistently, these all showed less subsurface damage than the previous specimens, with W + B modified alloy showing the least damage (only a 6 JLm thick a-case).
ACKNOWLEDGMENTS This work was supported by the US Department of Energy, through the Materials Science Division, Office of Basic Energy Sciences (BES), and the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Industrial Technolo-
Band Walloying additions
gies, Advanced Industrial Materials (AIM) Program under contract DE-AC05-960R22464 with Lockheed Martin Energy Research Corporation. Thanks to Mr C. A. Carmichael for melting the alloys, and to Mr J. W. Jones for TEM sample preparation. Dr R. Ramanujan wishes to thank Dr P. Mukhopadhyay (Head, Physical Metallurgy Section), Dr S. Banerjee (Head, Metallurgy Division) and Dr C. K. Gupta (Director, Materials Group) at the Bhabha Atomic Research Centre for their keen interest in this work.
12. 13. 14. 15. 16. 17. 18. 19.
REFERENCES I. Dimiduk, D. M., in Gamma Titanium A lumin ides, eds Y-W. Kim, R. Wagner and M. Yamaguchi. TMS, Warrendale, PA, 1995, p. 3. 2. Austin, C. M. and Kelly, T. J., in Gamma Titanium Aluminides, eds Y-W. Kim, R. Wagner and M. Yamagachi. TMS, Warrendale, PA, 1995, p. 21. 3. Huang, S. c., in Structural Intermetallics, eds R. Darolia, J. Lewandowski, C. T. Liu, P. Martin, D. Miracle and M. Nathal. TMS, Warrendale, PA, 1993, p. 299. 4. Kim, Y. W., in Gamma Titanium Aluminides, eds Y.-W. Kim, R. Wagner and M. Yamaguchi. TMS, Warrendale, PA, 1995, p. 637. 5. Kim, Y. W., JOM, 1994,46(7) 30. 6. Yamaguchi, M. and Inui, H., in Structural Intermetallics, eds R. Darolia, J. Lewandowski, C. T. Liu, P. Martin, D. Miracle and M. Nathal. TMS, Warrendale, PA, 1993, p. 127. 7. Beddoes, J., Zhao, L., Triantaffillou, J., Au, P. and Wallace, W., in Gamma Titanium Aluminides, eds Y.-W. Kim, R. Wagner and M. Yamaguchi. TMS, Warrendale, PA, 1995, p. 959. 8. Viswanathan, G. B. and Vasudevan, V. K., in Gamma Titanium Aluminides, eds Y.-W. Kim, R. Wagner and M. Yamaguchi. TMS, Warrendale, PA, 1995, p. 967. 9. Schwenker, S. W. and Kim, Y. W., in Gamma Titanium Aluminides, eds Y.-W. Kim, R. Wagner and M. Yamaguchi. TMS, Warrendale, PA, 1995, p. 985. 10. Liu, C. T., Maziasz, P. J., Clemens, D. R., Schneibel, J. H., Sikka, V. K., Nieh, T. G., Wright, J. and Walker, L. R., in Gamma Titanium Aluminides, eds Y.-W. Kim, R. Wagner and M. Yamaguchi. TMS, Warrendale, PA, 1995, p. 679. II. Wang, J. N., Schwartz, A. J., Nieh, T. G., Liu, C. T., Sikka, V. K. and Clemens, D. J., in Gamma Titanium
20. 21.
22.
23.
24.
25. 26. 27. 28. 29. 30. 31. 32.
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Aluminides, eds Y.-W. Kim, R. Wagner and M. Yamaguchi. TMS, Warrendale, PA, 1995, p. 949. Kim, Y.-W., Mat. Sci. Eng., 1995, A 192/193, 518. Mitao, S. and Bendersky, L. A., in Gamma Titanium Aluminides, eds Y.-W. Kim, R. Wagner and M. Yamaguchi. TMS, Warrendale, PA, 1995, p. 181. Martin, P. L., Mendiratta, M. G. and Lipsitt, H. A., Met. Trans., 1983, 14A, 2170. Ramanujan, R. V., Maziasz, P. J. and Liu, C. T., Acta Mater., 1996,44,2611. Liu, C. T., Schneibel, J. H., Maziasz, P. J., Wright, J. L. and Easton, D. S., Intermetallics 1996, 4, 429. Sastry, S. M. L. and Lipsitt, H. A., Metall. Trans., 1977, 8A,299. Yamaguchi, M. and Umakoshi, Y., Prog. Mater. Sci., 1990, 34(1), 1. Umakoshi, Y., Nakano, T. and Yamane, T., Mater. Sci. Eng., 1992, A152, 81. Kim, Y.-W., Acta Met., 1992,40, 1121. Takeyama, M., Kumagai, T., Nakamura, N. and Kikuchi, M., in Structural Intermetallics, eds R. Darolia, J. Lewandowski, C. T. Liu, P. Martin, D. Miracle and M. Nathal. TMS, Warrendale, PA, 1993, p. 167. Martin, P. L., Rhodes, C. G. and McQuay, P. A., in Structural Intermetallics, eds R. Darolia, J. Lewandowski, C. T. Liu, P. Martin, D. Miracle and M. Nathal. TMS, Warrendale, PA, 1993, p. 177. Kad, B. K., Hazzeldine, P. M. and Fraser, H. L., High Temperature Ordered Intermetallic Alloys V, eds I. Baker, R. Darolia, J. D. Wittenberger and M. H. Yoo. MRS Symp. Proc., Vol. 288, Pittsburgh, PA, 1993, p. 495. Wang, P. and Vasudevan, V. J., High-Temperature Ordered Intermetallic Alloys V, eds I. Baker, R. Darolia, J. D. Witten berger and M. H. Yoo. MRS Symp. Proc., Vol. 288, Pittsburgh, PA, 1993, p. 229. Bhowal, P. R., Konkel, W. A. and Merrick, H. F., in Gamma Titanium Aluminides, eds Y.-W. Kim, R. Wagner and M. Yamaguchi. TMS, Warrendale, PA, 1995, p. 787. Kim, Y.-W., J. Metals, 1989,41(7) 24. Livingston, J. D. and Cahn, J. W., Acta Metall., 1974, 22,495. Liu, C. T., White, C. L. and Horton, Jr, J. A., Acta Metall., 1985, 33, 213. Liu, C. T., in Structural Intermetallics, eds R. Darolia, J. Lewandowski, C. T. Liu, P. Martin, D. Miracle and M. Nathal. TMS, Warrendale, 1993, p. 365. Bartholomeusz, M. F. and Wert, J. A., Metall. and Mater. Trans., 1994, 25A, 2371. Ramanujan, R. V., Acta Metall., 1994,42,2775. Liu, C. T., Maziasz, P. J., Wright, J. L. and Clemens, D. R., unpublished data presented at the International Symposium on Advanced Materials and Technology for the 21st Century, JIM/TMS, Honolulu, HA, 13-15 December 1995; available on request.
PII:
(1997) 83-95
lntermerallics 5 © Elsevier Science Limited Printed in Great Britain. All rights reserved 0966-9795/97/$17.00
S0966-9795(96)00070-2
Effects of Band W alloying additions on the formation and stability of lamellar structures in two-phase y-TiAI P. J. Maziasz, R.
v. Ramanujan,* C. T. Liu &
J. L. Wright
Metals and Ceramics Division. Oak Ridge National Laboratory. PO Box 2008. Oak Ridge. Tennessee 37831. USA (Received 14 May 1996; accepted 25 June 1996)
The effects of additions of 140 appm Band 0·5 at.% W on the formation and stability of fully-lamellar structures in a Ti-47AI base alloy were studied. The fully-lamellar structure was formed by heat-treatment at 1400°C for I h (in vacuum) followed by furnace cooling. Stability was studied by aging these alloys for 168 h at 800, 1000 and 1200°C and making quantitative measurements of lamellar structure parameters (i.e. average lamellar spacing, £l!2-£l!2 spacing) using transmission electron microscopy (TEM) and hardness measurements. The B addition alone refines the lamellar structure relative to the binary TiAI alloy, but causes fragmented and discontinuous £l!2 lamellae. By contrast, W+B addition refines the lamellar structure and produces more uniform and continuous £l!, lamellae. The as-heat-treated Ti-47AI+W+B alloy is harder than the other t~o alloys. Aging for 168 h at 800°C does not change the fine initial lamellar microstructure of any of these alloys. Aging at 1000°C significantly coarsens the Ti-47AI alloy, causes some coarsening in the Ti-47AI+B alloy, but has little effect on the lamellar structure of the Ti-47AI+W+B alloy. Aging at 1200°C causes discontinuous coarsening and complete loss of the initial, fine, lamellar microstructure in all three alloys. Aging in vacuum produces a subsurface 'damage region' consisting of an £l!(Ti3AI) surface layer with another layer of coarsened y microstructure underneath. At 1000°C, this layer is greatest in the binary alloy and least in the Ti-47AI+W+B alloy. Specimens were 5·5% cold-forged and then aged at 1000°C to evaluate the effects of subsurface deformation caused during cutting of the specimens. Prior cold-deformation caused considerable recrystallization in the binary alloy, but almost none in the Ti-47AI+W+B alloy. The additions of Wand B to a Ti-47AI alloy refine the fully-lamellar structure obtained by heat-treatment, and make it more resistant to lamellar coarsening (and recrystallization if cold deformed) during aging at 800-1000°C. © 1997 Elsevier Science Limited. All rights reserved.
Key words: A. titanium aluminides (based on TiAI), B. microalloying, D. microstructure.
alloys provides much better high-temperature strength and creep-resistance than other structures (i.e. duplex), but the room-temperature ductility of fully-lamellar TiAl alloys has usually been low.4-9 Recent work, however, has shown that controlled processing which gives a refined colony size and very fine lamellar spacings in P/M materials can produce good ductility (1·4%) and high strength (YS = 971 MPa) at room temperature, as well as very good strength and creep-resistance at 760-S00°c. IO,I' The fully-lamellar microstructure consists of parallel laths of the (X2 (DO I9 structure) and y (Llo structure) phases forming colonies, with little or
INTRODUCTION
y-based titanium aluminides have been developed and are being tested for high temperature structural applications because they possess very low density (4 g/cm 3), good oxidation resistance,. and good creep resistance and high temperature strength. ' -3 The balance of room-temperature ductility and high-temperature strength is very sensitive to the microstructure. 1-6 Generally, the fully-lamellar structure of Ti-(47-4S)Al (at.%) *Permanent address: Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India. 83
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P. J. Ma::ias:: et al.
no y grains at the boundaries of those colonies. Lamellar spacings can range from > 0.5 mm to < 0.1 J.Lm. IO•12 Lamellar microstructures can be formed by heating the alloy into the a phase region, and then cooling through the a + y phase region to form the y lamellae. The residual a trapped between these growing y lamellae then transforms to the ordered 0'2 phase below the eutectoid temperature. Ti-47AI alloys are more resistant to discontinuous coarsening of their lamellar structures during aging than alloys with less Al. 13 Tungsten additions have been shown to be very beneficial for enhancing creep-resistance of simple and more complex Ti-(47-48)AI alloys.l4-l6 Recent studies of lamellar coarsening mechanisms have found that B+W additions to Ti-47AI alloys enhance their resistance to lamellar degredation at 800-1000°CY This was found not only in the bulk material, but also at surfaces that had experienced mechanical damage during cutting. One purpose of this paper is to present more detailed data showing the effects of Band W additions on the formation of the initial, fine lamellar structures in Ti-47 Al alloys, and to show the stability of such fine lamellar structures during subsequent aging. In particular, the behavior of the 0'2 component of the microstructure is emphasized. Another purpose is to study the effects of 5.5% cold-forging prior to aging to understand better the benefits of B+ W additions on subsurface damage resistance. Such information is vital for improving the 'damage tolerance' of machined TiAI components.
EXPERIMENTAL PROCEDURE Alloy ingots (2·5 cm in diameter and 5 cm long, weighing 175 gm) of nominal composition Ti-47AI, Ti-47AI-0·014B (at.%) (Ti-47AI+B) and Ti-47AI-0·5W-0·014B (Ti-47AI+W+B) alloys were prepared by arc melting and drop casting, and the difference in weight after melting was less than O· 5%. These ingots were then cut into slices, and specimens for aging or microstructural examination were made with a slow-speed saw. These specimens were then heat-treated in a vacuum furnace at 1400°C for 1 h in the a-phase region of the phase diagram6 and furnace cooled. This heattreatment homogenized the materials and established a fully-lamellar microstructure in all three alloys. Specimens were then aged at either 800, 1000 or 1200°C for 168 h and furnace cooled. To study better the effects of surface preparation on
sub-surface damage, some as-cast specimens of each alloy were gently ground with 180, 240, 400, 600 and 0 grit emery paper, and then heat-treated in vacuum at 1400°C for I h. They were then cold-forged at room temperature to a compression strain of 5-5
RESULTS
Bulk microstructure - as heat-treated The as-cast material of all the alloys had lamellar colonies (100-300 J.Lm) with about 5-15% of equiaxed y grains located mainly along colony boundaries (Fig. l(a». Heat-treatment of these
Band Walloying additions
85
Table 1. Effects of Band W on average lamellar spacing in TiAI alloys Quantitative lamellar microstructure data (nm) Alloy
Condition
Average spacing
ara2 spacing
y-width
a2 width
Ti--47AI Ti--47AI+B Ti--47AI+W+B Ti--47AI+B Ti--47AI Ti--47AI+B Ti--47AI+W+B Ti--47AI+B
I I I I I I I I
400 170 190 144 800-1950 350--400 190 >1900
520 330-500 420-560 340--400 >1300 1420 660 3700
200-650 70--400 140-720 120-630 520-2200 200-1200 150--440 2000--4000
60-120 50-100 50-130 35-150 220-1030 50-155 60-130 >1000
h at h at h at h at h in hat hat hat
1400°C" 1400°C" 1400°C" 1400°C" 1400°C" 1400°C" 1400°C" 1400°C"
+ + + + +
aged aged aged aged aged
168 168 168 168 168
h/800°C hll OOO°C hIlOOO°C hIlOOO°C h1l200°C
"Furnace cooled (fc).
Electron microscopy analysis of these initial lamellar structures does show relatively fine lamellar microstructures, with some differences related to alloy compositional effects that are not seen optically. The Ti-47AI binary alloy has an average lamellar spacing of about 400 nm, with about 520 nm spacing between the thinner U2 laths (Table 1). The Ti-47 AI+ B alloys have a finer total interlamellar spacing of about 170 nm and a smaller distance between U2 laths. However, the U2 lamellae in the B-modified alloy are not continuous as they are in the binary alloy, but many are short fragments that look like chopper fibers, as shown in Fig. 2 by both bright-field (BF) and dark-field (DF) photomicrographs with the U2 lamellae tilted about 10-12° from their edge-on orientation. The y (TiAI) and U2 (Ti3Al) phase lamellae are always observed to have the Sastry-Lipsitt crystallonamely graphic habit relationship, 17 {111}yll(0001)u2 and < 11 0> yll<1120>u2' as shown in Fig. 2(c). The Ti-47AI+B alloy has patches of very fine y+u2 lamellae and more yly twins instead of the wider y lamellae found in the binary alloy (Table 1). The Ti-47AI+W+B alloy also has a similar refined average lamellar spacing (190 nm) and U2 spacing compared to the binary alloy. However, the U2 lamellae are more uniform and continuous, and the average lamellar structure is more uniform (no fine lamellar patches and less yly twins in wide lamellae) relative to the Ti-47AI+B alloy (Fig. 3(a». Fig. 1. Optical microstructures of (a) Ti--47AI alloy, as-cast, showing lamellar colonies and equiaxed y grains, and (b) Ti--47AI+W+B, heat-treated for I hat 1400°C to produce a fully-lamellar microstructure.
as-cast alloys for 1 h at 1400°C in vacuum (furnace cooled) produced a fully-lamellar structure with much coarser colony (average size - 700 11m) (Fig. l(b».
Hardness - as-heat-treated Hardness measurements were also made on the as-heat-treated alloys to assess relative differences in strength (Table 2). The Ti-47AI+B alloy was about as strong as or slightly weaker (297 compared· to 309 dph, respectively) than the binary alloy, despite its refined lamellar spacing.
86
P. J. Ma=ias= et al.
zone axis: <101 >"{ II <1120> ~
Fig. 2. TEM of the fully-lamellar microstructure of as-heat-treated Ti-47AI-140 appm B alloy, showing (a) bright-field of the lamellae, with the £1'2 lamellae in dark contrast, and (b) dark-field of the £1'2 lamellae, and (c) selected area diffraction showing superimposed zone axis patterns from < 10 I >y//< 1120>£1'2. (c) was taken with the lamellae edge-on and parallel to the electron beam, whereas (a) and (b) were taken with a 10-12 0 tilt to show better the fragmented nature of the £1'2 lamellae.
However, the Ti-47AI+W+B alloy was clearly stronger than the other two alloys (331 dph). While W would be expected to contribute some solid-solution strengthening, the more continuous refinement of the a2 component of the lamellar microstructure is also probably a factor in the strength difference between the W + B- and B-modified alloys. Recent work has emphasized that the ara2 spacing is a major strength-determining factor in such fully-lamellar TiAI alloys.lo.16
Bulk microstructure - aging at 800-1200 o e Aging the fully-lamellar alloys at 800 0 e for 168 h produced little change in the microstructures. The
lamellar structure para~eters for the Ti-47AI+B alloy were measured after aging at 800, 1000, and 1200°C, whereas all three alloys were compared to determine alloying effects at 1000°C. The Ti-47Al+B alloy aged at 800°C showed similar lamellar structure parameters (average lamellar and ara2 spacing) to the initial structure in the as-heat-treated material (Table I and Fig. 4(a)). However, subtle changes did indicate the onset of coarsening mechanisms even at 800°C. For example, the finer a2 lamellar fragments present initially in this alloy appear to have dissolved, and groups of finer a2 lamellae are merging together, which is reflected in the wider range of a2 lath sizes (some thinner and some thicker) after aging.
87
Band Walloying additions Table 2. Hardness of heat-treated and aged TiAI alloys Alloy
Condition
Ti-47AI Ti-47Al+B Ti-47Al+W+B Ti-47AI Ti-47AI+B Ti-47Al+W+B Ti-47AI Ti-47Al+B Ti-47AI+W+B Ti-47AI Ti-47AI+B Ti-47AI+W+B
Ih at Ih at Ih at Ih at Ih at Ih at Ih at Ih at Ih at Ih at Ih at Ih at
1400°Ch 1400°Ch 1400°Ch 1400°Cb + aged 1400°Ch + aged 1400°Cb + aged 1400°Cb + aged 1400°Cb + aged 1400°Cb + aged 1400°Ch + aged 1400°Cb + aged 1400°Cb + aged
Hardness (dph," I kg)
168 168 168 168 168 168 168 168 168
hl800°C hl800°C hl800°C hllOOO°C hllOOO°C hllOOO°C hll200°C hl1200°C hl1200°C
307 297 331 316 305 349 324 309 344 267 293 291
"Average of four-five measurements on metallographically polished specimens. "Furnace cooled (fc).
Fig. 4. TEM of Ti-47Al+B alloy showing the changes in microstructure after aging for 168 h at (a) 800°C, (b) lOOO°C and (c) l200°e. Fig. 3. TEM of fully-lamellar Ti-47AI+W+B alloy showing (a) the initial lamellar structure and (b) the structure after aging at lOOO°C for 168 h.
These doublet and triplet clusters of finer a2 lamellae that are coarsening cause the average lamellae spacing number to decrease (Table 1), even though the overall microstructure shows
sIgns of early coarsening. The early stages of coarsening can be recognized better by noticing the 'Y lamellae size range, which significantly increases during aging at 800°e. Aging for 168 h at lOOO°C produces a large amount of continuous lamellar coarsening in the binary TiAI alloy, less in the B-modified alloy, and little or no coarsening in the W+B-modified
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Fig. 5. SEM of the electro-polished surfaces of TEM discs of (a) as-heat-treated Ti-47AI+B, (b) as-heat-treated Ti-47 AI+W+B, (c) Ti-47AI+B aged at lOOO°C, and (d) Ti-47 AI+W+B aged at lOOO°C. The a2 phase lamellae are visible in the secondary electron images because they protrude (polish slightly less than y) and are bright (higher Z than y).
alloy. As reported previously, the Ti-47AI alloy aged at 1000°C has a much more non-uniform microstructure, because it exhibits both continuous and discontinuous lamellar coarsening modes of instabilityY To the best of our knowledge, lamellar parameter measurements were made in colonies exhibiting continuous coarsening behavior. Both the average overall lamellar spacing and the 'Y lamellar size range show increases by a factor of 2-5 due to coarsening during aging, with the (X2 lamellae showing similar or larger changes (much thicker and spaced farther apart) (Table I). The Ti-47Al+B alloy shows much better resistance of its lamellar structure to continuous coarsening during aging at 1000°C than the binary alloy; however, the coarsening is much more evident at 1000°C than at 800°C (Figs 4(a) and 5(b), and Table 1). The average overall lamellar spacing of the Ti-47 AI+ B alloy nearly doubles during aging at 1000°C, but the (XZ-(X2 spacing almost triples. This can be seen more clearly in Fig. 5, showing SEM of the electropolished
surfaces of TEM disks in the as-heat-treated and aged conditions. These secondary electron images easily show the (X2 lamellae due to topographical (the (X2 lamellae polish a little slower than the adjacent 'Y) and Z (the (X2 is bright because it is the higher Z (atomic number) phase) contrast effects. Wider 'Y lamellae in this alloy have smaller, thinner 'Y fragments within them with heavy dislocation networks emanating from the ends. This suggests that once the (X2 lamellae dissolve, the 'Y/'Y boundaries are less stable and lamellar coarsening proceeds more rapidly. Consistently, the Ti-47AI+W+B alloy shows the best resistance to lamellar coarsening of these three alloys, and the lamellar structure parameters of this W + Bmodified alloy are almost unchanged during aging at 1000°C compared to the initial structure (Figs 3(a) and 3(b), and Table 1). In particular, the (X2-(X2 spacing increases only slightly and has only a few (X2 fragments, as shown in Fig. 5 via SEM, after aging relative to the initial as-heat-treated material. This demonstrates that W stabilizes the (X2 lamellae component of the microstructure against dissolution during aging, and that such (X2 lamellae dissolution is a critical first step in the processes that lead to continuous lamellar coarsening of these fine structures during aging. Finally, Fig. 4(c) shows the almost 10fold coarsening (with Ti-47Al+B as the example) of the lamellar structure that occurs during aging at 1200°C. Previously, optical microscopy of these same alloys showed similar behavior of both Ti-47AI+B and Ti-47Al+W+B at this temperature. 15 Hardness - aging at 800-1200°C
Hardness measurements were also made on metallographic specimens prepared from these same aged materials (Table '2). There is a slight increase in hardness of all three alloys after aging at 800 and 1000°C, but the relative strengths of the different alloys after aging remain the same as they were initially (Ti-47AI+W+B being the hardest). All of the alloys soften after aging at 1200°C, which completely coarsens the initially fine lamellar structures. After aging at 1200°C, the binary alloy is significantly softer than both the Ti-47AI+B and Ti-47AI+W+B alloys, which have similar hardnesses. The relative hardness values of the as-heat-treated alloys, and the changes in hardness observed in the different alloys after aging correlate consistently with the changes observed in their lamellar microstructures.
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Fig. 6. Optical microstructures of Ti--47AI alloy, showing the subsurface microstructure after (a) heat-treatment for I hat 1400°C, (b) aging for 168 h at 800°C, (c) aging for 168 h at 1000°C, and (d) aging for 168 h at l200°e.
Surface microstructure 800-1000 D C
unaged, and aged at
Studies of subsurface damage that developed in these alloys after aging were made previously,15 and that damage was suspected to be due to plastic deformation beneath the surfaces that were cut to prepare these specimens. Metallographic observations will be summarized and used as a comparison baseline for a more careful evaluation of cold-work effects on the microstructure during aging of these three alloys in the next section. The as-heat-treated specimens were sectioned prior to aging, with little or no detectable damage below most of the surfaces (Fig. 6(a)). After aging, these same surfaces showed a 'damage region' that penetrated into the bulk microstructure below the surface, with the depth of penetration varying with both aging temperature and alloy composition. The 'damage region' consisted of mainly small equiaxed 'Y grains at 800 D e, an a (Ti3Al) surface layer plus larger 'Y grains at lOOODe, and a thicker a layer with varying degrees of porosity at the external surface at 1200 D e (Figs 6(b)-(d) and Table 3). The a surface layer formed during aging on the Ti-47AI binary alloy is shown using backscattered electron (BSE) imaging in an electron-
Table 3. Sub-surface damage layer depth of aged TiAI alloysa Alloy
Condition
Ti--47AI Ti--47Al+B Ti--47AI+W+B Ti--47AI Ti--47AI+B Ti--47AI+W+B Ti--47AI Ti--47AI+B Ti--47AI+W+B
168 h at 168 h at 168 h at 168 hat 168 hat 168 h at 168 h at 168 h at 168 h at
800°C 800°C 800°C 1000°C 1000°C 1000°C l200°C l200°C l200°C
Layer thickness (ILm) 20-25 20-25 20-25 70-80 20-50 20-25 70-75 65-70 85-100
aAll heat-treated for 1 h at 1400°C in vacuum and furnace cooled (fc), then aged in vacuum.
microprobe in Fig. 7, and quantitative XEDS analysis confirms that this outer layer is Ti-25at.%AI, while the material deeper beneath the surface is Ti-47at.% AI. All of the alloys have a similar sub-surface damage layer that is about 25 JLm deep after aging at 800 D e. All the alloys have similar a surface layers that are 20-25 JLm deep after aging at lOOODe, but the depth of the equiaxed 'Y region beneath the a layer varies significantly with alloy composition. The binary alloy has the largest equiaxed 'Y region beneath the a layer (25 JLm), while the Ti-47AI+W+B alloy has none (Table 3 and Fig. 8). After aging at
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Fig. 9. Optical microstructures of 5·5% cold-pressed material aged for 168 h at 1000°C of (a) Ti-47AI alloy, and (b) Ti-47AI+W+B alloy. Note the lack of recrystallized grains in the Ti-47AI+W+B alloy. Fig. 7. Back-scattered image of the subsurface microstructure of Ti-4 7AI aged for 168 h at 1200°C taken in a scanning electron microprobe. The a (Ti3AI) phase appears bright in the picture and was identified using quantitative XEDS analysis (courtesy of D. L. Joslin, ORNL).
Fig. 8. Optical subsurface microstructures after aging for 168 h at 1000°C of (a) Ti-47AI+B and (b) Ti-47AI+W+B. Note only the a layer in the Ti-47AI+W+B alloy.
1200DC, all of the alloys have a thick (75-100 JLm) layer with a very coarse lamellar microstructure beneath it. The thicknesses of the (X layers are about the same on all three alloys aged at 1200°C, as one would expect. (X
Bulk microstructure - 5·5% cold-worked, and aged at lOOO°C The hypothesis was suggested previously that the reduced size of the sub-surface damage layer in the Ti-47Al+W+B alloy aged at 1000°C was due to the alloying elements Wand B making the TiAI alloys more resistant to recrystallization caused by
subsurface deformation and/or residual stresses due to cutting. To test this hypothesis, specimens of all three alloys were cold-forged about 5·5% and then aged for 48 and 168 h at 1000°C. Although all of the alloys had cracks, metallographic examination of the as-deformed specimens indicated that all had undergone significant plastic strain. All three alloys showed some development of fine equiaxed 'Y grains along intercolony boundaries after aging for 48 h. After 168 h, there were very significant differences between the three alloys, with the binary alloy showing the most recrystallization and equiaxed grain growth, and the Ti-47Al+W+B alloy showing little to none (Fig. 9). TEM examination of the Ti-47Al+B alloy showed very coarse, broken (X2 lamellae (Fig. 10) and considerably more continuous and discontinuous lamellar coarsening than was found anywhere in the same alloy aged without any prior deformation (cp. Figs 4(b) and 10).
Surface microstructure - 5·5% cold-worked, and aged at lOOO°C The carefully polished surfaces of these 5·5% coldpressed specimens exposed to aging in vacuum for 48 and 168 h at 1000°C were also examined for comparison with the previous subsurface 'damage regions' developed on cut and heat-treated specimens. The binary Ti-47 Al alloy developed an (X surface layer that was about 3-6 JLm thick right at the surface, with another layer of 'Yand (X2 grains that was about 10-12 JLm thick, for a total average damage region of 15 JLm after aging for 168 h (Fig. II(a)). The Ti-47Al+W+B alloy shows a 2-4 JLm thick (X layer after 48 h that grows to about 6 JLm after 168 h, but does not develop the discon-
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Fig. 10. TEM of the lamellar structure of the Ti-47AI+B aHoy, 5·5'Yo cold-pressed and aged for 168 h at IOOO°C.
tinuously coarsened Y+CI'2 region found in the binary alloy (Figs 11(b) and (c)). The surface behavior of the Ti-47AI+B alloy is close to that found in the binary, with a slightly thinner total damage layer thickness. Finally, it can be seen that there is some discontinuous coarsening adjacent to the crack seen in the Ti-47AI+W+B alloy in Fig. 11 (c). Most likely this reflects the effect of enhanced lamellar coarsening in what was the plastic zone initially associated with the crack. Degradation of the lamellar structure in that region is more severe than at the free surface because the careful polishing at that surface removed all such damaged material prior to aging. DISCUSSION
The previous paper on part of this work by Ramanujan et al. 15 categorized the various modes of continuous and discontinuous coarsening by which the lamellar microstructures in these alloys change during aging, and emphasized the changes in modes of lamellar instability as the aging temperature increased from 800 to 1200°e. These TiAI alloys exhibited primarily continuous coarsening of the initial fine lamellar structures at 800 and 1000°C, and discontinuous coarsening to form new and very coarse lamellar structures at 1200°e. The binary and Ti-47AI+B alloys also formed very large non-lamellar y grains which contain spherodized Cl'2 particles, whereas the Ti-47AI+W+B alloy did not. Ramanujan et al. noted that the additions of Wand B retarded the coarsening of the bulk lamellar structures in this Ti-47 Al series of alloys, and minimized the development of subsurface 'damage regions' beneath the cut surfaces
Fig. II. Optical subsurface microstructures of 5·5% coldpressed specimens aged at IOOO°C of (a) Ti-47AI aged for 168 h, (b) Ti-47AI+W+B aged for 48 h, and (c) Ti-47AI+W+B aged for 168 h.
of these specimens during subsequent aging, but those solute effects were not the main focus of that paper. In this paper, the discussion will focus in more detail on the effects that Band W have on the formation of the initial fully-lamellar structures in these Ti-47 Al alloys, and on the detailed behavior of the y and Cl'2 phase components of the overall lamellar structures during aging. It will also emphasize the role of cold-work and the effects of Band W additions on the subsurface damage that develops during aging.
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Relative to other studies of TiAI alloys with 40-50 at. (Yo AI, the initial lamellar structures formed in these Ti--47 Al alloys are relatively fine,18-24 although not as fine as the structures observed recently in P/M processed Ti--47 AI-2Cr-2Nb alloys.1O Following the classification of TiAI microstructures by Kim,20 the Ti--4 7Al alloys in this work that were heat-treated at 1400°C have a type I fully-lamellar (FL) structure, in which the y lamellae form in the c¥ phase grains as the material is cooled. Other work indicates that increased cooling rates can refine the lamellar structure of Ti--47AI and Ti--48AI alloys up to a point, but when the cooling rate becomes too high, massive transformation of the c¥ into y occurs.20.2I.24 In binary alloys, slow furnace cooling tends to produce lamellar structures with lamellar spacing of the order of 1 JLm or more. 21 ,24 Polysynthetically twinned TiAI has a lamellar structure with spacings of 1-2 JLm or slightly less, but that structure is dominated by y/y boundaries and the C¥2 lamellar are quite thick and spaced 5-10 /Lm apart. 6,19 Kim shows a lamellar spacing of 0·03-0·2 /Lm in a fully-lamellar Ti--47AI-I Cr-1V-2·5Mo alloy.20 However, to our knowledge, there are no systematic data on the effects of alloying on lamellar refinement in the same base TiAI alloy for the same heat-treatment/cooling conditions. 15 The effects that both Band B+ W additions have on refining the lamellar microstructure for the same heat-treatment/cooling rate conditions indicate that these elements are affecting how the y lamellae nucleate and grow to form this structure. Boron appears to increase the nucleation rate of y lamellae relative to the binary alloy, but it does not seem to have much effect on the way those individual y lamellae grow. The fact that there are so many fragmented C¥2 lamellae suggests that the y lamellae grow together rapidly during cooling to pinch-off the c¥ phase as the lamellar structure forms. Consistently then, adding W together with the B appears to retard the growth rate of the y lamellae after they nucleate, resulting in the more continuous and uniformly dispersed C¥2 lamellae that we observe in this work. Recent work on TiAI alloys containing W does not include detailed observations of C¥2 phase behavior within the general lamellar structure. 7,9,25 The differences in hardness of the as heattreated alloys are somewhat surprising in light of the differences observed in the initial lamellar microstructures of these alloys. The B modified alloy has about the same hardness or is slightly
softer than the binary Ti--47 Al alloy, despite its refined lamellar structure. Even though the C¥2 lamellae are fragmented, it is not clear why a finer dispersion of y/y interfaces would not increase the hardness, but softening effects associated with B additions to TiAI alloys have also been observed by others. 26 Compared to the amounts of B added by others (up to 1 at.%), the amount of B added in this study (0·014 at.%) is quite low. 3 By contrast, the Ti--47AI+W+B alloy is stronger than the binary alloy (331 dph compared to 307 dph), consistent with the refined lamellar structure that includes more continuous and uniformly dispersed C¥2 lamellae, and the solid-solution strengthening effects of W. 16 Such details of the lamellar structure are important to note, because recent work has indicated that both ultrafine lamellar structure and defect-free C¥2 lamellae within that structure contribute significantly to the high-temperature strength and creep-resistance of the FL structures in more complex TiAI alloys.IO,1l,16 The improved aging/coarsening resistance of the B- and W + B-modified alloys is particularly important because without such alloying effects enhancing stability, finer lamellar structures would be expected to be less stable rather than more stable during aging. One of the driving forces for discontinuous coarsening in the Livingston-Cahn treatment of general coarsening of lamellar structures (their examples are eutectics) for discontinuous coarsening, in which much coarser lamellar structures replaced finer structures, is a reduction of surface area to reduce the free energy of the system.27 Boron is well known to segregate to boundaries and interfaces in other intermetallic systems,28,29 but such segregation was not measured in this study. A comparison of the aging behavior of the Ti--4 7Al + B alloy to the binary definitely shows retarded coarsening kinetics at 1000°C, although there is considerable continuous coarsening due to dissolution of the C¥2 lamellae and instability of the y/y boundary structure. Relative to the other two alloys, the Ti--47AI+W+B alloy shows very little continuous coarsening of the initial fine lamellar structure during aging at 800 and 1000°e. The more continuous C¥2 lamellae of the initial as-heat-treated microstructure of the W+B modified alloy would be a positive factor in aging resistance of the microstructure, because broken or fragmented lamellae are unstable. Bartholomeusz and Wert 29 ,30 studied much coarser lamellar structures in Ti--44AI alloys, but they do show that terminal C¥2 lamellae dissolve as adjacent lamellae thicken due to the highly curved surface (short
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radius of curvature) at the terminal end affecting the solubility of Ti and Al in the adjacent TiAI matrix. Therefore, structures without such lamellar defects would be more stable because parallel lamellae without terminal defects have an infinite radius of curvature. However, this one factor alone is not sufficient to explain the beneficial effect of W on aging resistance, because the Ti--47 Al binary also has relatively defect-free (l'2 lamellae and yet it coarsens tremendously during aging. The stabilizing effects of W most likely also come from direct solid-solution effects that W has when it is present in both the (l'2 and I' lamellar phases. The microstructural data presented here show that the (l'2 lamellae in the Ti--47Al+W+B alloy are more resistant to dissolution, and show that the general lamellar structure is more resistant to the various modes of continuous 1'/1' lamellar coarsening. IS ,31 Tungsten is present in both the (l'2 and I' phase lamellae in the initial structure, with slightly more W in the (l'2 phase (0·9 at. % compared to 0·6% in the 1'), as shown previously by measurements of the compositions of individual lamellae using high-spacial resolution XEDS in this alloy. IS Similar measurements of Wand Al in individual lamellae of the Ti--4 7Al + W + B alloy aged for 168 h at 1000°C indicate that W is repartitioning to achieve equal concentrations (0· 7 at.%) in both phases while the Al difference between the (l'2 and I' phase lamellae is increasing (aging lowers the Al content of the (l'2 phase from 39 to 35 at.%, but has almost no effect on the I' phase composition). These compositional effects during aging are consistent with the (l'2 phase compositional differences above and below the eutectoid temperature, and would suggest that the microstructure forms with a metastable, nonequilibrium composition above the eutectoid temperature forced by the continuous cooling. During aging at lower temperatures, the microstructure appears to be approaching the equilibrium composition of the (l'2 and I' phases as the lamellar structure coarsens. If W diffusion is necessary before the (l'2 lamellae can dissolve and coarsen, and W is a slower diffusing species than Ti or AI, then that would help to explain how W retards coarsening of the lamellar structure. Tungsten also appears to generally retard the various modes of lamellar coarsening and microstructural instability that both result from the migration/propagation of I' boundaries and interfaces during aging at 800-1000°C. Such an effect could be related to W affecting the kinetics
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of Ti and Al diffusion in the I' phase, but more work would be necessary to verify such a mechanism. However, the suggestion that 1'/1' interfacial movement is more sluggish in the W-modified alloy during aging than it is in the alloys without W fits together with the argument made earlier that W retards the growth of I' lamellae during heat-treatment and cooling at higher temperatures to form the initial lamellar structure. The same structure-stabilizing effect of W is further supported by the lack of discontinuous coarsening or equiaxed I' grain formation beneath the surface of the Ti--47Al+W+B alloy specimens relative to the other alloys, and the recrystallization resistance found in the cold-worked and aged W + B modified alloy. There is little effect of W evident after the initial lamellar microstructure is completely removed during aging at 1200°C, because the microstructures of the Band W + B modified alloys look similar and have the same hardness values. This would suggest that W is most effective in TiAI processed/heat-treated to produce a fullylamellar structure, as suggested by creep studies of others in W -modified TiAI alloys.7,9 It is technologically significant that Band W+B additions can refine the lamellar microstructure even at relatively slow furnace cooling rates. In TiAI components with greater section thicknesses, cooling rate alone cannot be used to control and refine the structure. Since the mechanical properties of TiAI alloys are so sensitive to microstructural changes, alloying elements that minimize gradients in microstructure should also minimize properties gradients too. The addition of W+B together appear to be very beneficial for producing a refined FL structure that is less sensitive to cooling rate effects. Another technological benefit of the W + B addition is its minimization or elimination of subsurface damage layer evolution. This may be very important for resistance to mechanical failures in which cracks initiate at the surface, such as tensile straining, creep, or creep-fatigue. Analysis of the various damage layers formed on the aged specimens suggests that the (l'-case formation is more of a generic factor in these alloys that is affected by a loss of Al from the surface to the vacuum at a particular aging temperature. Indeed, in the 5·5% cold-pressed specimens, the careful surface preparation after heat-treatment at 1400°C and after the cold-deformation greatly reduced the (l'case formation in all of the alloys aged for 168 h at 1000°C. This demonstrates that the initial surface of the first group of aged specimens (cut and
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vacuum-heat-treated) lost some AI in the vacuum, even though it was not enough to form a visible a-case. The growth of the a-case during subsequent vacuum-aging then begins sooner and proceeds more rapidly at 1000°C compared to the cold-pressed specimens with all such surface affects from previous treatments removed. Recent work has shown that an a-phase surface layer significantly decreases room temperature ductility of a FL Ti-47AI-2Cr-2Nb alloy.32 The 'damage layer' of coarser equiaxed y grains with a2 particles is definitely affected by any deformation that occurs below the surface during cutting and by the alloy composition. The addition of W+B makes the Ti-47 AI alloys very resistant to such damage evolution with or without subsurface deformation. The binary alloy shows some discontinuous coarsening beneath the surface of the 5·5% cold-pressed specimen, despite all efforts to remove subsurface damage beforehand, but less than the as-heattreated material aged at 1000°C with its initial subsurface deformation intact. Ideally, it would appear to be important to remove any adverse surface effects of vacuum heat-treating or machining by polishing, and to add W + B to make the TiAI alloy more resistant to in-service damage layer evolution associated with such machined and/or heat-treated surfaces. Tungsten and boron additions together have a significant benefit of producing finer and more uniform lamellar structures initially in Ti-47 AI alloys heat-treated at 1400°C. These combined additions furthermore make the fine lamellar structure robust and resistant to coarsening during aging at 800 and 1000°C for 168 h.
CONCLUSIONS (1) The addition of B (140 appm) to Ti-47AI
refines the fully-lamellar structure produced by heat-treatment for I h at I 400°C, but with many a2 lamellae within that structure, A combination of W (0·5 at.%) and B additions produces a similarly refined lamellar structure, but with more uniformly continuous and defect-free lamellae. The Ti-47 AI +W + B was harder than the other two alloys. (2) Aging of these alloys for 168 h produced little effect at 800°C, but caused significant changes at 1000°C that depend strongly on alloy composition, and complete degradation of the microstructure by discontinuous coarsening
at 1200°C. All of the alloys aged at 800 and 1000°C were slightly stronger than the initial as-heat-treated material, with the Ti-47 AI+W+B alloy always being harder than the other two. However, all the alloys softened after aging at 1200°C, with the binary being much softer than the other two alloys. (3) After aging at 1000°C the binary alloy had a significantly coarser lamellar structure (average spacing - I-211m). The Ti-47AI+B alloy showed much less coarsening of the overall lamellar structure (average spacing - 0·35-0A 11m), but the a2-a2 spacing increased from O· 3-0· 5 11m to IA 11m due to dissolution and coarsening of this phase. The Ti-47AI+W+B alloy showed almost no change in the average lamellar spacing or the a2-a2 spacing after aging at 1000°C. (4) Aging of initially heat-treated (in vacuum) and cut specimens produced subsurface damage in all alloys and aging (in vacuum) conditions. All alloys showed a similar a-case that was about 20-25 11m thick at 800 and 1000°C, and 65-100 JLm thick at 1200°C. The binary Ti-47AI alloy had a coarse y grain layer below the a-case that was 50-60 JLm thick. That 'damage layer' was 25 JLm or less in the Ti-47AI+B alloy, and was not found at all III the Ti-47AI+W+B alloy. (5) Cold-pressing (5·5%) increased the lamellar degradation in all of the alloys aged at 1000°C for 168 h. The binary alloy showed extensive recrystallization into fine equiaxed "y grains, while the B-modified alloy showed less and W + B-modified alloy showed almost none. (6) The surfaces of the 5·5% cold-pressed and aged specimens were carefully polished to remove all damage from prior heat-treatment, deformation and specimen cutting. Consistently, these all showed less subsurface damage than the previous specimens, with W + B modified alloy showing the least damage (only a 6 JLm thick a-case).
ACKNOWLEDGMENTS This work was supported by the US Department of Energy, through the Materials Science Division, Office of Basic Energy Sciences (BES), and the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Industrial Technolo-
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gies, Advanced Industrial Materials (AIM) Program under contract DE-AC05-960R22464 with Lockheed Martin Energy Research Corporation. Thanks to Mr C. A. Carmichael for melting the alloys, and to Mr J. W. Jones for TEM sample preparation. Dr R. Ramanujan wishes to thank Dr P. Mukhopadhyay (Head, Physical Metallurgy Section), Dr S. Banerjee (Head, Metallurgy Division) and Dr C. K. Gupta (Director, Materials Group) at the Bhabha Atomic Research Centre for their keen interest in this work.
12. 13. 14. 15. 16. 17. 18. 19.
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