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The lamellar γ + β structure in Al-30T1-20V alloy
ScriptaIvictdlurgicaetMstaislia, Vol. 33.No. 1,pp. U-17,1595 cqyli@t 0 1995 Elsevicr scim Ltd PMtdltlthCUSAAllligtltSd 0956-716x/95 $9.50 + .oo
Pergamon
0956-716x(95)00157-3
THE LAMELLAR y+P STRUCTURE IN Al-3OTi-20V ALLOY G. Shao, P. Tsakiropoulos and A. P. Miodownik Department of Materials Science and Engineering University of Surrey Guildford Surrey, GU2 SXH, U. K. (Received December 12,1994) Iutroduction
Titanium ahuninide y(TiAl) based alloys have attracted great research interest recently due to their lower density and potential to operate at temperatures where nickel based superalloys are currently used. Most research on y based alloys has been on duplex a,+y alloys (l), where the coexistence of two intermetallic phases has not resulted in impressive improvements in alloy properties. The introduction of a disordered phase such as p, which is stable up to the melting temperature, could enhance the low temperature ductility of y based alloys. Previous studies have shown that the addition of V to Ti-Al alloys would tend to order the p phase to the B2 structure at lower temperatures (2,3). Since the B2 phase is even harder than y (2), the y+B2 structure is not expected to have improved ductility. Experimental phase equilibria study (2) and thermodynamic assessment (3) have shown that the disordered g phase can be retained to room temperature in alloys containing large amount of Al. This work has selected the Al-30Ti-20V ahoy to feature the as-cast microstructure of ingots of such alloys. Fxuerimental
Al-30at?hTi-20at%V ahoy ingots were prepared by IMI Titanium. The ahoy was arc melted and cast in a water cooled copper mould to give ingots of (40 x 70 x 25) mm3 in size. The chemical analysis of the ingots, also performed by IMI Titanium, gave contamination levels lower than: 25wppm Hydrogen 30wppm Nitrogen and 5OOwppmOxygen. TEM studieswere carried out on a JEOL 2000-t% and a PHILLIPS EM4OOT (equippedwithaLXNKANlOoEDXsystem)mi~. EDX analysis was performed with experimenti standard EDX spectrum profiles and Clint-Lorimer factors were determined experimentally. Results and Discussion
The free surface morphology of the Al-30Ti-20V ahoy ingot is shown in Figure la, which exhibits nonfacetted dendrites in a rectangular array. This morphology is characteristic of Q phase dendrites due to its cubic crystal structure. By comparison, the y dendrites have a facetted morphology (Figure 1b). A typical TEM image of the alloy ingot is shown in Figure 2a taken at [ 11lip, which contains thin layers of the prior S phase between thick y bands formed during post-solidification cooling. Since the orientation
13
14
THE LAMELLARy + p STRUCTURE
Figure 1. SEM images of the top ingot surfaces of Al-3OTi-20V (a), and Ti45Al morphologies of p (a) and y (b).
Vol.33, No. 1
(b) alloys, showing the characteristic
dendrite
relationship (OR) between p and y is about 7” off the [ 11l]P//[lO1], OR existing between B2 and y (3,4), the [ill], pattern (Figures 2b, 2c) does not contain a typical [lOI], pattern. However, the (llO),//(lll), plane pair OR is strictly obeyed. Tilting around ( 1lo), to [ 10 11, showed that the y bands are twin-related and the interface between y and p is (11 l),//( 1lo), (Figure 3). Order domain boundaries exist in the y lamellae due to the impingements of separately nucleated growing y lamellae. Figure 3a shows order domain boundaries such as the one between domains ’ 1” and “2” in Figure 3a, and two growing domains, “3” and “4” before impingement. The c-axis of adjacent order domains is 90” to one another around [lOOI,or [OlO],.The ~11 l>, pattern from the order domain boundary between “1” and “2” is shown in Figure 3d. The [ 11l]r pattern horn ’ 1’ is shown in Figure 3e. The [OOl] and [ 1lo] SAD patterns taken from the g phase exhibit diffuse maxima splitting around the { IOO}, superlattice positions for the ordered p(B2) phase (Figure 4a). The characteristics of this dilkse maxima splitting are dill&em from those of the diffuse o phase (5). Figure 4c shows the diflkse o streaking with dotted lines (5), together with the diffuse maxima splitting present in Figure 3a, the [OOl], pattern. It is clear that the diffuse streaking arising from the octahedral diffuse shells of the 0 phase (dotted lines) is 45 o to the streaking connecting every four difhtse maxima splitting around { 1001P in Figure 4a. Therefore, the above mentioned dihbse maxima splitting is not from the o phase. Comparing Figures 4b and 4d shows that very weak dilfuse o streaking is also present in the [ 1lo] a pattern in addition to the above mentioned diffuse maxima splitting around {loo},. DiI&se maxima splitting around superlattice positions has been observed in CuJ’d,, alloys containing strong short range ordering (SRO) (6). GyorQ and Stocks related the SRO diflkse maxima splitting to Fermisurf&e nesting (7). A complete quantitative description of SRO diffuse scattering is not available at present (8). Experimental evidence for SRO includes: _
2 diffuse maxima splitting around superlattice positions (6); ii) diffuse scattering corresponding to corrugated sheets in reciprocal space (9) which was interpreted by Sauvage and E. Parthe in terms of short range ordering of vacancies (10); and iii) superlattice spots broadening (11).
Vol.
15
33, No. 1
Figure 2. TEM image of y+p structure of Al-30Ti-20V ingot (a), corresponding SADP contains fi and y @) and MDP (c) fhtn p at
11111,.
Due to similarity with (7), it is suggested that the ditlkse maxima splitting observed in the Al-3OTi-20V alloy were caused by short range ordering in the B2 phase. It has been found by previous studies (2,3) that B2 is quite stable near the (a,+B2+y) region, while when the alloy compositions approach either the Ti-V edge or the Al-V edge, the tendency of S-B2 ordering is reduced The observation of SRO ditI%sescattering in this study supports the suggestion that P-B2 ordering in the Ti-Al-V system is a second order phase tran&nmation (3). EDX thin foil analysis showed that the p phase composition is AL,35T&,V,,4,and the y phase composition is Ab5,,T&,V0.i6.The g composition is close to the calculated p/B2 boundary at 1173K (3s). Conclusions
1. The as-cast Al-3OTi-20V alloy ingot has a lamellar y +S microstructure. The interface between the y and p phases is { 111>,I/{ 1lo},, which is different from the interface between B2 and y (3,4). Order domain boundaries form in y lamellae by the impingements of separately nucleated and growing lamellar y domains. 2. The p phase in the Al3OTi-20V alloy is short range ordered. The observation of SRO in the p phase comkms that the p-B2 ordering in T&U-V system is a second order transformation. The appearance of
16
THE LAMELLAR y + p STRUCTURE
Vol. 33, No. 1
Figure 3. (a) TEM image obtaioed by tihiog the specimen Corn [ll llp (tbe position in Figure 2) to [loll,, showing lame&r y+p structure. The arrow in (a) indicates [l lO]d/[l Ill,. @) IS . tb e corresponding SADP and (c) is a MDP from “1” in (a). The in&Ace. between p aody is(llO)p//(lll),.(d)isthe~lll~, MDPfiomtheorderdomainboundarybetween”l”and”2”in(a)and(e)isthe correspondent[lll],~om”1”.
a ductile p phase in the lame&r y+P structure is expected to enhance the low temperature ductility of the y based alloys. Ackuowledaement The authors would like to thank Professor J E Castle for the provision of research facilities Procurement Executive of the Mini&y of Defeme, UK for financial support.
and the
17
vol. 33, No. 1
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Cd) [l lo], are schematic diagram for (a) and (b), with dotted Figure4.SADPslicmthe~phsseinFig.2at[OOl]p(a)and[llO]p@).(c)and(d) lines showing the feature of diffuse o streaking and smaller points showing SRO d&se maxima splitting (5).
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Y. -W. Kim, J. Metals, 46(7), p.30 (1994). T. Abmedand H. M. Flower, Mater. Sci. Tecbn., 10, p.273 (1994). G. Shao, I? Tsakiropoulos and A P. Miodownik, 1994, INTERMETALLJCS, in press. B. J. Inkson, C. B. Boothboydand J. Humphreyx, Acta Metall. Matter., 41(10), p.2867(1993). G. Shao, A P. Miodownik and P. Tsakiropoulos, ‘Omega Phase Formation in Al-V and Ti-Al-V Alloys”, Submitted to Phil. Msg. A K. Ohshima and D. Watanabe, Acta Cryst., A29, p.520 (1973). B. L. Gvor@ and G. M. Stodcs, Phys. Rev. L&.50(5), p.374 (1983). J. W. E&y& “Monographs in Pm&al Electron M&&xpy in Materials Science”, Vol. 2, McMillao Press, New York (1975). J. Billiqbam, l? S. Bell and M. H. Lewis, Acta Cry& A28, p.602 (1972). M. Sauvage and E. Par&& Acta Cry&, A28, p.607 (1972). P. R Okamato and G. Thomas, Acta Met, 19, p.825 (1971).
Pergamon
0956-716x(95)00157-3
THE LAMELLAR y+P STRUCTURE IN Al-3OTi-20V ALLOY G. Shao, P. Tsakiropoulos and A. P. Miodownik Department of Materials Science and Engineering University of Surrey Guildford Surrey, GU2 SXH, U. K. (Received December 12,1994) Iutroduction
Titanium ahuninide y(TiAl) based alloys have attracted great research interest recently due to their lower density and potential to operate at temperatures where nickel based superalloys are currently used. Most research on y based alloys has been on duplex a,+y alloys (l), where the coexistence of two intermetallic phases has not resulted in impressive improvements in alloy properties. The introduction of a disordered phase such as p, which is stable up to the melting temperature, could enhance the low temperature ductility of y based alloys. Previous studies have shown that the addition of V to Ti-Al alloys would tend to order the p phase to the B2 structure at lower temperatures (2,3). Since the B2 phase is even harder than y (2), the y+B2 structure is not expected to have improved ductility. Experimental phase equilibria study (2) and thermodynamic assessment (3) have shown that the disordered g phase can be retained to room temperature in alloys containing large amount of Al. This work has selected the Al-30Ti-20V ahoy to feature the as-cast microstructure of ingots of such alloys. Fxuerimental
Al-30at?hTi-20at%V ahoy ingots were prepared by IMI Titanium. The ahoy was arc melted and cast in a water cooled copper mould to give ingots of (40 x 70 x 25) mm3 in size. The chemical analysis of the ingots, also performed by IMI Titanium, gave contamination levels lower than: 25wppm Hydrogen 30wppm Nitrogen and 5OOwppmOxygen. TEM studieswere carried out on a JEOL 2000-t% and a PHILLIPS EM4OOT (equippedwithaLXNKANlOoEDXsystem)mi~. EDX analysis was performed with experimenti standard EDX spectrum profiles and Clint-Lorimer factors were determined experimentally. Results and Discussion
The free surface morphology of the Al-30Ti-20V ahoy ingot is shown in Figure la, which exhibits nonfacetted dendrites in a rectangular array. This morphology is characteristic of Q phase dendrites due to its cubic crystal structure. By comparison, the y dendrites have a facetted morphology (Figure 1b). A typical TEM image of the alloy ingot is shown in Figure 2a taken at [ 11lip, which contains thin layers of the prior S phase between thick y bands formed during post-solidification cooling. Since the orientation
13
14
THE LAMELLARy + p STRUCTURE
Figure 1. SEM images of the top ingot surfaces of Al-3OTi-20V (a), and Ti45Al morphologies of p (a) and y (b).
Vol.33, No. 1
(b) alloys, showing the characteristic
dendrite
relationship (OR) between p and y is about 7” off the [ 11l]P//[lO1], OR existing between B2 and y (3,4), the [ill], pattern (Figures 2b, 2c) does not contain a typical [lOI], pattern. However, the (llO),//(lll), plane pair OR is strictly obeyed. Tilting around ( 1lo), to [ 10 11, showed that the y bands are twin-related and the interface between y and p is (11 l),//( 1lo), (Figure 3). Order domain boundaries exist in the y lamellae due to the impingements of separately nucleated growing y lamellae. Figure 3a shows order domain boundaries such as the one between domains ’ 1” and “2” in Figure 3a, and two growing domains, “3” and “4” before impingement. The c-axis of adjacent order domains is 90” to one another around [lOOI,or [OlO],.The ~11 l>, pattern from the order domain boundary between “1” and “2” is shown in Figure 3d. The [ 11l]r pattern horn ’ 1’ is shown in Figure 3e. The [OOl] and [ 1lo] SAD patterns taken from the g phase exhibit diffuse maxima splitting around the { IOO}, superlattice positions for the ordered p(B2) phase (Figure 4a). The characteristics of this dilkse maxima splitting are dill&em from those of the diffuse o phase (5). Figure 4c shows the diflkse o streaking with dotted lines (5), together with the diffuse maxima splitting present in Figure 3a, the [OOl], pattern. It is clear that the diffuse streaking arising from the octahedral diffuse shells of the 0 phase (dotted lines) is 45 o to the streaking connecting every four difhtse maxima splitting around { 1001P in Figure 4a. Therefore, the above mentioned dihbse maxima splitting is not from the o phase. Comparing Figures 4b and 4d shows that very weak dilfuse o streaking is also present in the [ 1lo] a pattern in addition to the above mentioned diffuse maxima splitting around {loo},. DiI&se maxima splitting around superlattice positions has been observed in CuJ’d,, alloys containing strong short range ordering (SRO) (6). GyorQ and Stocks related the SRO diflkse maxima splitting to Fermisurf&e nesting (7). A complete quantitative description of SRO diffuse scattering is not available at present (8). Experimental evidence for SRO includes: _
2 diffuse maxima splitting around superlattice positions (6); ii) diffuse scattering corresponding to corrugated sheets in reciprocal space (9) which was interpreted by Sauvage and E. Parthe in terms of short range ordering of vacancies (10); and iii) superlattice spots broadening (11).
Vol.
15
33, No. 1
Figure 2. TEM image of y+p structure of Al-30Ti-20V ingot (a), corresponding SADP contains fi and y @) and MDP (c) fhtn p at
11111,.
Due to similarity with (7), it is suggested that the ditlkse maxima splitting observed in the Al-3OTi-20V alloy were caused by short range ordering in the B2 phase. It has been found by previous studies (2,3) that B2 is quite stable near the (a,+B2+y) region, while when the alloy compositions approach either the Ti-V edge or the Al-V edge, the tendency of S-B2 ordering is reduced The observation of SRO ditI%sescattering in this study supports the suggestion that P-B2 ordering in the Ti-Al-V system is a second order phase tran&nmation (3). EDX thin foil analysis showed that the p phase composition is AL,35T&,V,,4,and the y phase composition is Ab5,,T&,V0.i6.The g composition is close to the calculated p/B2 boundary at 1173K (3s). Conclusions
1. The as-cast Al-3OTi-20V alloy ingot has a lamellar y +S microstructure. The interface between the y and p phases is { 111>,I/{ 1lo},, which is different from the interface between B2 and y (3,4). Order domain boundaries form in y lamellae by the impingements of separately nucleated and growing lamellar y domains. 2. The p phase in the Al3OTi-20V alloy is short range ordered. The observation of SRO in the p phase comkms that the p-B2 ordering in T&U-V system is a second order transformation. The appearance of
16
THE LAMELLAR y + p STRUCTURE
Vol. 33, No. 1
Figure 3. (a) TEM image obtaioed by tihiog the specimen Corn [ll llp (tbe position in Figure 2) to [loll,, showing lame&r y+p structure. The arrow in (a) indicates [l lO]d/[l Ill,. @) IS . tb e corresponding SADP and (c) is a MDP from “1” in (a). The in&Ace. between p aody is(llO)p//(lll),.(d)isthe~lll~, MDPfiomtheorderdomainboundarybetween”l”and”2”in(a)and(e)isthe correspondent[lll],~om”1”.
a ductile p phase in the lame&r y+P structure is expected to enhance the low temperature ductility of the y based alloys. Ackuowledaement The authors would like to thank Professor J E Castle for the provision of research facilities Procurement Executive of the Mini&y of Defeme, UK for financial support.
and the
17
vol. 33, No. 1
020
0 gj:
,p.
1::. . <$j)
.
I..._,’ ,,’
l/ oto
220
<$Zj:_;. ITO(fj.;, .. .’ ....,,’
Y_,,,’ .
.
20’0
(4 [O0 l]p
002 .
112
0’00
ii0
l
..-. :.. , ‘*.... :,’ , , .. ..,. ,’ .. I ,,I’ ‘..,: .,’
..-. ,:’ , ‘...) .:I .. , ( ,,;, ....’ ,i .._.’
.
.
222 0
2:o ,.-. ,:’ I ~..., .,.’ .. , , : .... I /’ ., .:. :.’
.
Cd) [l lo], are schematic diagram for (a) and (b), with dotted Figure4.SADPslicmthe~phsseinFig.2at[OOl]p(a)and[llO]p@).(c)and(d) lines showing the feature of diffuse o streaking and smaller points showing SRO d&se maxima splitting (5).
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Y. -W. Kim, J. Metals, 46(7), p.30 (1994). T. Abmedand H. M. Flower, Mater. Sci. Tecbn., 10, p.273 (1994). G. Shao, I? Tsakiropoulos and A P. Miodownik, 1994, INTERMETALLJCS, in press. B. J. Inkson, C. B. Boothboydand J. Humphreyx, Acta Metall. Matter., 41(10), p.2867(1993). G. Shao, A P. Miodownik and P. Tsakiropoulos, ‘Omega Phase Formation in Al-V and Ti-Al-V Alloys”, Submitted to Phil. Msg. A K. Ohshima and D. Watanabe, Acta Cryst., A29, p.520 (1973). B. L. Gvor@ and G. M. Stodcs, Phys. Rev. L&.50(5), p.374 (1983). J. W. E&y& “Monographs in Pm&al Electron M&&xpy in Materials Science”, Vol. 2, McMillao Press, New York (1975). J. Billiqbam, l? S. Bell and M. H. Lewis, Acta Cry& A28, p.602 (1972). M. Sauvage and E. Par&& Acta Cry&, A28, p.607 (1972). P. R Okamato and G. Thomas, Acta Met, 19, p.825 (1971).