- Email: [email protected]
Observations on the BCC→FCO transformation in a beta titanium alloy
Scripta METALLURGICA
Vol. 9, pp. 399-406, P r i n t e d in the U n i t e d
1975 States
Pergamon
Press,
Inc.
OBSERVATIONS ON THE BCC ~ FCO TRANSFORMATION IN A BETA TITANIUM AIJ-OY
T. J. Hesdley and H. J. Rack Sandia Laboratories Albuquerque, New Mexico 87115 (Received February
7, 1975)
Introduct ion De Angelis, Huang and Sargent (DHS) have recently suggested that when the metastable beta titanium alloy, RMI (38-6-44) is aged at 900°F (482°C) for 5-48 hours~ an ordered face-centered orthorhombic (FCO) phase (8')* precipitates within the beta (BCC) matrix (I). The detection of this long period superlattice precipitate was based on an analysis of reflections observed at normally "forbidden" positions within transmission electron diffraction patterns.
Williams (2)
has questioned the existence of such a ~' phase and has suggested that it may be an artifact introduced during sample preparation. Our examination of the aging behavior of BMI (38-6-44) indicates that the ~' phase is not present in this alloy after aging between 200°C-650°C (392°F-1202°F) for 30 minutes to lO00 hours. Experimental Procedure The chemical composition of the 0.625 in. (0.015 m) plate used in this investigation was (wt. pct.): Ti.
3.4 AZ, 8.3 V, 5.8 Cr, 3.9 Zr, 4.2 Mo, 0.06 Fe, 0.011 N, 0.105 O, 0.02 C, balance
Samples were solution treated at either 1700°F (927°C) or 1500°F (815°C) snd air or water
quenched respectively. lope described above.
Aging studies were then carried out within the time/temperature enveThin foils were prepared by standard techniques and examined in a
Phillips 200 Electron Microscope at lO0 KV. Results Table 1 slmmmrizes the results of a typical x-ray diffraetometer experiment.
The observed
diffraction peaks can he accounted for by or (}{CP) and ~ (BCC) phases, without recourse to an FCO phase.
Had sufficient volume fraction of the FCO phase been present, the 002, 020~ and
021 ~' reflections would be observed. Figure 1 is a selected area, electron diffraction pattern essentially equivalent to Figure 2 of Reference 1.
DHS indexed the matrix reflections as belonging to the
zone of the (FCO) 8' phase.
[i00]
laue
However, the pattern symmetry is equally that of (O01> zones in
@Using the notation of Reference I. Not to he confused with the (BCC) 8' phase-separation reported in some beta titanium alloys. 399
400
TRANSFORMATION
IN A B E T A T I T A N I U M
ALLOY
Vol.
9, No.
4
TABLE 1 Summsry of X-ray Diffraction Results for RMI (38-6-44) (1)
,(2) dobs.(A) 2.55 2.34
a(3)
8(3)
~,(4) 2.557 (llO)
2.557 (olo) 2.342 (ooe)
2.325 (OO2) 2.e66 2.e45
(1) (2) (3) (4)
2.266 (IIO)
2.245 (oil)
2.241 (~l) 2.225 (o2o) 2.oo7 (o21)
1500°F 815°C) 1 hr. WQ + 932°F (500°C) 16 hrs. - AC. Determined using Ni filtered, CuKo radiation. Calculated using aa,= 2.957 ~ ca = 4.678 A, a~ = 3.205 A. Calculsted using a~ = 3.125 A, b E' = 4.45 A, c8' = 4.65 from Reference 1.
FIG. 1 Electron Diffraction Pattern of BMI (38-6-44), Air quenched from 1700°F (927°C) and aged 72 hours at 932°F (500°C). The foil is tilted slightly from the [OO1] 8 orientation about [~lO] 8. the (BCC) ~ phase.
From the camera constant of our electron microscope, we found that the
d-spacings of the matrix reflections correspond to {llO] and {020} planes in the (BCC) ~ phase. Therefore the matrix pattern in Figure 1 has been indexed as the [OO1]~ Laue zone.
Vol.
9, No.
4
TRANSFORMATION
IN A B E T A T I T A N I U M
ALLOY
401
Spots 2 through 9 and certain of the satellite spots about the matrix reflections in Figure l were correctly indexed by D H S a s belonging to (ll20> Laue zones of the equilibrium (HCP) 6 phase.
In this alloy, a precipitates upon aging in the well-known Burgers orientation,
(O001)a I! (110)8 and [1120] a I! [lll]8. this phase (there are twelve possible).
DHS attributed the a reflections to two variants of However, dark field imaging of spots 2-9 showed that
four 6 variants actually reflect to these positions, Figure 2.
This is not surprising since
there are four a variants in the Burgers orientation having a (1120> zone oriented 5.25 ° from [00118 , namely:
(oooz)% II (llO)s; Ell~O]% II[~u]e with [h[o]% at (oool)62 II (ZlO)e; [11~o]:2 II[lil]swith [2[[0]62at (ooOl)c~3 II (%io)e; [11~o]~3 II[11118with [[2[0]~3at (OOOl) 4 II (~o)~;[l~O]%II [#b-] 8 with [2~'0]~4 at
5.25 ° from [00118 5.25 ° from [001] 8 5.25 ° from [00118 5.25 ° from [OO1] 8
The two patterns from a1 and a 2 superimpose as do those from 03 and a 4 (shown schematically in Figure 3).
Initially this superimposition appears inconsistent with Figure 2 where only one a
variant imaged in dark field on each spot.
However, the (ll20) a reciprocal lattice planes
do not themselves superimpose due to their respective 5.25 ° misorientations relative to [OO1] B. In each case, the two superimposed spots lie above and below the [O01]~ reciprocal lattice plane.
The specimen tilting employed in forming the dark field images of Figure 2 always
rotates the Ewald sphere toward the reciprocal lattice spot of one 6 variant and away from that of the other.
Therefore, only one variant images on a given spot.
Spots lO through 13 in Figure 1 were interpreted by DHS as split OlO superlattice reflections.
However, dark field imaging revealed that these are double diffraction spots originating
from regions where a and 8 overlap.
This is a common occurrence in the diffraction patterns
of foils containing precipitates (3)-
In Figure l, if the strong LlO~ reflection acts as a
primary beam and is rediffracted upon entering the a phase, then the 6 reflections 8 and 9 translate to positions ll and lB.
Likewise, with the strong l[O~ acting as a primary beam,
the ~ reflections 4 and 5 translate to positions lO and 12. from spot ll.
Figure 4 is a dark field image
Comparing this with Figure 2a, it is clear that the diffracted intensity in
Figure 4 origSmmtes from the regions where a and 8 overlap indicating double diffraction.
In
this manner, it was found that spots ll and 12 were double diffracted beams from the cl/~ overlap regions, as were spots l0 and 13 from 62/8 overlap regions. Figure 5 is the diffraction pattern obtained when the foil was tilted about [llO] 8.
In
this case, the llO B and llO~ spots are no longer intense reflections to act as primary beams in the a phase.
Hence spots lO-13 disappear as expected.
Now the strong llO~ and l'~O~ reflec-
tions act as primaries in a creating the double diffraction spots lh through 17.
By dark field
imaging, spots l~ and 16 were found to originate from 63/8 overlap regions, and spots 15 and 17 from ~4/B overlap regions.
402
TRANSFORMATION
IN A B E T A T I T A N I U M ALLOY
Vol.
a
b
4
r
c
d
FIG. 2 Dark field images of four ~ variants which diffract to spots 2-9 in Figure 1. (a) a I imaged on spots 4 and 8, (b) c~2 imaged on spots 5 and 9, (c) a~ imaged on spots 3 and 7, (d) a h imaged on spots 2 andS6. Mag. 30,000 X.
9, No. 4
Vol.
9, No. 4
TRANSFORMATION
O T~,
IN A B E T A T I T A N I U M
IIT'[I. lT112~
,T,,,,
O
.Tm_2,
i1112,
403
me~2
iP~
11112
ALLOY
,iT,,,
II1~1I!
i:--, ~
lilil~l
I,T2,,O
ITI2,
Ill3!
0 j; Matrix reflectiens ° o~reflectiens (4 variants) FIG. 3 Schematic diffraction pattern corresponding to Figure 1 with the (i120> Laue zones of the four ~ variants indexed.
Finally spots 18 and 19 in Figure 1 were not indexed by DHS, while spots 20 and 21 (also in Figure 5) were not present in their diffraction pattern.
Upon tilting the specimen some-
what further about [[1018, spots 20 and 21 disappeared from the patters Of Figure 1. 18-21 are also related to double diffraction effects.
Spots
That Is~ they appear at the forbidden
O001-type positions in the (ll20) Laue zones of the four a variants (see Figure 3).
Reflec-
tions commonly appear in this Laue zone at these positions because of double diffraction (4). For example, Figure 6 is a dark field image from spot 20 showing that the weak diffracted intensity originates from precipitates of the a 1 variant (comlmre with Figure 2a).
In the case
of spots 18-?_l, diffracted bea~s within the a precipitates are the source of double diffraction, not ~ matrix reflections.
Hence diffraction comes from the entire v o l ~
of a and not from
just the regions where a and ~ overlap. From the above results, it is possible to index the diffraction patterns in Figures l and 5 completely as shown in Figure 7.
The remaining satellite spots about the matrix reflec-
tions belong to other ~ variants (see, for example, reference 5).
404
TRANSFORMATION
IN A B E T A T I T A N I U M
ALLOY
Vol.
9, No. 4
5
FIG. 4 Dark field image from spot 3_I in Figure I illustrating double diffraction from the a/8 overlap regions. (Compare with Figure 22.) Mag. 30,000 X.
HG. 5 Same diffraction pattern as Figure l, except specimen tilted about [ll0]8. Note absence of spots 10-1B of Figure 1 and appearance of new spots 14-17.
0"~
Tof21
oo02r-,~-O00~z
o01T,.r2
T01T, ' 01"iTz ITo~""+4~41Toioi "oiio2
IO~,
0TIT~'~
°°°2x x xn°°l 0010T~
010111 01112
200~O.T012101122_
10(~. 0T112~ D'0"n3
oooi3 :-00014
0~0
00013_: °°°T4 x
~1~ O+O~lx x X0.O+~2
%
| ~'T 1o]oi 10~ °Tl°2
10T110 0TII2
00021+"~00022110,e 0112210121072~
O ji I d l 0 c t i a s •, a |MIocIkMIs (4 variants) x iieuble H f r | e t i N Spat D J L , m i I ~ DiffrMtklo
F'.IG. 6 Dark field image from spot 20 in Figures 1 and 5 showing weak diffracted intensity from vsriant. (Compare with Figure 20. )
~
• 30,000
X.
F~G. 7 Complete indexing of Figures i and 5 combined: [O01]A orientation superimposed with four (11~05a orientations, including double dlff ra ct ion.
Vol.
9, No.
4
TRANSFORMATION
IN A B E T A
TITANIUM
ALLOY
405
S 1}mmR
Selective dark field microscopy was used to account for all reflections in Figure 1. The extra spots at forbidden positions result from double diffraction and not from a long period superlattice.
The x-ray diffraction and transmission electron microscopy results are
complementary and show that aging of RMI (38-6-44) involves the precipitation of an (HCP) a phase from a (BCC) matrix.
No evidence for an FC0 phase has been observed.
Acknowled6ements The authors are indebted to S. F. Duliere for the x-ray diffraction results, and to F. Ao Greulich for preparing TEM specimens.
This work was supported by the Atomic Energy
Commission.
References i.
R. J. De Angelis, H. Huang and Gorden A. Sargent, Scripta Met. ~, 835 (1973).
2.
J. C. Williams, Titanium Science and Technology, p. 1433, Plenum Press, New York (1973).
3.
P. B. Hirsch, A. Howie, R. B. Nicholson, D. W. Pashley and M. J. Whelan, Electron Microscopy of Thin Crystals, p. 325, Butterworth and Co., London (1965).
4.
Ibid, p. ]_18.
5.
R. W. Carpenter and C. T. Liu, Scripta Met. 5, 255 (1971).
Vol. 9, pp. 399-406, P r i n t e d in the U n i t e d
1975 States
Pergamon
Press,
Inc.
OBSERVATIONS ON THE BCC ~ FCO TRANSFORMATION IN A BETA TITANIUM AIJ-OY
T. J. Hesdley and H. J. Rack Sandia Laboratories Albuquerque, New Mexico 87115 (Received February
7, 1975)
Introduct ion De Angelis, Huang and Sargent (DHS) have recently suggested that when the metastable beta titanium alloy, RMI (38-6-44) is aged at 900°F (482°C) for 5-48 hours~ an ordered face-centered orthorhombic (FCO) phase (8')* precipitates within the beta (BCC) matrix (I). The detection of this long period superlattice precipitate was based on an analysis of reflections observed at normally "forbidden" positions within transmission electron diffraction patterns.
Williams (2)
has questioned the existence of such a ~' phase and has suggested that it may be an artifact introduced during sample preparation. Our examination of the aging behavior of BMI (38-6-44) indicates that the ~' phase is not present in this alloy after aging between 200°C-650°C (392°F-1202°F) for 30 minutes to lO00 hours. Experimental Procedure The chemical composition of the 0.625 in. (0.015 m) plate used in this investigation was (wt. pct.): Ti.
3.4 AZ, 8.3 V, 5.8 Cr, 3.9 Zr, 4.2 Mo, 0.06 Fe, 0.011 N, 0.105 O, 0.02 C, balance
Samples were solution treated at either 1700°F (927°C) or 1500°F (815°C) snd air or water
quenched respectively. lope described above.
Aging studies were then carried out within the time/temperature enveThin foils were prepared by standard techniques and examined in a
Phillips 200 Electron Microscope at lO0 KV. Results Table 1 slmmmrizes the results of a typical x-ray diffraetometer experiment.
The observed
diffraction peaks can he accounted for by or (}{CP) and ~ (BCC) phases, without recourse to an FCO phase.
Had sufficient volume fraction of the FCO phase been present, the 002, 020~ and
021 ~' reflections would be observed. Figure 1 is a selected area, electron diffraction pattern essentially equivalent to Figure 2 of Reference 1.
DHS indexed the matrix reflections as belonging to the
zone of the (FCO) 8' phase.
[i00]
laue
However, the pattern symmetry is equally that of (O01> zones in
@Using the notation of Reference I. Not to he confused with the (BCC) 8' phase-separation reported in some beta titanium alloys. 399
400
TRANSFORMATION
IN A B E T A T I T A N I U M
ALLOY
Vol.
9, No.
4
TABLE 1 Summsry of X-ray Diffraction Results for RMI (38-6-44) (1)
,(2) dobs.(A) 2.55 2.34
a(3)
8(3)
~,(4) 2.557 (llO)
2.557 (olo) 2.342 (ooe)
2.325 (OO2) 2.e66 2.e45
(1) (2) (3) (4)
2.266 (IIO)
2.245 (oil)
2.241 (~l) 2.225 (o2o) 2.oo7 (o21)
1500°F 815°C) 1 hr. WQ + 932°F (500°C) 16 hrs. - AC. Determined using Ni filtered, CuKo radiation. Calculated using aa,= 2.957 ~ ca = 4.678 A, a~ = 3.205 A. Calculsted using a~ = 3.125 A, b E' = 4.45 A, c8' = 4.65 from Reference 1.
FIG. 1 Electron Diffraction Pattern of BMI (38-6-44), Air quenched from 1700°F (927°C) and aged 72 hours at 932°F (500°C). The foil is tilted slightly from the [OO1] 8 orientation about [~lO] 8. the (BCC) ~ phase.
From the camera constant of our electron microscope, we found that the
d-spacings of the matrix reflections correspond to {llO] and {020} planes in the (BCC) ~ phase. Therefore the matrix pattern in Figure 1 has been indexed as the [OO1]~ Laue zone.
Vol.
9, No.
4
TRANSFORMATION
IN A B E T A T I T A N I U M
ALLOY
401
Spots 2 through 9 and certain of the satellite spots about the matrix reflections in Figure l were correctly indexed by D H S a s belonging to (ll20> Laue zones of the equilibrium (HCP) 6 phase.
In this alloy, a precipitates upon aging in the well-known Burgers orientation,
(O001)a I! (110)8 and [1120] a I! [lll]8. this phase (there are twelve possible).
DHS attributed the a reflections to two variants of However, dark field imaging of spots 2-9 showed that
four 6 variants actually reflect to these positions, Figure 2.
This is not surprising since
there are four a variants in the Burgers orientation having a (1120> zone oriented 5.25 ° from [00118 , namely:
(oooz)% II (llO)s; Ell~O]% II[~u]e with [h[o]% at (oool)62 II (ZlO)e; [11~o]:2 II[lil]swith [2[[0]62at (ooOl)c~3 II (%io)e; [11~o]~3 II[11118with [[2[0]~3at (OOOl) 4 II (~o)~;[l~O]%II [#b-] 8 with [2~'0]~4 at
5.25 ° from [00118 5.25 ° from [001] 8 5.25 ° from [00118 5.25 ° from [OO1] 8
The two patterns from a1 and a 2 superimpose as do those from 03 and a 4 (shown schematically in Figure 3).
Initially this superimposition appears inconsistent with Figure 2 where only one a
variant imaged in dark field on each spot.
However, the (ll20) a reciprocal lattice planes
do not themselves superimpose due to their respective 5.25 ° misorientations relative to [OO1] B. In each case, the two superimposed spots lie above and below the [O01]~ reciprocal lattice plane.
The specimen tilting employed in forming the dark field images of Figure 2 always
rotates the Ewald sphere toward the reciprocal lattice spot of one 6 variant and away from that of the other.
Therefore, only one variant images on a given spot.
Spots lO through 13 in Figure 1 were interpreted by DHS as split OlO superlattice reflections.
However, dark field imaging revealed that these are double diffraction spots originating
from regions where a and 8 overlap.
This is a common occurrence in the diffraction patterns
of foils containing precipitates (3)-
In Figure l, if the strong LlO~ reflection acts as a
primary beam and is rediffracted upon entering the a phase, then the 6 reflections 8 and 9 translate to positions ll and lB.
Likewise, with the strong l[O~ acting as a primary beam,
the ~ reflections 4 and 5 translate to positions lO and 12. from spot ll.
Figure 4 is a dark field image
Comparing this with Figure 2a, it is clear that the diffracted intensity in
Figure 4 origSmmtes from the regions where a and 8 overlap indicating double diffraction.
In
this manner, it was found that spots ll and 12 were double diffracted beams from the cl/~ overlap regions, as were spots l0 and 13 from 62/8 overlap regions. Figure 5 is the diffraction pattern obtained when the foil was tilted about [llO] 8.
In
this case, the llO B and llO~ spots are no longer intense reflections to act as primary beams in the a phase.
Hence spots lO-13 disappear as expected.
Now the strong llO~ and l'~O~ reflec-
tions act as primaries in a creating the double diffraction spots lh through 17.
By dark field
imaging, spots l~ and 16 were found to originate from 63/8 overlap regions, and spots 15 and 17 from ~4/B overlap regions.
402
TRANSFORMATION
IN A B E T A T I T A N I U M ALLOY
Vol.
a
b
4
r
c
d
FIG. 2 Dark field images of four ~ variants which diffract to spots 2-9 in Figure 1. (a) a I imaged on spots 4 and 8, (b) c~2 imaged on spots 5 and 9, (c) a~ imaged on spots 3 and 7, (d) a h imaged on spots 2 andS6. Mag. 30,000 X.
9, No. 4
Vol.
9, No. 4
TRANSFORMATION
O T~,
IN A B E T A T I T A N I U M
IIT'[I. lT112~
,T,,,,
O
.Tm_2,
i1112,
403
me~2
iP~
11112
ALLOY
,iT,,,
II1~1I!
i:--, ~
lilil~l
I,T2,,O
ITI2,
Ill3!
0 j; Matrix reflectiens ° o~reflectiens (4 variants) FIG. 3 Schematic diffraction pattern corresponding to Figure 1 with the (i120> Laue zones of the four ~ variants indexed.
Finally spots 18 and 19 in Figure 1 were not indexed by DHS, while spots 20 and 21 (also in Figure 5) were not present in their diffraction pattern.
Upon tilting the specimen some-
what further about [[1018, spots 20 and 21 disappeared from the patters Of Figure 1. 18-21 are also related to double diffraction effects.
Spots
That Is~ they appear at the forbidden
O001-type positions in the (ll20) Laue zones of the four a variants (see Figure 3).
Reflec-
tions commonly appear in this Laue zone at these positions because of double diffraction (4). For example, Figure 6 is a dark field image from spot 20 showing that the weak diffracted intensity originates from precipitates of the a 1 variant (comlmre with Figure 2a).
In the case
of spots 18-?_l, diffracted bea~s within the a precipitates are the source of double diffraction, not ~ matrix reflections.
Hence diffraction comes from the entire v o l ~
of a and not from
just the regions where a and ~ overlap. From the above results, it is possible to index the diffraction patterns in Figures l and 5 completely as shown in Figure 7.
The remaining satellite spots about the matrix reflec-
tions belong to other ~ variants (see, for example, reference 5).
404
TRANSFORMATION
IN A B E T A T I T A N I U M
ALLOY
Vol.
9, No. 4
5
FIG. 4 Dark field image from spot 3_I in Figure I illustrating double diffraction from the a/8 overlap regions. (Compare with Figure 22.) Mag. 30,000 X.
HG. 5 Same diffraction pattern as Figure l, except specimen tilted about [ll0]8. Note absence of spots 10-1B of Figure 1 and appearance of new spots 14-17.
0"~
Tof21
oo02r-,~-O00~z
o01T,.r2
T01T, ' 01"iTz ITo~""+4~41Toioi "oiio2
IO~,
0TIT~'~
°°°2x x xn°°l 0010T~
010111 01112
200~O.T012101122_
10(~. 0T112~ D'0"n3
oooi3 :-00014
0~0
00013_: °°°T4 x
~1~ O+O~lx x X0.O+~2
%
| ~'T 1o]oi 10~ °Tl°2
10T110 0TII2
00021+"~00022110,e 0112210121072~
O ji I d l 0 c t i a s •, a |MIocIkMIs (4 variants) x iieuble H f r | e t i N Spat D J L , m i I ~ DiffrMtklo
F'.IG. 6 Dark field image from spot 20 in Figures 1 and 5 showing weak diffracted intensity from vsriant. (Compare with Figure 20. )
~
• 30,000
X.
F~G. 7 Complete indexing of Figures i and 5 combined: [O01]A orientation superimposed with four (11~05a orientations, including double dlff ra ct ion.
Vol.
9, No.
4
TRANSFORMATION
IN A B E T A
TITANIUM
ALLOY
405
S 1}mmR
Selective dark field microscopy was used to account for all reflections in Figure 1. The extra spots at forbidden positions result from double diffraction and not from a long period superlattice.
The x-ray diffraction and transmission electron microscopy results are
complementary and show that aging of RMI (38-6-44) involves the precipitation of an (HCP) a phase from a (BCC) matrix.
No evidence for an FC0 phase has been observed.
Acknowled6ements The authors are indebted to S. F. Duliere for the x-ray diffraction results, and to F. Ao Greulich for preparing TEM specimens.
This work was supported by the Atomic Energy
Commission.
References i.
R. J. De Angelis, H. Huang and Gorden A. Sargent, Scripta Met. ~, 835 (1973).
2.
J. C. Williams, Titanium Science and Technology, p. 1433, Plenum Press, New York (1973).
3.
P. B. Hirsch, A. Howie, R. B. Nicholson, D. W. Pashley and M. J. Whelan, Electron Microscopy of Thin Crystals, p. 325, Butterworth and Co., London (1965).
4.
Ibid, p. ]_18.
5.
R. W. Carpenter and C. T. Liu, Scripta Met. 5, 255 (1971).