Technical Field
[0001] This invention relates to titanium base alloys of the Ti₃Al (alpha-two) type which
have both good elevated temperature properties and sufficient low temperature ductility
to make them useful in an engineering sense.
Background Art
[0002] The present invention is an improvement on the alloys described in U.S. Patent No.
4,292,077, issued to the applicants herein and having common assignee herewith. As
indicated in the patent, the new alloys are comprised of aluminum, niobium and titanium.
The compositional ranges for the patented alloys were quite narrow since changes in
properties were discovered to be very sensitive to the precise composition. Generally,
the patented alloys contain titanium, 25-27 atomic percent aluminum and 11-16 atomic
percent niobium. The alloys have at least 1.5% tensile ductility at room temperature
and good elevated temperature creep strength, thus permitting their potential substitution
for certain nickel base alloys such as INCO 713C.
[0003] In an important embodiment of the prior invention, vanadium partially replaces niobium
in atomic amounts of 1-4%. This substitution desirably lowers the density of the alloy
but at the same time the favorable high temperature properties are retained. An optimum
atomic composition range for this embodiment is 24-26% aluminum, 10-12% niobium and
2-4% vanadium.
[0004] While the foregoing patented alloys meet the requirement of having creep rupture
life at 650°/380 MPa which is equal to INCO 713C on a density adjusted basis, the
alloys have less tensile strength at temperatures up to 400°C than does the commercial
beta processed alloy Ti-6-2-4-2 (by weight percent Ti-6Al-2Sn-4Zr-2Mo). Consequently,
compositional modifications of the patented alloys were evaluated to see if improvements
could be achieved. As the general field of titanium alloys indicates, there are many
potential alloying ingredients. But, as the prior work demonstrated, the composition
of useful Ti₃Al alloys is extremely critical. Many elemental additions which have
been common in other titanium alloys were previously shown to be of no advantage in
Ti₃Al alloy.
Disclosure of the Invention
[0005] An object of the invention is to provide Ti₃Al type alloys which have a superior
combination of creep rupture life and tensile strength at elevated temperatures in
the 600°C range, but which alloys at the same time have sufficient ductility to enable
their use at room temperature and their fabrication by conventional processes associated
with titanium base alloys.
[0006] According to the invention, new titanium base alloys contain by atomic percent 25-27
aluminum, 11-16 (niobium + molybdenum) and 0.5-4 molybdenum. Preferably they have
0.5-1.5 Mo. As especially preferred embodiment of the invention is the lighter weight
alloy containing vanadium in substitution for a portion of the niobium. Such an alloy
contains by atomic percent 25-27 Al, 11-16 (Nb + V + Mo), 1-4 (V + Mo), at least 0.5
Mo, balance titanium. More preferably, the light weight alloy contains 9-11 Nb, 1-3
V and 0.5-3 Mo, balance titanium.
[0007] The incorporation of molybdenum substantially increases high temperature ultimate
tensile strength and creep rupture properties, compared to the essential alloys of
our prior invention which did not contain molybdenum.
[0008] The foregoing and other objects, features and advantages of the present invention
will become more apparent from the following description of preferred embodiments
and accompanying drawings.
Brief Description of Drawings
[0009]
Figure 1 is a graph showing the comparative ultimate tensile strength-to-density ratio
for various known alloys, compared to the invention.
Figure 2 is a bar chart showing comparative stress rupture properties on a density
adjusted basis for the invention compared to various known alloys.
Best Mode for Carrying Out the Invention
[0010] The best mode of the invention is described in terms of atomic percent of elements.
Those skilled in the metallurgical arts will recognize the limitations on stating
the invention by weight percent and the utility of stating the invention by the preferred
atomic percent; they will be able to readily convert from atomic percents to exact
weight percents for particular embodiment alloys.
[0011] The alloys of the present invention are based essentially on the compositions which
we disclose in our U.S. Patent No. 4,292,077, the disclosure of which is incorporated
by reference. Those alloys contain a critical combination of Ti, Nb and Al. In the
patent we showed that the essential invention could be enhanced by including substituting
4% V for Nb, thereby lowering density. In making and disclosing the present invention,
we have used the light weight vanadium containing version of our prior invention.
Our work described herein shows that Mo is a particularly unique and valuable addition
to the essential Ti-Nb-Al alloys of our prior patent.
[0012] The alloys described herein were manufactured using conventional titanium base alloy
technology, basically vacuum arc melting and isothermal forging which is quite familiar
(albeit isothermal forging is a recent improvement). Alloys of the Ti₃Al composition
have been developed to the extent that large ingots, weighing up to 245 kg may be
procured on a routine basis from commercial sources. In the invention, the alloys
are cast, forged and heat treated. The procedures for manufacture and testing of forgings
are the same as those described in U.S. Patent 4,292,077.
[0013] An exemplary alloy demonstrating the invention is Ti-25Al-10Nb-3V-1Mo. (All compositions
hereinafter are in atomic percent unless otherwise stated.) The alloy has a density
of about 3% greater than that of Ti-25Al-10Nb-4V, which is 4.5 g/cc. The alloy was
isothermally beta forged (the cylindrical cast ingot pressed to a disk shape approximately
14% of the original ingot height) at a temperature of about 1120°C. This is about
40°C over the beta transus, estimated to be about 1080°C. Tables 1 and 2 show respectively
the tensile and creep rupture properties of the alloy.
Table 1.
Tensile Properties of Isothermally Beta Forged and Heat Treated Ti-25Al-10Nb-3V-1Mo
Alloy |
Specimen |
Temperature °C |
0.2% Yield Strength MPa |
Ultimate Tensile Strength-MPa |
El% |
RA% |
A |
25 |
825 |
1047 |
2.2 |
1.7 |
B |
260 |
831 |
1058 |
9.2 |
14.1 |
C |
427 |
729 |
950 |
12.1 |
16.9 |
D |
538 |
647 |
967 |
9.2 |
13.0 |
E |
650 |
640 |
835 |
9.1 |
14.3 |
Table 2.
Creep-Rupture Properties of Isothermally Beta Forged and Heat Treated Ti-25Al-10Nb-3V-1Mo
Alloy |
Specimen |
Test Conditions °C/MPa |
Time in Hours to |
|
|
0.2% El |
0.5% El |
1.0% El |
Rupture |
F |
650/380 |
2.8 |
31.1 |
184.5 |
* |
G |
650/380 |
1.4 |
12.0 |
66.3 |
222.8 |
H |
593/413 |
27.0 |
405.6 |
* |
* |
* Test terminated at 502 Hours without rupture |
[0014] Figure 1 shows how the ultimate tensile strength to density ratio of our new alloy
compares with those of a similar alloy lacking molybdenum and two commercial alloys,
alloy Ti-6-2-4-2 and nickel base alloy INCO 713C. It is seen that the new alloy provides
a significant improvement.
[0015] Figure 2 shows how the density-adjusted stress for 300 hr rupture life at 650°C for
the alloy containing molybdenum is substantially improved over the creep rupture
life for a similar alloy lacking molybdenum.
[0016] Generally, our alloys will be characterized in their optimally forged and heat treated
condition by a tensile ductility at room temperature of at least 1.5%, typically about
2.5%; an ultimate tensile strength of 1000 MPa at 25°C; and a 650°C/372 MPa creep
life of at least 150 hours, typically about 300 hours. They have stress-to-density
ratios of the order of 2 kPa/m³, compared to less than 1.5 kPa/m³ for the alloys of
our prior patent, and compared to even lower values for older alloys.
[0017] Our new alloys also have desirably increased dynamic elastic modulus compared to
other alloys, as indicated in Table 3. The Ti-25Al-10Nb-3V-1Mo 650°C modulus is almost
30% greater than the value for Ti-25Al-10Nb-4V, and a significant improvement over
commercial alloys as well. The modulus was measured by mechanically stimulating resonant
vibration of a beam of known dimensions and measuring the frequency response thereof.
Calculation is made from known dynamics relationships.
Table 3.
Dynamic Modulus of Selected Alloys (10⁷kPa) |
|
Temperature - °C |
|
20 |
315 |
650 |
Ti-6Al-2Sn-4Zr-2Mo |
11.9 |
10.4 |
8.6 |
Ti-25Al-10Nb-4V |
10.1 |
9.7 |
8.7 |
Ti-25Al-10Nb-3V-1Mo |
12.6 |
12.1 |
11.2 |
[0018] As much as 6-8% Mo may be included in our new alloys, since as Mo content rises,
creep strength and stiffness rise. However, density and oxidation resistance (necessary
for high temperature gas turbine use) decrease. Thus, for such alloys the Mo should
be limited to about 4% and preferably it is 0.5-1.5%. Our basic Ti-Nb-Al-Mo alloys
are useful, but they are ever more useful when V is used in place of Nb in accord
with out prior invention. But since V like Mo decreases oxidation resistance, the
total content of (V + Mo) should be maintained at less than 4%. Thus, out new alloys
will essentially consist of Ti, Al, Nb, Mo. They preferably will contain V. Tungsten
may substitute in part or whole for Mo, as indicated below. Other intentional additions
may be included in our essential alloys, such as less than 1% C or Si in replacement
of Ti.
[0019] Table 4 shows the lightest and heaviest embodiments of our invention in weight percent.
We provide this as a reference for the future.
Table 4.
Weight Percentages (w/o) for the Invention in Atomic Percentages (a/o) |
Alloy |
Element |
|
|
Al |
Mo |
Nb |
V |
Ti |
A |
a/o |
25 |
4 |
12 |
-- |
59 |
|
w/o |
13.5 |
7.7 |
27.3 |
-- |
56.5 |
B |
a/o |
27 |
0.5 |
10.5 |
-- |
62 |
|
w/o |
15.4 |
1.0 |
20.6 |
-- |
63 |
C |
a/o |
25 |
1.5 |
14.5 |
-- |
59 |
|
w/o |
13.5 |
2.9 |
26.9 |
-- |
56.7 |
D |
a/o |
27 |
0.5 |
10.5 |
-- |
62 |
|
w/o |
15.4 |
1.0 |
20.6 |
-- |
63.0 |
E |
a/o |
25 |
3.5 |
12.0 |
0.5 |
59 |
|
w/o |
13.6 |
6.7 |
22.4 |
0.5 |
56.8 |
F |
a/o |
27.0 |
0.5 |
7.0 |
3.5 |
62 |
|
w/o |
16.0 |
1.0 |
14.2 |
3.8 |
65.0 |
G |
a/o |
25 |
3 |
11 |
1 |
60 |
|
w/o |
13.7 |
5.9 |
20.8 |
1.0 |
58.6 |
H |
a/o |
27 |
0.5 |
9 |
0.5 |
63 |
|
w/o |
15.6 |
1.0 |
18.0 |
0.6 |
64.8 |
[0020] In our work consideration was given to other elements which might be substituted
in Ti-Nb-Al-V alloys to achieve the same results as molybdenum. We made the alloys
Ti-25Al-8Nb-X, where X was variously 1W, 1Ta, 1Hf, and 1V. We did not discern any
distinction between the ingredients, all the alloys having poor creep strength. In
addition, reference to Table 4 in our Patent 4,292,077 will show that there is no
consistent effect of Hf, Zr, or Sn in Ti-24Al-11Nb alloys. We made the alloys Ti-24Al-11Nb-Z,
where Z was variously 0.5Hf, 1Zr, (1Zr-0.5Si), 0.9C, 1.4Hf and (1.5Hf-0.9C), and found
that compared to Ti-24Al-11Nb the alloys had about the same or inferior creep properties,
and about the same tensile properties. Other beta stabilizers, such as iron, chromium
or nickel are unsuitable for use in the present invention because they form undesirable
phases after high temperature exposure. Their addition also reduces the high temperature
properties of our type of titanium alloys. Thus, our studies make us conclude that
molybdenum is unique in our invention, in combination with the narrow ranges of other
elements. Since tungsten is known to be metallurgically equivalent to molybdenum in
titanium alloys, it will be substitutional for molybdenum in the present invention.
However, the use of tungsten will result in an alloy with higher density and therefore,
less desirable density-corrected properties than those which result from the use of
molybdenum.
[0021] The properties of our molybdenum containing alloys were found to be sensitive to
microstructure. Based on the prior work, it was felt that the nature of the Widmanstatten
platelet array was the key micro-structural feature affecting properties. However,
in testing it was found that specimens were produced with coarse non-uniform beta
grain size. These test bars had associated with them lower tensile ductility, lower
fatigue life, and higher creep rupture strength than the other specimens. Analysis
showed that in our previous work alloys (Ti-25Al-10Nb-4V) had been redundantly upset
and redrawn on a conventional forging press. This working broke up the cast structure
and resulted in much finer uniform grain structure than resulted in some of the molybdenum
containing alloys. Consequently, we conclude that it is desirable with our new alloy
to provide some repetitious working prior to isothermally forging the billet to the
final desired shape. The desired microstructure will have an ASTM grain size of about
2-4 (0.15-0.20 mm nominal dimension).
[0022] The alloy made as described above is best used with limited time exposure at temperature
in the 565-675°C range. We have noticed some instability, in that yield strength increased
and ductility decreased after several hundreds of hours exposure. Further heat treatment
development may avoid the instability.
[0023] Generally, the heat treatment which the alloys of the present invention should be
given is similar to that disclosed previously in U.S. Patent No. 4,292,077. Solutioning
or forging should be conducted above the beta transus, followed by aging between 700-900°C
for 2-24 hours. The cooling rate from the solutioning or forging temperature should
be that which produces a fine Widmanstatten structure characterized by acicular alpha
two structures of about 50 x 5 x 10⁻⁶m dimension mixed with beta phase lathes, generally
as shown in Figure 7(b) of the referenced patent. The conditions necessary to achieve
this will depend on the size of the article, but generally cooling in air or the equivalent
will be suitable for most small articles. Of course, precautions should be taken
to protect the forgings from contamination from the environment, similar to steps
followed with the conventional alloys of titanium. An alternative heat treatment comprises
solutioning above the beta transus followed by quenching in a molten salt bath maintained
about 750°C, followed by air cooling.
[0024] Although this invention has been shown and described with respect to a preferred
embodiment, it will be understood by those skilled in the art that various changes
in form and detail thereof may be made without departing from the spirit and scope
of the claimed invention.
1. A titanium aluminum alloy consisting essentially by automatic percent of 25-27
aluminum, 11-16 (niobium + molybdenum 0.5-4 molybdenum, balance titanium.
2. The alloy of claim 1 having 0.5-1.5 molybdenum.
3. The alloy of claim 1 wherein 0.5-3.5 vanadium is substituted for niobium, the alloy
containing 1-4 (molybdenum + vanadium).
4. The alloy of claim 3 having 1-3 vanadium and 0.5-3 molybdenum.
5. The alloy of claim 3 having 3 vanadium and 1 molybdenum.
6. The alloy of claim 1 wherein up to 4 atomic percent tungsten is substituted for
molybdenum.
7. The alloy of claim 1 heat treated first at a temperature above the beta transus,
then cooled at a controlled rate, sufficient to produce a fine Widmanstatten structure.
8. The alloy of claim 8 further heat treated by aging at 700-900°C for 4-24 hours.
9. The alloys of claim 1 having a tensile ductility at room temperature of at least
1.5%.
10. The alloys of claim 1 having a creep stress to density ratio of 650°C of greater
than 1.6 kPa per kg per m³.
11. The alloy of claim 1 having a 650°C dynamic elastic modulus of greater than 9
x 10⁷ kPa.