Titanium aluminide which can be used at high temperature

Details Titanium aluminide which can be used at high temperature Titanium aluminide which can be used at high temperature

1998-03-04

2001-01-23

Sheehan, John (Department: 1742)

Metal treatment

Process of modifying or maintaining internal physical...

Heating or cooling of solid metal

C148S421000

Reexamination Certificate

active

06176949

ABSTRACT:

The invention related to the alloys predominantly formed of titanium and aluminum commonly known as titanium aluminides.
Titanium alloys are widely used in gas turbine engines but their applications remain limited because of the temperatures of use, which must not exceed 600° C. because, beyond this temperature, their mechanical strength rapidly decreases. During the last 20 years, a number of research studies have had the objective of developing titanium alloys which can be used at high temperatures by virtue of an ordered structure which confers increased strength on them. These new alloys, known as titanium aluminides, are mainly of the Ti
3
Al type (ordered &agr;
2
phase) and of the TiAl type (ordered &ggr; phase). Another ambition of these research studies was to be able also to at least partially replace nickel superalloys, which would be reflected by a large reduction in weight of the engines for the parts used at temperatures beyond which titanium alloys can be used. The main applications targeted by these new alloys relate to the HP compressor in turbomachines. Moreover, by being able to use a higher temperature, the compressor can operate with a better output, which has a favorable effect on lowering the specific consumption.
Studies have been carried out in particular on titanium aluminides of the Ti
3
Al type, characterized by a two-phase &agr;
2
(ordered hexagonal)+&bgr; (cubic) structure. In these alloys, the aluminum has a tendency to stabilize the &agr;
2
phase, whereas other elements which may be present, in particular niobium, vanadium, molybdenum and tantalum, have a tendency to stabilize the &bgr; phase.
U.S. Pat. No. 4,292,077 studies the influence of the composition of Ti—Al—Nb ternary alloys on their characteristics of use and provides an alloy, known as &agr;
2
, containing 24% aluminum and 11% niobium (Ti—24Al—11Nb according to the notation used in the continuation; all the concentrations are given here as atoms, except when otherwise indicated) as offering the best compromise between high-temperature creep strength, favored by aluminum, and ductility, favored by niobium. According to the inventors of the abovementioned patent, niobium can be replaced by vanadium to the level of 4%, which makes it possible to reduce the weight of the alloys while retaining the same standard of mechanical properties, indeed even while improving it.
Provision has also been made to improve the strength/ductility compromise by introducing both molybdenum and vanadium, the first of these constituents increasing both the tensile strength and the creep strength in comparison with the &agr;
2
alloy and the second making it possible to retain the ductility and to reduce the weight of the alloy. Thus U.S. Pat. No. 4,716,020 defines an alloy, known as Super &agr;
2
, containing 25% aluminum, 10% niobium, 3% vanadium and 1% molybdenum. This alloy, however, exhibits the major disadvantage of a low ultimate tensile stress. In addition, it is characterized by some structural instabilities which makes it lose its ductility when it is subjected for several hundred hours to a temperature within the range 565-675° C. U.S. Pat. No. 4,788,035 provides for reducing the amount of niobium and for introducing tantalum, in particular with the composition Ti—23Al—7Ta—3Nb—IV, which results in a particularly advantageous creep strength. However, no indication is given as regards the ductility at ambient temperature.
None of the above alloys possesses a combination of hot and cold strength and ductility, and of creep strength, sufficient to enable it to be used in gas turbines.
U.S. Pat. No. 5,032,357 described alloys having a niobium content of greater than 18% and possessing an orthorhombic phase, known as O, an ordered phase corresponding to the intermetallic Ti
2
AlNb compounds. In this phase, a crystallographic site is occupied exclusively by Nb, instead of being occupied without distinction by Ti and by Nb in the &agr;
2
phase.
The O phase was observed over a wide range of atomic compositions from Ti—25Al—12.5Nb to Ti—25Al—30Nb. For lower Al contents (between 20 and 24%), the alloys are two-phase &bgr;
0
+O and possess similar microstructures to those of the &bgr;+&agr;
2
alloys, although they are generally finer because of the slower kinetics of transformation. The &bgr;
0
phase corresponds here to the ordered structure of B2 type of the &bgr; phase. The orthorhombic alloys are thus divided into two groups: the O single-phase alloys, which are similar to the composition Ti
2
AlNb, and the &bgr;
0
+O two-phase alloys, which are substoichiometric in aluminum. The category of the O single-phase alloys, such as the Ti—24.5Al—23.5Nb alloy, is characterized by an increased creep strength. The category of the &bgr;
0
+O two-phase alloys, such as the Ti—22Al—27Nb alloy, is illustrated more particularly by their high strength, while retaining a reasonable ductility. Consequently, depending on a criterion of priority to creep or of priority to mechanical strength, the use of the two alloys Ti—24.5Al—23.5Nb (O) and Ti—22Al—27Nb (&bgr;
0
+O) has been recommended.
U.S. Pat. No. 5,205,984 furthermore provides for the partial substitution of the element vanadium by niobium for this novel category of orthorhombic alloys. The quaternary alloys obtained do not seem to be of particular advantage in comparison with the ternary alloys, taking into account in particular the known harmful influence, moreover, of vanadium on the oxidation resistance.
It turns out that the ternary orthorhombic alloys exhibit physical and mechanical characteristics which can limit their industrial development, such as a fairly high density (5.3) because of a high niobium content. In addition, these alloys undergo a pronounced loss in strength on prolonged annealing. An increase in the annealing time from 1 to 4 hours at 815° C. or else the use of a second annealing of 100 hours at 760° C. causes a loss of 300 MPa in the elastic limit of the Ti—22Al—27Nb alloy. Finally, the compromise is difficult to find between the cold ductility and the creep strength, whether by acting on the composition of the alloy or on the heat treatments to be applied to it.
One aim of the present invention is to produce titanium aluminides which possess specific tensile and creep strengths which are greater than those of the above alloys of the Ti
3
Al and Ti
2
AlNb categories, which can be used at temperatures of greater than 650° C. and which have a satisfactory ductility at 20° C.
Another aim of the present invention is to provide an alloy of the Ti
2
AlX type which possesses an excellent combination of tensile strength and creep strength up to 650° C. and which, at the same time, exhibits a high deformability at 20° C. to enable it to be manufactured and used.
These aims are achieved, on the one hand, by virtue of narrow ranges of alloy compositions and, on the other hand, by virtue of a transformation process which makes it possible to take advantage of these alloy compositions.
The invention is targeted in particular at an alloy of the Ti
2
AlX type composed at least essentially of the elements Ti, Al, Nb, Ta and Mo and in which the relative amounts as atoms of said elements and of silicon are substantially within the following intervals:
Al: 20 to 25%
Nb: 10 to 14%
Ta: 1.4 to 5%
Mo: 2 to 4%
Si: 0 to 0.5%
Ti: remainder to 100%.
In addition to the elements Ti, Al, Nb, Ta, Mo and Si, the alloy according to the invention can contain other elements, such as Fe, at low concentrations, preferably of less than 1%.
Optional characteristics of the alloy according to the invention, complementary or alternative, are stated hereinbelow:
It contains 21 to 32% of niobium equivalent as atoms. The niobium equivalent is obtained by adding, to the amount of niobium, the amounts of the other elements of the alloy favoring the &bgr; phase, modified by a coefficient corresponding to the &bgr;-gen power of the elements under consideration in comparison with niobium. Thus, as Ta and Mo have respectively &bgr;-gen powers equal to and trip

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