Patent Application
20150218675
1. Field of the Invention
The following invention relates to a TiAl alloy for use at high temperatures, in particular in the range from 750° C. to 900° C., and also to the production and use thereof.
2. Prior Art
Alloys based on intermetallic titanium aluminide compounds are employed in the construction of stationary gas turbines or aircraft engines, for example as material for turbine blades, since they have the mechanical properties necessary for use and additionally have a low specific gravity, so that the use of such alloys can increase the efficiency of stationary gas turbines and aircraft engines.
Accordingly, many TiAl alloys have already been developed, with use at present being made of, in particular, TiAl alloys which are based on the intermetallic γ-TiAl phase and are alloyed with niobium and molybdenum or boron and are therefore referred to as TNM or TNB alloys. Such alloys have titanium as main constituent together with from about 40 to 45 at. % of aluminum, about 5 at % of niobium and, for example, 1 at % of molybdenum and also small proportions of boron. The microstructure is characterized by a high proportion of γ-TiAl and likewise significant proportions of α2-Ti3Al, with further phases such as the β phase or B19 phase being able to occur in a relatively small proportion.
The known TNM or TNB alloys based on γ-TiAl usually have an equiaxial γ-TiAl microstructure, a lamellar microstructure or a duplex microstructure with equiaxial γ-TiAl grains and lamellar regions composed of γ-TiAl and α2-Ti3Al. Although such γ-TiAl alloys, especially those having lamellar microstructures, have very good overall mechanical properties up to 750° C., a deterioration in the mechanical properties, in particular a decrease in the creep resistance, occurs at higher temperatures because of the thermodynamic instability of the microstructure.
It is therefore an object of the present invention to provide an alloy which has a low specific gravity similar to that of the known γ-TiAl alloys and also comparable mechanical properties, in particular at high temperatures, with the use range preferably being broadened to temperatures in the range from 750° C. to 900° C. or 950° C. Such an alloy should be able to be produced and processed on an industrial scale without undue difficulty and also be able to be used reliably in stationary gas turbines and aircraft engines.
This object is achieved by a TiAl alloy having the features of claim 1, a process for producing a TiAl alloy having the features of claim 14 and also the use of the TiAl alloy having the features of claim 15. Further advantageous embodiments are subject matter of the dependent claims.
In the following, a TiAl alloy is an alloy whose main constituents are titanium and aluminum, so that the proportion of aluminum and titanium in at. % or % by weight is in each case greater than the proportion of any other alloy component. However, in at. % or % by weight, it is possible for the proportion of aluminum to be greater than the proportion of titanium, not only for the proportion of titanium to be greater than the proportion of aluminum as the designation TiAl appears to indicate. In addition, a TiAl alloy according to the invention is an alloy which is made up predominantly of intermetallic phases having the constituents titanium and/or aluminum.
The present invention accordingly proposes a TiAl alloy as high-temperature TiAl alloy comprising, apart from the main constituents titanium and aluminum, in particular a main constituent titanium, a proportion of aluminum of ≧30 at. % and in which the microstructure has a matrix composed of β phase in which precipitates of ω phase are embedded.
The term β phase refers to various morphologies of the β phase, e.g. β or β0. Analogously, the ω phase encompasses various morphologies such as ω0-B82, ω-D88 or ω″ transition phases.
The proportion by volume of the β phase and the ω phase should together be at least 55% by volume, preferably at least 75% by volume and in particular at least 80% by volume. The creep resistance, in particular, can be improved by a microstructure having a matrix of β phase with embedded ω precipitates, so that higher use temperatures compared to the known γ-TiAl alloys are possible. Owing to the matrix composed of β phase, the corresponding alloy can also be referred as β-TiAl alloy.
The proportion by volume of β phase to ω phase can be in the range from 1 to 4 to 4 to 1, in particular from 1 to 3 to 3 to 1.
The ω phase can precipitated with grain sizes in the range from 5 nm to 500 nm, in particular from 10 nm to 450 nm or from 25 nm to 400 nm, and be present in this form in the β matrix. In addition, the ω phase can also be present in, in particular, globular form at grain boundaries of the TiAl alloy, with grain boundaries of all possible microstructure constituents coming into question here.
For this purpose, the alloy can be subjected to at least one heat treatment lasting for from 1 to 100 hours at a temperature in the range from 20° C. below to 400° C. below the ω-solvus temperature, so that a thermodynamically stable microstructure is established. Precipitation of ω phases having small grain sizes in the nm range can, in particular, exert an advantageous influence on the strength properties.
The precipitation of the ω phase can also be carried out in such a way that the ω phase is present in at least two different grain size ranges in the microstructure, with a first grain size range being able to encompass grain sizes in the range from 5 nm to 100 nm and a second grain size range being able to encompass grain sizes in the range from 200 nm to 500 nm. Multistage aging heat treatments can be carried out for this purpose.
Various deformation mechanisms in the alloy can be suppressed as a function of the various grain sizes of the ω phases in order to increase the strength of the alloy. Thus, the ω precipitates having relatively large grain sizes can hinder cutting by means of dislocations, while the smaller ω precipitates can hinder climbing-over by the dislocations.
The ω phase can be present as semicoherent in spherical or cubic form in the β matrix, with the β matrix being able to have a network-like microstructure which makes a high creep resistance up to temperatures of 900° Celsius and more possible.
As alloy constituents, it is possible for one or more alloying elements from the group including niobium, molybdenum, tungsten, zirconium, vanadium, yttrium, hafnium, silicon, carbon and cobalt to be alloyed in. The alloy components niobium, molybdenum, tungsten, zirconium and cobalt are particularly advantageous since they stabilize the β phase. The alloy constituents niobium and molybdenum can, in particular, be provided in a ratio of from 1.8 to 1 to 5 to 1, preferably from 2 to 1 to 3 to 1, relative to one another in the alloy, so that the niobium content is always higher than the molybdenum content. The higher the proportion of niobium and molybdenum in the alloy, the higher can the ratio of niobium to molybdenum be chosen in order to promote precipitation of the ω phase. A relatively high proportion of niobium makes formation of the ω phase possible since niobium stabilizes ω phase formation while molybdenum essentially makes formation of the β phases possible.
The alloy components tungsten, zirconium, vanadium, yttrium and hafnium serve to form oxides and carbides which can form finely divided precipitates, so that these alloy constituents can not only aid mixed crystal stabilization but also contribute to the formation of precipitates to increase the strength of the alloy. Correspondingly, the alloy constituents tungsten, zirconium, vanadium, yttrium and hafnium can be at least partly interchanged. The same applies to the alloy constituents tungsten, vanadium and cobalt on the one hand and zirconium, yttrium and hafnium on the other hand.
The addition of cobalt can bring about a further increase in the creep resistance since the alloying element cobalt can reduce the stacking fault energy, so that splitting of dislocations occurs and makes climbing of the dislocations more difficult and thus increases the creep resistance.
The addition of silicon can improve the corrosion resistance of the alloy.
Accordingly, a β-TiAl alloy according to the invention can comprise from 30 to 42 at. % of aluminum, in particular from 30 to 35 at. % of aluminum, from 5 to 25 at. % of niobium, in particular from 15 to 25 at. % of niobium, from 2 to 10 at. % of molybdenum, in particular from 5 to 10 at. % of molybdenum, from 0.1 to 10 at. % of cobalt, in particular from 5 to 10 at. % of cobalt, from 0.1 to 0.5 at. % of silicon and from 0.1 to 0.5 at. % of hafnium, with titanium as balance. The individual alloy constituents should be selected within the content ranges indicated above so that they add up to a total of 100%. Accordingly, each content range indicated does not necessarily have to be completely exploited. Rather, this depends on which alloy constituents have already been selected in which proportion, so that the content ranges have a mutual influence on one another.
The TiAl alloy presented can be produced pyrometallurgically, with the melt being able to be drawn out as a single crystal or cast to form a polycrystalline product, so that the corresponding component composed of the β-TiAl alloy can be used as a single crystal, as a directionally solidified component or as a polycrystalline component.
In addition, powder-metallurgical production in which at least part of the alloy constituents, for example the alloying elements cobalt, tungsten, hafnium, vanadium and yttrium, can be mechanically alloyed is also possible.
To form the ω precipitates, the alloy can be subjected to single-stage or multistage aging heat treatments which are carried out in the temperature range from 20° C. below to 400° C. below the ω-solvus temperature at which the co phase goes into solution.
A corresponding TiAl alloy can, in particular, be used for components of stationary gas turbines or aircraft engines, for example for turbine blades.
| Number | Date | Country | Kind |
|---|---|---|---|
| 14154052.6 | Feb 2014 | EP | regional |
