Titanium aluminide intermetallic compositions

Abstract

Gamma titanium aluminide intermetallic compositions (gamma TiAl intermetallics) based on the TiAl (gamma) intermetallic compound. The gamma TiAl intermetallics contain chromium and niobium, as well as controlled amounts of carbon that achieve a desirable balance in room temperature mechanical properties and high temperature creep capabilities at temperatures approaching and possibly exceeding 1600 F. (about 870 C.).

Claims

1. A low pressure turbine blade of gas turbine engine comprising a titanium aluminide intermetallic composition based on a gamma TiAl intermetallic compound, the titanium aluminide intermetallic composition consisting of, by atomic percent, 46 to 50% aluminum, 1 to 3% chromium, 1 to 5% niobium, 160 to 470 ppm carbon, titanium and incidental impurities in amounts to yield the gamma TiAl intermetallic compound, wherein the titanium aluminide intermetallic composition exhibits a minimum room temperature ductility of not lower than 0.5%, and wherein the titanium aluminide intermetallic composition exhibits an average room temperature ductility of at least 1%.

2. The low pressure turbine blade according to claim 1, wherein the titanium aluminide intermetallic composition contains about 300 ppm carbon.

3. The low pressure turbine blade according to claim 1, wherein the titanium aluminide intermetallic composition is in the form of a casting and has a duplex microstructure containing equiaxed and lamellar morphologies.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a flow chart representing a method of processing castings formed of TiAl intermetallic compositions of this invention.

(2) FIG. 2 contains four graphs that plot fatigue creep resistance, room temperature and high temperature elongation, and crack growth threshold (Mc) of four experimental gamma titanium aluminide intermetallic compositions containing varying amounts of carbon between 160 and 500 ppm.

DETAILED DESCRIPTION OF THE INVENTION

(3) The present invention provides a gamma TiAl intermetallic composition that contains controlled amounts of chromium, niobium, and carbon to achieve a desirable balance of room temperature mechanical properties and high temperature creep capabilities that render the composition suitable for use in high temperature applications, including but not limited to the low pressure turbine section of a gas turbine engine.

(4) Mechanistically, carbon is known to increase the strength of TiAl intermetallic compositions by serving as an interstitial strengthening agent. According to the present invention, very controlled carbon additions are capable of promoting creep resistance properties without unacceptably decreasing room temperature ductility of gamma TiAl intermetallic compositions that contain 46 to 50 atomic percent aluminum, 1 to 3 atomic percent chromium, 1 to 5 atomic percent niobium. This advantageous balance of properties can be particularly achieved if the carbon level is about 160 to 500 ppm (about 0.016 to 0.05 atomic percent), more particularly about 160 to 470 ppm (about 0.016 to 0.047 atomic percent). The carbon additions can be introduced when preparing a primary or secondary melt, using virgin or revert/recycled materials of the gamma TiAl intermetallic composition.

(5) During investigations leading to the present invention, it was determined that, in gamma TiAl intermetallic compositions containing 1 to 3 atomic percent chromium and 1 to 5 atomic percent niobium, an inverse linear relationship exists between carbon content and room temperature ductility within a narrow carbon content range of 160 to 500 ppm. Concomitantly, the creep resistance of such compositions was observed to improve as the carbon content was increased over this range. On the basis of these relationships, it was further determined that controlled additions of carbon can result in improved creep resistance while maintaining adequate ductility to enable the design and manufacturing of components from such compositions, for example, when cast and processed to produce low pressure turbine blades of gas turbine engines.

(6) During the investigations, alloys containing four different levels of carbon were prepared: 160, 270, 420 and 500 ppm. The compositions were produced by melting ingots of the aforementioned 48-2-2 alloy in an induction skull melter, adding the controlled amounts of carbon to the melt, and then recasting the melt. Aside from their carbon contents, the nominal chemistries of the TiAl intermetallic compositions were, in atomic percent, about 48% aluminum, about 2% chromium, about 1.9% niobium, and the balance titanium and incidental impurities. Each composition was heat treated, hot isostatically pressed (HIPed), and tested for mechanical properties. The results of these tests are plotted in graphs in FIG. 2. As seen in the creep plot, creep resistance was observed to improve with carbon content, but room temperature and 1400 F. (about 760 C.) elongation decreased with carbon content. The crack growth threshold (K.sub.th) at 800 F. (about 425 C.) was acceptable at all of the tested carbon levels. The latter property is an important consideration for the gamma TiAl intermetallic composition of this invention, since it is a primary parameter of concern for long-term reliability of LPT blades and other components that are similarly subject to conditions that might promote crack propagation.

(7) Overall, the results of the investigation indicated that carbon contents within the ranges tested should provide a high temperature capability exceeding 1500 F. (about 815 C.), and likely about 1600 F. (about 870 C.) or more. Because a minimum room temperature ductility of 0.5% was determined to be a requirement for LPTB applications, the results from the investigated range further indicated that a preferred maximum carbon content for the gamma TiAl intermetallic composition of this invention is 470 ppm. In particular, the specimen containing a carbon level of 500 ppm was concluded to exhibit insufficient room temperature ductility to enable a gamma TiAl intermetallic composition based on the 48-2-2 alloy to be readily processable as an LPT blade. Because a nominal room temperature ductility of 1.0% was identified as desired for LPTB applications, the results of the investigation indicated that the tested carbon level of 270 ppm (0.027 atomic percent) provided a particularly desirable balance of properties. From this, it is believed that a nominal carbon content of about 300 ppm (0.03 atomic percent) was likely to provide an optimal balance between creep strength and room temperature ductility.

(8) Gamma TiAl intermetallic compositions of this invention can be processed according to a procedure represented in FIG. 1. As a nonlimiting example, following the production of a casting of the gamma TiAl intermetallic composition, a pre-HIP heat treatment can be performed at a temperature within a range of about 1800 to about 2000 F. (about 980 to about 1090 C.) for a duration of about five to twelve hours. Thereafter, the casting is cooled and transferred to a HIP chamber and then subjected to a high pressure HIP step (for example, 25 ksi (about 1720 bar) or more) at about 2165 F. for a duration of about three hours. The HIPed casting is then cooled, removed from the HIP chamber, and then subjected to a post-HIP solution treatment at a temperature of about 2200 F. for a duration of about two hours. While such a process is believed to be acceptable, a more preferable process is believed to be disclosed in U.S. Patent Application Ser. No. 61/614,751 filed Mar. 23, 2012, whose contents are incorporated herein by reference. The preferred process is particularly adapted to yield castings formed of gamma titanium aluminide intermetallic compositions that exhibit a desirable duplex microstructure containing equiaxed and lamellar morphologies that promote the ductility of the casting.

(9) While the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.