Niobium-silicide based composites resistant to high temperature oxidation

Abstract: A niobium-silicide refractory metal intermetallic composite adapted for use in a turbine component comprises: between about 19 atomic percent and about 24 atomic percent titanium; between about 1 atomic percent and about 5 atomic percent hafnium; up to about 7 atomic percent tantalum; between about 16 atomic percent and about 22 atomic percent silicon; up to about 6 atomic percent germanium; up to about 5 atomic percent boron; between about 7 atomic percent and about 14 atomic percent chromium; up to about 4 atomic percent iron; up to about 4 atomic percent aluminum; up to about 3 atomic percent tin; up to about 3 atomic percent tungsten; up to about 3 atomic percent molybdenum; and a balance of niobium. A ratio of the sum of atomic percentages of niobium and tantalum present in said niobium silicide refractory intermetallic composite to the sum of atomic percentages of titanium and of hafnium present in said niobium silicide refractory intermetallic composite has a value between about 1.4 and about 2.2. Chromium and iron together comprise between about 7 atomic percent and about 15 atomic percent of the niobium silicide refractory intermetallic composite, and silicon, germanium, and boron together comprise between about 16 atomic percent and about 22 atomic percent of the niobium silicide refractory intermetallic composite.

Description: The invention relates to Niobium (Nb) -silicide based composite compositions. In particular, the invention relates to Nb-silicide based composite compositions having properties that permit the Nb-silicide based composite compositions to find applications in turbine components. Turbines and their components (hereinafter "turbine components"), such as, but not limited to, aeronautical turbines, land-based, turbines, marine-based turbines, and the like, have typically been formed from nickel (Ni)-based materials, which are often referred to as Ni-based superalloys. Turbine components formed from these Ni-based superalloys exhibit desirable chemical and physical properties under the high temperature, high stress, and high-pressure conditions generally encountered during turbine operation. For example, turbine components, such as an airfoil, in modern jet engines can reach temperatures as high as about 1,150°C, which is about 85% of the melting temperatures (T m ) of most Ni-based superalloys. Because Ni-based superalloys have provided the level of performance desired in such applications, the development of suchNi-based superalloys has been widely explored. Consequently, the field has matured and few significant improvements have been realized in this area in recent years. In the meantime, efforts have been made to develop alternative turbine component materials. These alternate materials include niobium (Nb)-based refractory metal intermetallic composites (hereinafter "RMIC"s). Most RMICs have melting temperatures of about 1700°C. If RMICs can be used at about 80% of their melting temperatures, they will have potential use in applications in which the temperature exceeds the current service limit of Ni-based superalloys. RMICs comprising at least niobium (Nb), silicon (Si), titanium (Ti), hafnium (Hf), chromium (Cr), and aluminum (Al) have been proposed for turbine component applications. These silicide-based RMICs exhibit a high temperature capability that exceeds that of current Ni-based superalloys. Exemplary silicide-based RMICs are set forth in U.S. Patent 5,932,033, to M. R. Jackson and B.P. Bewlay, entitled "Silicide Composite with Nb-Based Metallic Phase and Si-Modified Laves-Type Phase" and U.S. Patent 5,942,055, to Jackson and Bewlay, entitled "Silicide Composite with Nb-Based Metallic Phase and Si-Modified Laves-Type Phase". Some known Nb-silicide based composites - including silicide-based RMIC materials - have adequate oxidation resistance characteristics for turbine component applications. These materials have compositions within the following approximate ranges: 20-25 atomic percent titanium (Ti), 1-5 atomic percent hafnium (Hf), and 0-2 atomic percent tantalum (Ta), where the concentration ratio (Nb+Ta):(Ti+Hf) has a value of about 1.4; 12-21 atomic percent silicon (Si), 2-6 atomic percent germanium (Ge), and 2-5 atomic percent boron (B), where the sum of the Si, B, and Ge concentrations is in the range between 22 atomic percent and 25 atomic percent; 12-14 atomic percent chromium (Cr) and 0-4 atomic percent iron (Fe), where the sum of the Fe and Cr concentrations is between 12 atomic percent and 18 atomic percent; 0-4 atomic percent aluminum (Al); 0-3 atomic percent tin (Sn); and 0-3 atomic percent tungsten (W). Other known Nb-based silicide composites - including silicide-based RMIC materials- possess adequate creep-rupture resistance for turbine component applications. These materials have compositions within the following approximate ranges: 16-20 atomic percent Ti, 1-5 atomic percent Hf, and 0-7 atomic percent Ta, where the concentration ratio (Nb+Ta):(Ti+Hf) has a value of about 2.25; 17-19 atomic percent Si, 0-6 atomic percent Ge, and 0-5 atomic percent B, where the sum of the Si, B, and Ge concentrations is in the range between 17 atomic percent and 21 atomic percent; 6-10 atomic percent Cr and 0-4 atomic percent Fe, where the sum of the Fe and Cr concentrations is in the range between 6 atomic percent and 12 atomic percent; 0-4 atomic percent Al; 0-3 atomic percent Sn; 0-3 atomic percent W; and 0-3 atomic percent Mo. In addition, some known Nb-silicide based composites - including silicide-based RMIC materials - have adequate fracture toughness for turbine component applications. Such materials contain greater than or equal to about 30 volume percent of metallic phases present in such components. Although the above Nb-silicide based composites and silicide-based RMIC materials possess beneficial mechanical and chemical properties, they do not adequately balance oxidation resistance properties with toughness and creep resistance properties. Thus, a single silicide-based RMIC alloy material composition that can provide adequate creep, oxidation resistance, and toughness for turbine component applications is currently not available. While the oxidation performance and creep-rupture resistance for turbine component applications of known RMICs are desirable, these materials and their properties may still be further improved for turbine component applications. For example, the chemistries and compositions of the RMIC material may be modified to enhance oxidation resistance for applications that subject the turbine component to high stresses at temperatures ranging from about 700°C to about 925°C (about 1300°F to about 1700°F) over extended periods of time. Therefore, what is needed is a material having a composition, chemistry, and properties that are suitable for various applications such as, but not limited to, turbine components, in which high stresses at elevated temperatures are encountered over long periods of time. More particularly, what is needed is a Nb-silicide based RMIC having improved oxidation resistance and creep resistance for use in turbine component applications in which high stresses at elevated temperatures are encountered over long periods of time. According to the invention there is provided a niobium silicide refractory intermetallic composite comprising: in atomic percent Titanium 19 to 24 Hafnium 1 to 5 Silicon 16 to 22 Chromium 7 to 14 Tantalum 0 to 7 Germanium 0 to 6 Boron 0 to 5 Iron 0 to 4 Aluminum 0 to 4 Tin 0 to 3 Tungsten 0 to 3 Molybdenum 0 to 3 Niobium Balance The invention also comprises a turbine component formed from such a niobium silicide refractory intermetallic composite The invention will now be described in greater detail, by way of example:- The niobium (Nb)-silicide based alloy composite of the present invention comprises Nb-silicide refractory metal intermetallic composites (hereinafter "RMICs") having compositions and chemical properties that provide the necessary balance among oxidation characteristics, creep resistance, and toughness. The Nb-silicide RMICs comprise between about 19 atomic percent and about 24 atomic percent titanium; between about 1 atomic percent and about 5 atomic percent hafnium; up to about 7 atomic percent tantalum; between about 16 atomic percent and about 22 atomic percent silicon; up to about 6 atomic percent germanium; up to about 5 atomic percent boron; between about 7 atomic percent and about 14 atomic percent chromium; up to about 4 atomic percent iron; up to about 4 atomic percent aluminum; up to about 3 atomic percent tin; up to about 3 atomic percent tungsten; up to about 3 atomic percent molybdenum; and a balance of niobium, wherein a ratio of a sum atomic percentages of niobium and tantalum present in the niobium silicide refractory intermetallic composite to a sum of atomic percentages of titanium hafnium present in the niobium silicide refractory intermetallic composite has a value of between about 1.4 and about 2.2 (i.e., 1.4<(Nb+Ta):(Ti+Hf)<2.2) , wherein chromium and iron together comprise between about 7 atomic percent and about 15 atomic percent of the niobium silicide refractory intermetallic composite; and wherein silicon, germanium, and boron together comprise between about 16 atomic percent and about 22 atomic percent of the niobium silicide refractory intermetallic composite. The atomic percent given for each element are approximate unless otherwise specified. The Nb-silicide based RMICs of the present invention exhibit oxidation and rupture resistance characteristics which are provided by the addition of Si, Cr and Al, and lesser amounts of Ti, Hf, and B. The Nb-silicide RMICs disclosed in the present invention can be used to form turbine components such as, but not limited to, buckets, blades, rotors, nozzles, and the like for applications in land-based turbines, marine turbines, aeronautical turbines, power generation turbines, and the like. Formation of a hexagonal M 5 Si 3 silicide (where M is titanium, hafnium, or combinations thereof), which has been found to be detrimental to creep resistance, is aided when the (Nb+Ta):(Ti+Hf) ratio of the Nb-silicide RMIC has a value of less than 1.5. Values for the ratio (Nb+Ta):(Ti+Hf) are reported in Table 1. Samples A2, A4, A21, A22, A24, A25, and A26, which represent the preferred Nb-silicide RMIC compositions of the present invention, exhibited radius changes due to oxidation of less than or equal to about 140 microns (about 5.5 mils) and less than or equal about 810 microns (about 32 mils) after 100 hours at temperatures in the range from about 982°C to about 1204 °C( about 1800°F to about 2200°F), and after about 73 hours at a temperature of about 1316 °C (about 2400°F), respectively. A weight change occurring with little change in radius in a refractory material alloy is indicative of oxidation attack at the ends of the sample. This type of oxidation leads to rounding of the sample edges, even though the degree of radial attack on the pin is small. The Nb-silicide RMICs of the present invention include constituents that reduce the oxide growth rate in Nb-silicide RMICs containing additional metallic and Laves-type phases. In the present invention, Laves-type phases preferably comprise up to about 20 volume percent of the Nb-silicide RMICs. Metallic phases preferably comprise at least 25 volume percent of the Nb-silicide RMICs. Maintaining a relatively high level of titanium in the Nb-silicide RMIC improves material performance. Increasing the titanium concentration, however, tends to increase creep rate. Thus, the composition of the Nb-based RMICs must be adjusted to balance desired creep rupture requirements with the reduced oxidation rate. Example 1: A series of Nb-silicide RMIC samples, as embodied by the present invention, were prepared by arc casting tapered disks about 2.03 cm (about 0.8") thick and with a diameter tapering from about 6.35 cm to about 7.62 cm (about 2.5" to about 3"). Pins having a diameter of about 0.30 cm (about 0.12") and a length of about 3.175 cm (about 1.25") were prepared by conventional machining processes, such as EDM and centerless grinding. The pins were then subjected to 100 hours exposure (hot time) with a total test exposure of about 117 hours in one-hour cycles. The heating cycles were followed with cooling to room temperature after each hour of hot time at either 982 °C (1800°F), 1095°C (2000°F), 1205 °C (2200°F), or 1315 °C (2400°F). Each Nb-silicide RMIC sample was weighed before testing, periodically during the tests, and after testing to determine an average weight change per unit area as a function of exposure time. Each sample was then cut at its approximate mid- section and prepared for metallographic evaluation of changes in diameter and in microstructure. Evaluation of the samples was not necessarily limited to a metallographic examination. Results of the weight change (listed in columns labeled 'wt') and metallographic measurements of diameter changes (listed in columns labeled 'mil') obtained at completion of the test are listed as a function of alloy composition for a series of Nb-silicide RMICs in Table 1. The atomic percentages listed in Table 1 are approximate. Values for the (Nb+Ta):(Ti+Hf) ratio are also provided. As can be seen from Table 1, the addition of boron and iron to the Nb-silicide RMICs of the present invention provide improves oxidation resistance over the temperature range of the tests. In addition, partial substitution of germanium for silicon in the Nb-silicide RMICs also improves oxidation resistance over the temperature range tested. Oxidation resistance of the Nb-silicide RMICs of the present invention over the temperature range tested is not degraded by the addition of either about 6 atomic percent tantalum or about 1 atomic percent tungsten. The presence of about 1 atomic percent molybdenum had a negative effect on the oxidation resistance of the Nb-silicide RMICs, and additions of about 1.5 atomic percent tin had a neutral effect on the high temperature oxidation resistance of the Nb-silicide RMICs.

Claims: A niobium silicide refractory intermetallic composite comprising: in atomic percent Titanium 19 to 24 Hafnium 1 to 5 Silicon 16 to 22 Chromium 7 to 14 Tantalum 0 to 7 Germanium 0 to 6 Boron 0 to 5 Iron 0 to 4 Aluminum 0 to 4 Tin 0 to 3 Tungsten 0 to 3 Molybdenum 0 to 3 Niobium Balance The niobium silicide refractory intermetallic composite of Claim 1, wherein a ratio of a sum of atomic percentages of niobium and tantalum present in said niobium silicide refractory intermetallic composite to a sum of atomic percentages of titanium and hafnium present in said niobium silicide refractory intermetallic composite has a value between about 1.4 and about 2.2; chromium and iron together comprise between about 7 atomic percent and about 15 atomic percent of said niobium silicide refractory intermetallic composite; and silicon, germanium, and boron together comprise between about 16 atomic percent and about 22 atomic percent of said niobium silicide refractory intermetallic composite. The niobium silicide refractory intermetallic composite of Claim 2, wherein said niobium silicide refractory intermetallic composite includes at least one metallic phase, said metallic phase comprising at least 30 volume percent of said niobium silicide refractory intermetallic composite. The niobium silicide refractory intermetallic composite of Claim 2, wherein said niobium silicide refractory intermetallic composite includes at least one Laves phase, said Laves phase comprising up to about 20 volume percent of said niobium silicide refractory intermetallic composite. A turbine component made of the niobium silicide refractory intermetallic composite of Claim 1. A turbine component made of the niobium silicide refractory intermetallic composite of Claim 2. The turbine component of claim 6, wherein said turbine component is a component selected from the group consisting of a bucket, a blade, a rotor, and a nozzle.

Inventors: BEWLAY BERNARD PATRICK, JACKSON MELVIN ROBERT, ZHAO JI-CHENG, BEWLAY, BERNARD PATRICK, JACKSON, MELVIN ROBERT, ZHAO, JI-CHENG
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 2001
Main classification: C22C29
IPC codes: F01D5, C22C29, C22C27