Complete coarse γ′ and greater than 90% eutectic γ-γ′ solutioning, without incipient melting, is demonstrated for the DS CM 247 LC superalloy. This unusual capability for this advanced Ni-base turbine blade and vane material results in considerable mechanical properties enhancement, with the DS alloy capability being near to current single crystal superalloys in the 345–207 MPa, 871°C–982°C (50–30 ksi, 1600°F–1800°F) operating condition. Microstructural features are detailed correlating strength and alloy stability.
Two complementary single crystal alloys have been developed from the MAR-M-247 composition, with the objectives of providing high creep-rupture strength, excellent oxidation resistance, good castability, practical solution heat-treatment ranges, high incipient melting points, and stable microstructures. The alloys, CM SX-2 and CM SX-3, are turbine blade and vane alloys, with CM SX-3 showing improved coated oxidation and corrosion resistance. Foundry performance characteristics studied using ten different single crystal casting processes to produce both solid and complex cored, thin-wall turbine blade and vane components were: “freckling” sensitivity, spurious grain formation, microporosity, and alloy/ceramic core reactions. Practical solution heat-treatment ranges (difference between the γ′ solvus and the incipient melting temperatures) have been established and vary from 45–50°F for CM SX-3 and 50–55°F for CM SX-2 measured without prior homogenization treatments. Extensive machined-from-blade (MFB) mechanical property work is reported. Alloy stability investigations were undertaken using prior tested MFB stress-rupture specimens. Environmental evaluations using both bare and coated single crystal specimens, subjected to separate cyclic/dynamic oxidation, and corrosion testing in burner-type rigs are also reviewed. A new γ′ microstructure/heat-treatment technology has been found to be particularly applicable to CM SX-2 and CM SX-3 alloys, because of their low γ/γ′ mismatch and suitable γ′ chemistry. This technology further increases the creep-rupture capability of both alloys by 10–40°F, depending on test temperature.
several derivative alloys at 103.5 MPa (15 KSI) and 1093'C (2000'F). Rupture lives of the modified SC alloys were significantly affected by both alloy purity and heat treatment. Critical aspects of vacuum induction refining, exothermic casting technology, alloy development and heat treatment, which contributed to this new class of turbine blades, are reviewed.
Directionally solidified (DS) and single-crystal (SX) superalloys and process technology are contributing to significant advances in turbine engine efficiency and durability. These gains are expected to arise from the development of higher creep strength and improved oxidation-resistant SX alloy compositions as well as from the development of SX casting and fabrication technology to utilize advanced transpiration-cooling schemes. This article provides a detailed discussion on the chemistry and castability of first- and second-generation DS and SX superalloys. It summarizes the chemistry modifications applied to MAR-M 247 to develop CMSX-2 with respect to function and objectives. The article also lists the nominal compositions of first- and second-generation DS and SX superalloys.
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