Development of the Single Crystal Alloys CM SX-2 and CM SX-3 for Advanced Technology Turbine Engines

Abstract

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.

Material Evaluation of Large Single Crystal and Directionally Solidified Bucket Castings for Advanced Land-Based Gas Turbines

“…The stress rupture data on hollow and solid buckets are summarized in Fig.6 of larson-miller type plot, comparing to the base line data where the tests were performed at 825 to 1050°c and maximum rupture times reached upto 3,000h in single crystal and 15,000h in directionally solidified castings. It is confirmed that the rupture strengths of machined-from-bucket specimens of hollow and solid buckets in both alloys well coincide with the base line data obtained from large slab specimens and they are as high as those of small castings reported previously (Harris et al, 1983) (Harris et al, 1984).…”

“…Authors have performed the development of large single crystal and directionally solidified bucket castings applicable for advanced land-based gas turbine of 1500°c class turbine inlet temperature, as well as advanced cooling technolgies for buckets and vanes (Matsuzaki et al, 1992). This paper presents the casting process study for the large single crystal buckets having complex serpentine internal cooling passages developed in this program and its application for large directionally solidified bucket with tip shroud, and the evaluation test results of long-term material properties including phase stability of bucket alloys, using CMSX-2 single crystal (Harris et al, 1983) and CM247LC directionally solidified alloys (Harris et al, 1984).…”

“…The long-term phase stabilty, especially at intermediate tempertures, is another important aspect in using single crystal and directionally solidified bucket alloys to land-based gas turbines, because typical brittle TCP phases such as sigma phase tend to be formed at such temperature range. The alloy chemistries of both CMSX-2 and CM247LC were controlled to be free from the TCP phases in alloy developments (Harris et al, 1983) (Harris et al, 1984). In this study, the phase stability of both alloys for the formation of TCP phases was examined on the long-term stress rupture specimens tested upto about 3,000 hours in CMSX-2 and about 15,000 hours in CM247LC at 825 to 1050°c.…”

New Advanced Cooling Technology and Material of the 1500°C Class Gas Turbine

“…15, where test specimens were sampled in longitudinal direction from various locations of slabs. Compared to typical conventional casting superalloy IN738LC and Mar-M247, both directionally solidified and single crystal slabs have much higher rupture strength which are almost the same levels as reported (Harris, K., 1983, Erickson, G.L. 1985.…”

“…Several steps of the bucket casting trial have been performed under cooperation with the casting supplier, in order to establish the production parameters for large size single crystal and directionally solidified buckets having complex serpentine internal cooling circuits of the 50 MW class and larger gas turbines. For bucket materials used in this study, CMS/C-2 single crystal (Harris et al, 1983) and CM247LC directional solidification superalloys (Erickson, 1985) were chosen because they have the highest level of creep and stress rupture strength among the same kind of superalloys. In these casting trials, various casting parameters have been adjusted, that is, o process parameters such as pouring temperature, mold temperature and withdrawal rate o core and mold materials selection o configurational modification for bucket producibility…”