Study on Heat Treatment Process, Microstructure and Properties of Low Re Second-generation Single Crystal Superalloy DD515

To address the high cost of rhenium in second-generation single-crystal superalloys, low-rhenium single- crystal alloy DD515 was selected as the experimental material with DD5 alloy as the control group. Single-crystal specimens were fabricated via directional solidification. Combined with optical microscopy, field-emission scanning electron microscopy, differential scanning calorimetry, high-temperature mechanical property tests and isothermal oxidation tests at 1100 ℃, the regulation law of heat treatment processes on the microstructure and properties of the DD515 alloy was investigated. Significant elemental segregation occurs in the as-cast DD515 alloy, whose eutectic content is approximately 2.9% higher than that of DD5 alloy. The initial melting temperature range of DD515 alloy is 1290 ℃-1295 ℃. As the solution temperature rises from 1285 ℃ to 1300 ℃ and the holding time extends from 2 h to 4 h, the volume fraction of residual eutectics in the alloy decreases, while the volume fraction of micropores increases correspondingly. The optimal heat treatment regime consists of solution treatment at 1295 ℃ for 2 h with air cooling, followed by two-stage aging at 1080 ℃ for 4 h(air cooling) and 900 ℃ for 4 h(air cooling). Under this treatment, the average size of the γ' phase is approximately 0.313 μm with a volume fraction ranging from 60% to 70%. The alloy exhibits tensile properties at 870 ℃ compare to DD5 alloy, achieves the maximum stress rupture life at 1093 ℃ under 158 MPa, and possesses oxidation resistance at 1100 ℃ equivalent to that of DD5 alloy. Considering the microstructure defects, process cost and service performance comprehensively, a single-step solution treatment at 1285 ℃-1295 ℃ for 2 h combined with two-stage aging is recommended for DD515 alloy. This work provides data support for the engineering application of the low-cost single-crystal superalloy.