Low Cycle Fatigue/Corrosion Interactions at 950 °C of AM1 Single Crystal Nickel-Based Superalloy

Abstract

Current nickel-based single crystal superalloys (SX) are mainly designed to increase significant mechanical loading at high temperatures. Therefore, the mechanical resistance is greatly dependant on the microstructure and on the potential metallurgical defects. Corrosion and oxidation at high temperatures may further induce a loss of load-bearing section and lower the overall mechanical performance of such single crystals. While the yet complex mechanical and corrosion mechanisms are relatively well established separately, little is known on their combined effects, let alone on as-cast (AC) versus fully heat-treated (FHT) microstructures. This paper shows that the low cycle fatigue (LCF) at 0.5 Hz, R_σ = 0.05 and 950 °C is lowered when the AM1 nickel-based single crystal superalloy is pre-corroded with 1 mg/cm² Na₂SO₄ at 950 °C. The degradation increases with increasing pre-corrosion time due to the formation of a porous, brittle corrosion layer that favours the number of crack initiation sites, which are subsequently assisted by hot corrosion and oxidation. In addition, AM1 FHT shows better LCF fatigue resistance than AM1 AC, due to a better creep resistance of the FHT microstructure under these conditions.