Influence of Trace Amounts of Sulfur on the Microstructure and Mechanical Properties of Directionally Solidified Ni-Based Superalloy GTD-111

Abstract

The influence of trace amounts of sulfur (S) on the microstructural evolution, tensile and creep properties in directionally solidified (DS) Ni-based superalloy GTD-111 were systematically investigated. Doping of S in the range of < 1 to 154 ppm resulted in the formation of a Ti2SC-type carbosulfide phase near the coarse eutectic γ/γ and η (eta, Ni3Ti) phases in interdendritic regions and grain boundaries (GB). The morphology of the Ti2SC was found to change from discrete particles to curved and elongated (film-like) shapes with increasing S content. The measurement of GB line fractions revealed that the GB fraction of Ti2SC increases with S content and reaches about 20% in 154 ppm S alloy. Tensile test results showed that the tensile properties at room temperature were not influenced by S doping, while the tensile elongation at 650-980 oC significantly decreased with increasing S content. The creep life and rupture elongation were also found to decrease with S content. The normalized creep life of 154 ppm S alloy was only in the 69%-74% range, compared to that of < 1 ppm S alloy. Careful microstructural observation of the fracture surface and longitudinal section of the creep ruptured 154 ppm S alloy revealed that Ti2SC plays a significant role in crack formation at the matrix γ/ Ti2SC interface, and leads to brittle facet-like traces on the fracture surface. It was concluded from these results that the formation of film-like Ti2SC in high S alloys might be responsible for the reduction in creep life and rupture elongation of DS GTD-111 alloy.