Effect of Grain Boundary Morphology on the High Temperature Tensile Properties of Developed Ni-Based Superalloy (LESS)

In order to investigate the mechanical properties of the developed wrought γ'-hardened Ni-based superalloy, LESS alloy (Low Eta Sigma Superalloy), and to understand the deformation behavior, tensile tests were carried out from room temperature to 800oC in comparison with IN 740H alloy. Like other superalloys, the tensile properties of the LESS alloy were found to be dependent on temperature. As the temperature rose between room temperature and 700oC, the ultimate tensile strength (UTS) decreased slightly, and then decreased sharply. The yield strength (YS) of the LESS alloy was found to be about 40% higher than that of IN 740H alloy from room temperature to 700oC. On the basis of microstructure observations, the high yield strength of the LESS alloy is thought to result from the dense grain boundary, where the fine carbides (MC, M23C6) and the γ' phase form a continuous film structure, which more effectively suppressed dislocation movements during deformation. It was found that the deformation mode of the LESS alloy changed from γ'- shearing to mixed mode (γ'-shearing + climb) at around 700~800oC. Although the main fracture mode of the LESS alloy was ductile dimple fracture, elongation decreased with temperature due to the local brittle region caused by micro-twins.