Effect of Ta on microstructural evolution and mechanical properties of a solid-solution strengthening cast Ni-based alloy during long-term thermal exposure at 700 °C

Abstract

Effect of Ta on microstructural evolution and mechanical properties of a solid-solution strengthening cast Ni-based superalloy, IN617B alloy, considered as a potential candidate for castings in 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants, has been studied in this paper. Detailed observation reveals the alloys with different contents of Ta have similar solution treated microstructure. During long-term thermal exposure at 700 °C, Ta addition refines γ′ particles and enhances the thermal stability of γ′ phase by decreasing the γ-γ′ lattice misfit. With increasing Ta content the solubility of Mo in matrix decreases, which promotes the transformation of film-like M23C6 at grain boundaries (GBs) into blocky M6C carbide with the reaction of M23C6→−Cr+Mo,TaM6C. Consequently, the morphology of GB carbide changes from continuous film structure to dispersed block structure. In addition, local concentration of metal elements leads to the precipitation of trace amount of σ phase which is not affected by Ta concentration. Nearly all the tensile properties at 700 °C of the alloy after long-term thermal exposure are improved noticeably with the increase of Ta content. But, the plasticity of the alloy with 2.0 wt. % Ta addition decreases remarkably due to the excessive Ta content leading to severe M6C coarsening at GBs. The alloy achieves optimal mechanical properties with about 1.0 wt. % Ta content.