The Application of Grain Boundary Engineering to a Nickel Base Superalloy for 973 K (700 °C) USC Power Plants

Abstract

The microstructure of a Ni-base superalloy Inconel740H designed for the ultra-supercritical coal-fired power plants has been enhanced via grain boundary engineering. Single-step thermomechanical processing treatments were carried out to optimize the grain boundary character distribution (GBCD) and assessed based on the fraction of low-Σ (Σ ≤ 29) coincidence site lattice (CSL) boundaries, f csl, as well as on the interrupted high-angle grain boundary (HAGB) network. Solution-annealed samples were compressed by 3, 6, 10, and 15 pct at room temperature followed by annealing at 1373 K (1100 °C) for between 5 and 40 minutes. For samples deformed to strains of values of less than 10 pct, deformation-induced grain boundary migration occurs and high values of f csl, with large cluster sizes, are obtained. For samples deformed to strains larger than 10 pct, static recrystallization dominates, resulting in decreased value of f csl. The highest f csl value (≈80 pct) was obtained in the sample annealed at 1373 K (1100 °C) for 20 minutes after 6 pct cold deformation, in which the HAGB network was substantially interrupted. The triple junction distributions of samples before and after GBE were also studied. The introduction of large amounts of CSLBs after thermomechanical-processing treatment increased both fractions of J2 and J3 type junctions (triple junctions containing 2 or 3 CSL boundaries), therefore leading to a significant increase in the resistant triple junction fraction, defined as f J2 /(1 − f J3 ). In addition, the thermal stability of the GBCD-optimized microstructure was confirmed to be stable at 1023 K (750 °C) for 500 hours without significant decrease in f csl.