Microstructure and High-Temperature Tensile Behavior of Ni-Based Superalloy EP741NP for Aerospace Applications

This paper presents the details on predicted and experimental microstructural evolution and high-temperature tensile behavior of a hot isostatically processed (HIPed) and heat-treated Ni-based superalloy EP741NP for aerospace applications. The phase prediction carried out using JMatPro® software from the composition of the alloy as a function of heat treatment has been found to agree with the results of characterization by scanning and transmission electron microscopy. Prediction has revealed the evolution of secondary and tertiary gamma prime (γ′) with phase fraction of ~46% during solutionization at 1483 K (1210 °C) for 8 h and ~12% during the course of aging treatment at 1144 K (871 °C) for 32 h, respectively. A uniform distribution of secondary γ′ with typical octocube morphology within a size range of 0.5–0.8 µm and a sparse distribution of tertiary γ′ of fine spherical particles with average size < 0.1 µm have been observed during study of microstructure. Electron backscattered diffraction (EBSD) studies showed epitaxial growth for γ′ phase within a grain with similar crystallographic orientation. Tensile tests conducted at room temperature and in the temperature range from 823 K to 1123 K (550 °C to 850 °C) have shown that yield strength (YS) and ultimate tensile strength (UTS) remain almost unchanged up to temperature 923 K (650 °C) due to microstructural stability of strengthening γ′ precipitate. Thereafter, it starts decreasing because tertiary precipitates cannot hinder grain boundary motion at high temperature, first because of their size and more importantly because they are usually dissolved back into the matrix at high temperature, which assist the annihilation and rearrangement of dislocations leading to reduction of dislocation density. Fractographic studies have revealed that the fracture mode is found to be mostly mixed in nature (both ductile and brittle fracture).