Precipitation strengthening and mechanical behavior of advanced superalloy

Abstract

Inconel 740, a relatively new member of Ni-based superalloy family, has been developed for the applications to steam boiler tubing in ‥Advanced Ultra-Supercritical (A-USC)… fossil power plants. Since the target steam condition of the new boiler was 375 bar and 700 �aC, this alloy was designed to have excellent performances at 700-750 �aC; i.e., good microstructure stability, high resistance to coal ash corrosion and oxidation, and high creep-rupture strength. Like many other Ni-based superalloys, Inconel 740 is primarily strengthened by the precipitation of very fine gamma prime (�^�S) (having an ordered face-centered cubic structure) in the disordered gamma (�^) matrix. However, limited studies have been performed on the detailed relation between precipitation strengthening and mechanical performance in this new superalloy. With this in mind, here I have attempted to analyze the influences of (1) thermo-mechanical treatment and (2) isothermal aging on the relation. First, the influences of thermo-mechanical treatment on the precipitation behavior and its strengthening in Inconel 740 were investigated using the specimens on which a different level of stresses were applied at 700 �aC. Microstructural analysis revealed that, with increasing applied stress, the fraction of gamma prime precipitates continuously increased but their size was not significantly changed and remained in the range of 20-70 nm, which could be explained by considering the possible effects of the stresses on the Gibbs free energy barrier for heterogeneous nucleation. Measurement of nanoindentation hardness showed that there was no strength degradation by the thermo-mechanical treatment. With these results, the role of precipitates in the hardness variation with the stress was successfully analyzed in terms of the governing precipitation strengthening mechanisms. Second, the influence of isothermal aging on the microstructures and hardness in Inconel 740 was explored using the specimens aged at 810 �aC for different times. As aging time increased, the size of gamma prime precipitates continuously increased while their fraction remained almost constant. Nanoindentation experiments revealed that the overall hardness increased till the aging time of 100 h and then decreased with the aging time. Estimation of phase hardness by applying a simple rule-of-mixture showed that, with aging time, the hardness of gamma matrix decreased and that of gamma prime precipitates increased. Based on the above results, the change in mechanical behavior with aging was discussed in terms of the solid solution and precipitation strengthening effect.