Oxidation-induced recrystallization and damage mechanism of a Ni-based single-crystal superalloy during creep

Abstract

The creep tests for Ni-based single-crystal superalloy are carried out under 980 °C, and the microstructure at the crack tip is characterized at micro- and nano- scales. Numerous γ’ phase dissolution, nucleation and growth of fine grains are observed near the crack. In addition, γ’ phase did not effectively hinder the nucleation and growth of recrystallization grains at the oxidation-affected zone. Instead, recrystallization promoted the degradation of the γ’ phase to dissolve along the recrystallized grain boundaries. Combining the sub-grain boundary growth model proposed by Winning et al. and the discontinuous recrystallization model suggested by D.G. Cram, a recrystallization and oxidative damage model for Ni-based single-crystal superalloy is proposed. • Local polycrystallization or recrystallization will greatly damage the high-temperature properties of the alloy. From the point of view of Ni-based single-crystal superalloy design, polycrystallization during medium-high temperature creep is very rare and should be strictly avoided. • Many researches have paid attention to the recrystallization of polycrystalline Ni-based superalloys. However, there is no report about the appearance of polycrystallization region in Ni-based single crystal superalloys during creep. Additionally, systematic research and explanation on the mechanism of discontinuous recrystallization of Ni-based SC superalloy are also very lacking. • In this paper, it is found for the first time that in the process of medium-high temperature (980 °C) creep under low stress level, polycrystallization region and a large amount of γ’ phase dissolve appeared at the crack tip, which leads to a sharp decline in the properties of the alloy. Researchers believed that the γ’ phase is the main factor that hinder the recrystallization of Ni-based single-crystal alloy. Nevertheless, this paper propose that the local strain and depletion of solute atoms makes the sub-grain boundaries tend to become the nucleation of recrystallization.