Study of precipitation-assisted stress relaxation and creep behavior during the ageing of a nickel-iron superalloy

Abstract

To study the stress relaxation and creep behavior of a nickel-iron base superalloy during the ageing process, solution annealed microstructures were subjected to loading experiments at various temperatures. Neutron diffraction (ND) measurements were carried out during the loading process to record the elastic lattice strain response of the γ, γ″ and γ′ phases. The results show that over time, increased length contraction occurred at all three temperatures and the sample tested at 720 °C experienced the most significant shrinkage. During the precipitation of the γ″ phase, the chemical composition of the γ matrix will change as the niobium atoms migrate to the form γ″ precipitates, which directly contributes to a change in the lattice parameter of the matrix and governs the samples' volume shrinkage. The creep strain at 720 °C was positive, while negative creep could be seen at 500 °C and 620 °C. The observed negative creep at 500 °C and 620 °C was caused by the samples' shrinkage, and the creep strain at 720 °C was positive because creep plastic strain is relatively large and thus dominates the measured creep deformation. The stress relaxation results exhibited a significant increase with the increase in test duration at 500 °C, 620 °C and experienced a normal decrease at 720 °C. Stress increase during the relaxation tests is a manifestation of the phenomenon of “negative creep”. The precipitation induced volume shrinkage was found to be the key reason for the observed negative creep and abnormal stress relaxation.