Precipitation and strengthening behavior of η phase in polycrystalline Ni-based superalloy

Abstract

η phase has the potential to strengthen the alloy at high temperatures due to their stability compared to γ′ phase. Recently, the role of η phase with needle-like shape in creep resistance is insufficient studied. Moreover, the common effect of Ti/Al ratio and C content on microstructure and creep resistance in Ni-based superalloys is rarely investigated. The microstructure, particularly η phase precipitation and its role in 815 °C/379 MPa creep property for polycrystalline Ni-based superalloy were investigated by changing Ti/Al ratio and the C content in this research. The spherical γ′ phase evenly distributes within γ matrix of low Ti/Al ratio alloy. The size and volume fraction of γ′ phase slightly decrease in high Ti/Al ratio alloy, and η phase precipitates in the alloy. The creep life of the alloy has increased by 46.4 %. The crystallographic relationships are observed as (3 $\bar{3}\bar{3}$)M₂₃C₆//(004)η and (1 $\bar{1}\bar{1}$)γ′//(004)η, with the lattice misfits of 1.27 ± 0.05 % and 0.28 ± 0.03 %, respectively. The increased C content results in a reduction of η phase and a slight decrease in creep life of high Ti/Al ratio and C content alloy. γ′ phase impedes dislocation motion through coherency strengthening, order strengthening and Orowan bypass mechanisms. Additionally, the bent η phase occurs during creep deformation and coordinately deforms with γ matrix, γ′ phase layer could impede dislocation motion and buffer stress fluctuation to a certain extent. The specific crystallographic relationship between M₂₃C₆ carbide and η phase leads to the localized strengthening. The stress increment induced by η phase is decreased from 242.8 MPa to 52.2 MPa with the volume fraction of η phase decreasing from 0.98 ± 0.05 % to 0.21 ± 0.04 %.