The effect of Fe addition on γ′ precipitates during thermal exposure in Ni-Co-based single crystal superalloys
Wenchao Yang, Huajing Zong, Yang Chen, Pengfei Qu, Haijun Su, Yin Dong, Jun Zhang, Lin Liu
Abstract
The effect of Fe addition on γ′ precipitates in Ni–20Co-xFe-12Cr-7.5Al-7.5Ti-1.5 W (where x = 0 or 4 at.%) single crystal superalloys during thermal exposure process was systematically studied. The results showed that the alloys presented a good microstructure stability and no any harmful TCP phases were formed in the long-term aged microstructures (such as, 1000 °C for 1000 h). And, the Fe addition had an obvious influence on the volume fraction of γ′ precipitates, γ/γ′ lattice misfit, and the morphology of γ′ precipitates. The higher Fe content enabled the γ matrix to contain more Al and Ti elements, resulting in a lower volume fraction of γ′ precipitates. Simultaneously, the higher content of Al and Ti in γ matrix enlarged the lattice constant of γ matrix and decreased the γ/γ′ lattice misfit from 0.30% to 0.11%, which further induced the morphology of γ′ precipitates changing from the cuboidal shape to the spheroidal shape. Furthermore, the coarsening behavior of γ′ precipitates was analyzed and a corresponding mechanism was proposed that the alloy with Fe addition had a higher coarsening rate of γ′ precipitates during thermal exposure process compared to the alloy without Fe addition, which might result from a lower diffusion activation energy and a larger γ/γ′ interfacial energy.