Microstructure, phase stability and element partitioning of γ-γ′ Co-9Al-9W-2X alloys in different annealing conditions

Abstract

The phase stability, γ′ precipitate morphology and element partitioning behavior in dependence on temperature and alloying composition have been systematically investigated in Co-9Al-9W-2X alloys using electron microscopy, energy-dispersive X-ray spectroscopy (EDX) and high-energy X-ray diffraction (HEXRD). The results show that the decomposition of the γ′ phase is promoted by adding some quaternary elements which increase the γ′ solvus temperature (except V). A larger lattice misfit in the Mo, V, Ti, Nb or Ta containing alloys can explain that coarser γ′ particles with a more cuboidal morphology were found in these alloys. Additional alloying elements and the annealing temperature show a stronger influence on the partitioning behavior of W between the γ and γ′ phases compared to that of Co and Al. The partitioning tendency towards the γ′ phase decreases with increasing temperature for all investigated alloying elements except Cr.