Effect of chromium content on microstructural evolution of CoNiAlW superalloy

Effects of chromium (Cr) content on types and morphologies of various phases of a newly developed Co-Ni-Al-W superalloy (Co-30Ni-6W-9Al-3Ti-1Ta) have been investigated in this study. Microstructural examinations were carried out by field-emission scanning electron microscope (FESEM), electron backscatter diffraction (EBSD), high-resolution transmission electron microscope (HRTEM), and differential thermal analysis (DTA). Round-cornered cubic and cuboidal gamma' was developed in these new Co-based alloys after solutionizing at 1300 degrees C for 24 h and aging them at 900 degrees C for 96h. Addition of 6 at.% Cr caused the formation of a new phase called mu with a stoichiometric formula Co7W6 within the gamma matrix. Increasing the Cr content to 10 at.%, the volume fraction of mu phase increased from 0.5 to 3.7%. Further increase in Cr content to 14 at.% caused the formation of another phase, beta, with a stoichiometric formula CoAl within the matrix. In addition, the presence of Cr significantly influenced the gamma' morphology and altered it from cuboidal to round corner cuboidal gamma'. This was attributed to the reduction of gamma/gamma' lattice misfit due to Cr addition. The formation of mu-Co7W6 phase has a pronounced effect on the solvus temperature. Both melting and gamma' solvus temperatures reduced due to the partitioning of elements such as W to mu phase by increasing Cr-content. A Kurdjumov-Sachs orientation relationship was also established between the gamma/gamma' phases and beta. Simulation of pole figures shows that diffusive transformations can have lattice distortive components, which is expected for martensitic transformations.