Effect of forming element additions on the homogenization behavior and formation of hierarchical microstructures in Ni-based superalloys

Here, we study the homogenization behavior and microstructure of seven Ni-Al-Ti alloys with quaternary additions of gamma forming elements 4Cr, 4Co, 4Ru, 4Mo, 4Hf, 4 W and 2Re. To design a homogenization treatment, the as-cast microstructure is analyzed revealing the diffusion distances x between dendrite cores and interdendritic regions. The temperatures for homogenization are determined using differential scanning calorimetry (DSC) and Thermo-Calc simulations, to be between 1150 and 1275 degrees C. The time to achieve homogenization is modelled based on the residual segregation index delta utilizing diffusion distance, homogenization temperature and diffusion data. Electron probe micro analyzer (EPMA) measurements show that our predictions match for the 4Cr, 4Co, 4Ru, 4 W and 2Re alloys while the 4Hf alloy shows insufficient homogenization. Transmission electron microscopy (TEM) reveals a two-phase gamma/gamma' microstructure after 750 degrees C / 24 h, whereby the 4Co and 4Ru alloys form hierarchical microstructures. We observe gamma plates in the 4Co alloy and gamma spheres in the 4Ru alloy. Ru in the 4Ru alloy is involved in stabilizing the morphology of gamma spheres. We provide a straightforward method for the design of homogenization treatments of Ni-based superalloys and demonstrate an alloy design pathway for tailoring the phase stability of hierarchical microstructures.