Effects of rhenium and high-angle grain boundaries upon the elemental distribution and microstructure of Ni-based single-crystal superalloys

The effects of rhenium (Re) and high-angle grain boundaries upon the elemental distribution and microstructure of as-cast Ni-based single-crystal superalloy blades are investigated herein. The addition of Re reduces the coarsening of the gamma dendrite and gamma' phases. Further, the degree of partitioning of the element Al in the gamma' phase decreases with Re addition, while that of the elements Ta, Ti, Ni, and Mo increases. The addition of Re greatly decreases the concentration gradient, and greatly increases the diffusion distance, of the alloying elements in the region close to the gamma/gamma' phase boundary. As the grain boundary angle increases from 35 degrees to 45 degrees, a Re-enriched sigma phase is directly precipitated from the gamma' phase at the grain boundary. In addition, both the amount of sigma phase and the concentration of Re in that phase increase with increasing grain boundary angle.