Compositionally graded nano-sized borides in a directionally solidified nickel-base superalloy

Abstract Boron is known to influence the grain boundary microchemistry and thereby improve the mechanical performance of superalloys; however, the form in which boron exists at the grain boundary is debatable. We show from electron microscopy characterization that in a directionally solidified Ni-base superalloy boron is present in the form of nearly spherical nano-sized metal borides. We also report the existence of a gradient in composition of Cr, and W within the nano-sized boride precipitates located along the grain boundaries. These borides are present discretely and suppress the agglomeration of mostly reported M23C6 carbides at the columnar grain boundaries. This could be a possible reason for the reported improvement in transverse creep properties of boron modified directionally solidified superalloys.