Boride particles in a powder metallurgy superalloy

Abstract

Using optical and electron metallography, the composition, morphology, and distribution of M₃B₂ borides in as–hipped (hot isostatically pressed) samples of the powder metallurgy superalloy Nimonic AP1 have been determined. Two types of boride are present depending on the HIP temperature. Hipping below the boride solvus results in low–aspect ratio particles, distributed both inter– and intragranularly. Hipping above the boride solvus produces high–aspect ratio particles which are exclusively intergranular. A small difference in both lattice parameter and composition has been measured. Electron energy loss spectroscopy of the particles has confirmed the presence of boron, and laser ion–induced mass analysis has indicated a low carbon level. The higher susceptibility to edge cracking during forging of material hipped above the boride solvus is related to the boride morphology. Studies of the subsequent recrystallization of the forged samples have indicated that necklace formation is neither inhibited nor accelerated by the presence of grain boundary borides. Intragranular boride particles have a similar influence to cuboidal γ′ in controlling the subgrain size within the warm worked grains.

MST/210