The Local Compositional Order and Dislocation Structure of Grain Boundaries in Ni3Al

Abstract

The local compositional order and dislocation structure of grain boundaries in Ni3Al, with and without boron, were examined using electron microscopy techniques. Lattice imaging studies showed that small angle twist, tilt and mixed boundaries and large angle (near Σ = 5) [001] twist boundaries are ordered up to very close to the interface plane. A compositionally disordered region ~1.5 nm thick is present in the vicinity of a large angle general boundary in boron-doped Ni3Al, while similar boundaries in boron-free material are ordered. Image simulations were performed and it was shown that the experimental observations of a locally disordered region cannot be explained as being an artifact. Dislocations with Burgers vectors that correspond to anti-phase boundary (APB)-coupled superpartials were found in small angle [001] twist boundaries in both boron-free and boron-doped Ni3Al and a small angle [011] tilt boundary in boron-doped Ni3Al. The APB energies determined from the dissociation of the boundary dislocations were smaller than reported for bulk Ni3Al. For small angle twist boundaries the presence of boron reduced the APB energy at the interface until it approached zero.