Interfacial structure of delta phase in Inconel 718 and the selection of precipitate habit planes
We investigated the structure and defects associated with interphase boundaries between a γ (fcc) matrix and plate-shaped precipitates of the δ (orthorhombic) phase in Inconel 718. Based upon transmission electron microscopy (TEM) observations, the average habit plane was confirmed to be (111)γ which is consistent with previous reports. A parallel array of misfit dislocations with Burgers vector b=1/6[112̅]γ, (designated M1) are always observed lying along the [11̅0] direction. Another array of misfit dislocations appears in some regions of the interface with Burgers vector b=1/6[21̅1̅]γ (designated M2). These dislocations also lie along the [11̅0] direction. Irregular ledges were identified on the interface and are believed to contribute to the thickening of δ plates. Dislocations in the matrix were also characterized. Most matrix dislocations have a 1/2[1̅01]γ Burgers vector. The growth ledges in the habit plane of a single δ plate have a variety of effective Burgers vectors. A geometric matching approach based upon near-coincident sites was employed to explain the interfaces structure of interphase boundaries in Inconel 718, as well as fdc/bcc in Ni-45wt% Cr. In both cases, the conjugate plane is the plane with the highest areal density of near-coincident sites over a small region while the average habit plane is determined by the continuity of near-coincidence sites over a large area. The M1 interfacial dislocations in the γ/δ interface accommodate misfit in the habit plane whereas M2 dislocations do not and are probably a by-product of the dissociation of matrix dislocations. In the fcc/bcc system, the habit plane is not parallel to the conjugate plane and the partial dislocations associated with matrix stacking faults improve matching in the habit plane even though their Burgers vector lies out of this plane.