Interfacial structure and growth mechanisms of lath-shaped precipitates in Ni-45 wt% Cr

The interfacial structure of bcc laths precipitated from a nickel-rich fee matrix were studied using transmission electron microscopy and high-resolution electron microscopy (HREM). 50 precipitates and more than 200 defects in their vicinity were analysed. HREM images indicate the parallel conjugate planes of the Kurdjumov-Sachs orientation relationship are continuous across the broad face of the laths. In agreement with earlier studies, the (121)f habit plane contains structural ledges, irregularly spaced dislocations but no regular array of misfit dislocations. 87% of dislocations in the habit plane have Burgers vectors in the conjugate plane. These dislocations are found in the risers of growth ledges and must climb as the ledges migrate. The dislocations accommodate misfit in the conjugate planes of the orientation relationship while misfit perpendicular to these planes is compensated by elastic strain. Stacking faults found extending into the matrix from the precipitate habit plane accommodate elastic strain and appear to aid the formation of growth ledges. On the side facet of the precipitate lath, two types of linear defect are found: misfit dislocations spaced 12nm apart with -5 [111]f Burgers vectors, and structural ledges parallel to the misfit dislocations spaced 1.5 nm apart. The misfit dislocations are in a sessile orientation with respect to growth of the side facet.