INTERFACIAL STRUCTURE AND GROWTH MECHANISMS OF LATH-SHAPED PRECIPITATES IN NI-45 WT-PERCENT CR

The interfacial structure of bcc laths precipitated from a nickel-rich fee matrix were studied using transmission electron microscopy and hi gh-resolution electron microscopy (HREM). 50 precipitates and more tha n 200 defects in their vicinity were analysed. HREM images indicate th e parallel conjugate planes of the Kurdjumov-Sachs orientation relatio nship are continuous across the broad face of the laths. In agreement with earlier studies, the (1 (2) over bar 1)(f) habit plane contains s tructural ledges, irregularly spaced dislocations but no regular array of misfit dislocations. 87% of dislocations in the habit plane have B urgers 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 orient ation relationship while misfit perpendicular to these planes is compe nsated by elastic strain. Stacking faults found extending into the mat rix from the precipitate habit plane accommodate elastic strain and ap pear to aid the formation of growth ledges. On the side facet of the p recipitate lath, two types of linear defect are found misfit dislocati ons spaced 12 nm apart with 1/3[1 (1) over bar 1](f) Burgers vectors, and structural ledges parallel to the misfit dislo$iions spaced 1.5 nm apart. The misfit dislocations are in a sessile orientation with resp ect to growth of the side facet.