Anisotropic Growth of the Primary Dendrite Arms in a Single‐Crystal Thin‐Walled Nickel‐Based Superalloy

Abstract

The influence of the withdrawal rate and wall thickness on the primary and secondary dendrite arm spacings of single‐crystal specimens manufactured by the Bridgman process is investigated. Thin‐walled specimens with wall thicknesses of 0.4, 0.8, 1, and 2 mm are compared with conventional cast single‐crystal cylindrical specimens with a diameter of 15 mm of the nickel‐based superalloy MAR M247LC. It is found that the average primary dendrite arm spacing decreases slightly with decreasing wall thickness, whereas the secondary dendrite arm spacing is relatively independent of the cast wall thickness. The slight decrease in primary dendrite arm spacing relates to an ≈ 0.2 mm wide boundary zone next to the ceramic mold/cast metal interface that is not only present in the thin‐walled specimens, but also within cylindrical single‐crystal specimens. Within this boundary zone, the primary dendrite arm spacing is anisotropic. While the spacing of two dendritic cores sitting aside each other (y‐direction) remains at the bulk value, the spacing of two dendritic cores sitting in line with the interface normal (x‐direction) is significantly reduced.