Influence of directional solidification variables on the cellular and primary dendrite arm spacings of PWA1484

A series of directional solidification experiments have been performed to elucidate the effects of thermal gradient G and growth velocity V on the solidification behavior and microstructural development of the multicomponent Ni-base superalloy PWA 1484. A range of aligned as-cast microstructures were exhibited by the alloy: (i) aligned dendrites with well developed secondary and tertiary arms; (ii) flanged cellular dendrites aligned with the growth direction and without secondary arms; and (iii) cells with no evidence of flanges or secondary arms. The role of the imposed process parameters on the primary arm spacings that developed in the Bridgman-grown samples were examined in terms of current theoretical models. The presence of secondary arms increases the spacings between dendrites and leads to a greater sensitivity of λ1 on G−1/2V−1/4. The exponent of V was analyzed and found to depend upon the imposed gradient G. High withdrawal velocities and low thermal gradients were found to cause radial non-uniformity of the primary dendrite arm spacing. Such behavior was associated with off-axis heat flows.