During the production of single-crystal superalloy blades, a kind of channel-type defect, named “quasi-freckle”, was found on the casting surface, which is similar to typical freckles in macroscopic appearance but different in microstructure. In the as-cast microstructure of the quasi-freckle channels, the γ/γ’ eutectic is significantly accumulated and can be dissolved during the solution heat treatment. Since no disoriented grains were detected, the quasi-freckles have a basically identical crystal orientation with the matrix. The quasi-freckle channels actually appear as thermosolutal convection traces in the directional solidification process of single-crystal casting. Because the convection was not strong enough to break dendrite arms, the single-crystal consistency of the castings was not destroyed. However, with the deterioration of the solidification condition and the increase in solutal convection, quasi-freckles often develop into typical freckle defects.

Ni-W alloys are promising heterogeneous seed materials for directional solidification of Ni-based single-crystal superalloys. In this study, Ni-20 wt.% W (Ni-20W) seeds were used to initiate DD419 castings. Their solidification characteristics, melt-back behavior, interfacial microstructure, and orientation transfer were compared with those of DD419 homogeneous seeds. In two thermal cycles, Ni-20W had liquidus temperatures of 1492.6 and 1493.6 °C, with a critical nucleation undercooling of 14.5 °C in both cycles. DD419 had liquidus temperatures of 1388.8 and 1394.1 °C and undercoolings of 44 and 37 °C, respectively. Metallography and paired Laue measurements of one plate casting were consistent with orientation inheritance from Ni-20W to DD419. The software-reported differences in Theta and Alpha were 0.2° and 2.7°, respectively; the software-reported R value was 3.52. In the same-batch turbine-blade comparison, visible interfacial stray grains were observed on the examined starter-region surfaces of all six Ni-20W-seeded blades. No comparable features were observed on the examined surfaces of the three DD419 homogeneous-seeded blades. The maximum observed melt-back lengths were approximately 12 mm for Ni-20W and 45 mm for DD419. These observations indicate that orientation inheritance in the plate specimen did not ensure a defect-free starter region in the blade comparison. The two seed materials also showed markedly different melt-back behavior under the same nominal casting schedule.