Sliver defect formation in single crystal Ni-based superalloy castings

• Published in: Materials & design 2020-11, Vol.196, p.109138, Article 109138
• Main Authors: Xu, Wenliang, Wang, Fu, Ma, Dexin, Zhu, Xintao, Li, Dichen, Bührig-Polaczek, Andreas
• Format: Article
• Language: English
• Subjects: Dendritic solidification; Directional solidification; Single crystal Ni-based superalloys; Sliver defect
• ISSN: 0264-1275; 1873-4197
• DOI: 10.1016/j.matdes.2020.109138

Sliver is a typical grain defect in single crystal Ni-based superalloy castings produced by Bridgman directional solidification. However, its formation mechanism has yet to be fully understood. In this study, the formation of sliver defects was investigated with respect to the defect morphological features, the microstructure of defect initiation points, and the solidification conditions. Experiment results showed that sliver defects were originated from fragmentation of dendrite trunks and could developed to various morphologies. The misorientation range of sliver grains was between 3.2° to 12°. Microstructural analysis results indicated that the dendrites fragmented abruptly without significant plastic deformation. And the dendrite fragmentation could be facilitated by oxide inclusions and solidification pores. It was also found that the sliver defects were prone to occur at the interface where primary dendrite converge to the mold inner wall. Based on the low misorientation level of sliver grains in the experimental observation, the critical solid volume fraction range for fragmentation was suggested to be 0.6–0.8. The dendrite fragmentation was attributed to the non-uniform stress distribution in dendrite during solidification. In addition, the effect of airfoil geometry of a typical blade casting and defect prevention methodology were discussed. [Display omitted] •During directional solidification of SX castings, dendrite fragmentation arose the formation of sliver defect.•The macroscopic and microscopic non-uniform stress played significant roles in the dendrite fragmentation.•The possible solid fraction range for fragmentation was suggested to be 0.6–0.8.•The sliver occurrence may be correlated to the formation of converging interface between dendrite tips and mold wall.