Yttrium-induced morphological evolution of primary MC carbides in a second-generation Ni-based single crystal superalloys

Evolution of the morphology of primary MC carbides with the doping of yttrium with different content from 0 to 0.5 wt% in a typical second-generation nickel based SX superalloys (DD5) was investigated in this work. The deep-etched samples indicate that the morphology of MC carbides evolves from script-like to faceted octahedral block with the increase of yttrium content to 0.5 wt%. Meanwhile, STEM analyses demonstrated that yttrium significantly disturbed the orientation relationship between primary MC carbides and matrix. These phenomena were attributed to the ultra-low surface tension property of yttrium, which has a spontaneous segregation propensity at the phase interface during solidification process, and utterly alters compositional and temperature fields in the liquid at the front of carbide/matrix interface.