The refinement mechanism of intermetallic refiners Cr/Co-Fe-Nb in IN939 superalloy under thermally controlled solidification (TCS) process

The elevated pouring temperatures characteristic of nickel-based superalloys usually result in a diminished efficacy of intermetallic grain refiners in their refinement capabilities. The Thermally Controlled Solidification (TCS) technique employs an extremely low pouring temperature, thereby enabling the practical utilization of such refiners. This study elucidates the grain refinement mechanism of CrFeNb and Co₃FeNb₂ intermetallic refiners in the IN939 superalloy under TCS conditions. After the addition of refiners, the average grain size markedly decreased from 6981 μm to 129 μm. The in-situ formation of the C14 Laves phase on refiner particles serves as the primary heterogeneous nucleation site. A specific crystallographic orientation relationship (OR) between the C14 Laves phase and the γ matrix was identified through Kikuchi patterns and electron backscatter diffraction (EBSD) analysis: [101]_γ // [101¯0]_Laves, ₍1̅11)_γ // (2̅112)_Laves. Edge-to-Edge Matching (E2EM) calculations corroborate the superior nucleation capability of the C14 Laves phase. Moreover, the refiners possess high stability and effectively homogenize the microstructure of complex castings. This study introduces a refinement mechanism for intermetallic refiners and offers theoretical insights to inform the development and application of innovative grain refiners for nickel-based superalloys.