Quantitative and Qualitative Assessment of As-Cast Microstructure and Microporosity of γ-γ′ Co-based Superalloys

The gamma-gamma & PRIME; Co-based alloys have attracted extensive attention as a new class of materials for high temperature applications. The work is focused on quantitative and qualitative microstructural characterization of selected gamma-gamma & PRIME; type Co-based superalloys obtained by gravity casting. The four Co-based alloys were prepared via vacuum induction melting. The microstructure was analyzed by means of light microscopy and scanning electron microscopy. The microstructural parameters such as porosity and secondary dendrite arm spacing (SDAS) were evaluated by image analysis. The new Co-based superalloys are characterized by proper as-cast microstructure owing to low SDAS and limited microporosity. The values of secondary dendrite arm spacing are comparable for all alloys. The W-free alloys containing Mo and Nb exhibit substantially lower shrinkage porosity compared to those of Co-Al-W and Co-Ni-Al-W. Due to crystallization of Mo and Nb rich phases at lower temperatures, these alloys are characterized by higher solidification range.