Effect of Ta and Ti on the solidification characteristics of novel γ′-strengthened Co-base superalloys

The effects of Ta and Ti on the microstructure, segregation and solidification path of Co-7Al-8W-base alloy were investigated using directional solidification quenching experiments and isothermal solidification quenching experiments. The results showed that the as-cast microstructure of the Co-7Al-8W alloy (Base alloy) is the γ single phase. The Al element slightly segregates to the interdendritic regions, the W element slightly segregates to the dendritic core, and Co has almost no segregation. The solidification path of the Co-7Al-8W alloy is L→L1+γ→γ. With the addition of 1 at.% Ta, there is no change in the segregation of elements, the microstructure and the solidification path of the alloy. The Ta element strongly segregates to the interdendritic regions in the Co-7Al-8W-1Ta alloy. With the addition of 4 at.% Ti, the Ti element strongly segregates to the interdendritic regions, and the segregation of Al and W elements increases. Blocky γ′ phase and (β+γ′)e eutectics form in the interdendritic regions. The solidification path of the Co-7Al-8W-4Ti alloy is L→L1+γ→L2+γ+(β+γ′)e→L3+γ+(β+γ′)e+γ'→γ+(β+γ′)e+γ'. After the addition of 1 at.% Ta+4 at.% Ti, the segregation of Al, Ta and Ti decreases, while the segregation of W increases. Laves phase, blocky γ′ phase and (β+γ′)e eutectics form in the interdendritic regions. The solidification path of the Co-7Al-8W-1Ta-4Ti alloy is L→L1+γ→L2+γ+Laves→L3+γ+Laves+(β+γ′)e→L4+γ+Laves+(β+γ′)e+γ'→γ+Laves+(β+γ′)e+γ'. The above results could provide an experimental basis for composition optimization and solidification process control of novel γ′-strengthened Co-base superalloys.