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-7A1-8W-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 gamma 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-4Ti alloy is L -> L-1+gamma -> L-2+gamma+(beta+gamma')(e) -> L-3+gamma+(beta+gamma')(e)+gamma'->gamma+(beta+gamma')(e)+gamma', After the addition of 1 at.% Ti, 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 gamma' phase and (beta+y')(e) eutectics form in the interdendritic regions. The solidification path of the Co-7Al-8W-4Ti alloy is L -> L-1+gamma -> L-2+gamma+(beta+Y')(e) -> L-3+gamma+(beta+gamma')(e)+gamma' ->gamma+(beta+gamma')(e)+y', 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 gamma' phase and ((beta+y')e eutectics form in the interdendritic regions. The solidification path of the Co-7A1-8W-1Ta-4Ti alloy is L -> L-1+gamma -> L-2+gamma+Laves -> L-3+gamma+Laves+(beta+gamma')(e) -> L-4+gamma+Laves+(beta+gamma')(e)+gamma' -> gamma+Laves+(beta+gamma')(e)+gamma'. The above results could provide an experimental basis for composition optimization and solidification process control of novel gamma'-strengthened Co-base superalloys. (C) 2018 Elsevier B.V. All rights reserved.