Relationship between η phase formation and solidification rate in directionally solidified IN792 + Hf alloy

Abstract

The effect of solidification rate on the formation of η phase in a directionally solidified IN792 + Hf superalloy was studied. A Cr rich ‘precipitate free zone’ adjacent to the eutectic (γ + γ′) was noticed in the quenched directionally solidified microstructure of the alloy. The formation of the precipitate free zone was due to the solidification of eutectic (γ + γ′) which consumed a large amount of the γ′ forming elements and increased the ratio of (Ti + Hf + Ta + W)/Al in the residual liquid. The high ratio of (Ti + Hf + Ta + W)/Al in the final residual liquid induced the formation of η phase and hence η phase always appeared at the boundary between the eutectic regions and the dendrite core regions. The ratio of (Ti + Hf + Ta + W)/Al was decreased by the diffusion of elements between the residual liquid and the surrounding area. A lower solidification rate favoured the back diffusion and decreased the ratio of (Ti + Hf + Ta + W)/Al in the residual liquid, and hence reduced the formation of η phase.