Effect of Zr on solidification segregation behavior of K417G alloy and its anomalous effect during rapid cooling process

Although Zr is classified as a beneficial element in superalloys, it noticeably increases the solidification segregation, worsens the as cast microstructure and lowers the melting point of the superalloys, but the detailed information about the behavior of Zr during the solidification remains unknown till now. In this paper, some evidences supporting the assumption that a film of Zr formed at the solid/liquid interface during solidification have been found, including the precipitation of ZrC, Zr2(SC) and Ni–Zr phases at the grain boundary or eutectic (γ+ γ′) front, and the concentration of Zr at the crack surface formed at the final stage of the K417G alloy solidification. The Zr-rich film at the solid/liquid interface markedly lowers the solidification rate of K417G alloy, and decreases the content of Al, Ti, Cr and Mo in the dendrite core. As a result, the precipitation of (γ+ γ′) eutectic is largely increased. Under water quenching conditions, the solidification of the final liquids contains two sequential eutectic reaction, including the (γ+ γ′) at the external part of the liquid pool and the (γ+ carbide) reaction at the central part of the liquid pool. Zr inhibits the second reaction and increases the crack formation tendency, implying that it would damage the weldability of the alloy.