Eta phase and boride formation in directionally solidified Ni-base superalloy IN792+Hf

A series of directional solidification experiments have been conducted to elucidate the formation mechanism of eta and Cr-rich phases in the Ni-base superalloy IN792 + Hf. Both eta and Cr-rich phases were found to be the final solidification products developed from the remaining liquid after gamma/gamma' eutectic reaction. The (Ti + Ta + Hf)/Al ratio in the residual liquid played a significant role in the nucleation of eta phase. During the solidification of gamma/gamma' eutectic, the continual increase of (Ti + Ta + Hf)/Al ratio in the residual liquid eventually led to the completion of gamma/gamma' eutectic reaction and caused the nucleation of eta phase. The results of electron probe microanalysis and transmission electron microscopy revealed that the Cr-rich phase was Cr, Mo, and W containing M5B3 and M3B2 type borides. The formation of these boride phases was found to be strongly influenced by the formation of gamma/gamma' eutectic. Because of the limited solubility of Cr, Mo. and W in gamma' phase, these elements were enriched in the residual liquid during the solidification of gamma/gamma' eutectic. In addition, boron would preferentially segregate into liquid due to its very limited solubility in both gamma and gamma' phases so that the possibility of boride formation in the residual liquid ahead of the gamma/gamma' eutectic was increased. A modified Scheil model was adopted to explain the influence of solidification rate on the formation of eta phase and borides, and the results were discussed.