Freckle formation and thermodynamic assessment for Nb-bearing superalloys

Abstract

IN718, IN625 and IN706 are wildly used Nb-bearing cast-wrought superalloys. With the ever increasing demand for large ingot size to meet new design for aggressive jet engine and industrial land base engine, the solidification macrosegregation, i.e. freckle, becomes more and more concerned in recent years. The freckle is believed to be caused by instable convective flow in the mush zone, and the well-adopted approach to evaluate freckle formation tendency is through the calculation of Rayleigh Number (Ra). However, it is difficult to calculate Ra due to complex solidification behaviors of multi-components superalloys. In this paper, the freckles formed in Nb-bearing superalloy were examined first. The thermodynamic calculation approach was then used to assess liquid composition as a function of temperature during the solidification under Scheil solidification condition. The relative Ra numbers for these alloys were calculated based on the results of thermodynamic calculation. Finally, this thermodynamic approach was evaluated through comparison with the calculation from semi-experimental results. The Ra numbers obtained by thermodynamic approach are in good agreement with ingot size capability of industry melting shops, which is believed to be limited mainly by freckle defect.