A combined solubility product/new PHACOMP approach for estimating temperatures of secondary solidification reactions in superalloy weld metals containing Nb and C

Abstract

Solubility data are combined with the new PHACOMP calculation procedure and solute redistribution equations to propose a simple method for estimating temperatures of the secondary solidification reactions that occur in superalloy weld metals. The variation in the primary solid composition with temperature is calculated with the Scheil equation for substitutional elements, while the lever law is used for C. The calculated compositions are used to determine the temperature-dependent variation in the solubility parameter and the metal d-level parameter, which is used in the new PHACOMP routine. The solubility parameter and metal d-level profiles are compared to temperature-dependent critical values to predict temperatures of secondary solidification transformations involving carbides and topologically close-packed phases such as Laves. The general approach is applied to experimental Ni-base and Fe-base alloys containing systematic variations in Nb, Si, and C. The procedure correctly predicts formation of the NbC phase prior to the Laves phase and reveals the effect of nominal alloy content on reaction temperature. Calculated reaction temperatures are in reasonable agreement with experimentally measured values, particularly when considering the relatively large range of material constants that are needed to make the calculations.