Modeling solute redistribution and microstructural development in fusion welds of Nb-bearing superalloys

Abstract

Solute redistribution and microstructural evolution have been modeled for gas tungsten arc fusion welds in experimental Ni base and Fe base Nb-bearing superalloys. The multi-component alloys were modeled as a ternary system by grouping the matrix (Fe, Ni, Cr) elements together as the “solvent” to form the γ component of the γ-Nb–C “ternary system”. The variation in fraction liquid and liquid composition during the primary L→ γ and eutectic type L→( γ+NbC) stages of solidification were calculated for conditions of negligible Nb diffusion and infinitely fast C diffusion in the solid phase. The proposed model is based on modifications of solute redistribution equations originally developed by Mehrabian and Flemings. Results of the calculations were superimposed on the pseudo-ternary γ-Nb–C solidification surfaces to predict the solidification reaction sequences along with the total and individual amounts of γ/NbC and γ/Laves eutectic-type constituents which form during solidification. Comparison was made to experimentally measured values, and reasonable agreement was found. The model results permit quantitative interpretations of composition–microstructure relations in these Nb-bearing experimental alloys and should provide useful insight into comparable commercial alloy systems as well.