MODELING SOLUTE REDISTRIBUTION AND MICROSTRUCTURAL DEVELOPMENT IN FUSION WELDS OF NB-BEARING SUPERALLOYS

Solute redistribution and microstructural evolution have been modeled for gas tungsten are fusion welds in experimental Ni base and Fe base Nb-bearing superalloys. The multi-component alloys were modeled as a t ernary system by grouping the matrix (Fe, Ni, Cr) elements together as the ''solvent'' to form the gamma component of the gamma-Nb-C ''terna ry system''. The variation in fraction liquid and liquid composition d uring the primary L --> gamma and eutectic type L -->(gamma + NbC) sta ges of solidification were calculated for conditions of negligible Nb diffusion and infinitely fast C diffusion in the solid phase. The prop osed model is based on modifications of solute redistribution equation s originally developed by Mehrabian and Flemings. Results of the calcu lations were superimposed on the pseudo-ternary gamma-Nb-C solidificat ion surfaces to predict the solidification reaction sequences along wi th the total and individual amounts of gamma/NbC and gamma/Laves eutec tic-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 compos ition-microstructure relations in these Nb-bearing experimental alloys and should provide useful insight into comparable commercial alloy sy stems as well. (C) 1998 Acta Metallurgica Inc. Published by Elsevier, Science Ltd. Ali rights reserved.