The use of new PHACOMP in understanding the solidification microstructure of nickel base alloy weld metal

The weld metal microstructures of five commercial nickel base alloys (HASTELLOYS* C-4, C-22, and C-276, and INCONELS* 625 and 718) have been examined by electron probe microanalysis and analytical electron microscopy. It has been found that solidification terminates in many of these alloys with the formation of a constituent containing a topologically-close-packed (TCP) intermetallic phase(i.e., σ, P, Laves). Electron microprobe examination of gas-tungsten-arc welds revealed a solidification segregation pattern of Ni depletion and solute enrichment in interdendritic volumes. New PHACOMP calculations performed on these segregation profiles revealed a pattern of increasingMd (metal-d levels) in traversing from a dendrite core to an adjacent interdendritic volume. In alloys forming a terminal solidification TCP constituent, the calculatedMd values in interdendritic regions were greater than the criticalMd values for formation ofσ as stated by Morinagaet al. Implications of the correlation between TCP phase formation andMd in the prediction of weld metal solidification microstructure, prediction of potential hot-cracking behavior, and applications in future alloy design endeavors are discussed.