Analysis of precipitation in cast alloy 718

Precipitation in alloy 718, a Ni-Cr-Fe based superalloy with Nb additions, was studied as a function of heat treatment with specific reference to grain boundary precipitation. Three different heat treatments were studied; a low temperature precipitation treatment (1 h at 650 ° C) typical of age hardening, a high temperature precipitation treatment (1 h at 930 ° C) typical of solution anneal and a homogenization treatment (1 h at 1100 ° C). It was observed that grain boundary precipitation was dependent on the location of the boundary relative to the dendritic solidification pattern of the alloy. The same grain boundary exhibited a change in precipitation behavior as it passed from a dendritic core into the interdendritic region. In general, niobium carbides (NbC) were observed to be located on the grain boundaries in the Nb-depleted core region, while Nb-rich precipitates which included Ni₃Nb (δ), Ni₃Nb (γ″), laves, and NbC were observed in the Nb-enriched interdendritic region. The grain boundaries in the interdendritic region contained δ precipitated in a discontinuous manner.