On the development of nano and microscale precipitates upon isothermal aging of nickel base superalloy 59

Alloy 59 is a Ni base alloy, which has a nominal chemical composition of 59% Ni, 16% Mo, 23% Cr and 1% Fe. Exposure of the alloy to high temperature effects can result, depending on the prevailing thermal conditions, in the precipitation of second phases, thus profoundly affecting the alloy properties. Accordingly, the present work was initiated with the purpose of investigating microstructural evolution of the alloy, focusing on the nanoscale and microscale second phase precipitation brought about by isothermal aging. The aging treatment was carried out at two different temperatures (700 and 900 °C) for time intervals varying between 1 and 100 h. Electron microscopy observations have revealed the presence, in both the as-received and aged alloy, of a large population of nanoscale precipitates corresponding to the Ni(Mo,Cr)2 phase. In addition, the aged specimens were found to contain microscale Ni2(Mo,Cr)2 particles, formed at the grain boundaries as well as within the grains. Transmission electron microscopy (TEM) observations, made on specimens taken from the heat affected zone of a welded plate, have revealed the absence of microscale precipitates, and microstructural features thus remained similar to those of the as-received alloy. Accordingly, the susceptibility of the alloy to pit corrosion within the heat affected zone cannot be linked to the formation of Mo–Cr rich precipitates.