Phase transformations during long-term ageing of NiFeCr alloys in the temperature range 450–600 °C

Phase equilibria in a selected phase region of the ternary NiCrFe system were investigated at temperatures between 450 and 600 °C. The experimental study, based on microstructural observation and electrical resistivity measurements, focused on the phase transformations occurring in a series of ternary alloys with 30%–67% Ni, 1%–51% Fe and 17%–32% Cr during ageing for up to 32 000 h. Those materials, which take on short-range order during earlier stages of ageing, undergo three metallurgical changes at very slow rates: long-range order based on Ni2Cr, occurring at lower temperatures; the precipitation of the embrittling b.c.c. α′-Cr phase and, possibly, of the tetragonal σ-(FeCr) phase, occurring especially at higher temperatures. The degree of order and the ordering rate, which decrease with increasing temperature, fall with Cr concentration and are markedly reduced by increasing amounts of Fe. The amount and size of grain boundary precipitates increase with time and/or temperature of ageing, and with the Fe concentration.