Effects of composition on degree of order and ordering kinetics of Ni-Cr based superalloys

The ordering behaviour of different Ni-Cr base alloys, isothermally aged in the 450-600°C temperature range for up to 32000 h, was investigated by electrical resistivity measurements, lattice parameter determinations by X-ray diffraction and TEM observations. Degree of order and ordering kinetics were observed to depend on chemical composition, ageing time and temperature : SRO develops in all alloys during cooling from solution temperature and early stages of ageing with resistivity increments by up to 7% and lattice contractions by up to 0.05%. After 30000 - 32000 h ageing at temperatures below 500°C SRO transform to LRO in Brightray S, Nimonic 80A and Inconel 690 with sharp resistivity decreases of up to 16% and lattice contractions of up to 0.12%, whereas in the other alloys examined in this work only SRO structure was observed.