Atomic ordering in the Ni-Cr-Fe system

The ordering transformation based on the Ni2Cr superlattice was studied by monitoring the variations in lattice parameter, electrical resistivity and microhardness in a series of ternary Ni-Cr-Fe alloys (30-67 at.% Ni, 17-32 at.% Cr and 1-51 at.% Fe) after long-term exposures at temperatures between 450 and 600 °C. The microstructural evolution of the alloys at interrupted annealing times was observed and then correlated to the variations in the physical properties. The degree of order and ordering kinetics depend on alloy composition, time and temperature of aging. Short-range order develops in all alloys during the first hours of aging. The degree of short-range order increases with decreasing temperature, increasing Ni and decreasing Fe concentration. In alloys approaching the composition Ni2Cr with additions of up to 5 at.% Fe this structure transforms to long-range order upon aging below the critical temperature for aging durations of the order of thousands of hours. The transformation kinetics depend on temperature and are markedly delayed by Fe addition. The degree of long-range order falls with increasing Cr concentration. In an alloy approaching the Ni2Cr atomic ratio with a 10 at.% Fe addition, long-range order forms after 32 000 h aging at 475 °C.