The influence of Cr additions on the ordering of Fe-Cr-Ni alloys near the Ni3Fe composition

Abstract

A study of ordering in (Ni3Fe)1-xCrx alloys (0<x<17%) is described. The critical temperature for ordering (Tc) has been determined using electron diffraction to detect long-range order (LRO). Low-temperature (<450 degrees C) anneals were performed with a 1 MeV electron microscope, using electron irradiation to increase the ordering rate. Mossbauer spectroscopy was used to characterise the short-range order (SRO) in annealed unirradiated samples. Tc was found to decrease rapidly with increasing Cr content, falling from 502 degrees C in near-Ni3Fe material, to about 450 degrees C at a Cr concentration of 6%. No LRO was detected in samples with >or=12% Cr. The Mossbauer spectra of the ordered alloys contain a contribution attributed to Fe atoms each with a Cr neighbour. The magnitude of this component is smaller than it would be in a random alloy, implying that Cr atoms preferentially occupy Fe sites in Ni3Fe. In the alloys with the higher Cr contents little evidence of SRO was detected. A very slight increase in the mean hyperfine field on aging these alloys may indicate the formation of Fe-rich regions, as previously proposed by Binnatov and co-workers (1975).