Two-phase coexistence in Fe-Ni alloys synthesized by ball milling

We used mechanical alloying with a Spex 8000 mixer/mill to synthesize a series of Fe100-xNix alloys from x=0 to x=49. The Spex mill was modified so that it could also operate at a reduced milling intensity, and we compared the alloys synthesized after long times with the normal and reduced milling intensities. X-ray diffractometry and Mossbauer spectrometry were used to measure the volume fractions of the bcc and fee phases in the alloys, and to determine the chemical compositions of the individual phases. We found that the composition ranges of the bcc and fee single phase regions were extended well beyond their equilibrium ranges. At the higher milling intensity, we found that the bcc phase was destabilized with respect to the fee phase, and the two-phase region shifted to lower Ni concentrations. For those alloys with coexisting bcc and fee phases, we present evidence that the chemical compositions of the two phases are nearly the same. We explain the destabilization of the bcc with milling intensity as originating with a higher defect density in the bcc alloys than in the fee alloys. We argue that this defect density is not homogeneous throughout the alloy, however, and the distribution of defect enthalpies can explain the two-phase coexistence in the as-milled alloys. (C) 1996 American Institute of Physics.