Precipitation in copper-iron alloys

Abstract

The growth of coherent iron precipitates on ageing an alloy of 1 at. % iron in copper has been followed with the Mössbauer technique through the initial stages, where small numbers of iron atoms cluster together, to the state where the particles are large enough to behave as gamma iron. The growth is accompanied by changes in the isomer shifts, electric field gradients, and the hyperfine magnetic fields acting at the iron nuclei, and by changes in the magnetic moments localized on the iron atoms. Six iron near neighbours seem to cause an iron atom to lose its magnetic moment, but it remains highly susceptible, so that large moments are induced when the matrix orders magnetically. In small clusters (< 12 atoms) all or neary all the iron atoms have magnetic moments and they are strongly coupled parallel, leading to giant moment behaviour in applied magnetic fields. The spectra are consistent with the spherical or near-spherical growth of precipitates expected in this system. Cold rolling reduces the cluster size for all stages of growth, except for the larger gamma iron particles, where a transformtion to ferromagnetic body-centred cubie iron occurs.