Phase transformations in a Co-Cu-Ni alloy

The 7Co-70Cu-23Ni (at.%) alloy aged at 823 K is located inside the miscibility gap of the ternary phase diagram. Its unmixing studied by anomalous small and wide-angle Xray scattering, transmission electron microscopy, dilatometry and thermomagnetometry, proceeds in two steps: a 1-2 h transient period then a coarsening regime obeying a 0.25 power kinetic law, in which three families of spherical GP zones enriched in Co and Ni aligned along the soft )left angle bracket)1 0 0(right angle bracket)f.c.c. directions of the Cu enriched matrix form, then moires appear for long ageing duration. The slope of the tie-line (delta)CCo/(delta)CNi is equal to 0.5. The determination of the partial atomic displacements showed that the Co and Ni enriched precipitates are contracted while the Cu enriched matrix is dilated. The zones have a complex nanostructure revealed by two Curie temperatures related to a Co enriched heart surrounded by a Co depleted region. The size of the zones and the surrounding displaced regions evolve similarly versus time.