Coherent Modulated Structure during the Martensitic hcp-fcc Phase Transition in Co and in a CoNi Alloy

Diffuse neutron-scattering experiments on the martensitic fcc-hcp phase transition in single crystals of pure cobalt and Co-32% Ni reveal the presence of satellites around (111) fcc reflections within a temperature range where the fcc and hcp phases coexist. The satellites indicate a modulation of the (111) fcc lattice planes with a wavelength $\lambda =6\mathrm{}{d}_{111}$. This result can be explained by the occurrence of an intermediate structure where platelike nuclei of the hcp phase are coherently inserted into the fcc matrix.