Structure of nanometer-sized Au–Cd alloy particles near equiatomic compositions at room temperature

Nanometer (nm)-sized Au–Cd alloy particles with a composition of Au–44.6at.% Cd on average were prepared by a high vacuum electron beam deposition method, and their structure at room temperature was studied by lattice imaging and Fourier transform techniques in transmission electron microscopy, and electron diffraction. The Au–Cd alloy particles exhibited polyhedral shapes with facets, and possessed basically the B2 type structure similar to that of the β2 phase Au–Cd alloys near equiatomic compositions in bulk. It was thus seen that the high temperature β2 phase becomes stable in the nm-sized particles at room temperature. It was found, however, in some β2 phase particles that modulated regions were formed a few nm in size with periods two and six times as large as the {110} planar spacing of the B2 lattice. The Fourier transform revealed that the modulated regions with twofold and six-fold periods have, respectively, definite crystal structures of the β2′ and γ2′ martensites in bulk. The modulated regions thus seemed to be the nuclei of these martensites. These martensite nuclei were coherent with each other and with the surrounding B2 matrix lattice. No clear interfaces between these martensites and the matrix were present, other than the common close-packed planes with which these martensites and the matrix are connected.