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 met hod, 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 beta (2) phase Au-Cd alloys near equiatomic compositions in bulk. I t was thus seen that the high temperature beta (2) phase becomes stable in the nm-sized particles at room temperature. It was found, however, in some beta (2) phase particles that modulated regions were formed a few nm in siz e with periods two and six times as large as the {110} planar spacing of th e B2 lattice. The Fourier transform revealed that the modulated regions wit h twofold and six-fold periods have, respectively, definite crystal structu res of the beta (2)' and gamma (2)' martensites in bulk. The modulated regi ons thus seemed to be the nuclei of these martensites. These martensite nuc lei were coherent with each other and with the surrounding B2 matrix lattic e. No clear interfaces between these martensites and the matrix were presen t, other than the common close-packed planes with which these martensites a nd the matrix are connected. (C) 2001 Elsevier Science B.V. All rights rese rved.