Effect of surrounding environment on atomic structure and equilibrium shape of growing nanocrystals: gold in/on SiO2

We report on the equilibrium shape and atomic structure of thermally-processed Au nanocrystals (NCs) as determined by high resolution transmission electron microscopy (TEM). The NCs were either deposited on SiO2surface or embedded in SiO2layer. Quantitative data on the NCs surface free energy were obtained via the inverse Wulff construction. Nanocrystals inside the SiO2layer are defect-free and maintain a symmetrical equilibrium shape during the growth. Nanocrystals on SiO2surface exhibit asymmetrical equilibrium shape that is characterized by the introduction of twins and more complex atomic defects above a critical size. The observed differences in the equilibrium shape and atomic structure evolution of growing NCs in and on SiO2is explained in terms of evolution in isotropic/anisotropic environment making the surface free energy function angular and/or radial symmetric/asymmetric affecting the rotational/translational invariance of the surface stress tensor

Displacive Phase Transformations in Spangold 18 Carat Gold Alloys

The displacive phase transformations in an Au-32 at% Cu-22 at% Al gold alloy are discussed. This alloy, and others similar to it, is used to produce SpangoldTM jewellery pieces. Complex textures of intersecting laths are produced on polished surfaces of the alloys by thermal treatment, giving the material a glittering appearance. Dilatometry of both loaded and unloaded samples indicated that the Ms and Mf temperatures were, depending on prior treatment, in the range of 30 to 15°C and 20 to -10°C respectively. The As and Af temperatures also depended on prior treatment, and were in the range of 55 to 80°C and about 85 to 95°C, respectively. The unloaded specimen exhibited a closed hysteresis loop on a graph of change of length with temperature. Martensite laths of the loaded specimen became aligned along the tensile axis during thermal cycling. X-ray diffraction revealed that both the parent phase and the martensitic phase possessed orthogonal lattices. Although a face centred cubic disordered phase is presumed to be present close to the melting point of the alloy, it cannot be preserved down to room temperature by quenching