38 research outputs found
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
Stability of the tetrahedral motif for small gold clusters in the size range 16–24 atoms
Effects and Sensitivity Analysis of Cracking Parameters on the Fatigue Crack Propagation
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
The effect of nickel on the martensitic-type transformations of Pt3Al and TiPt
The effect of nickel on two classes of martensitic-type transformations in platinum systems has been studied. The first transformation is Ll2 to DO'c in the Pt3Al system and the second is B2 to B19 in the TiPt system. The microstructures after transformation in the two systems are very different. The product of the Pt3Al transformation has a twinned microstructure, typical of cubic-to-tetragonal transformations. The product of the TiPt transformation is lath-like, although the morphology can be altered using heat treatments. The parent phase in the TiPt system is not retained at room temperature, whereas the parent phase in the Pt3Al transformation can be stabilised to room temperature. A great variation in hardness and transformation temperature is seen in each system as the composition is varied about the stoichiometric ratio, which has the lowest hardness. The Pt3Al transformation temperature has been reported to range from around room temperature to 1000°C. The TiPt transformation temperature can range from 1000 to 1080°C. The effect of nickel additions on these alloys also has a marked effect on the parent and product phase stability, and hence the microstructure and resulting hardness. The effect on the Pt3Al phase is complex, as nickel appears to stabilise the parent phase. The hardness varied in the region of 350 to 500 HV10. For the TiPt phase, the hardness values were generally found to increase with the nickel additions increasing from 250 to about 600 HV10. The addition of 20 at.% nickel decreases the transformation temperature from around 1000°C to about 600°C