Near atomic scale studies of electronic structure at grain boundaries in Ni3Al

Calculations of Cu L2,3 fine structure at grain boundaries

The segregation of impurities to grain boundaries in metals and alloys has been known for some time to make changes in ductility. Examples of this effect are the embrittlement of copper by the addition of bismuth and the ductilization of Ni3Al by boron impurities. The mechanism by which these dramatic changes in mechanical properties arise is still largely unknown. It has been suggested that embrittling elements draw charge from neighbouring metal atoms while impurities that enhance ductilty act in the opposite way. Changes in the electronic states can be detected as changes in the energy loss spectrum when a small probe in a FEG STEM is moved across the boundary. Recent work by Muller has shown significant differences between the Ni L3 spectrum from grain boundaries in Ni3Al with and without boron. Bruley has shown that a “white line”, indicative of empty Cu d states, appears in the Cu L3 edge from Cu atoms near boundaries where bismuth has segregated.

Structure and bonding at the atomic scale by scanning transmission electron microscopy

A new generation of electron microscopes is able to explore the microscopic properties of materials and devices as diverse as transistors, turbine blades and interfacial superconductors. All of these systems are made up of dissimilar materials that, where they join at the atomic scale, display very different behaviour from what might be expected of the bulk materials. Advances in electron optics have enabled the imaging and spectroscopy of these buried interface states and other nanostructures with atomic resolution. Here I review the capabilities, prospects and ultimate limits for the measurement of physical and electronic properties of nanoscale structures with these new microscopes.

Effect of swift heavy ions in Ni-Al nanocrystalline films studied by X-ray absorption spectroscopy

X-ray absorption spectroscopic measurements have been used to compare the electronic structures of swift heavy ions (100 MeV Si ions) irradiated and pristine Ni-Al nanocrystalline films. Results from X-ray diffraction (XRD), X-ray absorption near-edge structure (XANES) spectra at Al K-, and Ni L(2,3)-edges and extended X-ray absorption fine structure (EXAFS) at Ni K-edges are discussed. The observed XRD peaks indicate the improvement of crystalline nature and Al(111) clustering after the swift heavy ion interactions. While the XANES spectra at Ni L(2,3)-edges show decrease in the intensity of white line strength, the Al K-edge shows increase in intensity after irradiation. Above results imply that swift heavy ions induce low Z (i.e., Al) ion mass transport, changes in Al sp-Ni-d hybridization, and charge transfer. EXAFS results show that crystalline nature is improved after swift heavy irradiation which is consistent with XRD results.

Method of linearizing the 3d L3/L2 white line ratio as a function of magnetic moment

We have developed a parameterization method which linearizes the relationship between local magnetic moment and the 3d L3/L2 "white line" ratio as observed in electron energy loss spectroscopy or X-ray near edge absorption spectroscopy. We first establish that the parameterization linearizes an existing theoretical result for ratio versus moment. We then test our method on data sets for which a white line ratio has been previously published by other authors, who have studied a series of compounds using a consistent deconvolution procedure. Finally, we apply our linearization method to the observed ratios of a series of 3d transition metals, and to the Cr L edges for a Au(x)Cr(1 - x) alloy. In addition we obtain, for the first time, experimental results on the Au L3 and L2 edge white lines of this alloy system. These results are consistent with a model in which the large local moment in this system is not limited to Cr dopants, but extends into the gold matrix.