Track D Social Science, Human Rights and Political Science

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138414/1/jia218442.pd

Atomic step contrast from forbidden reflections

Diffraction contrast from weak 1/3 {422} reflections, observed in 〈111〉 diffraction patterns from gold, has been explained in terms of atomic steps on the (111) surface. It has been assumed from structure factor considerations that these reflections extinguish at an integral number n of complete unit cells, i.e. after every third (111) atomic layer. We show that this assumption is not valid unless the crystal is oriented with this reflection in the exact Bragg orientation. As the crystal is tilted away from the Bragg condition, extinction of the 1/3 {422} beam no longer occurs when the crystal is an integral number of unit cells thick. For tilts of about 5° from the zone axis, extinction may occur for instance after every sixth (111) layer. This phenomenon is explained using a kinematic scattering argument based on the multislice approach, where the phase of the scattered amplitude between successive layers depends on tilt from the 〈111〉 zone axis. Experiment is correlated with dynamical many-beam multislice calculations

A TEM and EDX study of cavities formed in tin by xenon ion implantation

A detailed transmission electron microscopy and EDX study has been carried out on cavities formed in tin by xenon ion implantation. Computer simulation of dark field images has identified thickness fringe contrast due to pyramidal {112} faceting on the otherwise {001} surfaces. Quantitative EDX measurements have shown that all the detectable xenon is associated with the cavities as a consequence of the high homologous implantation temperature, and that the larger of these are considerably underpressurized with respect to equilibrium pressure. These cavities have been shown to shrink when annealed, due to thermal emission of vacancie

Site Selection and Pseudo-Clustering Behaviors of Alloying Elements in Aluminum-Lean γ-TiAl Intermetallics

Site selection and pseudo-clustering behaviors of the various M alloying elements in Al-lean Ti50Al50-X M (X) (X = 1, 2, 3, 4, and 5 at. pct) intermetallics have been investigated by means of the ordering energy-dependent and long-range-order forced fast Monte Carlo simulation method. The ordering energies have been calculated via pseudopotential approximation in the electronic theory of alloys up to the third coordination sphere (CS) taking the anisotropic nature of tetragonal L1(0)-type structure of gamma-TiAl into account. It was shown that the site occupation characteristics of the M alloying element atoms in gamma-TiAl intermetallics are governed by the relative magnitude of partial ordering energies between Ti-M and Al-M atomic pairs. However, the sign of partial ordering energies of these atomic pairs at the first CS becomes important in determining the clustering behavior and controls the dissolution modes of alloying element atoms in the gamma-TiAl matrix. The pseudo-clustering behavior of alloying elements reveals three dissolution modes, namely, random dissolution (mode I), planar clustering in two dimensions (mode II), and three-dimensional (3-D) clustering (mode III) of the M occupant atoms