Oleg M. Barabash

Polychromatic Microdiffraction Characterization of Defect Gradients in Severely-Deformed Materials

This paper analyzes local lattice rotations introduced in severely deformed polycrystalline titanium by friction stir welding. Nondestructive three-dimensional (3D) spatially resolved polychromatic X-ray microdiffraction, is used to resolve the local crystal structure of the restructured surface from neighboring local structures in the sample material. The measurements reveal strong gradients of strain and geometrically necessary dislocations near the surface and illustrate the potential of polychromatic microdiffraction for the study of deformation in complex materials systems.

Spatially resolved distribution of dislocations and crystallographic tilts in GaN layers grown on Si(111) substrates by maskless cantilever epitaxy

The spatial distribution of strain, misfit and threading dislocations, and crystallographic orientation in uncoalesced GaN layers grown on Si(111) substrates by maskless cantilever epitaxy was studied by polychromatic x-ray microdiffraction, high resolution monochromatic x-ray diffraction, and scanning electron microscopy. Tilt boundaries formed at the column/wing interface depending on the growth conditions. A higher lateral to vertical growth ratio suppressed sidewall deposition and was found to produce larger lattice tilts in the GaN films. Two kinds of crystallographic tilts are observed in the films. The measurements revealed that the free-hanging wings are tilted upward at room temperature in the direction perpendicular to the stripes. Finite element simulations of the thermally induced part of the wing tilt are presented. Moreover, a misorientation between the GaN(0001) and the Si(111) planes is observed in the parallel to the stripe direction. Its origin is discussed with respect to the strain of ...

Evolution of dislocation structure in the heat affected zone of a nickel-based single crystal

Using polychromatic microbeam synchrotron diffraction together with electron and optical microscopy, we studied dislocation structure changes of Ni—based single crystal superalloy during impulse heating up to the melting temperature. It is shown that the distribution of the thermal gradient is not monotonic. The maximum value of the thermal gradient is observed in the heat affected zone near the fusion line. Depending on the temperature, the formation of dislocation is accompanied by the partial or complete dissolution of γ′ particles in the matrix. Dislocations form and multiply due to thermal gradients, and their arrangement correlates with temperature gradient field and with the dissolution and reprecipitation of γ′ particles.

Deformation in the heat affected zone during spot welding of a nickel-based single crystal

Constrained melting and solidification and associated thermal cycling during welding produces extensive microstructural changes. Such changes are observed during spot welding of single-crystal Ni-based superalloy TSM-75. Analysis of Laue diffraction patterns attained from the heat-affected-zone indicates the presence of a high density of dislocations. Dislocation rearrangement results in local rotations around the [100] direction, which is perpendicular to the temperature gradient. These dislocations exist over a large fraction of the heat-affected zone. The analyses also showed that the dislocation density was maximal near the fusion line and gradually decreases with an increase in the distance from the fusion line. The dislocation density increased with the cooling rate.

In-situ observation of stress-induced stochastic twin boundary motion in off stoichiometric NiMnGa single crystals

In-situ X-ray microdiffraction is used to illuminate the physics of non-uniform stochastic motion of type II twin boundaries in NiMnGa twinned crystals during external stress field loading. Asymmetry between tensile and compressive loading and a large hysteresis loop were found. The formation of local strained regions precedes each boundary movement. The location of strained regions adjusts to the position of the twin boundary. Abrupt motion of the boundary correlates with corresponding spikes at the load/displacement curve.