17 research outputs found
Correlated subgrain and particle analysis of a recovered Al-Mn alloy by directly combining EBSD and backscatter electron imaging
Correlated analysis of (sub)grains and particles in alloys is important to
understand transformation processes and control material properties. A
multimodal data fusion workflow directly combining subgrain data from electron
backscatter diffraction (EBSD) and particle data from backscatter electron
(BSE) images in the scanning electron microscope is presented. The BSE images
provide detection of particles smaller than the applied step size of EBSD down
to 0.03 m in diameter. The workflow is demonstrated on a cold-rolled and
recovered Al-Mn alloy, where constituent particles formed during casting and
dispersoids formed during subsequent heating affect recovery and
recrystallization upon annealing. The multimodal dataset enables statistical
analysis including subgrains surrounding constituent particles and dispersoids'
location with respect to subgrain boundaries. Among the subgrains of
recrystallization texture, Cube{001}\left subgrains experience an
increased Smith-Zener drag from dispersoids on their boundaries compared to
CubeND{001}\left and P{011}\left
subgrains, with the latter experiencing the lowest drag. Subgrains at
constituent particles are observed to have a growth advantage due to a lower
dislocation density and higher boundary misorientation angle. The dispersoid
size per subgrain boundary length increases as a function of misorientation
angle. The workflow should be applicable to other alloy systems where there is
a need for analysis correlating grains and grain boundaries with secondary
phases smaller than the applied EBSD step size but resolvable by BSE imaging
Orientation dependent pinning of (sub)grains by dispersoids during recovery and recrystallization in an Al-Mn alloy
The recrystallized grain size and texture in alloys can be controlled via the
microchemistry state during thermomechanical processing. The influence of
concurrent precipitation on recovery and recrystallization is here analyzed by
directly correlating (sub)grains of P, CubeND or Cube orientation with
second-phase particles in a cold-rolled and non-isothermally annealed Al-Mn
alloy. The recrystallized state is dominated by coarse elongated grains with a
strong P, weaker CubeND and even weaker Cube texture. The correlated data
enables orientation dependent quantification of the density and size of
dispersoids on sub-boundaries and subgrains in the deformation zones around
large constituent particles. A new modified expression for the Smith-Zener drag
from dispersoids on sub-boundaries is derived and used. The results show that
the drag on (sub)grain boundaries from dispersoids is orientation dependent,
with Cube subgrains experiencing the highest drag after recovery and partial
recrystallization. The often observed size advantage of Cube subgrains in Al
alloys is not realized due to the increased drag, thereby promoting
particle-stimulated nucleation (PSN). Relatively fewer and larger dispersoids
in deformation zones around large particles give a reduced Smith-Zener drag on
PSN nuclei, thus further strengthening the effect of PSN. Observations
substantiating the stronger P texture compared to the CubeND texture are a
higher frequency of P subgrains and a faster growth of these subgrains. The
applied methodology enables a better understanding of the mechanisms behind the
orientation dependent nucleation and growth behavior during recovery and
recrystallization with strong concurrent precipitation in Al-Mn alloys. In
particular, the methodology gives new insights into the strong P and CubeND
textures compared to the Cube texture
Characterization of ferroelectric domain walls by scanning electron microscopy
Ferroelectric domain walls are a completely new type of functional interface, which have the potential to revolutionize nanotechnology. In addition to the emergent phenomena at domain walls, they are spatially mobile and can be injected, positioned, and deleted on demand, giving a new degree of flexibility that is not available at conventional interfaces. Progress in the field is closely linked to the development of modern microscopy methods, which are essential for studying their physical properties at the nanoscale. In this article, we discuss scanning electron microscopy (SEM) as a powerful and highly flexible imaging technique for scale-bridging studies on domain walls, continuously covering nano- to mesoscopic length scales. We review seminal SEM experiments on ferroelectric domains and domain walls, provide practical information on how to visualize them in modern SEMs, and provide a comprehensive overview of the models that have been proposed to explain the contrast formation in SEM. Going beyond basic imaging experiments, recent examples for nano-structuring and correlated microscopy work on ferroelectric domain walls are presented. Other techniques, such as 3D atom probe tomography, are particularly promising and may be combined with SEM in the future to investigate individual domain walls, providing new opportunities for tackling the complex nanoscale physics and defect chemistry at ferroelectric domain walls
Vertically Aligned GaAs Nanowires on Graphite and Few-Layer Graphene: Generic Model and Epitaxial Growth
By utilizing the reduced contact area of nanowires, we
show that
epitaxial growth of a broad range of semiconductors on graphene can
in principle be achieved. A generic atomic model is presented which
describes the epitaxial growth configurations applicable to all conventional
semiconductor materials. The model is experimentally verified by demonstrating
the growth of vertically aligned GaAs nanowires on graphite and few-layer
graphene by the self-catalyzed vapor–liquid–solid technique
using molecular beam epitaxy. A two-temperature growth strategy was
used to increase the nanowire density. Due to the self-catalyzed growth
technique used, the nanowires were found to have a regular hexagonal
cross-sectional shape, and are uniform in length and diameter. Electron
microscopy studies reveal an epitaxial relationship of the grown nanowires
with the underlying graphitic substrates. Two relative orientations
of the nanowire side-facets were observed, which is well explained
by the proposed atomic model. A prototype of a single GaAs nanowire
photodetector demonstrates a high-quality material. With GaAs being
a model system, as well as a very useful material for various optoelectronic
applications, we anticipate this particular GaAs nanowire/graphene
hybrid to be promising for flexible and low-cost solar cells