61 research outputs found
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
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
The evolution of precipitate crystal structures in an Al-Mg-Si(-Cu) alloy studied by a combined HAADF-STEM and SPED approach
This work presents a detailed investigation into the effect of a low Cu
addition (0.01 at.%) on precipitation in an Al-0.80Mg-0.85Si alloy during
ageing. The precipitate crystal structures were assessed by scanning
transmission electron microscopy combined with a novel scanning precession
electron diffraction approach, which includes machine learning. The combination
of techniques enabled evaluation of the atomic arrangement within individual
precipitates, as well as an improved estimate of precipitate phase fractions at
each ageing condition, through analysis of a statistically significant number
of precipitates. Based on the obtained results, the total amount of solute
atoms locked inside precipitates could be approximated. It was shown that even
with a Cu content close to impurity levels, the Al-Mg-Si system precipitation
was significantly affected with overageing. The principal change was due to a
gradually increasing phase fraction of the Cu-containing Q'-phase, which
eventually was seen to dominate the precipitate structures. The structural
overtake could be explained based on a continuous formation of the thermally
stable Q'-phase, with Cu atomic columns incorporating less Cu than what could
potentially be accommodated.Comment: 13 pages, 10 figures, 2 table
Effect of the nanostructuring by high-pressure torsion process on the secondary phase precipitation in UNS S32750 Superduplex stainless steel
In this work, the precipitation and the morphology of secondary phases after severe plastic deformation (SPD) processing followed by an isothermal treatment was investigated. High-pressure torsion (HPT) was the SPD process carried out on superduplex 2507 (UNS S32750) stainless steel material under P = 6 GPa at room temperature. At this high strain levels (ε up to 170) samples have shown grain size decrease and strained microstructure with high dislocation density and nanostructure features. After a short isothermal treatment at 830 °C, the sigma phase and chromium nitrides were revealed as the main secondary phases identified by scanning and transmission electron microscopy and element analysis by energy dispersive spectroscopy. Scanning precession electron diffraction and automated crystal orientation mapping have been carried out in order to confirm the precipitation of the secondary phases. In fact, the results provide evidence that the precipitation of chromium nitrides seems to be the preferred nucleation site for sigma phase at higher deformation strain, in addition to the intergranular precipitation of sigma. Both the sigma phases nucleated integranularly and besides chromium nitrides are randomly orientated
Optimizing compositional and atomic-level information of oxides in atom probe tomography
Atom probe tomography (APT) is a 3D analysis technique that offers unique
chemical accuracy and sensitivity with sub-nanometer spatial resolution.
Recently, there is an increasing interest in the application of APT to complex
oxides materials, giving new insight into the relation between local variations
in chemical composition and emergent physical properties. However, in contrast
to the field of metallurgy, where APT is routinely applied to study materials
at the atomic level, complex oxides and their specific field evaporation
mechanisms are much less explored. Here, we perform APT measurements on the
hexagonal manganite ErMnO3 and systematically study the effect of different
experimental parameters on the measured composition and atomic structure. We
demonstrate that both the mass resolving power (MRP) and compositional accuracy
can be improved by increasing the charge-state ratio (CSR) working at low laser
energy (< 5 pJ). Furthermore, we observe a substantial preferential retention
of Er atoms, which is suppressed at higher CSRs. We explain our findings based
on fundamental field evaporation concepts, expanding the knowledge about the
impact of key experimental parameters and the field evaporation process in
complex oxides in general
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In Situ Heat-Induced Replacement of GaAs Nanowires by Au.
Here we report on the heat-induced solid-state replacement of GaAs by Au in nanowires. Such replacement of semiconductor nanowires by metals is envisioned as a method to achieve well-defined junctions within nanowires. To better understand the mechanisms and dynamics that govern the replacement reaction, we performed in situ heating studies using high-resolution scanning transmission electron microscopy. The dynamic evolution of the phase boundary was investigated, as well as the crystal structure and orientation of the different phases at reaction temperatures. In general, the replacement proceeds one GaAs(111) bilayer at a time, and no fixed epitaxial relation could be found between the two phases. The relative orientation of the phases affects the replacement dynamics and can induce growth twins in the Au nanowire phase. In the case of a limited Au supply, the metal phase can also become liquid.The Research Council of Norway is acknowledged for the support to the Norwegian Micro- and Nano-Fabrication Facility, NorFab (197411/V30), the FRINATEK program (214235), and the NORTEM project (197405). G.D. and C.D. acknowledge funding from ERC under Grant 259619 PHOTO EM. C.D. acknowledges financial support from the EU under Grant 312483 ESTEEM2
Observation of Electric-Field-Induced Structural Dislocations in a Ferroelectric Oxide
Dislocations are 1D topological defects with emergent electronic properties. Their low dimensionality and unique properties make them excellent candidates for innovative device concepts, ranging from dislocation-based neuromorphic memory to light emission from diodes. To date, dislocations are created in materials during synthesis via strain fields or flash sintering or retrospectively via deformation, for example, (nano)-indentation, limiting the technological possibilities. In this work, we demonstrate the creation of dislocations in the ferroelectric semiconductor Er(Mn,Ti)O3 with nanoscale spatial precision using electric fields. By combining high-resolution imaging techniques and density functional theory calculations, direct images of the dislocations are collected, and their impact on the local electric transport behavior is studied. Our approach enables local property control via dislocations without the need for external macroscopic strain fields, expanding the application opportunities into the realm of electric-field-driven phenomena.publishedVersio
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