18 research outputs found
Automating Dislocation Characterization in 3D Dark Field X-ray Microscopy
Mechanical properties in crystals are strongly correlated to the arrangement
of 1D line defects, termed dislocations. Recently, Dark field X-ray Microscopy
(DFXM) has emerged as a new tool to image and interpret dislocations within
crystals using multidimensional scans. However, the methods required to
reconstruct meaningful dislocation information from high-dimensional DFXM scans
are still nascent and require significant manual oversight (i.e.
\textit{supervision}). In this work, we present a new relatively unsupervised
method that extracts dislocation-specific information (features) from a 3D
dataset (, , ) using Gram-Schmidt orthogonalization to represent
the large dataset as an array of 3-component feature vectors for each position,
corresponding to the weak-beam conditions and the strong-beam condition. This
method offers key opportunities to significantly reduce dataset size while
preserving only the crystallographic information that is important for data
reconstruction
In-Situ Visualization of Long-Range Defect Interactions at the Edge of Melting
Connecting a bulk material's microscopic defects to its macroscopic
properties is an age-old problem in materials science. Long-range interactions
between dislocations (line defects) are known to play a key role in how
materials deform or melt, but we lack the tools to connect these dynamics to
the macroscopic properties. We introduce time-resolved dark-field X-ray
microscopy to directly visualize how dislocations move and interact over
hundreds of micrometers, deep inside bulk aluminum. With real-time movies, we
reveal the thermally-activated motion and interactions of dislocations that
comprise a boundary, and show how weakened binding forces inhomogeneously
destabilize the structure at 99% of the melting temperature. Connecting
dynamics of the microstructure to its stability, we provide important
opportunities to guide and validate multiscale models that are yet untested
An Online Dynamic Amplitude-Correcting Gradient Estimation Technique to Align X-ray Focusing Optics
High-brightness X-ray pulses, as generated at synchrotrons and X-ray free
electron lasers (XFEL), are used in a variety of scientific experiments. Many
experimental testbeds require optical equipment, e.g Compound Refractive Lenses
(CRLs), to be precisely aligned and focused. The lateral alignment of CRLs to a
beamline requires precise positioning along four axes: two translational, and
the two rotational. At a synchrotron, alignment is often accomplished manually.
However, XFEL beamlines present a beam brightness that fluctuates in time,
making manual alignment a time-consuming endeavor. Automation using classic
stochastic methods often fail, given the errant gradient estimates. We present
an online correction based on the combination of a generalized finite
difference stencil and a time-dependent sampling pattern. Error expectation is
analyzed, and efficacy is demonstrated. We provide a proof of concept by
laterally aligning optics on a simulated XFEL beamline
Correlating Chemical Reaction and Mass Transport in Hydrogen-based Direct Reduction of Iron Oxide
Steelmaking contributes 8% to the total CO2 emissions globally, primarily due
to coal-based iron ore reduction. Clean hydrogen-based ironmaking has variable
performance because the dominant gas-solid reduction mechanism is set by the
defects and pores inside the mm-nm sized oxide particles that change
significantly as the reaction progresses. While these governing dynamics are
essential to establish continuous flow of iron and its ores through reactors,
the direct link between agglomeration and chemistry is still contested due to
missing measurements. In this work, we directly measure the connection between
chemistry and agglomeration in the smallest iron oxides relevant to magnetite
ores. Using synthesized spherical 10-nm magnetite particles reacting in H2, we
resolve the formation and consumption of w\"ustite (FeO) - the step most
commonly attributed to agglomeration. Using X-ray scattering and microscopy, we
resolve crystallographic anisotropy in the rate of the initial reaction, which
becomes isotropic as the material sinters. Complementing with imaging, we
demonstrate how the particles self-assemble, subsequently react and sinter into
~100x oblong grains. Our insights into how morphologically uniform iron oxide
particles react and agglomerate H2 reduction enable future size-dependent
models to effectively describe the multiscale iron ore reduction
X-ray induced grain structure dynamics in Bi2Se3
Grain rotation in crystals often results in coarsening or refinement of the
grains that modify the mechanical and thermal properties of materials. While
many studies have explored how externally applied stress and temperature drive
grain structure dynamics in nano-polycrystalline materials, the analogous
studies on colossal grains have been limited, especially in the absence of
external force. In this work, we used X-ray free electron laser pulses to
irradiate single-crystalline bismuth selenide (Bi2Se3) and observed grain
boundary formation and subsequent grain rotation in response to the X-ray
radiation. Our observations with simultaneous X-ray diffraction and
transmission X-ray microscopy demonstrate how intense X-ray radiation can
rapidly change grain morphologies of initially single-crystalline material.Comment: 20 pages, 8 figures including 3 supplemental figure
Transonic Dislocation Propagation in Diamond
The motion of line defects (dislocations) has been studied for over 60 years
but the maximum speed at which they can move is unresolved. Recent models and
atomistic simulations predict the existence of a limiting velocity of
dislocation motions between the transonic and subsonic ranges at which the
self-energy of dislocation diverges, though they do not deny the possibility of
the transonic dislocations. We use femtosecond x-ray radiography to track
ultrafast dislocation motion in shock-compressed single-crystal diamond. By
visualizing stacking faults extending faster than the slowest sound wave speed
of diamond, we show the evidence of partial dislocations at their leading edge
moving transonically. Understanding the upper limit of dislocation mobility in
crystals is essential to accurately model, predict, and control the mechanical
properties of materials under extreme conditions
Simultaneous Bright- and Dark-Field X-ray Microscopy at X-ray Free Electron Lasers
The structures, strain fields, and defect distributions in solid materials
underlie the mechanical and physical properties across numerous applications.
Many modern microstructural microscopy tools characterize crystal grains,
domains and defects required to map lattice distortions or deformation, but are
limited to studies of the (near) surface. Generally speaking, such tools cannot
probe the structural dynamics in a way that is representative of bulk behavior.
Synchrotron X-ray diffraction based imaging has long mapped the deeply embedded
structural elements, and with enhanced resolution, Dark Field X-ray Microscopy
(DFXM) can now map those features with the requisite nm-resolution. However,
these techniques still suffer from the required integration times due to
limitations from the source and optics. This work extends DFXM to X-ray free
electron lasers, showing how the photons per pulse available at these
sources offer structural characterization down to 100 fs resolution (orders of
magnitude faster than current synchrotron images). We introduce the XFEL DFXM
setup with simultaneous bright field microscopy to probe density changes within
the same volume. This work presents a comprehensive guide to the multi-modal
ultrafast high-resolution X-ray microscope that we constructed and tested at
two XFELs, and shows initial data demonstrating two timing strategies to study
associated reversible or irreversible lattice dynamics
Analytical methods for superresolution dislocation identification in dark-field X-ray microscopy
Abstract
We develop several inference methods to estimate the position of dislocations from images generated using dark-field X-ray microscopy (DFXM)—achieving superresolution accuracy and principled uncertainty quantification. Using the framework of Bayesian inference, we incorporate models of the DFXM contrast mechanism and detector measurement noise, along with initial position estimates, into a statistical model coupling DFXM images with the dislocation position of interest. We motivate several position estimation and uncertainty quantification algorithms based on this model. We then demonstrate the accuracy of our primary estimation algorithm on synthetic realistic DFXM images of edge dislocations in single-crystal aluminum. We conclude with a discussion of our methods’ impact on future dislocation studies and possible future research avenues
Extensive 3D mapping of dislocation structures in bulk aluminum
Thermomechanical processing such as annealing is one of the main methods to tailor the mechanical properties of materials, however, much is unknown about the reorganization of dislocation structures deep inside macroscopic crystals that give rise to those changes. Here, we demonstrate the self-organization of dislocation structures upon high-temperature annealing in a mm-sized single crystal of aluminum. We map a large embedded 3D volume ([Formula: see text] [Formula: see text]m[Formula: see text]) of dislocation structures using dark field X-ray microscopy (DFXM), a diffraction-based imaging technique. Over the wide field of view, DFXM's high angular resolution allows us to identify subgrains, separated by dislocation boundaries, which we identify and characterize down to the single-dislocation level using computer-vision methods.We demonstrate how even after long annealing times at high temperatures, the remaining low density of dislocations still pack into well-defined, straight dislocation boundaries (DBs) that lie on specific crystallographic planes. In contrast to conventional grain growth models, our results show that the dihedral angles at the triple junctions are not the predicted 120[Formula: see text], suggesting additional complexities in the boundary stabilization mechanisms. Mapping the local misorientation and lattice strain around these boundaries shows that the observed strain is shear, imparting an average misorientation around the DB of [Formula: see text] 0.003 to 0.006[Formula: see text]