206,820 research outputs found
Microstructure Controlled Shear Band Pattern Formation and Enhanced Plasticity of Bulk Metallic Glasses Containing in situ Formed Ductile Phase Dendrite Dispersions
Results are presented for a ductile metal reinforced bulk metallic glass matrix composite based on glass forming compositions in the Zr-Ti-Cu-Ni-Be system. Primary dendrite growth and solute partitioning in the molten state yields a microstructure consisting of a ductile crystalline Ti-Zr-Nb β phase, with bcc structure, in a Zr-Ti-Nb-Cu-Ni-Be bulk metallic glass matrix. Under unconstrained mechanical loading organized shear band patterns develop throughout the sample. This results in a dramatic increase in the plastic strain to failure, impact resistance, and toughness of the metallic glass
Large supercooled liquid region and phase separation in the Zr–Ti–Ni–Cu–Be bulk metallic glasses
Results of calorimetric, differential thermal analysis, and structural measurements are presented for a series of bulk metallic glass forming compositions in the Zr-Ti-Cu-Ni-Be alloy system. The calorimetric data for five alloys, prepared along the tie line between phase separating and nonphase separating compositions, show that the transition from phase separating to nonphase separating behavior is smooth. The bulk glasses near the center of the tie line exhibit large supercooled liquid regions: Delta T approximate to 135 K, the largest known for a bulk metallic glass
Off-diagonal magnetoimpedance in field-annealed Co-based amorphous ribbons
The off-diagonal magnetoimpedance in field-annealed CoFeSiB amorphous ribbons
was measured in the low-frequency range using a pick-up coil wound around the
sample. The asymmetric two-peak behavior of the field dependence of the
off-diagonal impedance was observed. The asymmetry is attributed to the
formation of a hard magnetic crystalline phase at the ribbon surface. The
experimental results are interpreted in terms of the surface impedance tensor.
It is assumed that the ribbon consists of an inner amorphous region and surface
crystalline layers. The coupling between the crystalline and amorphous phases
is described through an effective bias field. A qualitative agreement between
the calculated dependences and experimental data is demonstrated. The results
obtained may be useful for development of weak magnetic-field sensors.Comment: 19 pages, 6 figure
In situ transmission electron microscopy studies of shear bands in a bulk metallic glass based composite
In situ straining transmission electron microscopy (TEM) experiments were performed to study the propagation of the shear bands in the Zr56.3Ti13.8Cu6.9Ni5.6Nb5.0Be12.5 bulk metallic glass based composite. Contrast in TEM images produced by shear bands in metallic glass and quantitative parameters of the shear bands were analyzed. It was determined that, at a large amount of shear in the glass, the localization of deformation occurs in the crystalline phase, where formation of dislocations within the narrow bands are observed
Fitting Photometry of Blended Microlensing Events
We reexamine the usefulness of fitting blended lightcurve models to
microlensing photometric data. We find agreement with previous workers (e.g.
Wozniak & Paczynski) that this is a difficult proposition because of the
degeneracy of blend fraction with other fit parameters. We show that follow-up
observations at specific point along the lightcurve (peak region and wings) of
high magnification events are the most helpful in removing degeneracies. We
also show that very small errors in the baseline magnitude can result in
problems in measuring the blend fraction, and study the importance of
non-Gaussian errors in the fit results. The biases and skewness in the
distribution of the recovered blend fraction is discussed. We also find a new
approximation formula relating the blend fraction and the unblended fit
parameters to the underlying event duration needed to estimate microlensing
optical depth.Comment: 18 pages, 9 figures, submitted to Ap
Structure of AlSb(001) and GaSb(001) Surfaces Under Extreme Sb-rich Conditions
We use density-functional theory to study the structure of AlSb(001) and
GaSb(001) surfaces. Based on a variety of reconstruction models, we construct
surface stability diagrams for AlSb and GaSb under different growth conditions.
For AlSb(001), the predictions are in excellent agreement with experimentally
observed reconstructions. For GaSb(001), we show that previously proposed model
accounts for the experimentally observed reconstructions under Ga-rich growth
conditions, but fails to explain the experimental observations under Sb-rich
conditions. We propose a new model that has a substantially lower surface
energy than all (nx5)-like reconstructions proposed previously and that, in
addition, leads to a simulated STM image in better agreement with experiment
than existing models. However, this new model has higher surface energy than
some of (4x3)-like reconstructions, models with periodicity that has not been
observed. Hence we conclude that the experimentally observed (1x5) and (2x5)
structures on GaSb(001) are kinetically limited rather than at the ground
state.Comment: 6 pages, 6 figure
Glassy dynamics of partially pinned fluids: an alternative mode-coupling approach
We use a simple mode-coupling approach to investigate glassy dynamics of
partially pinned fluid systems. Our approach is different from the
mode-coupling theory developed by Krakoviack [Phys. Rev. Lett. 94, 065703
(2005), Phys. Rev. E 84, 050501(R) (2011)]. In contrast to Krakoviack's theory,
our approach predicts a random pinning glass transition scenario that is
qualitatively the same as the scenario obtained using a mean-field analysis of
the spherical p-spin model and a mean-field version of the random first-order
transition theory. We use our approach to calculate quantities which are often
considered to be indicators of growing dynamic correlations and static
point-to-set correlations. We find that the so-called static overlap is
dominated by the simple, low pinning fraction contribution. Thus, at least for
randomly pinned fluid systems, only a careful quantitative analysis of
simulation results can reveal genuine, many-body point-to-set correlations
Influence of magnetic viscosity on domain wall dynamics under spin-polarized currents
We present a theoretical study of the influence of magnetic viscosity on
current-driven domain wall dynamics. In particular we examine how domain wall
depinning transitions, driven by thermal activation, are influenced by the
adiabatic and nonadiabatic spin-torques. We find the Arrhenius law that
describes the transition rate for activation over a single energy barrier
remains applicable under currents but with a current-dependent barrier height.
We show that the effective energy barrier is dominated by a linear current
dependence under usual experimental conditions, with a variation that depends
only on the nonadiabatic spin torque coefficient beta.Comment: 8 pages, 4 figure
Path sampling for lifetimes of metastable magnetic skyrmions and direct comparison with Kramers' method
We perform a direct comparison between Kramers' method in many dimensions --
i.e., Langer's theory -- adapted to magnetic spin systems, and a path sampling
method in the form of forward flux sampling, as a means to compute collapse
rates of metastable magnetic skyrmions. We show that a good agreement is
obtained between the two methods. We report variations of the attempt frequency
associated with skyrmion collapse by three to four orders of magnitude when
varying the applied magnetic field by 5 of the exchange strength, which
confirms the existence of a strong entropic contribution to the lifetime of
skyrmions. This demonstrates that in complex systems, the knowledge of the rate
prefactor, in addition to the internal energy barrier, is essential in order to
properly estimate a lifetime.Comment: 5 pages, 5 figures (main text), 8 pages including supplemental
materia
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