91 research outputs found
Nonlinear Ultrasonic Properties of As-Quenched Steels
We have investigated the effect of carbon content on the nonlinear ultrasonic parameter β and the longitudinal phase velocity v L in a series of martensitic steel specimens. The specimens were measured in the as-quenched state to insure that the carbon was present primarily as an interstitial in the martensite. Experimentally, β increased with increasing mass percent carbon (or hardness), while v Lremained virtually the same for all specimens. Therefore we conclude that β is sensitive to microstructural variations between the specimens, but v L is not. X-ray diffraction experiments indicate that the dislocation density in the specimens is high (∼1011/cm2) and increases with increasing carbon content. These results support the hypothesis that the observed increase in β may be attributed to dislocations affected by internal stresses in the quenched specimens
Nonlinear acoustic and microwave absorption in glasses
A theory of weakly-nonlinear low-temperature relaxational absorption of
acoustic and electromagnetic waves in dielectric and metallic glasses is
developed. Basing upon the model of two-level tunneling systems we show that
the nonlinear contribution to the absorption can be anomalously large. This is
the case at low enough frequencies, where freqeuency times the minimal
relaxation time for the two-level system are much less than one. In dielectric
glasses, the lowest-order nonlinear contribution is proportional to the wave's
intensity. It is negative and exhibits anomalous frequency and temperature
dependencies. In metallic glasses, the nonlinear contribution is also negative,
and it is proportional to the square root of the wave's intensity and to the
frequency. Numerical estimates show that the predicted nonlinear contribution
can be measured experimentally
Nonlinear acoustic and microwave absorption in disordered semiconductors
Nonlinear hopping absorption of ultrasound and electromagnetic waves in
amorphous and doped semiconductors is considered. It is shown that even at low
amplitudes of the electric (or acoustic) field the nonlinear corrections to the
relaxational absorption appear anomalously large. The physical reason for such
behavior is that the nonlinear contribution is dominated by a small group of
close impurity pairs having one electron per pair. Since the group is small, it
is strongly influenced by the field. An external magnetic field strongly
influences the absorption by changing the overlap between the pair components'
wave functions. It is important that the influence is substantially different
for the linear and nonlinear contributions. This property provides an
additional tool to extract nonlinear effects.Comment: correction : misspelled name in references correcte
Dislocation Kinks in Copper: Widths, Barriers, Effective Masses, and Quantum Tunneling
We calculate the widths, migration barriers, effective masses, and quantum
tunneling rates of kinks and jogs in extended screw dislocations in copper,
using an effective medium theory interatomic potential. The energy barriers and
effective masses for moving a unit jog one lattice constant are close to
typical atomic energies and masses: tunneling will be rare. The energy barriers
and effective masses for the motion of kinks are unexpectedly small due to the
spreading of the kinks over a large number of atoms. The effective masses of
the kinks are so small that quantum fluctuations will be important. We discuss
implications for quantum creep, kink--based tunneling centers, and Kondo
resonances
The Cause of ‘Weak-Link’ Grain Boundary Behaviour in Polycrystalline Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10 Superconductors
The detrimental effects of grain boundaries have long been considered responsible for the low critical current densities (J_c) in high temperature superconductors. In this paper, we apply the quantitative approach used to identify the cause of the 'weak-link' grain boundary behaviour in YBa2Cu3O7 [1], to the Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10 materials that we have fabricated. Magnetic and transport measurements are used to characterise the grain and grain boundary properties of micro- and nanocrystalline material. Magnetisation measurements on all nanocrystalline materials show non-Bean-like behaviour and are consistent with surface pinning. Bi2Sr2CaCu2O8: Our microcrystalline material has very low grain boundary resistivity (ρ_GB), which is similar to that of the grains (ρ_G) such that ρ_GB≈ρ_G=2×〖10〗^(-5) Ωm (assuming a grain boundary thickness (d) of 1 nm) equivalent to an areal resistivity of ρ_G=2×〖10〗^(-14) Ωm^2. The transport J_c values are consistent with well-connected grains and very weak grain boundary pinning. However, unlike low temperature superconductors in which decreasing grain size increases the pinning along the grain boundary channels, any increase in pinning produced by making the grains in our Bi2Sr2CaCu2O8 materials nanocrystalline was completely offset by a decrease in the depairing current density of the grain boundaries caused by their high resistivity. We suggest a different approach to increasing J_c from that used in LTS materials, namely incorporating additional strong grain and grain boundary pinning sites in microcrystalline materials to produce high J_c values. Bi2Sr2Ca2Cu3O10: Both our micro- and nanocrystalline samples have ρ_GB/ρ_G of at least 10^3. This causes strong suppression of J_c across the grain boundaries, which explains the low transport J_c values we find experimentally. Our calculations show that low J_c in untextured polycrystalline Bi2Sr2Ca2Cu3O10 material is to be expected and the significant effort in the community in texturing samples and removing grain boundaries altogether is well-founded
Atomistic simulations of dislocation mobility in Al, Ni and Al/Mg alloys
Dislocation velocities and mobilities are studied by Molecular Dynamics
simulations for edge and screw dislocations in pure aluminum and nickel, and
edge dislocations in Al-2.5%Mg and Al-5.0%Mg random substitutional alloys using
EAM potentials. In the pure materials, the velocities of all dislocations are
close to linear with the ratio of (applied stress)/(temperature) at low
velocities, consistent with phonon drag models and quantitative agreement with
experiment is obtained for the mobility in Al. At higher velocities, different
behavior is observed. The edge dislocation velocity remains dependent solely on
(applied stress)/(temperature) up to approximately 1.0 MPa/K, and approaches a
plateau velocity that is lower than the smallest "forbidden" speed predicted by
continuum models. In contrast, above a velocity around half of the smallest
continuum wave speed, the screw dislocation damping has a contribution
dependent solely on stress with a functional form close to that predicted by a
radiation damping model of Eshelby. At the highest applied stresses, there are
several regimes of nearly constant (transonic or supersonic) velocity separated
by velocity gaps in the vicinity of forbidden velocities; various modes of
dislocation disintegration and destabilization were also encountered in this
regime. In the alloy systems, there is a temperature- and
concentration-dependent pinning regime where the velocity drops sharply below
the pure metal velocity. Above the pinning regime but at moderate stresses, the
velocity is again linear in (applied stress)/(temperature) but with a lower
mobility than in the pure metal.Comment: PDF, 30 pages including figures, submitted to Modelling Simul. Mater.
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