159 research outputs found
Simulations of cubic-tetragonal ferroelastics
We study domain patterns in cubic-tetragonal ferroelastics by solving
numerically equations of motion derived from a Landau model of the phase
transition, including dissipative stresses. Our system sizes, of up to 256^3
points, are large enough to reveal many structures observed experimentally.
Most patterns found at late stages in the relaxation are multiply banded; all
three tetragonal variants appear, but inequivalently. Two of the variants form
broad primary bands; the third intrudes into the others to form narrow
secondary bands with the hosts. On colliding with walls between the primary
variants, the third either terminates or forms a chevron. The multipy banded
patterns, with the two domain sizes, the chevrons and the terminations, are
seen in the microscopy of zirconia and other cubic-tetragonal ferroelastics. We
examine also transient structures obtained much earlier in the relaxation;
these show the above features and others also observed in experiment.Comment: 7 pages, 6 colour figures not embedded in text. Major revisions in
conten
Kinetics of coherent order-disorder transition in
Within a phase field approach which takes the strain-induced elasticity into
account, the kinetics of the coherent order-disorder transition is investigated
for the specific case of alloy. It is shown that a microstructure
with cubic precipitates appears as a transient state during the
decomposition of a homogeneous disordered solid solution into a microstructure
with tetragonal precipitates embedded into a disordered matrix. At
low enough temperature, favored by a weak internal stress, only
precipitates grow in the transient microstructure preceding nucleation of the
precipitates that occurs exclusively at the interface of the solid
solution with the precipitates. Analysis of microstructures at
nanoscopic scale shows a characteristic rod shape for the
precipitates due to the combination of their tetragonal symmetry and their
large internal stress.Comment: 2 postscript figures and 1 JPG pag
Spin density wave dislocation in chromium probed by coherent x-ray diffraction
We report on the study of a magnetic dislocation in pure chromium. Coherent
x-ray diffraction profiles obtained on the incommensurate Spin Density Wave
(SDW) reflection are consistent with the presence of a dislocation of the
magnetic order, embedded at a few micrometers from the surface of the sample.
Beyond the specific case of magnetic dislocations in chromium, this work may
open up a new method for the study of magnetic defects embedded in the bulk.Comment: 8 pages, 7 figure
Patterned nanostructure in AgCo/Pt/MgO(001) thin film
The formation of patterned nanostructure in AgCo/Pt/MgO(001) thin film is
simulated by a technique of combining molecular dynamics and phase-field
theory. The dislocation (strain) network existing in Pt/MgO is used as a
template whose pattern is transferred to AgCo phase in spinodal decomposition,
resulting in regular arrays of Co islands that are attracted by the
dislocations. The influence of various factors, such as component concentration
and film thickness, is studied. It is found that the spinodal decomposition of
AgCo in this system is mainly characterized by a competition between a
surface-directed layer structure and the strain-induced patterned structure,
where the patterned Ag-Co structure only dominates in a small range near the
interface (less than 10 atomic layers). However, if the interlayer diffusion
can be minimized by controlling film growth conditions, it is shown that the
patterned structure can be formed throughout the entire film.Comment: 8 pages, 12 figure
Recommended from our members
ON LONE ELECTRON PAIR MECHANISM OF SUPERCONDUCTIVITY IN CERAMIC MATERIALS
Recommended from our members
ON LONE ELECTRON PAIR MECHANISM OF SUPERCONDUCTIVITY IN CERAMIC MATERIALS
Recommended from our members
Progress report on DOE research project [Thermodynamic and kinetic behavior of systems with intermetallic and intermediate phases]
A theoretical investigation was made of the coherent displacive phase transformation between two equilibrium single-phase states producing several orientation variants of the product phase. The research was focused on a behavior of coherent systems (martensitic systems, metal and ceramic, and ferroelectric systems) with defects. The computer simulation demonstrated that randomly distributed static defects may drastically affect the thermodynamics, kinetics, and morphology of the transformation. In particular, the interaction of the transformation mode with the defects may be responsible for appearance of two new fields in the phase diagram: (i) the two-phase field describing the tweed microstructure, which consists of the retain parent phase and the variants of the product phase and (ii) the single-phase field describing the tweed microstructure, which consists of the variants of the product phase. These new fields can be attributed to the pre-transitional states observed in some of th e displacive transformations. The microstructure evolution resulting in formation of the thermoelastic equilibrium is path dependent. This unusual behavior is expected in systems with a sharp dependence of the transition temperature on the defect concentration
- âŠ