15,063 research outputs found
How do the grains slide in fine-grained zirconia polycrystals at high temperature?
Degradation of mechanical properties of zirconia polycrystals is hardly
discussed in terms of solution-precipitation grain-boundary sliding due to
experimental controversies over imaging of intergranular amorphous phases at
high and room temperatures. Here, the authors applied the techniques of
mechanical spectroscopy and transmission electron microscopy (TEM) to shed
light on the amorphization of grain interfaces at high temperature where the
interface-reaction determines the behaviour of fine-grained zirconia
polycrystals. They present mechanical spectroscopy results, which yield
evidences of an intergranular amorphous phase in silica doped and high-purity
zirconia at high temperature. Quenching of zirconia polycrystals reveals an
intergranular amorphous phase on TEM images at room temperature.Comment: 12 pages, 3 figure
Domain Patterns, Texture and Macroscopic Electro-mechanical Behavior of Ferroelectrics
This paper examines the domain patterns and its relation to the macroscopic electromechanical behavior of ferroelectric solids using a theory based on homogenization and energy minimization. The domain patterns in different crystalline systems are classified, the spontaneous strain and polarization for single crystals and polycrystals are characterized, and the optimal texture of polycrystals for high-strain actuation is identified. The results also reveal why it is easy to pole PZT at compositions close to the 'morphotropic phase boundary'
Geometry of polycrystals and microstructure
We investigate the geometry of polycrystals, showing that for polycrystals
formed of convex grains the interior grains are polyhedral, while for
polycrystals with general grain geometry the set of triple points is small.
Then we investigate possible martensitic morphologies resulting from intergrain
contact. For cubic-to-tetragonal transformations we show that homogeneous
zero-energy microstructures matching a pure dilatation on a grain boundary
necessarily involve more than four deformation gradients. We discuss the
relevance of this result for observations of microstructures involving second
and third-order laminates in various materials. Finally we consider the more
specialized situation of bicrystals formed from materials having two
martensitic energy wells (such as for orthorhombic to monoclinic
transformations), but without any restrictions on the possible microstructure,
showing how a generalization of the Hadamard jump condition can be applied at
the intergrain boundary to show that a pure phase in either grain is impossible
at minimum energy.Comment: ESOMAT 2015 Proceedings, to appea
Stress-Induced Phase Transformations in Shape-Memory Polycrystals
Shape-memory alloys undergo a solid-to-solid phase transformation involving a change of crystal structure. We examine model problems in the scalar setting motivated by the situation when this transformation is induced by the application of stress in a polycrystalline material made of numerous grains of the same crystalline solid with varying orientations. We show that the onset of transformation in a granular polycrystal with homogeneous elasticity is in fact predicted accurately by the so-called Sachs bound based on the ansatz of uniform stress. We also present a simple example where the onset of phase transformation is given by the Sachs bound, and the extent of phase transformation is given by the constant strain Taylor bound. Finally we discuss the stressâstrain relations of the general problem using MiltonâSerkov bounds
On the electronic structure of the charge-ordered phase in epitaxial and polycrystalline La1-xCaxMnO3 (x = 0.55, 0.67) perovskite manganites
In this work the charge transport properties of charge ordered (CO)
La1-xCaxMnO3 (LCMO) (x= 0.55, 0.67) epitaxial thin films and polycrystals are
discussed following the recent controversy of localised electron states vs.
weakly or de- localised charge density wave (CDW) states in CO manganites. The
transport properties were investigated by current vs. voltage, direct current
resistivity vs. temperature, local activation energy vs. temperature,
magnetoresistance and admittance spectroscopy measurements, which all indicated
a localised electronic structure in the single CO phase. Delocalised charge
anomalies observed previously may be restricted to phase separated materials.Comment: Physical Review B, to be publishe
On calculation of effective galvanomagnetic characteristics of inhomogeneous metals. Exact solution for the longitudinal effective conductivity of polycrystals of metals in high magnetic fields
In the framework of the perturbation theory an expression suitable for
calculation of the effective conductivity of 3-D inhomogeneous metals in
uniform magnetic field is derived. For polycrystals of metals with closed
Fermi surfaces in high magnetic fields the perturbation series defining the
longitudinal and the hall elements of the perturbation series can be summed
allowing us to obtain the exact expression for the leading terms of all these
elements of the effective conductivity tensor.Comment: 12 page
Multiple scattering theory for polycrystalline materials with strong grain anisotropy: theoretical fundamentals and applications
This work is a natural extension of the authors previous work, Multiple
scattering theory for heterogeneous elastic continua with strong property
fluctuation, theoretical fundamentals and applications, which established the
foundation for developing multiple scattering model for strongly scattering
heterogeneous elastic continua. In this work, the corresponding multiple
scattering theory for polycrystalline materials with randomly oriented
anisotropic crystallites is developed. As applications in ultrasonic
nondestructive evaluation, we calculated the dispersion and attenuation
coefficient of one of the most important polycrystalline materials in
aeronautics engineering, high temperature titanium alloys. The effects of grain
symmetry, grain size, and alloying elements on the dispersion and attenuation
behaviors are examined. Key information is obtained which has significant
implications for quantitatively evaluating the average grain size, monitoring
the phase transition, and even estimating gradual change in chemical
composition of titanium components in gas turbine engines. For applications in
seismology, the velocities and Q-factors for both hexagonal and cubic
polycrystalline iron models for the Earth uppermost inner core are obtained in
the whole frequency range. This work provides a universal, quantitative model
for characterization of a large variety of polycrystalline materials. It also
can be extended to incorporate more complicated microstructures, including
ellipsoidal grains with or without textures, and even multiphase
polycrystalline materials. The new model demonstrates great potential of
applications in ultrasonic nondestructive evaluation and inspection of
aerospace and aeronautic structures. It also provides a theoretical framework
for quantitative seismic data explanation and inversion for the material
composition and structural formations of the Earth inner core.Comment: 37 pages, 16 figure
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