Concise Dictionary of Materials Science Part 9 ppt

silica
Silicon dioxide SiO
2
. Interatomic bond in silica is partially covalent and
partially ionic (see electronegativity). It has three polymorphic modifica
-
tions: cristobalite, tridymite, and quartz, with the transformation temper
-
atures 1470 (cristobalite ↔ tridymite) and 867°C (tridymite ↔ quartz).
In all of the modifications, Si atoms are arranged at the centers of tetra
-
hedra formed by O atoms.
simple lattice
See primitive lattice.
single crystal
Body consisting of one crystal only. There are no grain boundaries
in single crystals, although subboundaries and sometimes twin boundaries
can be found.
single-domain particle
Magnetic particle whose minimum linear size is smaller
than the domain wall thickness; because of this, it consists of one magnetic
domain. If several domains were present in such a particle, the particle’s
free energy would be increased. In the particle, the energy of the magnetic
poles at its surface is the lowest in the case of the largest pole spacing.
Thus, in a single-domain particle of an elongated shape, the orientation
of its magnetization vector is determined not only by its magnetic crys
-
talline anisotropy, but also by its shape anisotropy. If elongated single-
domain particles are oriented predominately along the same direction in
a body, the latter possesses a magnetic texture and excellent hard-magnetic
properties.

single slip
Dislocation glide motion over a single slip system characterized by
the maximum Schmid factor.
sintering
Procedure for manufacturing dense articles from porous particulate
compacts (porosity in green compacts usually is between 25 and 50 vol%)
resulting from spontaneous bonding of adjacent particles. The main driv
-
ing force for sintering is a decrease of an excess free energy associated
with the phase boundaries. Sintering is fulfilled by firing the compacts
at high temperatures (up to ∼0.9 T
m
), and is always accompanied by their
shrinkage and densification (i.e., a decrease in porosity). Shrinkage
evolves primarily through coalescence of neighboring particles under the
influence of the capillary force in the neck between the particles. The
pore healing also contributes to shrinkage. Densification during sintering
is accomplished by both the surface diffusion and the grain-boundary
diffusion. It is essential for densification that the pores remain at the grain
boundaries, because the pores inside the grains can be eliminated by slow
bulk diffusion only, whereas the grain-boundary pores “dissolve,” via the
splitting out of vacancies and their motion to sinks, by much more rapid
grain-boundary diffusion. Thus, the theoretical density can be achieved
in cases in which the abnormal grain growth is suppressed and the rate
of normal grain growth is low (for details of microstructure evolution in
the course of sintering, see solid-state sintering). Sintering can be accel
-
erated in the presence of a liquid phase (see liquid-phase sintering) or by
pressure application during firing (see hot pressing).
size distribution

Histogram displaying the frequency of grains (or particles) of
different sizes. The shape of grain size distribution after normal grain
© 2003 by CRC Press LLC
silica
Silicon dioxide SiO
2
. Interatomic bond in silica is partially covalent and
partially ionic (see electronegativity). It has three polymorphic modifica
-
tions: cristobalite, tridymite, and quartz, with the transformation temper
-
atures 1470 (cristobalite ↔ tridymite) and 867°C (tridymite ↔ quartz).
In all of the modifications, Si atoms are arranged at the centers of tetra
-
hedra formed by O atoms.
simple lattice
See primitive lattice.
single crystal
Body consisting of one crystal only. There are no grain boundaries
in single crystals, although subboundaries and sometimes twin boundaries
can be found.
single-domain particle
Magnetic particle whose minimum linear size is smaller
than the domain wall thickness; because of this, it consists of one magnetic
domain. If several domains were present in such a particle, the particle’s
free energy would be increased. In the particle, the energy of the magnetic
poles at its surface is the lowest in the case of the largest pole spacing.
Thus, in a single-domain particle of an elongated shape, the orientation
of its magnetization vector is determined not only by its magnetic crys
-

talline anisotropy, but also by its shape anisotropy. If elongated single-
domain particles are oriented predominately along the same direction in
a body, the latter possesses a magnetic texture and excellent hard-magnetic
properties.
single slip
Dislocation glide motion over a single slip system characterized by
the maximum Schmid factor.
sintering
Procedure for manufacturing dense articles from porous particulate
compacts (porosity in green compacts usually is between 25 and 50 vol%)
resulting from spontaneous bonding of adjacent particles. The main driv
-
ing force for sintering is a decrease of an excess free energy associated
with the phase boundaries. Sintering is fulfilled by firing the compacts
at high temperatures (up to ∼0.9 T
m
), and is always accompanied by their
shrinkage and densification (i.e., a decrease in porosity). Shrinkage
evolves primarily through coalescence of neighboring particles under the
influence of the capillary force in the neck between the particles. The
pore healing also contributes to shrinkage. Densification during sintering
is accomplished by both the surface diffusion and the grain-boundary
diffusion. It is essential for densification that the pores remain at the grain
boundaries, because the pores inside the grains can be eliminated by slow
bulk diffusion only, whereas the grain-boundary pores “dissolve,” via the
splitting out of vacancies and their motion to sinks, by much more rapid
grain-boundary diffusion. Thus, the theoretical density can be achieved
in cases in which the abnormal grain growth is suppressed and the rate
of normal grain growth is low (for details of microstructure evolution in
the course of sintering, see solid-state sintering). Sintering can be accel

-
erated in the presence of a liquid phase (see liquid-phase sintering) or by
pressure application during firing (see hot pressing).
size distribution
Histogram displaying the frequency of grains (or particles) of
different sizes. The shape of grain size distribution after normal grain
© 2003 by CRC Press LLC
T
Taylor factor
Quantity averaging the influence of various grain orientations on
the resolved shear stress, τ
r
, in a polycrystal:
σ = Mτ
r
(M is the Taylor factor, and σ is the flow stress). The averaging is fulfilled
under the supposition that the deformations of the polycrystal and its
grains are compatible. Reciprocal Taylor factor can be used for polycrys
-
tals instead of Schmid factor, whose magnitude is defined for a single
grain only. In a nontextured polycrystal with FCC structure, reciprocal
Taylor factor is 0.327.
temper carbon
In malleable irons, graphite clusters varying in shape from flake
aggregates to distorted nodules.
tempered martensite
Microconstituent occurring in quenched steels upon the
tempering treatment at low temperatures. Due to the precipitation of
ε
-

carbides, the lattice of tempered martensite is characterized by a tetra-
gonality corresponding to ∼0.2 wt% carbon dissolved in the martensite.
See steel martensite.
tempering of steel martensite
Alterations in the phase composition under the
influence of tempering treatment. They are the following. Up to ∼200°
C, as-quenched martensite decomposes into tempered martensite and ε-
(or η-) carbide (in low- to medium-carbon steels) or χ-carbide (in high-
carbon steels). Above ∼300°C, cementite precipitates from the tempered
martensite, whereas the latter becomes ferrite and the ε- and η- (χ-)
carbides dissolve. In steels alloyed with carbide-formers, the alloying
elements inhibit the carbon diffusion and displace all the previously men
-
tioned phase transitions to higher temperatures. In addition, at tempera-
tures ∼600°C, the diffusion of the substitutional alloying elements
becomes possible, which leads to the occurrence of special carbides
accompanied by cementite dissolution. The phase transformations
described are accompanied by the following microstructural changes in
martensite and ferrite. Crystallites of tempered martensite retain the shape
of as-quenched martensite. Ferrite grains, occurring from tempered mar
-
tensite, do not change their elongated shape and substructure until coars-
© 2003 by CRC Press LLC