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 . 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 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. 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.