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The varistor voltage increases with increasing number of ZnO grain boundaries between the electrodes.
Thus, to fabricate varistors with low breakdown voltages it is necessary to reduce the number of ZnO grain boundaries between the electrodes.
It has been reported that adding only Ba or Ti to ZnO promotes grain growth enabling large ZnO grains to be obtained [4,5].
Addition of 0.25 mol% Ba did not promote grain growth, but Ba addition above approximately 0.35 mol% did promote grain growth.
However, Ba is uniformly distributed at grain boundaries despite the non-uniform distribution of Bi at grain boundaries.

The TRC strip (Fig. 1a) had a coarse dendritic columnar grain structure with an average grain size of 600 mm.
The Hunt map in Fig 2(b) predicts that TRC of Az31 alloy with adequate grain refiner addition produces a fully equiaxed grain structure.
A physical approach to grain refinement by MC process has been developed to provide grain refinement without addition of chemical grain refiners.
In the MC-TRC process, Mg-alloy melt has been intensively sheared to enhance nucleating particles with adequate number density, suitable size and distribution.
In the case of MC-TRC process is the solidification driven process, which can be controlled by the number density of potent nucleating oxide particles.

TNTZ, which was subjected to aging treatment at 723 K for 259.2 ks in vacuum followed by water quenching, subjected to HPT processing at rotation numbers (N) of 1 to 20 under a pressure of around 1.25 GPa at room temperature.
The materials composed of nanosized grains with high angle grain boundaries exhibit better mechanical properties and higher hardness values as compared to coarse–grained materials.
In this study, the grain size, namely grain diameter for TNTZAHPT was measured quantitatively using TEM bright field images.
As similar to TNTZHPT, the non-etched band of TNTZAHPT is thinner at low rotation numbers and in the central regions.
The thickness of the non-etched band increases with the equivalent strain (εeq) as a function of the number of rotations (N) and the distance from the center. 3.

The ECAP processed material after three passes exhibit superior LCF lives at low strains presumably due to higher strength and large refinement of grains.
The work pieces can be pressed a number of times using the same die because the cross-section remains constant.
Refinement of grain size occurs because of the increasing strain.
As a result, the structure and mechanical properties of materials processed by ECAP are affected by the number of passes and the processing route [2 - 4].
The superior performance of the material after three passes may be attributed to higher strength and large refinement of grains (see Fig. 3).

Denomination Magnetism Aided Machining (MAM) comprises a number of relatively new industrial machining processes (mainly finishing and surface improving) developed presently, too.
v f \ Al2O3+Fe grains grains a) b) Pole Workpiece Al2O3+Fe grains Fig. 1.
The consequences based on the experimental results are: • The natural (mined) Al2O3+Fe grains have sharper cutting edges than the TiC+Fe grains produced by sintering, thus the material removal rate (MRR) is higher using the former type of grains
For finishing cuts finer grains (Wz = 100…300 μm) are recommended
The grist or flour fills the cavities of the grains, and this way the grains glide over the fine surfaces of the workpieces without scratching them while the sharp burrs will be removed.

In both cases, the ultrafine grains were developed in the samples.
The elongation was not affected significantly by the number of ARB cycles in both cases.
However, such ultrafine grains are not developed in the sample of CR-ARB.
Tensile strength of the samples processed by the CR-ARB increased with the number of ARB cycles.
Lee: Ultrafine Grained Materials 2, Edited by Y.

One of the reasons is that many microstructural factors near grain boundaries contribute to the mechanical properties, e.g. the width of PFZ, size of grain boundary precipitates and so on.
On the grain boundaries, furthermore, grain boundary precipitates are observed in all the investigated alloys.
The quantitative analysis in Fig.4 reveals that whereas the addition of Ag results in a high number density in the later stage, in the Cu-added alloy the number density is higher than in the Al-Zn-Mg ternary alloy in the early stage and subsequently decreases.
Ag-added Cu-added Al-Zn-Mg ternary 10 0 10 1 10 2 A.Q. 0 5 10 15 20 Number density, N ×1023 /m3 Aging time, t /ks Fig.4 Changes in number density of nanoclusters and η' precipitates formed in the grain interiors of the Al-Zn-Mg ternary, Ag-added and Cu-added alloys aged at 433K.
The quantitative analysis of the 3DAP results reveals that whereas the addition of Ag results in a high number density in the later stage, in the Cu-added alloy the number density is higher than in the Al-Zn-Mg ternary alloy in the early stage and subsequently decreases.This is in agreement with the hardness results, and suggests that Cu increases the rate of nucleation, while Ag increases the density of nucleation sites.

The gradual reduction of grain size with increasing ultrasonic output power can be attributed to an increase of the nuclei number formed in the initial crystallization period.
The average grain size was 120µm.
With increasing the treating time, the grain size of the alloy was reduced.
In this way, a large number of nuclei can be produced during the expansion stage.
Thus, the grains of the Mg-3.0wt.

There are few Fe and Mn element in liquid alloy, limited number of small particles in the formed compounds is the reason for no grain refinement.
It is indicated that Mg-Al phase composition at the grain boundary changed.
Grain refinement appears, which is available from Fig. 3.
As shown in Fig. 2, TRT process led to the grain of sample No. 2 coarsening.
Consequently, grain refines the same as sample No.3.

The main aim of the present paper is 1) to identify some general trends in dependency of eutectic temperature on the chemical nature of components, 2) to describe of a number of specific sintering parameters of the multiphase eutectiс ceramics, as well as 3) to understand the dependency of eutectic temperature on the number of components in a given system.
Fig. 5 Microstructure of solidified eutectic in system LaB6-B4C-W2B5 Fig. 6 Decrease of melting point (liquid phase formation temperature) with increasing number of components in a system Analysis of the experimental data on the decrease in Teut with increasing number of components in a system shows liquid formation temperature that is almost half of that for individual phases.
As noted above, there is a correlation between the temperature of liquid formation, on one hand, and a number of components in a system (n≥2), on the other hand.
Minimization of the grain size in a sintered material at equivoluminar compositions confirms the screening effect: pure component phases after sintering show grain diameters of ~12 to 18 μm (growth factor 120 to 180), whereas two-phase sintered materials have grain diameters of ~2 to 4 μm (growth factor 20 to 40).
As the number of component phases n increases, grain growth intensity is decreased at all sintering temperatures considered (see Fig. 7).