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What’s more, the number of grains and nucleation rate increase along with the increase of the ultrasonic power.
The dimensions of grains decrease with the rise of the ultrasonic power.
What’s more, the number of grains and the nucleation rate increase along with the increase of the ultrasonic power.
The power of crystallization which is related to form new grains increases a lot.
Such the powerful shock wave acting on grains can cause huge energy fluctuation and fuses or breaks the growing grains, making the close range molecules changing to remote range molecules, which is conducive to form new grains.

A large number of microcracks along grain boundaries of the equiaxed structure solder developed with increasing cycling, while for the dendrite structure solder, cyclic deformation took place along the direction of the maximal shear stress during fatigue tests and microcracks initiated and propagated along shear deformation bands.
The equiaxed microstructure of the Sn-3.8Ag-0.7Cu solder was composed of equiaxed tin-rich grains and needles or nodules of Ag3Sn phase and Cu6Sn5 intermetallic, as shown in Fig.1(a).
The size of most of the equiaxed β-Sn grains of the solder were in the range of 10~30 um.
For the SnAgCu solder with equiaxed structure, although some deformation in some grains was observed, a large number of microcracks developed along the grain boundaries, as shown in Fig.4(a).
Hence, the grain boundary sliding might be involved in development of the microcracks.

However, the achievement of full density depends on a number of factors including the shape, size and plasticity of the particles, hydrostatic pressure applied, processing temperature, deformation rate and actual plastic strain experienced by the particles.
In the case of Al, it was thought to be related to easier recrystallisation at room temperature and decreasing number of pores acting to pin grain boundaries at the higher pressure [25].
The stability of the material was attributed to the presence of pores which pinned grain boundaries and thus prevented grain growth.
Although Fig. 5 Compressive stress versus strain curves showing increases in both strength and strain to fracture with increasing number of passes in Al consolidated from nanosized particles by ECAP at 400°C.
A number of promising materials with both enhanced strength and good ductility have been achieved.

Texture weakening or randomization are desirable approaches to get high performance Mg alloy and a small number of cases having weak or non-basal textures have been reported recently for the Mg alloy sheets and extrusions[3-6].
Grains shown in Fig.2(d) are large deformed unrecrystallized grains.
Fig. 6(a) shows several long elongated grains (LEGs) and besides those grains the microstructure appear lots of fine equiaxed grains (FEGs).
With extrusion temperature increasing the number of LEGs decreases but that of FEGs increasing.
At 665K the elongation has been improved, which is ascribe to the homogenous grain distribution and the fine grain size.

Fig. 2 Grain boundary defect model for (Gd,Co,Nb)-doped SnO2 varistors.
Breakdown electrical field of varistors can be expressed by the following equation [7]: EB =n • Vg, (2) where n is the average grain number per unit length, Vg, the breakdown voltage of a grain boundary.
It is found that the resistance of grain boundaries decreases significantly with increasing Gd2O3 concentration.
All the above results and analyses indicate that the average grain size plays a very important role in the variation of electrical properties and Gd2O3 concentration is the substantial reason for the variation of average grain size.
It is not clear how CoGd2O4 inhibits the SnO2 grain growth up to now and this problem needs to be further studied.

Today a number of advanced technologies, such as precision casting technology and deep drawn or rolled materials are becoming popular in the market.
The average grain size was 15 µm.
The grains clearly became finer at the material closer to the die cavity.
Although the average grain size was deceased to 5.6 µm at 40 mm, the microstructure consists of very fine grains and large grains in a manner of the so-called necklace structure.
On the other hand, at the corner the non-recrystallized grains decreased and the recrystallized grains and the region increased.

With increasing the number of rolling steps, the content of CNTs coated on the Cu powder surface increased because of the increment of the flat surface area of flaky Cu rolled powder.
CNTs distributed at primary particle boundaries were effective to prevent the grain coarsening by their pinning effects, and this grain refinement was the main strengthening factor of the Cu-CNT composite via rolling process.
These raw materials are shown in Figure 1.The grain size of Cu powder was about 3μm.
The grain growth was promoted during sintering even though dispersed with Cu2O particles.
Fig.7 Relationship between yield stress and grain size of extruded materials.

Introduction A large number of different processes have now been proposed for the severe plastic deformation (SPD) of metals, including equal channel angular extrusion (ECAE) [1,2], high-pressure torsion [3], and accumulated roll-bonding (ARB) [4,5].
It has been suggested that the transition to a continuous microstructural evolution is a result of the presence of a large number of high angle boundaries (HABs) in the system, with a figure of 70% HABs often quoted as the transition threshold [9].
The most direct parameter for following the microstructural evolution during annealing is perhaps the average grain size, or more usefully, the grain/cell size distribution.
During conventional recrystallization, nucleation and growth of new grains within a deformed matrix will lead to a bimodal grain size distribution.
For the ECAE ε ≈ 1 sample, annealing results clearly in discontinuous recrystallization, with a large number of non-randomly located nuclei.

Fine-grained concretes are inferior to powder concretes not only in resistance, but also in durability (Fig. 4, 5).
This is primarily due to the large difference in conductivity, which increases as the grain size of the aggregate increases [10].
In cases where fine-grained concrete of the composition C:S=1:3 (by weight) is used, not all aggregate particles are covered with a cement shell [11], so there is a significant number of large pores in the contact zone between the aggregate and the concrete matrix, through which aggressive ions penetrate deep into the sample.
Concrete compositions with various types of aggregates and fillers can be arranged in a row as the corrosion resistance of concrete based on them decreases: powder concrete with quartz filler → powder concrete with marble filler → fine-grained concrete on marble filler → fine-grained concrete on quartz sand
Fine-grained concrete on marble aggregate is more resistant to magnesian corrosion than sulphate one

After the extrusion, the alloy presents a bimodal grain structure consisting of fine equiaxed dynamically recrystalized grains with about 1mm grain size and elongated strips of unrecrystallized grains.
Average grain size increased to 5mm after the heart treatment.
The maximum number of cycles was defined as 107 cycles.
Number of cracks observed at specimen surface was summarized in Fig.4.
Fig. 4 Number of cracks observed at the specimen surface during fatigue test.