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INFLUENCE OF GRAIN SIZE ON THE THERMAL DIFFUSIVITY OF Pt-SnO2 CERAMIC M.M.
The grain coarsening is mainly attributed to sintering [6].
In bigger grain size samples, phonons would encounter less number of grain boundaries, presenting a longer apparent mean free path.
As shown in Fig.4, the inclusion of 0.5 wt% Pt in SnO2 (Sample B) inhibits the grain growth as it produces smaller average grain size.
This can be attributed to the reduction in the number of grain boundaries blocking the heat flow path.

The results revealed that the change of microstructure plays an important part in the toughness properties at different locations of the HAZ. the CGHAZ (coarsen grain HAZ) microstructure contains granular bainite with the large effective grain size and a number of rod-like M/A constituents, which provide the nucleation sites and linear propagation path for micro-crack, therefore, this location becomes a local brittle zone.
Granular bainite grains have large morphology domains and some prior austenite grain boundaries are retained due to accelerated cooling.
A number of rod-like or granular-like M/A (martensite/austenite) constituents are dispersive distribution in granular bainite.
Due to fine and irregular shaped grains, it is different to calculate average grain size of the FGHAZ according to the optical micrograph.
Each quasi-polygonal ferrite grain has its unique crystallographical orientation domain; therefore, the image quality map and corresponding grain color map exhibit that ferrite grains have very fine crystallographical size.

Grain sizes were measured by intercept method.
When bubbles collapse acoustic streaming develops in the melt, distributing the nuclei into the surrounding liquid producing a significant number of nuclei in the molten alloy, thus promoting heterogeneous nucleation.
When cavitation develops, the shock waves generated by bubbles collapse lead to fragmentation of dendritic cells in the mushy zone, which are re-distributed throughout the melt by acoustic streaming, increasing the number of solidification nuclei.
So the turning point of grain size is 660 ºC.
And because of the high pressure pulses caused by the ultrasonic cavitation, the wettablity of large number of impurity particles, such as Al2O3 based intermetallic particles, will be improved by eliminating the gas absorbed on the particles and fill the melt in the micro cracks of the particles.

Plasticity of Copper with small grain size Y.
It was observed that at intermediate grain size stress relaxation shifts from unit to partial dislocations both nucleated at grain-boundaries and finally in the extreme refinement of the grains a radical change occurs with relaxation at the grain-boundary regions.
The stress concentration depends on the number of dislocations in the pile up ; combining the pile up length to the grain size leads to the Hall-Petch law:τs ∝∝∝∝ Gb d , where τs is the yield strength.
(b) Ultrafine grains have grain size below the critical dislocation mean free path, though (c) pile-up of dislocations and grain-boundaries sources still exist.
The grains are separated by low, high angle grain boundaries and twins.

Not only the number of particles on heterogeneous nucleation is increased, but also it can control the growth of grain.
When addition content of Al-Ti-C-Ce is 0.2%, grain size had a new phenomenon what is local grain coarsening, but the number of second phase particles were decreased.
The number of equiaxed grains decreased.
At the same time, the number of point structure decreased.
However, when addition content is 0.3%, as shown in Fig. 3(d), the number of second phase particles which precipitated from the grain boundary were increased.

The three-stage bolt forming using the ultra-fine grained specimens was carried out to check formability of the material.
The improved mechanical strength achieved by the ECAE is due to grain refinement since strength of materials was known to be proportional to the reciprocal of the square root of the grain size according to the empirical Hall-Petch equation [4].
Recently, high strength M4 bolts were manufactured using the ultra-fine grained AA6061 processed by the ECAE without heat treatment and formability of the ultra-fine grained specimen was investigated by Choi et al. [9].
In experiments of the ECAE, three different routes A, Bc and C up to four passes were applied to investigate the effect of processing routes and the number of passes on microstructural change and its homogeneity.
However, Vickers microhardness value was more homogeneous in the ultra-fine grained bolt than in the conventional one.

Discussion The number of microcracks nucleated by each microstructure for the tested temperatures for the rolling direction is shown in Table 2, which also shows that the number of microcracks decreases with test temperature.
Therefore, a smaller number of microcracks will be developed in a smaller volume of the strained material.
Number of microcracks for each fracture initiation microstructure in the rolling direction.
At –60ºC the number of microcracks decreased about 60%.
Number of microcracks for each fracture initiation microstructure in the transverse direction.

We measured the grain sizes by counting the number of grains intercepting each line using the following equation.
(number of cycles to failure = 11,050,521).
We counted the number of grains within the ODA.
This number includes grains crossing the ODA boundary.
(number of cycles to failure = 21,345,292).

At above 880℃, the grains grow owing to the heating.
The number of grain boundaries decreases, and the boundaries becoming flat and form angles close to 120ºwith the increasing holding temperature.
Under the temperature of 1100℃, the average grain size is less than 250μm; from 1100℃ to 1250℃, the grain size grows gradually; and there is an abrupt growth at 1250℃, as the grain size increases rapidly.
Consequently, the grain size increases tremendously.
At 1220℃, obvious grain growth tendency can be observed.

There are 3 stages: reordering of dislocations and formation of sub grains, nucleation of grain boundaries and finally grain growth.
This P2 grain boundary peak is absent in single crystals and the peak position depends on the grain size.
Starting with a single crystal, the introduction of a small number of dislocations by cold deformation leads to different relaxation mechanisms.
The sub grain boundaries disappear and the misorientation between adjacent grains increases.
At the same time the dislocation density in the grains decreases as grain boundaries absorb residual dislocations.