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There are number of refinement mechanisms proposed over the years for grain refinement.
The cavitation induced dendrite fragmentation hypothesis assumes that the shock waves generated from the collapse of bubbles lead to fragmentation of dendrites, which are redistributed through acoustic streaming, thereby increasing the number of crystals.
When ultrasonic intensity is increased to 4 kW/cm2 increased grain refinement is observed and grain size is reduced further to 17 μm.
This hypothesis assumes that the shock waves generated from the collapse of cavitation bubbles cause fragmentation of the dendrites, which are then redistributed through acoustic streaming, thereby increasing the number of crystals and hence refining the grain size.
As a result of grain refinement, the grain boundary area is increased.

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 microstructure of ultrafine-grained steels was analysed before and after nitriding.
HPTD can be used to obtain materials with ultrafine-grained (UFG) structure with mean grain size of less than 1 μm and predominantly large-angle grain boundaries [1-3].
Prior to ion nitriding, samples of martensitic and austenitic steels were treated by heat to obtain a homogeneously grained microstructure.
The presence of a larger number of grain boundaries, vacancies and dislocations in crystalline structure of UFG steel has a stimulating effect on the diffusion of nitrogen, which is confirmed in [9-11].
The presence of a significant number of grains, vacancies and dislocations in the crystalline structure of UFG steels has a stimulating effect on nitrogen diffusion.

The optimized relaxing time on grain refinement is 60 s.
The linear intercept method was used to measure mean austenite grain diameters.
The austenite grain size becomes smaller with the decrease of reheating temperature.
A small grain sized austenite has a relatively large number density of grain boundary nucleation sites so bainite dominates the microstructure, whereas a relatively large number density of intragranular nucleation sites leads to a microstructure consisting of predominant acicular ferrite [13].
It is therefore proposed that lath-like or plate-like acicular ferrite grains formed earlier effectively divide prior austenite grains into smaller and separate regions.

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.

The fibrous grains are aligned close to the billets extrusion direction.
Showing; the grain sizes obtained, using the mean linear intercept λx parallel and λy perpendicular to the main direction of alignment, grain aspect ratio, equivalent circular diameter (ECD) grain size after grain reconstruction from the EBSD data, the standard deviation of the ECD grain size distributions normalised with respect to the mean diameter (σSD/dm), the percentage of HAGB area, and the mean boundary misorientations.
Overall, this leads to a slightly larger average ECD grain size than for Route A and a grain aspect ratio close to one (Table 1).
The average 'grain sizes' determined in table 1 for this alloy are therefore somewhat misleading, as they are dominated by the larger numbers of small grains present, which are particularly fine in the case of the 90° die.
Altering the die angle from 120 to 90°, changes the ideal shear strain per extrusion cycle from 1.15 to 2, but reduces the total number of cycles to obtain the same strain (from 15 to 9 in this case).

The result shows that the Al-3Ti-0.5B master alloy which was prepared by adding mixture of Ti sponge and KBF4 power into molten aluminum contains a large number of granular TiB2 phase and blocky TiAl3 phase.
Addition of grain refiners is the most common and effective method for refining the grain structure during the solidification process [1-3].
Compared with B alloy, the number of TiB2 particles in A alloy is more.
Figure 5.a) shows that commercial purity Al without grain refiner exhibits coarse equiaxed grains and columnar grains.
The macrostructure of Al is composed of fine equiaxed grains after the addition of grain refiner as shown in Figure 5.b) and Figure 5.c) respectively.

Microstructure Evolution in Fine-Grained Microalloyed Steels K.R.
In the present steel, ferrite grain sizes of as low as 2µm have been obtained in this way.
Such process models currently exist for plain carbon and a number of high strength low alloy (HSLA) steels of up to 550MPa minimum yield strength [4].
Similar trends were also observed for the larger austenite grain size of 32 and 53µm.
Similar trends for ferrite refinement were observed for larger austenite grain sizes.

The aim of adding yttrium was to control b grain growth above the b transus by grain boundary pinning.
In the present case, strengthening of the b texture, occurring during b grain coarsening resulted in strengthening of particular b texture components, which increases the likelihood of a texture modification by selective growth of a variants on the common (110) b grain boundaries into unoccupied large b grains. 1.
While EBSD provides the ability to combine macroscopic texture information with information on the microstructural scale, the requirement of very large EBSD maps to capture a sufficient number of b grains in combination with a small step size in order to capture microstructure information, makes this methodology very time consuming.
It seems that the extensive b grain growth observed in conventional Ti-6Al-4V has resulted in preferential growth of favourably oriented b grains belonging to potentially the original cast texture.
As discussed in [1], large b grains allow relatively free growth of a variants from b grain boundaries with two adjacent b grains having a common (110) normal.

The grain boundary microstructure ( the type, the frequency of grain boundaries and the connectivity of different type of grain boundaries ) was analyzed by SEM-EBSP-OIM for all the specimens.
The initial grain structure is drawn in red lines.
It is reasonable to imagine that the grain boundary connectivity may control the grain growth through the interaction between initially existed grain boundaries and newly formed interphase boundaries during phase transformation.
As shown in Figure 4, significant abnormal grain growth was observed and the magnitude of abnormal grain growth increased with increasing the number of cycling.
The grain size of abnormally grown grains was almost ten times larger than the initial grain size ( d0 =59-65µm).