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The development of a coarse grained microstructure during the recrystallization has been noted and discussed by a number of authors but, the mechanism of grain control remains uncertain.
It appears that two factors: the peculiar distribution of high- (HAGB) and low- (LAGB) angle grain boundaries in the as-consolidated state, and the uniformity of the fine scale microstructure makes the nucleation of recrystallization difficult because the grain boundary junctions are strong pinning points, restricting the bowing of grain boundaries [7].
The grain structures in the tensile and compressed areas of sample 4 are presented in Fig.5.
It should be emphasized that the sub-micron grains are not low-misorientation cell structures typical in aluminum alloys, but true grains with large misorientations (see Fig. 5).
Subsequent heat-treatment leads to recrystallization into a very coarse grained microstructure [4].

In this strain range, grain size (thickness) results from both the convection and the migration of grain boundaries.
Grain size, or rather grain thickness, is determined by the combination of convection and migration of the grain boundaries [4].
However, a number of metallographic investigations carried out on b titanium alloys and an aluminium alloy have shown that the measured average thickness Hexp is systematically larger than H [4].
In fact the true origin of average grain growth is the disappearance of grains when boundaries impinge together.
At intermediate strains, grains become strongly serrated in association with the generation of a large number of subgrain boundaries.

With an increase of deposition time, the Sn/S atomic ratio and grain size and the number of crystallites in the layers increased while the dislocation density and bulk resistivity decreased.
Fig. 2(b) gives the variation of the number of crystallites, CA with the deposition time.
This could be due to an improvement in the crystallinity of the layers corresponding to larger grain sizes thereby decreasing the grain boundaries and consequently a decrease in the dislocation density.
Fig. 2: Change of (a) crystallite size and (b) number of crystallites and dislocation density with deposition time.
The increasing the deposition time could increase the crystallite size and the number of crystallites whereas the dislocation density and the bulk resistivity decreased.

Statistical Analysis of Histograms of Grain Size Distribution in Nanostructured Materials Processed by Severe Plastic Deformation A.
Studies have shown that bulk metal materials with structure, in which crystallites with sizes less than 100 nm, limited by high-angle boundaries, predominate, can be formed only by a limited number of methods.
In all these works, the results are represented as histograms of size distribution of grains.
The analysis of the histograms of grain size distribution was carried out in two stages.
Ditenberg, On the limiting minimum size of grains formed in metallic materials produced by high pressure torsion, Phys.

In order to achieve finer grains and better mechanical properties, multi-pass FSP was conducted to prepare fine-grained magnesium alloys.
Dadashpour et al. [7] investigated the effect of heat treatment and number of passes on microstructure and properties in AZ91C alloy.
The as-FSP alloy exhibited an increase in ultimate tensile strength (UTS) and elongation as the number of passes increasing due to finer grains and more dissolution of β phase (Mg17Al12).
After FSP, microstructures in SZ of AZ61 alloy are greatly refined into equiaxed grains with an average grain size of about 2.0 μm as shown in Fig. 1(c).
Mostafapour, Effect of heat treatment and number of passes on the microstructure and mechanical properties of friction stir processed AZ91C magnesium alloy, J.

The fine NbC precipitated in the crystal and grain boundary enhanced strength of grain at high temperature, which resulted plastic deformation or slip at high temperature were mainly concentrated in the grain boundary, and high temperature plasticity of HAZ were decreased.
Test Materials and Methods Two commercial seamless steel tubes of TP347H stainless steel were selected as test materials, with specifications of OD76 × 6(mm) and OD57 × 9(mm), numbers D and X, respectively.
Number C Si Mn Cr Ni Nb S P Tube X 0.040 0.31 1.48 18.00 10.77 0.61 0.001 0.026 Tube D 0.041 0.44 1.45 17.57 9.05 0.72 0.002 0.026 Fig.1 Heat Cycle Curve of Simulated HAZ of Welded Joint.
Under optical microscope, twin austenite structure and grain grade 4-6 were observed for the section N of X6 sample, and win austenite structure and grain grade 5-7 for section N of D7 sample.
Grain strength is enhanced by the precipitation of fine NbC in the grain boundary and grain boundary under the welding thermal cycle, plastic deformation and slip at high temperature mainly concentrate on the grain boundary, which lead to the decrease of high temperature plasticity of thermal cycle area.

Two related alloys were tested: one inoculated with boron grain refiner Ti-44Al-7.5Nb-2.7B (at.%); and the other Ti-45.5Al-8Nb (at.%) without grain refinement.
Microstructure engineering of specimens inoculated with boron grain refiner.
They can serve as the nuclei of primary crystallites which grow further into equiaxed grains.
The added refiner has the effect of creation a large number of sites for heterogeneous nucleation of bcc β(Ti), thus preventing the lateral columnar growth.
The equiaxed grains form a random pattern, as shown in Fig.7.

It can be seen from Figure 1 without the pulsed electric field processing of solidification, center grains are coarse , the direction of growth are disorder, right side are mostly small equiaxed grains.
It can be seen from Figure 2 that the pulsed electric field processing of samples solidified, center grains are coarse, the right end are much more tightly packed columnar grains, the direction of growth of columnar crystal are same in the cooling direction.
When the liquid metal was connected with the anode of the pulse, Fe, Mn, Cr atoms will rapidly close to the solid - liquid interface at the role of electric field moment, yet Si atoms will be away from the solid - liquid interface, with the number of solute atoms declining, the number of solute atoms are fewer, nucleation rate has dropped, the resulting grain size to slightly larger.
Thus by the pulsed electric field treated samples, the number of martensite are increased, the number of ε → γ transformation are increased after heat back, which increased the amount of shape recovery. (2) Solidification of pulsed electric field treated are most columnar crystals, most non-treated solidification are nearly equiaxed.
Pulsed electric field is benefit to Fe-Mn-Si-Cr alloys grain refinement.

The CNA method uses a ternary number, ijk to denote an inter-atomic bond.
In the ternary number system, i designates the number of common neighbors shared by two atoms, j denotes the number of bonds among those common neighbors, and k indicates the longest path in these bonds.
Within the grain size regime investigated, the yield stress increases with increasing grain size, revealing an inverse Hall-Petch effect.
A number of theories have been proposed to explain the Hall-Petch break-down[18], a recent argument put forward by Louchet [19] and co-workers seems particularly appealing.
However, a peak fraction exists for each grain size with the actual peak value depending on the initial grain size.

The results have indeed shown that IFC can shorten the HP and reduce austenite grain growth and its variation across thick plate.
This technique has been successfully adopted in a number of industrial plate mills.
The prior austenite grain sizes from the as-quenched PSC samples were revealed by etching in saturated picric acid and grain sizes were measured by the linear intercept method.
Fifthly, the steel B with Ti and Nb microalloying elements had the finest prior austenite grain size after 3 roughing passes, as shown in the Fig. 5, which indicates that the Ti restricted the austenite grain growth during the high temperature deformation.
The refinement of austenite grain size is due to the IFC lowered austenite grain growth temperature and to a much shorter waiting time during the HP.