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The LM micrographs of ARB samples in Fig. 2a,b show gradual grain flattening with increasing number of ARB cycles.
The size of the lamellae does not change significantly with the increasing number of cycles in AA8006 specimens.
Moreover, in contrast to AA8006 alloy, in AA5754 alloy lND shows a decreasing tendency with the number of ARB cycles.
Grain subdivision by low-angle boundaries (LAGB) and a progressive conversion of LAGB into high-angle grain boundaries occurs with increasing number of cycles in all alloys.
On the contrary, the alloy AA5754 exhibits a steady increase in hardness with increasing number of ARB cycles.

Grain Boundary Diffusion.
Last 60 years special attention in material science is paid to grain boundaries.
Certainly, the main effect is connected with GB structure, determined by orientation of one grain to another.
The typical distribution of ln P value is presented at Fig. 3 b (N is number of GB with given value of P, step is equal to 0.5).
For chemical reaction at grain boundary: nAb+mBb=AnBmb the equilibrium constant can be written as : or for dilute solution.

Material The nickel-based superalloy Alloy 718 (material number 2.4668) used in this study was produced by VDM-Metals via multiple vacuum melting steps.
EBSD analysis prior to loading (IPF map in Figure 4a) provides grain orientation information and characterizes the grain boundary type.
Figure 6 shows an example of the evaluation procedure, presenting an SEM image of different grain boundaries and enabling measurement of oxidation at various grain boundary types.
Twin grain boundaries (CSL3) thus show no discernible oxidation attack, while the attack on the other grain boundary types varies.
Acknowledgements Financial support by the Deutsche Forschungsgemeinschaft (DFG, project number 526257118, project title: Scale-Bridging Microstructure-Sensitive Assessment of Intergranular Cracking during High-Temperature Dwell-Time Fatigue of Polycrystalline Superalloys) is gratefully acknowledged.

Young's modulus tends to decrease and Hall-Petch low fails to describe correlation between grain structure and hardness for submicro-grained and nanocrystalline iron.
Hall-Petch coefficient, ky, decreases as grain size decreases within submicro-grained and, then, nano grained sections and it takes even negative value in nano grained section modified by nitrogen.
Introduction Nano-crystalline materials with a large number of grain boundaries and triple junctions have been found exhibit interesting combination of physical and mechanical properties, making them of growing attention to researches employed in scientific and engineering applications.
According to "composite model" of Mughrabi [11] the former effect could be resulted from the large number of grain boundaries and, thus, increased volume fraction of the triple junctions for which mechanical properties are different from those of grain interior.
Young's modulus for nc α-Fe[N] increases when grain size is limited to less than 20 nm whereas the opposite is true for α-Fe[N] refined to larger grain size.

Different scales of gray in the image indicate different grain orientations, rather than the phase difference at atomic numbers, as in the backscattered electron image.
In these two cases, the sample with higher current is accompanied with more incident electrons, which leads to more significant differences in the number of backscattered electrons among grains with different orientations, and therefore more obvious contrast differences.
When the accelerating voltage is 1kV, the differences in the number of backscattered electrons are very small among grains with different orientations because of the small number of generated backscattered electrons; hence, many of the grains cannot be distinguished (Fig. 7a).When the accelerating voltage is 2kV, the distinction among the number of backscattered electrons for grains with different orientations becomes quite obvious, leading to an obvious channel effect contrast (Fig. 7b).When the accelerating voltage is increased to 5kV, the differences among grains with different orientations decreases (Fig. 7c).When the accelerating voltage is increased to 10kV, the channel effect contrast continues to attenuate (Fig. 7d).When the accelerating voltage is increased to 15kV and 20kV, the channel effect contrast actually strengthens compared with that of 10 kV (Figs. 7e and 7f).
However, there are still a small number of grains that can only be clearly distinguished by EBSD, but not by the channel effect contrast images (such as grains 1–9).
The interfacial angles between the unidentified grains are very small, such as 8° between grains 1 and 2, 4° between grains 3 and 4, 7° degrees between grains 5 and 6, and 2° degrees between grains 7 and 8.

Coarse grain (CG) Titanium with original grain size of 150 μm had been pressed by ECAP at 425oC by 4, 8 and 12 passes, respectively.
There is uniformity in the arrangement of grains in sample and the signal of elongation of grains on rolling direction.
It can be inferred from the results that the hardness of Ti increase proportionally to the number of processing pass, from 228 of 4 passes to 311 of 12 passes.
This indicates the uniformity in the grain arrangement and the significant increase of grain density of Ti during ECAP.
Acknowledgements This work was financially supported by the Vietnam National Foundation for Science and Technology Development (Nafosted), Grant Number 107.02-2010.10.

After 4th pass, the avarage grain size decreased from initial approximate size 7 µm to 200 nm, whereby the average grain size was changeless after subsequent deformations.
Introduction Present scientific research is intensive oriented on ultra-fine grained structures formation (UFG with grain diameter 1µm-200 nm), and nanoscale structures (NSG with grain diameter ≤ 200nm) in polyedric metallic materials, attained through the use of SPD.
Initial Cu structure is coarse grained, with average grain size 7 µm and low yield strength and tensile strength, but with sharply defined reduction of area (82%).
The initial material grain shape is polygonal and uniform.
With the growing number of ECAP passes, the dimple size decreases and the quantity of dimples increase.

The strain is calculated as εeq = 2π·r·n/(t·√3), where r, n and t are the distance from the torsion axes, the number of applied revolutions and the mean thickness of the sample, respectively.
Fig. 3a and 3b show that the fine-grained microstructure after HPT was completely transformed into substructurefree, fully recrystallized grains with polygonal grain shape having an average grain size of 10µm and 2 µm, respectively.
By analyzing the grain size distribution a significant bimodality is observed with a first maximum at 75 nm (fine grains) and a maximum at 500 nm (large grains).
GC is known as a process in which smaller grains are eliminated, large grains grow and the grain boundaries assume a lower energy configuration.
In the fine-grained region the number of favorable nucleation sites and the driving force (reduction in grain boundary energy) is very high.

With increase in Cr content, good grain refinement and less hardness have been observed.
Albuquerque et al. [7] carried out a non-destructive evaluation of grain size influence on the mechanical properties of a Cu-Al-Be shape memory alloy with and without grain refiners and shown that the addition of grain refiners increases the stress and strain of the alloy.
The microstructural change with the formation of small grains was observed under OM.
· The microstructure analysis shows the grain refinement in the alloy with considerable reduction in grain size, otherwise the grain size of ternary alloy was around 20 times larger
· The reduction in the grain size shows the improvement in the hardness

The hot deformation has a critical influence on the number of packets that forms.
Group Number Variants Number {111}γ plane Group 1 1,2,3,13,14,15 (111) Group 2 5,6,7,16,17,18 (-1-11) Group 3 8,9,10,19,20,21 (-111) Group 4 10,11,12,22,23,24 (1-11) Table 1: The 24 KS (or GT) variants and their partition into four groups.
The variant numbers are based on [4].
The reconstructed EBSD maps of 2 prior γ grains of sample 0.2 (grain 1) and sample 0.8 (grain 2) are represented in figures 3a and 3b, respectively.
However, the hot deformation is shown here to have a critical influence on the number of groups that forms.