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Samples were pressed through various numbers of repetitive passes up to a maximum of 8 passes corresponding to an imposed strain of ~8.
Furthermore, this result is not consistent with published data for many other alloy systems: for example, experiments showed the measured 0.2% proof stress increased significantly after processing by ECAP for a number of commercial aluminum-based alloys having submicrometer grain sizes [21].
First, the average grain size was reduced initially to ~0.5 µm.
Although it is relatively easy to introduce an ultrafine grain size into a metal through processing by SPD, these extremely small grains are generally not stable at elevated temperatures unless precipitates are present to restrict the grain boundary mobility.
The latter type of cavitation failure is typical of a number of superplastic alloys and the overall results are consistent with conventional superplasticity occurring in materials with grain sizes in the range of ~2-5 µm [27].

.%) alloy is mainly attributed to the fine grains and the less segregation phases at grain boundary.
Consequently, the number of nuclei is increased, the growth of grains is further restricted, and the structure with fine equiaxed crystal grain can be obtained eventually [13].
From the SEM photograph in Fig. 2(a), the grain boundaries or interdendritic region can be seen obviously, the grain boundary segregation is seriously, and the catenoid or spherical precipitates distribute in the grain boundary.
In addition, the number of segregation in Fig. 2(b) is sharply less than that in Fig. 2(a).
The reason may be attributed to the grain refinement and the augment of the grain boundary proportion .

φ=0° φ=45° φ=65° (a) (b) An investigation of the nucleus density only in one plane does not give sufficient information about the absolute nuclei number because grains can nucleate and grow also in other directions.
The nucleus number distribution, in general, has also a tendency to increase with the band size.
For bands with 20 to 30 µm length average number was about 2-4 nuclei, for larger bands from 4 to 6.
Therefore, the number of nuclei has been taken not per grain, but per band length.
In the first case, if several separated bands are introduced instead of one large deformed Cube grain in the simulated deformed microstructure, the number of possible nucleation sites on the Cube-S boundaries is increased, which increases the volume fraction of RX Cube grains.

Compared to other SPD methods, HPT has a number of advantages and is promising in obtaining true nanocrystalline structure in pure metals whereas other techniques, such as, for example, equal-channel angular pressing (ECAP) enable to attain only submicrocrystalline structure.
Average grain sizes were determined with an accuracy of ± 2%.
The grain boundaries are mostly wide and curved, and complicated diffraction contrast and moiré pattern in grains bulk testify the presence of high internal stresses.
Dark-field images (with electron diffraction patterns) of Nb specimens (at the radius middle) after 5 (a) and 10 (b) HPT at room temperature The steady-state deformation and saturation of grain refinement at SPD was reported in a number of publications (see, e.g., [5-9]), and different explanations of this phenomenon were suggested.
At higher annealing temperatures, beginning from 500 0C, an intensive grain growth is observed, and grain sizes grow to as large as 1 mm or even larger.

The described process of calculation is used by microstructures of particular planes: · addition of points of intersection boundaries of area orientated grain boundaries of microstructure with parallel (horizontal) experimental lines labeled as (Pp), in the case of microstructure Fig.4, where the number of points of intersection is 491; · addition of points of intersection boundaries of area orientated grain boundaries of microstructure with vertical experimental lines (P0) is 391; · calculation of points of intersection of surface grains with parallel experimental lines related to the length unit of an experimental line (Pl)p with the unit [mm'1] according to the relation · calculation of points of boundary intersection of surface grains with parallel experimental lines related to the length unit of the experimental line (PL)O with the unit [mm-1] according to a relation.
When the value of microstructure orientation is equal to 1 and the number of cross-sections of surface grain boundaries with parallel and vertical experimental lines is same, it means the structure is not orientated.
Orientation of grain boundaries - the _ first draw 057 x 5 mm.
Orientation of grain boundaries - the second draw 050 x 3.75 mm.
Orientation of grain boundaries - the third draw 044 x 3 mm.

EBSD results showed that after five cycles of ARB, sheets were found to contain ultra-fine grains with high fraction of high angle grain boundaries.
Fig. 2 shows that the mean spacing of the lamellar boundaries decreased by increasing the number of the ARB cycles, but, the fraction of high angle boundaries increased during the ARB process continuously.
Fig. 3 shows the weight loss of the ARBed 6061 aluminum sheets versus number of ARB cycles.
Variation of weight loss of the ARBed 6061 Al alloy as a function of number of ARB cycle (under applied load of 4.2 N).
- A coarse grain structure formed under subsurface which had a strain incompatibility regarding to the fine grain structure and non-equilibrium ultra-fine grains (ARB structure)

The strain rate sensitivity is found to increase slightly in ultrafine-grained Cu compared with conventional Cu, but decrease in ultrafine-grained Fe and Ti relative to their coarse-grained counterparts.
Such UFG metals usually contain a large number of dislocations in a heavily deformed microstructure.
The dislocations produced during SPD have organized into grain boundaries and the dislocation density is low inside the grains.
In actuality, it only means that the number of dislocations that get stored into the nc/UFG microsctructure during deformation is no longer significant to appreciably elevate the flow stress over the strain range tested.
Note that a large fraction of high-angle grain boundaries are indeed present to facilitate grain boundary sliding.

Using random number select transition particles and their transfer direction and compared with transfer time and particles falling transition interval time.
The main influence of the deposition rates is the number of transition.
Along with the increase of Si content, Si hinder grain growth trend is more obvious, especially when the content of 8%, the size of grain was a substantial decrease.
Si mainly is in the hole or the interstitial position of crystal lattice when the Si content is small; Si presents at the boundary of grain massively when the Si content is large, to barrier TiN crystal grain growing up, and to gather flaws in the around of Si, to increase the crystal grain quantity, so it is advantageous to the crystal grain refinement.
Si will cause grain refinement and the crystal grain arranges anomalous.

Microstructural observation showed that the increasing in mixing time has increased the number of AC particles to become agglomerated.
The black particles indicate AC particles which located at the grain boundaries.
The AC particles such as showed in Fig. 2(a) had been well-structured along the grain boundaries.
Reinforcement concentration between the grains will hold the grains and preventing from grain dislocation and slow down the grain growth [12].
It can be observed that the number of AC particles become agglomerated with the increasing of mixing times where the size of AC particles increased with the increasing of mixing time.

Both HPT and ECAP are capable of imparting Von Mises strains up to two figures, depending on the number of turns, passes, or die geometry.
In this respect, Figure 15 of that paper [6] summarizes a large number of investigations correlating flow stress with grain size, over the range of d = 5 - 1000 nm.
However, this mechanism has also been considered as probable in a number of cases in which grain size was in the S-mc/coarse range.
However, the number of recent investigations in S-mc materials pointing out to grain boundary sliding as an operative mechanism appears to be growing; for instance internal friction Figure 3.
The authors show that grain size reduction led to a decrease in grain boundary diffusion activation energy, thus shifting the onset of grain boundary sliding to lower temperatures.