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Figure 1 (a) is a photograph of the sample sintered at a temperature of 450 ℃, it can be seen , the number of small second phase , the formation of voids at the grain boundaries more loose tissue of the large particles and between the particles the binding force is not high .
Figure 1 (b) is a photograph of the sample sintered at a temperature of 500 ℃, it can be seen, significantly increased the number of the second phase, a continuous distribution on the grain boundaries, but still significant voids phenomenon, which may be due to sintering temperature slightly above the eutectic temperature of the material, there eutectic liquid, appear transient liquid phase sintering, the grain boundary diffusion is not fully, resulting sintered structure is not dense.  
Figure1 (c) photograph of the sample sintered at a temperature of 550 ℃, it can be seen, the second phase the number continues to grow, there is almost no voids exist, the higher the density of the material.
Most La and Al react to form Al11La3 who looks like the needle, in turn, increases dispersion of the grain boundary phase; Needle (Al, Mg) 11 la3 phase forming nucleation first , adsorbs on α- Mg , stopping α- Mg grain's growth due to its pinning effect, and then we get fine grains and reinforced matrix.
This is consistent with strengthening mechanism of refining the grain of the magnesium alloy at room temperature, also can use Hall-Petch formula (1) to explain, namely, grain size, the greater the compressive strength is smaller, the smaller the grain size, grain diameter, the greater the compressive strength

The addition of carbide-forming elements (such as Cr) leads to an increase in the number of grains containing shear bands after warm rolling, a change that can improve the formability of the steel after annealing [3-6].
Bulk texture Deformed grains Recrystallized grains Figure 3.
The common feature for both steels is that the grains nucleated at ferrite grain boundaries were larger than those nucleated within the grains (predominantly at the in-grain shear bands).
For both steels, the grain sizes of recrystallized γ-fiber grains were slightly larger than of α-fiber grains.
Calculations are based on the numbers of nuclei observed.

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.

Generally the grain size should be ≤10µm.
Some coarse-grained alloys can also present superplasticity except fine-grained structural superplasticity.
Recrystal grains grown obviously, grain boundary is distinct owing to high tensile temperature and long tensile time.
The formation of finer grain consumes distortion energy, reduces dislocation density, loosens stress concentration caused by grain-boundary sliding, provides new, slidable, large angle grain boundaries and benefits grain-boundary sliding [4~5].
The grains are finer and the fracture is even.

The as-forged Ti-45Al-5Nb-0.3Y alloy is comprised of a large number of dynamic recrystallization (DRX) γ grains, curved and broken lamellae, and a small amount of remnant lamellae.
Furthermore, each of grains is comprised of lamellar microstructures (TEM image is shown in Fig.2b).
Metallographic examination shows that, after high temperature forging, the majority zone of the asforged Ti-45Al-5Nb-0.3Y alloy is comprised of a large number of dynamic recrystallization (DRX) γ grains, curved and broken lamellae, and a small amount of remnant lamellae (Fig.4a).
The DRX γ grain size reaches 1~2µm.
After holding at 1320℃,1340℃ for 30min and at 1370℃ for 15min, TiAl samples contain γ and α2 grains, and when it is furnace cooled to room temperature, the phase transformation of α → L (α/γ) → L(α2/γ) takes place in α grains.

In the text, SSVs of an orientation x are numbered x1, ..., x4, i.e. the basic variant and three other ones, generated respectively by two-fold axes 2NDL , 2TDL , 2RDL .
Journal Title and Volume Number (to be inserted by the publisher) 3 4 Title of Publication (to be inserted by the publisher) Figure 2b shows a much less typical case of nucleation in a band with the orientation close to the Cube component.
In the foils prepared from ~10%recrystallized material two types of new grains were observed: small, isolated grains and much larger grains formed complex sets of recrystallization twins.
The orientations of small grains - with the area at least 102 times smaller than the area of the largest grain observed - are distributed around the deformation texture components.
Small, isolated grains could be found in the neighborhood of complex recrystallization twin sets. 6 Title of Publication (to be inserted by the publisher) Differences in the size of new grains can be related to the characteristics of orientation relationships between a new grain and the deformed matrix; quite different for small and large grains.

Kimura, Mechanical properties of ultra fine grained steels, J.
Tsuzaki, On Annealing Mechanisms Operating in Ultra Fine Grained Alloys, in M.
Also, the characteristic length of dislocation becomes grain size dependent in submicrocrystalline/nanocrystalline materials, i.e. the grain size strengthening should vanish as the grain size decreases down to nanoscale [[] E.
This suggests that the grain size strengthening becomes less effective as the grain size decreases to submicron range.
Acknowledgements The financial support received from the Ministry of Education and Science, Russia, under Grant No. 14.575.21.0003 (ID number RFMEFI58414X0003) is gratefully acknowledged.

Nanocrystalline materials with high strength have been reported in large numbers.
Additionally, nanocrystalline materials are characterized by a large volume fraction of grain boundaries and triple junctions [1].
When the grain size is reduced to the nanometer range, the dislocation density within the grain is decreased in particular for materials with average grain sizes less than 30nm [2].
This result suggests that fabricated bulk nc-Ni-W had inhomogeneous grain size.
Bulk nc-Ni with a grain size of about 60 nm exhibited an ultimate strength of 1006 MPa and good ductility of 8.8 %. 3.

The size of VC decreases and its number density increases by lowering transformation temperature, corresponding to the larger hardness increase.
Both specimens transformed at temperatures above 873K consist of pearlite and small amounts of proeutectoid ferrite along prior austenite grain boundaries and inside austenite grains.
Black region is untransformed austenite now transformed to martensite during quenching and bright region is grain boundary ferrite grains.
As shown in Fig. 4, a large part of ferrite grains hold near K-S relationship with one side of austenite grain and there is no ferrite grain holding near K-S relationship with both sides of austenite grains.
With lowering transformation temperature, dispersion of VC becomes finer in size and higher in number density

For efficient production of ultra-fine grain structure in the alloys with γ'- phase via SPD, it is necessary to have a large number of incoherent γ/γ' boundaries prior to deformation.
Annealing twins were present in some grains.
Recrystallized grains of submicron sizes were revealed near these areas, as well as in the original grain boundaries.
A lesser number of passes (by a factor of two) was required to generate refined structure in Allvac 718Plus than in Alloy 718.
These factors narrowed the MF temperature range and, correspondingly, decrease the number of passes needed to generate structures with the same refined grain sizes.