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The 200 peer-reviewed articles in this “Nanomaterials by Severe Plastic Deformation” special collection are a convincing demonstration of the relevance of bulk ultrafine grained and nanostructured materials, produced by severe plastic deformation, to a wide range of researchers and engineers., The total number of articles in this edition, larger than that in the 2008 edition, shows that this community is, in fact, growing.
The coverage includes all aspects of NanoSPD: Principles of SPD Processing, Microstructural Evolution and Grain Refinement, Mechanical Properties of SPD Materials, Functional and other Properties of SPD Materials, Innovation and Applications.

After 50% reduction and above, an equiaxed, fine grained structure mainly surrounded by high-angle boundaries was uniformly formed with dislocation substructures, where the dislocation density in the grains is relatively low.
Introduction Decreasing a grain size is a promising strategy to increase both strength and toughness of metals.
A large number of low-angle boundaries are also present within the blocks.
With increasing applied strain, a martensite lath structure was gradually changed into an equiaxed fine grained structure containing dislocation substructures.
A quite uniform fine grained structure with a large amount of high-angle boundaries was obtained only after 50% compression and above.

Grain refinement remains the only option to increase strength without introducing harmful side effects.
Numbers indicate applied pass strain.
Partial austenite recrystallization after each finishing pass was expected to provide only limited austenite grain refinement and a relatively coarse final ferrite grain size.
The depth of the X trough correlated with the final ferrite grain size.
Influence of Al on grain refinement in as-rolled V-microalloyed steels.

The effect of grains boundaries self-orientation caused by grains deformation was evaluated using stereology.
But the orientation is not the same as deformation and so a correlation between the grain deformation and grain orientation was used.
In an undeformed state, the structure is isotropic, the grains have isometric dimension and grain boundaries are not oriented.
From the specific number (number to unit of length) of parallel test lines intersections with grain boundaries (PL)P and perpendicular lines ones (PL)O, the total specific surface area (SV)TOT of grains and the planar oriented part of specific surface area (SV)OR of grains were estimated.
From measured grain boundary orientation which is proportional to grain boundaries deformation degree the local plastic deformation was estimated.

These materials can be produced through a number of chemical and physical processing routes, the best results, so far, have mainly been attained by Severe Plastic Deformation (SPD) processes.
The grain size distributions and the average grain size were obtained after measuring individually more than 250 grains in each case.
The grains were equiaxial with a homogeneous grain size distribution [7].
Especially in the 0.55 %C milled powder, a great number of carbon atoms are present in the ferrite grain boundaries and also interstitially solved in the ferrite, occupying the dislocation cores [11,12].
In this case, the absence of carbon leads to the rapid growth of some grains and a little number of grains are clearly larger than 1 µm.

The SPS-950 consists mainly of submicrometer-sized grains (Fig. 3a), whereas the PLS-1350 exhibits larger grain sizes, usually of several micrometer (Fig. 3b).
Generally, domain wall motion induces the degradation of Qm and thereby, higher Qm value shown in the SPS-950 was attributed to pinning the movement of the aligned domains on the large number of grain boundaries resulting from the fine grain size [7].
In addition, it is interesting to note that the Kp value is the same for both the SPS-950 (fine grains) and the PLS-1350 (larger grains), because generally as grain size decreases, the randomly oriented domains are difficult to be arranged [8].
Domain structures depending on the grain sizes in the SPS-950. was domain size�(grain size) m , which leaded to a parabolic scaling (m�1/2) [9].
Most grains are found to consist of ferroelectric domains whos e structure is similar to that in bulk, with single-domain in a few small grains.

The AFM results for all ratios indicate that as the number of pulses increase, the higher the resulted grain size.
The average grain size was between (58.82 nm) to (95.75nm).
It's seen that as the number of pulses increase, the average grain size increase due to cluster formation.
Figure 1 below demonstrates the AFM results while Table 2 sums up the grain sizes.
The AFM results for all ratios indicate that as the number of pulses increase, the higher the resulted grain size.

The main factors in producing of ALFA using the powder metallurgy method are the process of compacting and its large grains.
This is consistent with the analysis of variance; the greatest significant influence is grains size factor, with 74% the percent contribution.
The compressive strength will be increase that is caused by increase of compacting pressure and large grain size.
The main factor which influences compressive strength is the grain size.
The smaller the grain size, the higher compressive strength of ALFA. 3.

However, hot rolling is a very complex process, which requires a strict control over a number of different parameters.
C)Recrystallized grain size.
As the pass number is increased, the critical strain increases.
In Fig.7b, large grains characteristic of the deformed, unrecrystallized austenite are apparent with a few smaller recrystallized grains often located at three grain intersections on prior austenite grain boundaries.
In this figure, the evolution of average austenite grain size during rolling is graphically represented against pass number.

As for the properties of grain boundaries the compensation effect was observed in grain boundary diffusion [1], migration [2,3,5] and solute segregation [5].
To fulfill the condition 4, the grain boundary concentration is defined by the tangent to the Gibbs energy of the grain boundary parallel to Fig. 1 Gibbs energy in grain boundary segregation.
(9) In general, d∆H ch(Ψj) ≠ 0 and d∆Sch(Ψj) ≠ 0 are real numbers and therefore, there must exist a range of the changes of the variables Ψj within which the constant temperature Tc is defined as j PT N j j ch j PT N j j ch ch ch c ji ji S H S H T Ψ         Ψ∂ ∆∂ Ψ         Ψ∂ ∆∂ = ∆ ∆ = Ψ≠Ψ = Ψ≠Ψ = ∑ ∑ d d d d ,, 1 ,, 1
Let us comment here this connection for grain boundary segregation.
Tc is 930 K in the grain boundary segregation in α-Fe but 1386 K in the grain boundary migration in Fe-6at%Si alloy [5]).