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Introduction Wrought aluminium alloys have a number of desirable properties such as low density, high specific strength and excellent formability that make them particularly attractive in various applications such as aviation and automotive [1].
In the chemical refinement, chemical elements such as Ti and Sc are added as grain refiners to Al alloys, reducing the grain size [3].
Grain sizes measured using the linear intercept method (ASTME112-10) [15] and the average grain size of 7 samples reported.
In the case of the ultrasonic agitation, the grains are finer at the centre of the ingot, whilst the grains are larger towards the edge.
Therefore, the grains do not refine uniformly.

Therefore the used definition “the dynamic superplasticity” reflects consecutive change of states which happens in material with initial varying grain size structure under the changing temperature-rate conditions: initial varying grain size → equiaxed fine-grained structure (4…7 microns) formed in the temperature-rate conditions of superplasticity → coarse-grained at further increase in strain rate.
Formation of fine-grained structure and its dependence on temperature and strain rate in the range of phase transitions, create conditions for realization of the mechanism of grain boundary slipping, characteristic for superplasticity.
Statement of the specified task is based on a large number of the experimental data generalized in [1] and relating to alloys as: AMg5 (AlMg5, 5056), 1561, D18T (2117), 1980, B95 (AlZnMgCu1.5, 7075), AK4 (2618), AK6 (AlCuMg0.5, 2117), AK8 (AlCuSiMn 2014).
Valiev, Grain Boundaries and Properties of Metals, Metallurgiya, Moscow, 1987
Lyashenko, A model of grain boundary sliding during deformation, Technical Physics Letters. 38 (11) (2012) 972–974

Most grains became much finer and coarse grains were broken into fine grains.
In addition, by increasing the number of PITS-ECAPT passes, micro-hardness of the sample was slightly increased and almost remained the same.
Most particles were contacted with each other, both the number and the size of pores were reduced, thus the compact density was slightly improved.
By increasing the number of ECAPT passes, as the powder compact was almost approached to the theoretical compacted density, the mechanical properties were increased slightly.
As the number of passes increased, larger accumulated strains were sufficient to reduce dislocations by annihilation and rearrangement within grains, so the dislocation density was nearly constant, i.e., a dynamic balance between working hardening and grain recovery was occurred.

Then the alloy was annealed in order to obtain samples of a given grain size.
An average grain size is about 100 nm (Fig. 1).
At higher magnifications one could see that the alloy was not fully recrystallized and there was quite a number of defects inside the grains.
Also quite a number of low angle boundaries was found inside some grains.
The average grain size was 100 nm.

The as-cast ingots were cold-rolled into sheets 1 mm thick, which were annealed for 1.8 ks at 350℃, resulting in a fully recrystallized grain structure with a mean grain size of 41 µm.
boundaries, while the elongated grains are often subdivided by low-angle boundaries.
This map shows the presence of a number of boundaries with misorientation angles less than 2°, which further subdivide the grains into smaller units.
The dislocation density is, in general, higher in the elongated grains than in the equiaxed grains, as can be seen in Fig. 2(a).
Micrographs taken under such conditions were used for dislocation density measurements and it was found that the density within the equiaxed grains is 1.0 ×1013 m -2 and 3.5×1013 m -2 in the elongated grains.

In the Mg-1Zn-1Nd alloy, grain coarsening is accompanied by a bimodal grain size distribution, whereas in the Mg-4Zn-1Nd alloy, the grain coarsening leads to a uniform grain size distribution.
However, magnesium has limited formability at room temperature due to the insufficient number of slip systems, and thus a strong basal texture after rolling.
The Mg-4Zn-1Nd alloy shows a uniform grain size distribution with an average grain size of ~50 μm, while the Mg-1Zn-1Nd alloy shows a bimodal grain distribution and the average grain size is an order of magnitude smaller, i.e. only ~5 μm.
Precipitates can be found inside the recrystallized grains and also close to the grain boundaries.
It is well-known that solute segregation can decrease the grain boundary mobility and slow down the grain growth rate.

The grain size distributions after the tests are plotted in Fig. 1.
For the ultimate grain size distribution, D is always around 2.5 [6-10].
Therefore, in this condition, the Bpi is defined as the area between initial grain size distribution and ultimate grain size distribution up to 0.5 mm particle size and the Bt is defined as the area between initial grain size distribution and current grain size distribution up to 0.5 mm particle size, as shown in Fig. 3.
The conclusion further verifies that the number of broken particles increases with the increasing confining pressure during compression.
The number of the largest particles decreases with the increase in confining pressures, and the number of smaller particles increases with the increase in confining pressures.

The increasing versatility of testing machines, like dilatometry with easily variable temperatures, in addition to the growing expenses that go along with increasing the number of experiments for high cost materials, leads to the question whether performing all those experiments is really justified.
Reducing the number of experiments by 50 % during materials characterization therefore appears feasible.
The grain sizes after cooling are measured metallographically.
Similar spacing is used for the matrices covering grain growth, srx kinetics, grain size after drx and srx respectively.
While in classical models grain growth can only start after full srx, Fig. 8 shows that grain growth starts while srx is still in progress.

Finally, the original coarse equiaxed grains were replaced by new recrystallized grains and the average grain size is smaller than the original grains.
Fig. 6 Grains size and number variation: (a) one pass; (b) two pass The grain size and number variation are shown in Fig. 6 after pressing one (Fig. 6(a)) and two (Fig. 6(b)) pass.
Fig. 6(a) shows, after one pass, the microstructure was coarse equiaxed grain, average size of the grains is 2.9 and the number of grain is 84.
Fig. 6(b) shows that after pressing two pass, the grain size reduced drastically; the average size of the grains is 2.7 and the number of grain is 94, which is small equiaxed grains and distributed randomly.
Under all conditions except the involving grain refinement by ECAP, the increased number of grains was determined mainly by the recrystallization nucleation process.

The following equation is realized up to 0.2µm grain size in the relation between yield strength σy and grain size d: σy [MPa]= 100+600×d[µm]-1/2.
In the photograph (a), it is found that dislocations nucleate at grain boundary and a fairly large number of dislocations move even though the applied stress is less than yield strength.
The grain refinement below 1µm is not so easy but ultra fine grained (UFG) iron has been fabricated by consolidation of mechanically milled iron powder.
The grain size of commercial low carbon steels is around 10µm, thus the contribution to grain refinement strengthening can be estimated at 140MPa (white arrow).
Fig. 13 schematically illustrates the grain boundary.