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This was attributed to the development of very thin layers of a Zn-rich grain boundary phase, which leads readily to the occurrence of enhanced grain boundary sliding at room temperature.
Homogeneous coarse-grained (CG) microstructure with an average grain size of 50 μm was observed in the annealed material.
Summary CP Cu and Al 6063 alloy were subjected to ECAP and ECAP-PC processing for varying number of passes.
Langdon, Twenty-five years of ultrafine-grained materials: Achieving exceptional properties through grain refinement, Acta Mater. 61 (2013) 51-59
Langdon, Enhanced ductility of nanocrystallien and ultra-fine grained metals, Rev.

The mechanisms of grain refinement are discussed.
The backscatter electron images in Fig. 2 show the general features of microstructural evolution with the increased number of CFAE passes.
EBSD maps of CFAE processed IF steel sheet samples, showing evolution of deformation structure with the increased number of passes: a) 1, b) 2, c) 3, d) 4, e) 6 and f) 8.
Mechanisms of grain refinement.
Summary IF steel sheets have been processed by continuous frictional angular extrusion to various total numbers of passes.

The grain diameter reduced from 45µm to 2.8 µm after 6 passes of ECAE.
The refinement in grain size causes dislocation pile- up at the grain boundary, which impedes dislocation movement and increases the yield stress of the material.
His study also included the optimal choice of the processing route and number of passes.
Their results demonstrated that the grain size of the alloy gradually reduced with the increase of the pass number and the mechanical properties were significantly improved after the ECAE.
Fig.8 shows Scatter plot of the Vickers Microhardness across the flow direction for different number of passes.

The AISI serial numbers include steel type 1006(6A), 1008(8A), 1010, 1015, 1018, and 1022 [2~3].
This is mainly because the twisted and deformed grains are gradually recovered and part of second crystal grains develop and combine to greater crystal grains.
In one of them, ASTM grain numbers for wire materials of 6A and 8A are greater than that of 1T wire materials.
Table 3 The integrated table of the average grain size for each set of wire material Wire material number Position 1 Position 2 Position 3 Position 4 ASTM grain number Rolling 1T 9.3 9.1 8.9 9.3 9.1 Rolling 6A 8.9 9.0 9.0 9.2 9.0 Rolling 8A 8.9 9.3 9.5 9.3 9.3 Annealing 6A 9.2 8.7 8.8 9.1 9.0 Annealing 8A 6.8 7.7 7.4 7.4 7.4 Conclusions This study makes comparisons and analyses on the wire rod materials of extra low carbon steels and low carbon steels on mechanical properties, micro-structures, crystal grain sizes.
ASTM grain numbers for wire materials of 6A and 8A are greater than that of 1T wire materials.

One efficient approach is processing NC material that combine grains with bimodal distribution in which NC grains provide high strength, whereas coarse grains can enhance ductility.
As such the total density of dislocation can be described by , where and are the volume fraction of the coarse grains and NC grains, respectively, and are the density of stored dislocation in the coarse grains and NC grains[9], respectively.
The total back stress stemmed from all dislocations accumulated along the grain boundaries[10] is expressed by : (3) where is the number of dislocations stopped at the grain boundaries, and,is the density of nano/microcracks ,is the maximum number of dislocation that can be emitted from the nano/microcracks tip along one slip plane.
To calculate the maximum number of lattice dislocations that can be emitted along one slip pline , we use the following calculation procedure as introduced by Ovid’ko and Sheinerman[11].
Fig. 1 Dependence of the maximum number of edge dislocation that can be emitted from the crack tip along one slip plane on the grain size d in bimodal copper It has been proved that the number of nano/microcracks in BNC materials increases during plastic deformation, which leads to the density of nano/microcrack being sensitive to the applied load.

Although the number of triple junctions depends on the number of boundaries, all peculiarities in the behavior of polycrystals during grain growth have been solely attributed to the motion of grain boundaries so far.
The grain boundary GB12 was vicinal to a twin boundary 60° <111>, and the other two grain boundaries were random high-angle grain boundaries.
Grain 2 Grain 3 Grain 1 Fig. 7.
The normal grain growth is driven by curvature, particularly of grain boundaries at conventional grain size, but predominantly by triple line curvature at nanometer grain size dimension.
The triple line excess energy is the difference between the total energy of a polycrystalline volume and the total energy of a single crystal with the same number of atoms plus the excess energy due to the grain boundaries present.

Because of these large strains, materials grains get finer.
Their results indicate that for route BC, it was kept constant when the number of passages N was increased.
The grain size was measured according to the ASTM standard E112.
The yield and total strains are increased by increasing the pass numbers.
The results of experiments indicated that the strength of the extruded materials increased by increasing the numbers of ECAE passes and hence the grain refinement.

Stress Generation During Grain Boundary Interdiffusion L.
We show that the inequality of intrinsic grain boundary diffusion coefficients of the two components leads to plating out of additional material at the grain boundary in the form of extra material wedge, which generates an elastic stress field in the vicinity of the grain boundary.
As pointed out by Hwang et al., “…the mass gained or lost by the unequal diffusion in the grain boundary is compensated for by a mass flow in a direction perpendicular to the grain-boundary plane…” [8].
We will further suppose that a total number of atoms of both components per unit area of the GB is constant: ; ; , (3) where d is the GB thickness.
Mishin: Fundamentals of grain and interphase boundary diffusion.

Stress induced macroscopic separation of the material along the grain boundaries is mostly attributed to grain boundary weakening due to intergranular precipitation and/or impurity segregation [1,2].
The composition of grain boundaries was studied by Auger electron spectroscopy (AES) using a Microlab 310F VG.
It consists of twinned austenite grains separated by straight boundaries.
(5) Owing to a number of measured and theoretically assessed quantities of various uncertainty levels, a possible deviation ∆ i γcoh = ∆ i γcoh (∆f, ∆ktd , ∆kmd, ∆k * , ∆RS) of the intergranular fracture energy was determined following the standard error analysis.
The former mechanism was observed in ASS due to grain boundary precipitation of carbide particles.

However, a limited number of studies were dealt with examination of superplastic behavior of Al alloys subjected to ECAP and followed by rolling [5,6].
Most of grains contain no well-defined substructure.
However, former ribbons retain as chains of elongated grains.
In the alloy subjected to warm rolling the static annealing leads to grain coarsening with retaining grain aspect ratio (AR), defined as the ratio of the grain dimension in the longitudinal direction to that in the transverse direction (Fig.4(c)).
Acknowledgments The financial support received from the Ministry of Education and Science, Russia, under Grant No. 14.578.21.0097 (ID number RFMEFI57814X0097) is gratefully acknowledged.