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The average initial grain size before deformation was about ∼140 µm.
Their number and the boundary misorientations increase with further deformation, finally leading to the development of new fine grains surrounded by HABs at large strains.
The mechanism of gradual increase in the boundary misorientation in the last stage, i.e. 4≤ε≤12, can be associated with increase in the number of lattice dislocations evolved by usual intrinsic slip and the absorption of accommodation dislocation that originates from incompatibility between neighboring grains.
ECAP results in grain refinement at all of the pressing temperatures.
Sakai: Ultrafine grained Materials IV, edited by Y.T.

It is observed that there shows no evidence of reentrant grain flow, and the grain structure of both in the a) b) Fig.1 Optical micrograph of the alloy forging along the thickness: (a) T/2, and (b) T/8.
And in addition to aging precipitates inside grains and on the grain boundaries, there are special coarse precipitates inside the grain in the forging.
In the grain boundary map, coarse blue lines represent high angle grain boundaries (>15°), fine green lines (5~15°) and fine red lines (2~5°) represent low angle grain boundaries (2~15°).
A large number of size grains with high angle boundaries in big size elongated grains shows that the small size grains may be recrystallized grains, which are dominant in the core (T/2) of the alloy forging (as shown in Fig.4(a)).
The grain boundaries of recrystallized grains are as the weak interface, they are easy to become the path of crack propagation, which seriously affect the elongation and fracture toughness of the alloy forging.

Based on these results, a mechanism of grain growth by SIBM was suggested.
The relationship between the temper rolling reduction and average grain diameter; triangle : before temper rolling, full circles: after the final annealing following temper rolling. 0 50 100 150 200 250 0 5 10 15 20 25 30 Reduction (%) Grain Diameters (µm) Journal Title and Volume Number (to be inserted by the publisher) 3 Fig. 3 .
This texture change means that Goss grains grow more preferentially through SIBM than any other grains.
The driving force for this grain growth mechanism is usually considered to be the stored energy gradient between neighboring grains, which is known to depend also on grain orientations [1].
Journal Title and Volume Number (to be inserted by the publisher) 5 From the discussions above, D-Cube grains are thus expected to be more homogeneous than Goss ones, as it is the case for single crystals.

For example, pure aluminium is known to be a soft ductile metal, but the fracture toughness is very low in absolute numbers.
Both UFG-metals show similarities such as the elongated grain structure and the inclination of the grains with respect to the shear direction, which is not noticeable in TEM-microhgraphs.
The grain sizes are around 200 nm to 300 nm in both cases.
The hardness increases linearly with the radius for different number of rotations.
High toughness cannot simply be referred to a reduction in grain size focusing on the given bcc examples.

The change of size and shape of dimples fracture from larger and irregular shape dimples in initial sample to much smaller and grains-look like dimples in ECAPed sample indicating the grain refinement occurred after ECAP.
Introduction The grain refinement is one of the strengthening method of the metal alloys based on the Hall Petch formula which relates the strength with the inverse of the grain sizes, the finer the grains the stronger the alloys.
However, small number of larger dimples still can be observed in a small area.
It is clear from Fig. 4 that the small size dimples have more regular shape (looks like grains), where the initial voids might initiate at grain boundary as the strain concentrator.
The change of size and shape of dimples fracture from larger and irregular shape dimples in initial sample to much smaller and grains-look like dimples in ECAPed sample indicating the grain refinement occurred after ECAP.

T. (001)- and (212)-oriented recrystallized grains mainly nucleate, and (001)-oriented grains mainly grow up.
Although the nucleation of (111)-oriented grains was also observed, the size of such grains is at most of μm order and is smaller than that of (001)-oriented grains.
In contrast, (111)-oriented grains hardly grow with increasing storage time, although the number of nucleated grains increases.
Except (111)- and (001)-oriented grains, (212)- and (112)-oriented grains were also observed.
The grains nucleated in the growth direction of recrystallized grains and (001)-oriented grains mainly grew up to a maximum grain size of approximately 40 μm.

Grain growth and coalescence was prevalent in the early deformation stage, while grain boundaries were impaired and replaced by dislocation interactions when 24%.
Furthermore, the plastic deformation mechanism would change from dislocation activities to grain boundary sliding and/or diffusion when the grain size is decreased below a certain length scale [2].
For undeformed samples (Fig. 3(a)), the grains are equiaxed and separated mainly by high-angle grain boundaries.
Counting about 400 grains from a number of TEM images, an average grain size of 90 nm with a lognormal distribution from 30 to 200 nm can be obtained.
At 24% (Fig. 3(c)), significant changes occurred in both grain size and morphology.

According to Callister [10] the particle growth is a result of the movement of grain boundaries, which is conducted through two processes: grain boundary diffusion and migration of grain boundaries.
Both processes promote the densification, but the grain boundary migration that occurs at a higher temperature promotes faster grain growth.
Measurements of average grain size and interfacial area per unit of volume were carried out (Figure 5) [13], counting the number of intersections between the grain boundary and straight lines with known length, which were designed on the image with the program ImageJ of free access.
The number of intersections per image was more than 400, aiming to achieve better measurement accuracy.
(Numbers denote the logarithm of frequency).

After rolling to 20% pre-deformation degree, a large number of twins and dislocation tangles appear in microstructure, the twins are narrow and crowded together, there are some dislocation tangles in a part of twins, as shown in Fig. 7 (a).
Shear leads to the emergence of a large number of twins which make the make the structure produces certain orientation.
Dislocation tangles lead to the increase of the sub-grain boundaries, at last lead to grain crush.
After remelting at 875°C, the numbers of the twins and dislocation tangles significantly reduce, in Fig. 7(b).
The increasing of liquid phase makes effect of dendrites fusing and solid grain spheroidization better, grain size smaller, and solid grain rounder.

The average grain size was measured by the OIM software, and the average (sub)grains size was measured by the mean linear intercept method.
The number of dislocations emitted by sources was less than the number of dislocations consumed for the formation of new GNBs and increasing misorientation of GNBs existed at e~0.22.
Upon further strain the spacing between GNBs decreases and the number GNBII increases (Fig. 2e).
It is obvious that at high strains, the number of dislocations emitted by sources was significantly higher than the number of dislocations consumed for the formation of new deformation-induced boundaries or increasing misorientation of GNBs existed at e~0.91.
The fraction of new fine grain with an average size of ~1 µm increases to ~19%.