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The mechanism for grain refinement is incomplete dynamic recrystallization.
The SAD patterns were arc-like, which suggests that a number of the high-angle boundaries (>15°) exist within the selected areas of Fig. 2b [5].
However, the grain size was not uniform.
The grain with diameter of 2.30 µm was observed in Fig. 5(b).
Lath martensite is a type of fine grained structure subdivided by a number of high-angle boundaries.

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 austenite grain in the HAZ is considerably coarser when compared to the grain size of the parent metal.
Since grain coarsening in the HAZ is accompanied by some loss in toughness, limits have been placed on the recommended weld heat input to constrain grain growth in the HAZ region.
In the case of prior-austenite grain boundaries, the number and size of the M/A particles have the most important influence on the Charpy impact results.
Increasing the number and size of M/A particles at the grain boundaries leads to the increase of the statistical probability of cracking and the crack propagation.
Under dynamic fracture, the failure takes place in a very short time, and thus there is no sufficient time for a crack to find a weak plane and eventually the crack propagates from one grain to the near neighboring grain and extends to the prior-austenite grain boundaries.

- Grain size – grain size distribution was calculated under assumption that the grain boundary misorientation limit is set to 15°.
In the first one number of EBSD points is divided by number of grains to calculate average grain size.
The second grain size statistics we will call “grain size (A)”.
Grain sizes.
Using typical grain size calculations (number of EBSD points divided by number of grains), the average grain size drops down from 140µm2 for initial state to 80µm2 for 17% deformed material (in hard direction) or 40µm2 for 16% deformed material (in soft direction).

The gradual reduction of grain size with increasing ultrasonic output power can be attributed to an increase of the nuclei number formed in the initial crystallization period.
The average grain size was 120µm.
With increasing the treating time, the grain size of the alloy was reduced.
In this way, a large number of nuclei can be produced during the expansion stage.
Thus, the grains of the Mg-3.0wt.

The number of total atoms was set to be 731640.
Thus, the MD calculations indicated that grain boundaries act not only as barriers for dislocation motion but also as dislocation sources, which implies that grain refining increases the number of dislocation sources.
The effect of increasing in the number of dislocation sources on the BDT behaviour will be discussed next, basing on the dislocation shielding theory.
Michot[22] pointed out that the BDT temperature depends on the number of dislocation sources and spacing along a crack front.
It is therefore concluded that the decrease in the BDT temperature by grain-refining is due to not the decrease in dislocation mobility with respect to short range barriers but the increase in the number of dislocation sources around the crack.

A number of special techniques of plastic working has been successfully utilised for processing the ultrafine grained metallic materials [5].
The grain/subgrain sizes were measured perpendicular to the rolling axis by a linear intercept method.
Appearance of individual equiaxed annealed grains takes place upon heating to temperatures of T ³ 500°C.
At 400°C, no changes in the hardness and the transverse grain/subgrain size take place in the both steels during annealing.
Such a uniform mixture of grains of different phases impedes effectively any discontinuous grain growth during annealing.

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.

In addition, in a number of cases, the specimen fractures rapidly following the localization of deformation.
Precisely this fact allows one to realize large deformations of metals using SPD processes and to achieve submicron grain refinement.
After a certain number of TE passes, standard tension test samples with a diameter of 5mm were cut from the billets.
This effect is associated with the emergence of new paths of internal stress relaxation (for example, via grain boundary sliding [9]).
Commercially pure titanium has higher ductility in its UFG state than in the coarse grained state.

It is revealed that a bimodal grain structure, i.e. ultrafine grains accompanied by micron-sized grains was developed after 4 passes.
After three passes, as shown in Fig. 1(c), there was a significant increase in the fraction of HAGBs and a large number of equiaxed grains formed along the grain boundaries between elongated large grains, so it can be concluded that most of these smaller grains were formed by the fragmentation of the elongated grains and some of them were further refined from the fine grains formed in the first and second pass.
After four passes, as shown in Fig. 1(d), a large number of ultrafine equiaxed grains formed along the HAGBs of the remaining coarse grains.
The above results further demonstrates that a bimodal grain structure comprising of ultrafine grains (with grain sizes less than ~1 μm) and micron-sized grains was achieved in the present study.
Acknowledgements The authors would like to acknowledge the financial support from Research Council of Norway, under the FRINATEK project ‘BENTMAT’ (Project number 10407002) and China Scholarship Council (201406080011).