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The strengthening mechanism belongs to grain refining strengthening.
As can be seen, the as-extruded alloy displayed a very fine grain structure with the average grain size of about only 2 mm.
While for the as-extruded Mg-1.5Zn-0.6Zr-0.2Sc alloy, as shown in Fig. 4(c) and (d), the fracture surface was nearly all occupied by a large number of dimples.
With the fine grain structure, more grains can be aroused to satisfy the deformation coordination under external forces.
First, the as-extruded alloy has much refined grain structure.

It is well known that shear strain plays a critical role in the grain refinement [5-7].
Due to this, grain refinement can be achieved both in the surface layer and in the core of the processed material [8].
Due to necessity of application very fine mesh with large number of elements, these solutions are computationally very costly.
Concentrations of high values of compressive stresses are located near a large accumulation of strong perlite grains.
However, the number of localizations due to the high accumulation of strong perlite grains is growing.

In this case, the error in determining of the mean square displacements of impurity atoms can be reduced by introducing of a large number of impurity atoms.
Mishin, Atomistic modeling of point defects and diffusion in copper grain boundary, Interface Science. 11 (2003) 131-148
Plimpton, Molecular-statics and molecular-dynamics study of diffusion along [001] tilt grain boundaries in Ag, Physical Review B. 51 (1995) 4523-4529
Starostenkov, Mechanisms of grain-boundary diffusion in two-dimensional metals, Technical Physics Letters. 31 (2005) 650-652
Kaygorodova, The causes of formation of the triple junctions of grain boundaries containing excess free volume in fcc metals at crystallization, Solid State Phenomena. 249 (2016) 3-8

Our previous investigations showed that SPD of the Ti-6Al-4V ELI alloy with initial globular coarse-grained structure led to the formation of a number of substructures associated with the formation of new grains, subgrains and twins [6,7].
Some grains of former globular α-phase have small colonies of very thin (less than 50 nm) micortwins (Fig. 3b).
The α-grains are about 8 µm in diameter in the cross section (Fig. 5a), and 20 µm long in the longitudinal section. β-phase is located along boundaries of α phase grains.
[4] USA Patent # 6,399,215, IIC7 C22 C014/00; C22 F001/18, Ultrafine-grained titanium for medical implants.
Valiev, Microstructures and mechanical behavior of ultrafine-grained Ti-6Al-4V alloy produced by ECAP, In: Ultrafine Grained Materials III, TMS, eds Y.T.

The former starts from 1960s [1], and the research indicates it can refine the grain microstructure obviously by applying the continuous current during the solidification [2]-[4].
After the 4V voltage is applied, we observe the cross section of the positive and negative polarity for the sample in Fig.2b and 2c, and detect that the sample after the treatment forms the grain with similar shape and even size, and the grain boundary is clear.
The cause is that the Fe-C-S alloy itself includes a large number of elements which can form FeS, and it will form MnS for the pig iron includes a little amount of the element Mn.
For the current will select the solid phase pathway preferentially, which will melt the protruding and coarse grain of the solid-liquid interface front, to solidify the solid-liquid interface evenly and uniformly, so that the structural grain is even and refined.
Viscous shear as an agent for grain refinement in cast metal [J] ,Trans Metal Soc AIME,1961,221: 419-420

The experimental results show that the existence of undissolved ferrite can not only change the microstructure, but also increase the impact fatigue life The impact fatigue life elongates with increasing of amounts of undissolved ferrite The grain can be fined by using subcritical quenching process and the area of phase boundaries can also be greatly increased because of undissolved spheroidal carbide.
The stress magnitude can be decreased in the main stress plain because not all of the micro cracks could connected with the main crack. (2) The grain number obtained by using subcritical quenching process is finer than that obtained by using the normal quenching process.
So the grain boundaries are greatly increased and the phase boundaries are also greatly increased because of so many small dispersed ferrite particles in existence.
The impact fatigue strength could be increased because the relationship between the grain diameter and strength is still accord to Petch's formula.
Though the existence of undissolved ferrite has bad effect of lower strength, it can also increase the dual-phase structure strength for finer grain number.

The grains can be fined by using subcritical quenching process and the area of phase boundaries can also be greatly increased because of undissolved spheroidal carbide.
The stress magnitude can be decreased in the main stress plain because not all of the micro cracks could connect with the main crack. (2) The grain number obtained by using subcritical quenching process is finer than that obtained by using the normal quenching process.
So the grain boundaries are greatly increased and the phase boundaries are also greatly increased because of so many small dispersed ferrite particles in existence.
The bending fatigue strength could be increased because the relationship between the grain diameter and strength is still following Petch's formula.
Though the existence of undissolved ferrite has bad effect of lower strength, it can also increase the dual-phase structure for finer grain number.

The welding process can also induce age hardening and precipitation of number of phases.
This was estimating to contain 4-5 grains of ferrite and 10-15 grains of austenite.
The EBSD map of selected grains of austenite and ferrite were pole figured to identify the cracking planes of these grains.
The different colours in the same grains are the regions of different crystallographic orientation within the austenite grains.
About 10-12 grains of each phase were pole figured.

Because micro-beam white X-ray diffraction patterns are sensitive to grain orientation, the diffraction pattern changes significantly across grain boundaries during stage scanning.
The crystal orientation obtained for each grain was used to estimate the elastic constants of grains with a bcc structure.
In particular, there were stress concentrations near grain boundary triple points.
In other words, the Young’s modulus was low in [100]-oriented grains and high in [111]-oriented grains.
Grain images for the unloaded and tensile-loaded samples were obtained by analyzing a large number of Laue patterns.

High-frequency impacts are overlapped by ultrasonic vibrations that modifies the structure of the steel surface layer: grain fragmentation occurs; the number of structural crystalline defects increases [8, 9].
The metallographic analysis has shown that the annealed steel comprised ferrite grains 10 ÷ 20 µm in size and round micron-sized carbide Cr23C6 grains located at the interface of ferrite grains (Fig. 1 a).
As TEM analysis showed, the original ferrite grains with random dislocation substructure which are located in the plastic deformation zone (Fig. 1 c), form submicron-sized grains with net dislocation substructure (Fig. 1 b).
Due to the lower number of defects in steel having the equilibrium original structure, nitrogen diffusion basically occurred by the grain volume that retards nitriding.
This increase can be connected both with the increase of nitrogen-strengthened ferrite in the surface layer and fine-dispersed nitride precipitation on defects the number of which has grown after the ultrasonic finishing.