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Because the twin nucleates at the grain boundary, stress concentration is necessary to be accommodated by dislocation absorption into the grain boundary at low strain rates.
Extrinsic grain boundary dislocations move and engender grain boundary sliding (GBS) with low thermal assistance.
If the basal slip system is activated at low stresses, then the number of the independent slip system is only two, where the von Mises condition is violated and where it might engender low formability.
Therefore, SRS for AZ31 Mg alloy can be described as follows: (1) lattice dislocations move toward grain boundaries; (2) they pile up at grain boundaries; (3) dislocation absorption occurs time-dependently to accommodate stress concentration there; (4) grain boundaries slide by the motion of extrinsic grain-boundary dislocations.
The Role of Grain Boundaries, phys. stat. sol.

Through statistical analysis for grains number, it can be drawn that the higher the cutting speed, the more serious grains refinement.
Grain boundaries at the area of 227μm×174μm are outlined using IPP software, the grain equivalent diameters are measured and the number of grains is counted.
Statistical analysis for grains size and numbers in metallographic image is shown in Fig.8.
With the increase of cutting speed, the numbers of large grains and medium grains continue reducing while the small grain numbers increase in FGH95 alloy machined surface.
With the increase of cutting speed, the number of large grains and medium grains continue decreasing while the small grain numbers increase for machined surface of FGH95 superalloy.

Some Remarks on the Magnitude of the Chemical Diffusivities at Moving Grain Boundaries Paweł Zięba Polish Academy of Sciences, Institute of Metallurgy and Materials Science 25 Reymonta St., 30-059 Cracow, Poland nmzieba@imim-pan.krakow.pl Keywords: Grain boundary diffusion, grain boundary movement, discontinuous reactions Abstract.
The number of atoms, which have to diffuse across the boundary increases with the further boundary movement giving a contribution to the formation of the solute profile.
After a certain time, the number of atoms becomes so large that it may cause the stop of the boundary to enable all the excessive atoms to enter the solute depleted α lamella.
Gust: in Fundamentals of Grain and Interphase Boundary Diffusion (John Wiley & Sons Ltd.
Kolobov: Did you observe the formation of dislocations on the grain boundaries during the process of migration of grain boundaries.

Therefore, as the grain size decreases, the strength of the material increases.
From fig 4, we can see that the grain refinement takes place as the number of high angle boundaries increase with the number of passes and this is also an evidence for the attainment of high tensile properties obtained earlier.
The dislocation cell structure as depicted by the TEM micrographs shown in fig 4 clearly indicates that the dislocation density increases with the increase in number of passes which ultimately leads to the grain refinement and pore size reduction.
Variation of pore diameter with number of passes From figure it can be seen that the diameter of pore decreases with increase in number of passes and also the pore diameter reduction is more in pass 2A and 3A than that of 2C and 3C.
· The grain size decreases with the increase in number of passes thereby providing increase in tensile strength after each pass.

As expected on the base of the results of the wedge-shaped samples, the addition of the novel grain refiner leads to much finer primary α-Al grains and levels the spatial variation of the grain size throughout the whole thickness of the sample.
Other advantages of the addition of the novel grain refiner to the Al-Si cast alloys is the reduction of the number of primary silicon particles in the microstructure and the refinement of the eutectic phase as it can be seen in Fig. 5 which reports the micrographs of the Al10Si alloy without and with the addition of the novel grain refiner.
Moreover, the addition of the novel grain refiner makes the grain size far less dependent on the cooling rate employed to solidify the material.
StJohn, Grain Refinement of Aluminum Alloys: Part I.
Hogan, Al3Ti and Grain Refinement of Aluminum, J.

Introduction Ultrafine grained steels and alloys with a grain size below one micron are considered as advanced structural materials, which exhibit superior mechanical properties [1].
The longitudinal grain boundaries among ferrite lamellas are characterised by a wide variety of their angular misorientations from low-angle subboundaries to high-angle grain boundaries.
The numbers indicate the boundary misorientations in degrees.
The grain refinement during the cold rolling is accompanied by a drastic increase in the dislocation density in grain/subgrain interiors.
This annealed microstructure looks like the cold rolled one consisting of highly elongated austenite/ferrite grains with an almost random grain boundary misorientation distribution and high dislocation density in grain/subgrain interiors.

The number of grain boundaries along the propagation direction of cracks and the amount of substructures after heat treatment increase with the decline of thicknesses of 2124 alloy plates, while the grain size is reverse.
Different plate thickness is due to different deformation during the hot padding, so grain size and number of grain boundaries appear obvious different in different plates, and maybe it is because of the difference of the microstructure determines the propagation rate of fatigue crack, which ultimately affect the fatigue life of the alloy.
Besides, coarse Al6 (Mn) phases are also observed inside the grain and on the grain boundaries [9].
It is reasonable that the number of boundaries along the propagation direction of cracks increases with the decline of thicknesses of alloy plates.
The smaller plates thickness are, the smaller grains size is, the more number of grain boundary and substructure and the higher the fatigue life of alloy is, that is to say, the better fatigue performance of the alloy is.

The casting sample made without ultrasonic vibration exhibited coarse acicular grains of about 315 μm average grain size (Fig.2a).
Fig. 2 The microstructure of the samples treated at ultrasonic power of 0 kW(a), 0.6 kW(b), 1.2 kW(c), 1.8 kW(d) and 2.4 kW(e) According to the ultrasonic cavitation effect, a strong shock wave produced by the collapsed hole causes nonlinear mechanical damage to the cavitation bubbles near the solid phase, and repeated high-speed impacts lead to local damage to the solidphase, increasing the number of grains.
For non-wetting particles, the cavitation effect can easily produce, so in solidification front a continuous cavitation will repeatly generate, the formation of local cavitation heat pulse will continuously impact solidification front, the interface of particles will partially ablate, thereby the number of grains increases and the size of grain decreases, and the refined grain structure leads to increased hardness.
Thus, the growth rate of the crystal grains increases and the crystal grains are refined.
Ultrasonic treatment causes coarse grains to be refined, the number of large grains decreases significantly, and the size of grains is more homogenous.

AGG, also referred to as secondary recrystallization[4], typically results in microstructures of bimodal grain sizes, containing a small number of very large grains called abnormal grains.
Grain growth need grain boundary energy, which comes from deformation energy.
When some grains inner have large dislocation density and no CSL boundaries with high grain boundary migration, the grains may grow rapidly due to the high grain boundary energy.
Influence of the primary recrystallization texture on abnormal grain growth of goss grains in grain oriented electrical steel [J].
Influence of the primary recrystallization texture on abnormal grain growth of goss grains in grain oriented electrical steel [J].

If parallel Widmanstätten austenite laths form in two adjacent ferrite grains, zig–zag ferrite grain boundaries arise.
Detailed analysis reveals that coarse ferrite grains are fragmented into a number of subgrains with misorientation less than 2°.
Fig. 3a shows a straight ferrite grain boundary.
Precipitates occur along grain/subgrain ferrite boundaries and also inside ferrite grains.
In the case of the nucleation of Widmanstätten austenite laths on both sides of the ferrite grain boundary, zig–zag ferrite grain boundaries arise because flat facets at ferrite grain boundaries are not parallel due to misorientation between the grains. 3.