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As soon as atoms move from one grain to another because of energy and/or orientation gradients, the grain boundary starts to move leaving behind a micro zone of relatively small dislocation density and hence small strain energy.
Here, grain boundary movement and inhomogeneous recovery take place concurrently.
Theory Diffusion plays a very important role in recovery, grain growth, and recrystallization.
This is the number of atoms diffusing through and perpendicular to a unit cross sectional area of solid per unit of time, N Number of lattice sites per unit volume, D Diffusivity of vacancies or interstitials, c Concentration of vacancies or interstitials.
With beginning motion of the grain boundaries recrystallization is initiated.

This allows us to conclude that the increase in the number of cycles does not automatically imply an improvement of the mechanical properties and an optimum number must be determined judicially.
Variation of hardness as a function of the number of cycles. 2.
Processed with a number of cycles equal to 20, the material becomes stronger and tougher.
Kim, Wear properties of high pressure torsion processed ultrafine grained Al-7% Si Alloy, Materials & Design, Vol. 53, pp. 373-382, 2014
Hong, Ultra-fine grained bulk aluminum produced by accumulative roll-bonding (ARB) process, Scripta Materialia, Vol. 39, n. 9, pp. 1221-1227, 1998

A number of theoretical models and mechanism postulate that the rumpling process is related to the cyclic plastic strains in the bond coat driven by the growth strains in oxide combined with thermal expansion mismatch among oxide and bondcoatings as well as substrate [1-3].
The ridges on the low-Pt coating grain boundaries were typical as others observation[7].
The small grain size and presence of PtAl2 induced the compact roughening or waviness oxide growth.
The ridges which formed on the grain boundaries for low-Pt coating in Fig.2a could not be observed in Fig.3a.
The polished low-Pt coating diminished the ridges formation on the top surface of grain boundaries.

However, the decreased grain sizes can lead to an increased rate of hydrogen absorption by metal polycrystals.
Samples of the alloy in the initial fine-grained (FG) state were previously annealed at 1023 K for 1 hour.
Individual grain and subgrains are visible on the dark-field image (Fig. 1, b).
Quite a number of reflections uniformly spaced on a circle are observable in the microdiffraction image of such a structure recorded from the area of 1.4 µm2 (Fig. 1, a).
Such a type of microdiffraction is typical for ultrafine-grained materials with high misorientations between the elements of grain/subgrain structure, nonequilibrium grain boundaries, and internal fields of compressive stresses.

It is thus not possible to detect if all grains of a given orientation behave the same and how the growth kinetics of a single grain varies with time.
On average, however, it is confirmed that cube grains grow faster than other grains in AA1050 cr 90% annealed at 270°C.
So far 5 grains have been analyzed.
Some of the recrystallizing grains are marked by circles and numbers. b) detector image from a detector with good spatial resolution placed as near as possible to the sample - used for filming the growth of a single grain.
[29] E.M.Lauridsen et al.: Recrystallization and Grain Growth.

Most frequently, the following three mechanisms of fatigue crack nucleation were observed in the investigated duplex stainless steel: (i) transgranular in ferrite grains (Fig. 2), (ii) intergranular at phase boundaries due to very localized plastic deformation of adjacent ferrite grains and (iii) intergranular at phase boundaries due to high grain boundary normal stresses without any plastic deformation of adjacent ferrite grains.
FIB-tomography in combination with high resolution SEM revealed that no subsurface obstacle (grain boundary, phase boundary or inclusion) caused the observed crack arrest in the midst of the first grain.
Eq. 3 is valid for the case of a dislocation pile-up against a single grain boundary, such as found in the ferrite grain α1 in Fig. 2 in form of slip traces.
The dislocation dipoles increase the stored energy stepwise until a critical value is reached, at which crack initiation occurs after a number of load cycles Ni, according to (3) where G is the shear modulus, ws the specific fracture energy that is necessary to generate the fracture surface, ν Poisson’s ratio, d the grain diameter, Δτ the shear stress range and τf the frictional shear stress.
Successively the crack converges to the grain boundaries at the lower and right side in Fig. 3e.

Abnormal grain growth was found in pure BaTiO3 ceramic with 37.30 mm of average grain size.
The potentiality of PTC barium titanate ceramic is demonstrated not only by the number of applications but also by the expertise acquired on structure-properties relationships in this material [5].
From the SEM micrographs, pure BaTiO3 ceramic shows highly dense of irregular shape grains with the average grain size of about 37.30 mm, as illustrated in Fig. 3(a).
As increasing Bi2O3 content to 1.0 vol%, the grain growth is observed and highly dense grains are revealed.
The grain size of ceramics in this system strongly depends on Bi2O3 content.

Introduction There are a number of environments for which materials are subjected to dynamic loading [1-4].
In the laboratory environment, dynamic loading can be examined in a number of ways.
Each iteration resulted in a doubling of the number of layers in the composite and a factor of two reduction in individual layer thickness.
The average values of the layer thickness were estimated based on the number of layers and the overall sample thickness.
Some of the nanovoids formed at the vertical grain boundary, while most of them nucleate in the grain interior.

Finally, when the grains are large enough, they will interact via their thermal or solute layers [6].
The number of austenite nuclei (N) per unit volume has been calculated from the experimental work by estimating the irregular grain size, or diameters (R) of the grains including ferrite, pearlite and cementite in several positions of the specimens.
Austenite and eutectic grains are shown in fig. 5 and 6 respectively.
Fig. 4: Growth of eutectic grains Fig. 7: CR1 effect on austenite nuclei number. 0 500 1000 1500 2000 25000,6 0,8 1 1,2 1,4 1,6 1,8 2 2,2 Liquidus nucleation undercooling [k] Number of austenite nuclei [1E6/m 3 ] 0 500 1000 1500 2000 2500 0 1 2 3 4 5 6 7 8 9 CR1 [k/s] Number of austenite nuclei [1E6/m 3 ] Fig.5: Austenite grains (dia 30mm) X100 Fig.6:Eutectic cells Fig. 7: Effect of CR1 on eutectic cells number 1296.8-) 1 Ln(CR 1749.2N = (10) 162.59liq TLn( 13763.9N )∆ = (11) 0.8172 1 CR 0.1401 liq T + =∆ (12) 3929.4 1 974.11 − = CR g N (13) The eutectoid structure can be either of stable ferrite plus graphite, and/or metastable pearlite plus cementite, depending on the diffusion rate, the lamellar spacing of the austenite dendrites, the alloying elements and the cooling rate [22].
The relation between the number of austenite nuclei or the number of eutectic cells and cooling rate before solidification starts, CR1 are deduced.

And the ingot central also has a large number of pores, only the lower part of the ingot organization is uniform, as shown in Fig. 6.
The grains are multiform with a mass of pore.
The grain sizes are smaller and the pole between grain boundary is less with nanoparticles refining.
Further study with 0.1wt.% pretreated SiC nanoparticles reinforced is shown as Fig. 7c, composites with uniform grains and small grain boundary can be obtained.
The second route can fabricate composites with uniform grains and small grain boundary.