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However, despite efforts made over a number of years, the control of the industrial casting process remains unsatisfactory; resulting in a significant number of the components produced being rejected due to solidification defects.
These boundaries can be due to a number of different causes.
When the local undercooling is sufficient to exceed the undercoolability of the used alloy, new grains may nucleate to generate macroscopic stray grain defects [2, 3].
The CMSX6 samples reveal a prominent low tendency for stray grains.
In this case, the formation of stray grains is avoided.

Numerical Study of Grain Boundary Diffusion in Nanocrystalline Materials D.
This allows us to analyze diffusion profiles for different geometrical situations such as a single boundary, square grains with the grain size of 80 nm and 25 nm and geometries comprising differently oriented boundaries of the average length of 30 nm .
The numbers indicate the x and y coordinate respectively.
In most experiments on nanocrystalline materials the GB diffusion length Lgb is larger than the average grain size d.
Gust: Fundamentals of Grain and Interphase Boundary Diffusion (Wiley, Chichester, 1995) [3] L.

The casted material had a columnar grain structure with a grain size of several millimeter.
The band structure in grain C continues through the grain boundary into the neighbouring grain A (Fig. 2).
C CD grain B CD c) B A grain C CD grain A CD b) a) d) Fig. 2: Band structure near the grain boundary AC comprising several groups of parallel microbands.
Formation of new grains.
In this study, the specimen contains just some few grains and also the deformation conditions are not exactly defined due to low grain number, friction and specimen geometry.

Before the ECAP process, the materials were annealed at 500˚C for 1hr (grain size: 100µm).
The average size of fine grains is about 250 nm.
Once this specific number of cycles had been exceeded, both the number and area of the damaged regions showed a significant rise.
The number and area of damaged regions including SBs slowly increased with further cycling up to a specific number of cycles, depending on the material and stress amplitude.
Once this specific number of cycles had been exceeded, both the number and area of the damaged regions showed a significant rise.

Takao et al. [7] found two features of slip band in a grain of carbon steel: it was nearly equal to the grain size at early stage of fatigue process; and its length did not change during the fatigue process.
N and Ni mean number of cycles and fatigue crack initiation cycles, respectively.
In this study, fatigue crack initiation is defined when the fatigue crack in a grain reaches a grain boundary and starts growing in Mode I.
Therefore the grain size can be defined as the initial crack size in carbon steels.
(2) Prediction error from empirical σw1 value increase with increase in grain size.

the coarse grains IF steel sheet (CG-IF) and the ZP.
Since the NSUFG/ZP interface has a great number of the structural defects for preferential nucleation sites, great many of nuclei can form on the NSUFG/ZP interface and then immediately connect to each other with enough diffusion mass transfer due to the grain boundary and dislocation diffusion, so that the smooth and thick reaction layer is formed.
On the other hand, small number of nuclei which are formed on the CG/ZP interface with small number of nucleation sites, slowly grow and take much time to connect to each other because of limited mass transfer, resulting in the thin reaction layer in the shape like stone wall as shown in Fig.3.
In the NSUFG layer, the Zn element can penetrate into α-Fe through quite large amounts of grain boundaries by help of the grain boundary diffusion.
This is explained by the increase in driving forces and number of nucleation sites due to the NSUFG structures and the large contribution of rapid diffusion such as grain boundary and dislocation diffusion to the reactions

As seen in the OIM maps, after the first pass, the as-casted microstructure is stretched into fibrous structures, but it still has some residual coarse grains as illustrated a dark area and mark as G in the Fig 3a where a large number of LAG are found.
After two passes, the coarse grains are divided into fine equiaxed grains (~ 5μm) that are surrounded by HABs, and no residual coarse grain is found in the entire region.
Unlike the process at low speed, even after the first pass of Conform process, the well-developed sub-grain boundaries with small number of tangled dislocations are produced (see Fig. 4a), indicating that recrystallization happened during initial passes at high speed.
Moreover, a large number of precipitates are observed in the Fig. 4(c)-(f).
And the number of new recrystallization grains with straight grain boundary (see Fig. 6(d) and (e)) increases, which can be postulated that the continuous dynamic recrystallization induced by strain may occurred and cause the grain refinement, which may be another reason for the rising of the YS and the ductility.

Individual grains or group of grains undergo large relative rotations.
At low strain rate enhanced grain growth occurs.
Grain boundary sliding as well as grain boundary migration occurs extensively.
iv) Grain boundary orientation: The grain boundaries between adjacent matrix grains should be high energy (i.e. high angle or disordered).
vii) Mobility of grain boundaries: - Mobility of grain boundaries develops at triple points as well as at other obstructions along grain boundary.

To increase the strength of cement stone it is necessary to reduce the number and size in the microstructure of the coarse blocks of the portlandite, hydroaluminate, hydrosulphuret of calcium and to increase the number of microfine components, which is achieved by reduction of water-cement ratio and compact distribution of output cementing grains in the system “cement-water” through the use of modifiers.
The test sample is characterized by a fairly dense microstructure with a large number of pores, which are either already overgrown with hydrate new formations, or are in the stage of overgrowth.
To increase the strength of the cement stone, it is advisable to reduce the number and size of large-crystal blocks of portlandite, which is achieved by reducing the water-cement ratio and compact placement of output grains in the cementing system with hyperplasticizing modifiers.
Technogenic sands as effective filler for fine-grained fibre concrete, Journal of Physics: Conference Series. 1118 (2018) 012020
Vatin, Fine-grained concrete with combined reinforcement by different types of fibers, MATEC Web of Conferences. 245 (2018) 03006

During the initial deformation stage, the dislocation density and the number of sub-grain structures increased gradually, and many dislocations formed tangles, resulting in work hardening.
The dislocations can be found both at grain boundaries and in grains.
At the same time, the number of sub-grain structures increases obviously, which enhances the effect of hindering on dislocation motion.
Comparing with the sheet of 80% reduction, the number of dislocations appeared at grain boundaries and in grains decrease greatly, besides, the tangle phenomenon of dislocations was significantly reduced.
With the increasing of cold rolling reduction, the dislocation density and the number of sub-grain structure in alloy sheets increase and a large number of dislocation tangles are formed, leading to work hardening.