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With an increase of ECAP number up to 3, a great number of parallel shear bands can be observed, while, grains are inclined at an angle of 90º to transverse direction (Fig. 7c).
This is because repeated deformation increases the number and misorientation of the boundaries of deformation bands.
Many researchers [18] [46] mentioned that, by increasing the number of passes, the dislocation density inside the sub-grains becomes saturated and then a significant reduction is observed inside the cells.
It can be observed that the hardness varies with the number of passes in a manner expected for large strain deformation.
The obtained microstructures are refined by the interaction of shear bands and their increase in number.

Due to the ultrafine grains (or domains or particles) and a high density of grain boundaries (or generally interfaces) in nanostructured materials, many properties and performance of the materials are expected to be significantly varied with respect to their coarse-grained counterparts.
Ductility of nanostructured metals is found to decrease at smaller grain sizes, even for those ductile metals in coarse-grained forms.
Electrical conductivity Electrical resistivity increases with a decreasing grain size for metals due to the large scattering of electrons at grain boundaries.
Beside the grain size effect, the resistivity is also sensitive to the microstrain of the nano-metals which is related to the grain boundary structure.
Chemical reactivity Chemical reactivity of nanostructured metals is considerably enhanced due to the large number of grain boundaries.

Ultrafine-grained materials obtained by various techniques of SPD demonstrate unusual mechanical behavior and a number of unique properties often not achievable in coarse-grained materials [4-6].
It was shown in a number of studies that grain boundaries as an element of structure playing a determinative role in such materials markedly differ from those in common polycrystals, and are referred to as “non-equilibrium” or deformation-modified boundaries [7-16].
Thus, in the hafnium bronze more uniform fragmentation of structure with increasing true strain is observed, contrary to the commercially pure copper, in which the average grain sizes increased and the microhardness decreased with the increasing number of revolutions because of the development of relaxation processes [19].
Plasticity and grain boundary diffusion at small grain sizes.
Mössbauer Spectroscopy of Grain Boundaries in Ultrafine-Grained Metal Materials.

This resulted in higher stresses and strains making grain boundary susceptible to IGSCC.
A number of TEM micrographs and selected area diffraction (SAD) patterns were recorded and analyzed.
Twins and shear bands are linear features which have been observed to terminate at the grain boundaries.
Since the bands terminate at the grain boundary preventing the transmission of strain to the adjacent grain, local stresses as well as strain are expected to increase at the grain boundary and the grain boundary also gets disrupted as shown in fig. 3.
Such increased localised stresses and strains at the grain boundary and more dislocations at grain boundaries are expected to make these regions the preferred path for crack growth in simulated BWR conditions water in the non-sensitized condition.

fragmentation of grains.
The microstructure of the SZ is characterized by fine and equiaxed α-Mg grains with an average grains size of about 2.5 µm.
However, both the number and the intensity of Mg5Gd-type phase diffraction peaks decreases considerably after FSP, indicating the significant dissolution of the Mg5Gd-type phase into the matrix.
The SZ was characterized by fine and uniform recrystallized grains.
The average grain size was refined to 2.5±0.9 µm (Fig. 4c).

The results showed that β grains with the average size of about 305 μm and the discontinuous grain boundary α phase along the β grain boundary were obtained for the samples deformed at 881℃.
However when the deformation temperature increased to 896℃ the average size of β grain increased to 510 μm, and the continuous grain boundary α phase along the straight β grain boundary were obtained.
The fracture mechanism analysis revealed that the fracture mode of fine β grain and discontinuous grain boundary α phase is the transgranular fracture, while for the coarse β grain and continuous grain boundary α phase is the intergranular fracture.
During deformation at 881℃, the average β grain size of about 305μm and the discontinuous GBα phases along the β grain boundary were obtained.
Fig. 5 shows the fracture surface and crack of TC18 titanium alloy deformed at 896℃. the macro fracture morphology of the sample deformed at 896℃was relatively flat, there were a large number of deep rock sugar block tearing edges in the crack initiation area, and there were also a large number of dimples (Fig. 5a), which is a typical ductile-brittle mixed fracture.

The particle is W7 AI2O3 with primary mean grain size of 6.3μm.
Results and Discussion Fig.3 and 4 show the variations with grains size of the total number and active number of particles in two-body machining, respectively, in grinding zone.
The active particles number with grain size shows a similar trend.
Wheel workpiece nozzle Abrasive slurry Fig.2 The set-up diagram of abrasive jet machining Fig.3 Variation of the number of particles in grinding zone with grain size Fig.4 Variation of the active particles number in grinding zone with grain size Fig.5 Compared diagram in theoretical and actual material removal rate Fig.5 shows the theoretical and actual MRR.
The total number of particles and active particles number models in grinding zone were founded and simulated.

Grain interaction tensor.
The grain interaction tensor Υ embodies the elastic coupling mechanism among grains.
Effects of grain shape.
A stress analysis was first performed by considering four grain interaction mechanisms (so far neglecting any grain shape effect).
An interesting feature of the software is the possibility, given the relatively low number of fit (i.e. adjustable) parameters, to carry out a detailed error analysis.

Alloy development and grain size control
After isothermal forging the grain size is reduced to about 120µm because the borides formed on casting are broken up and pin grain boundaries during recystallisation and during grain growth.
The grain size found in ingots of Ti46Al8Nb1B is about 150µm and in this case appropriate thermomechanical processing can lead to grain refinement to produce grains of about 100µm [5].
This type of microstructure is developed because the massive gamma is full of crystal defects which increase the number of potential sites for the precipitation of alpha which can precipitate on all four {111} planes in the gamma, yielding the complex type of microstructure shown in Figure 2.
It is likely that pre-yield cracking will not be an issue in this type of alloy since it is very fine grained, with a grain size of about 10µm.

Since the ceramics densification process and grain growth is a contradiction, the preparation of micro-nano grained ceramics with high relative density remains an challenge[2-4].
The change of grain size needs to be further proved by the SEM.
Table 2 is the relative density, grain size and other parameters.
Through the longitudinal comparison, it is found that only using the 400 nm powders in the 400 nm series, the ceramics grains are not fully developed, still spheroids, and a large number of pores are present around the grains, indicating a low degree of densification (Fig. 4a).
Utilizing the combination of 400 nm+200 nm, the ceramics grains are fully developed, the morphology is a polyhedron with clear boundaries, and coexisting large-sized grains and small-sized grains (Fig. 4b).