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The production of fine grained materials by SPD, led to a large number of investigations focusing on the microstructure development and related to mechanical properties.
Compared to other SPD processes the HPT technique offers a large number of advantages, as stated in [4].
Transmission electron microscopy (TEM) was used to evaluate the ultrafine grain microstructure evolution with respect to position on deformed disc and the number of turns at corresponding εeq.
The ufg grain substructural features in disc edge support this selective growth of fine grains, as is seen in Fig. 4.
Specific behaviour resulted as strain was increasing with number of turn.

In order to correlate the grain boundary microstructure to IGSCC, and in general, improve the understanding to IGSCC of Alloy 132 weld metal, the grain boundary microstructure as well as IGSCC behavior of Alloy 132 was studied in this work.
Some correlations were found between grain boundary chemistry and IGSCC susceptibility in these studies.
Efforts were made to find a correlation between IGSCC behavior and the grain boundary microchemistry.
The strain rate and charging time were 2.08×10 -7 s-1 and 192hrs Fig. 5 The fracture surface of the hydrogen-charged specimen 123456789101112 0 10 20 30 40 50 60 70 Atomic concentration (at%) Facet number 123456789101112 0 5 10 15 20 25 30 Atomic concentration (at%) Facet number 123456789101112 0 1 2 3 4 5 Atomic concentration (at%) Facet number (a) (b) (c) Fig. 6 Variations (∆) and average (●) of the concentrations of (a) Ni, (b) Cr and (c) P on the IG facets plotted with those average values from TG surface (dashed line) SCC Behavior of Alloy 132 in simulated BWR environment.
The segregation of phosphorous at the grain boundary is around 1-4at%.

According to the mechanical and microstructure test results, increasing in welding pass number causes reduction in grain size and increasing in average hardness of HAZ.
Also inter-pass slag inclusion defect occurred in high number of passes.
For example increasing welding current intensity causes increasing in heat input which leads to grain growth in HAZ and then reduces mechanical properties of weldments.A105 and A106parts joined together with TIG method by changing current intensity and pass numbers, then macro and microscopic evaluations, hardness, tensile, impact and bending tests accomplished.
Due to the increase of heat input with increasing of current from 80 to100am, the grain size of the HAZ in the S3 increased then hardness decreased from 149 in S2 to 141HV in S3.
(b) is clear that the HAZ grain size in the S4(80amps-5pass) (S2) is smaller than in S2(80amps- 3 passes ).

The main reason is that the size of the inherent crack in the material depends on the grain size.
Hexagonal boron nitride (h-BN) ceramics is one of the ceramic materials affected by grain size.
The flexural strength and fracture toughness of samples were tested at room temperature by a three-point bending method (AG-20KNIC, SHIMADZU, Japan), and the number of samples tested in the same condition was eight.
This also shows that the larger grain size is favorable for the directional alignment of flaked h-BN grains.
Furthermore, the local agglomeration of the grains is observed in h-BN ceramics, and the small h-BN grains did not achieve the better filling effect.

The various grains are depicted with different colors.
If one compares the size of both systems to a perfect lattice containing the same number of atoms, one can Fig. 4 System temperature and size evolution with time.
Diffusion at grain boundaries.
A distinction has been made between atoms located at grain boundaries (less than 8 Å from a grain boundary) and bulk atoms.
This preliminary study will be extended further with other configurations (number of atoms, grains) to confirm the observed trend.

With rising annealing time, the number of low angle boundary (0～15o) decreased due to the mergence of grain with sub-boundary.
It can been seen that the average grain size increased with rising annealing time The abnormal growth of partial grains resulted in the increase of average grain size.
Large number of boundaries fell into the range of more than 15° in misorientation angles in all IF steels.
The number of low angle boundary (0～15°) increased and that of high angle boundary decreased (45～65o) with extending annealing time.
With rising annealing time, the number of low angle boundary (0～15o) decreased due to the mergence of grain with sub-boundary.

Contrast to the conventional IF steel, the super fine grain steel has super fine grains and gives excellent press-formability such as low yield strength, high r-value (the plastic strain ratio) .
The fine precipitates are hardly observed close to the precipitation free zone(PFZ) besides the small numbers of coarser precipitates.
The little precipitates were apparently observed to be located in one side of grain boundary.
It is inferred that precipitates nearby the grain boundaries is to be formed during the grain growth after recrystallization because the shape of this region is intimate to the grain boundary of recrystallized grains.
Therefore, it is inferred that the arrays of coarse precipitates parallel to the grain boundaries as shown in Fig.4 which shows the trace of the grain boundaries.

All the grain size values are slightly above 200 nm, except for the HPT-processed specimen having a grain size of about 160 nm.
The higher the number of passes, the lowest the thermal stability of the microstructure.
Figure 2b shows the dislocation density and the twin boundary frequency as a function of the number of ECAP passes.
These volumes may then act as the embryos for the new grains formed by recrystallization thereby reducing the time required for grain nucleation.
(b) The variation of dislocation density and the twin boundary frequency with the number of ECAP passes for Ag.

The surface layer of the hardness indentations was removed by acid solution to observe microstructure beneath the indentations, where a large number of bending, delamination and kinking grains were found.
It is worth noting that a number of buckling grains can be observed.
The formation of numerous cavities or voids resulted from the deformation of Ti3SiC2 because it possesses small number of independent slip systems.
A number of dislocations and stacking faults can be observed in the basal planes under TEM.
A large number of basal plane dislocations and planar defects exist in grains and at grain boundaries.

The microstructure of ultrafine-grained steels was analysed before and after nitriding.
HPTD can be used to obtain materials with ultrafine-grained (UFG) structure with mean grain size of less than 1 μm and predominantly large-angle grain boundaries [1-3].
Prior to ion nitriding, samples of martensitic and austenitic steels were treated by heat to obtain a homogeneously grained microstructure.
The presence of a larger number of grain boundaries, vacancies and dislocations in crystalline structure of UFG steel has a stimulating effect on the diffusion of nitrogen, which is confirmed in [9-11].
The presence of a significant number of grains, vacancies and dislocations in the crystalline structure of UFG steels has a stimulating effect on nitrogen diffusion.