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Upon extrusion, the as-cast coarse grains underwent pronounced grain refinement and the second phases were broken up and formed stringers in the extrusion direction.
The grain size was measured using the software of Image-Pro Plus 5.0 with the grain number of more than 1000.
The average grain size of dynamic recrystallized (DRXed) grain for the Mg alloy extruded at 270 ºC, 300 ºC and 330 ºC was about 1.47, 2.50 and 4.46 μm, respectively.
Fig. 2 IPF figures of the Mg-Zn-Ca alloy extruded at (a) 270 ºC, (b) 300 ºC, (c) 330 ºC It has been reported that high extrusion temperature result in large nucleation and migration rate of grain boundaries [5], a higher nucleation rate during extrusion process will increase the fraction of DRXed grains, and the higher migration rate of grain boundaries can lead to a rapid grain growth during extrusion, which is responsible for the large grain size at 330 ºC.
Fig. 4 Pole figures of the Mg-Zn-Ca alloy extruded at (a) 270 ºC, (b) 300 ºC, (a) 330 ºC ED (b) (c) (a) (0001) (10-10) (a) 0.0 0.1 0.2 0.3 0.4 0.5 0.00 0.02 0.04 0.06 0.08 0.10 0.12 Number fraction Schmid factor m=0.2057 0.1 0.2 0.3 0.4 0.5 0.00 0.02 0.04 0.06 0.08 0.10 0.12 Number fraction Schmid factor m=0.2384 0.0 0.1 0.2 0.3 0.4 0.5 0.00 0.02 0.04 0.06 0.08 0.10 0.12 Number fraction Schmid factor m=0.2386 (c) (b) The tensile properties of the Mg alloys were strongly dependent on their microstructure including grain size, texture, second phase, etc.

Mg17Al12 is formed as discontinuous precipitates at grain boundaries, which might promote the grain boundary sliding during elevated temperature applications [3].
One effective way to hinder the migration and sliding of grain boundaries at elevated temperature is to form strong precipitates with high melting points at the grain boundaries as well as at the grain interior.
Generally, Mg2Sn precipitates forms both at the grain boundaries and at grain interior during aging.
The general idea of increasing the strength is by increasing the number density of particles, i.e. increase in precipitate nucleation rate.
It is clear that the number density and the vol. % precipitate increase with increasing aging time.

The size of pores after the irradiation was between 0.2~1.2 μm, and mostly distributed at 0.3μm ~0.6 μm; The pores at grain boundary of two adjacent grains was less, the pores at grain boundary were distributed by the way of triangle or quadrilateral.
Fission gases and fission fragments were produced during fuel assemblies being irradiated, and the modality and number of pores would change due to fission outcome.
a) D2 b) D4 Fig. 2 SEM micrographs of specimens Specimens after etched grain boundaries were visible, grain size was about 5μm~30μm, grain size greater than 20μm and less than 10μm were few, mostly grain size distributed at 10μm ~15μm, The pores at grain boundary of two adjacent grains was less, the pores shape distribute at grain boundary presented either triangular or square (Fig 3 arrow or circle diagramming), this viewpoint correspondence match with José R.A.
Table 1 Measurement results of pore size Number of fuel rod Number of specimens Biggest diameter（μm） Average diameter（μm） Pore density（106/cm2） 1# D1 3.293 0.662 0.248 D2 2.297 0.628 0.168 D3 1.732 0.569 0.120 2# D4 1.218 0.427 0.379 D5 1.714 0.482 0.360 D6 1.870 0.442 0.337 Pre-irradiated fuel rod - 1.299 0.329 2.425 Conclusion a) There were cracks in fuel pellets, most micro-cracks were trans-granular crack.
The size of pores at grain boundary of two adjacent grains was less, the pores at grain boundary were distributed by the way of triangle or quadrilateral.

The number density of Z phase measured at 550 o C was lower that that at 600o C, indicating that the preferential recovery of martensitic lath structure around prior austenite grain boundary is not remarkable at 550 o C in contrast with 600 o C.
The MX carbonitrides are located at PAGB, lath, block and packet boundaries and inside lath grains before creep [16].
Additionally, grain boundary diffusion may accelerate the Z phase formation.
Figure 7 demonstrates change of number density of Z phase particle during creep exposure.
The stress can influence grain boundary diffusion relates to the Z phase formation around PAGB.

The ECAP was conducted at a temperature 643 K in accordance with processing routine BC (sample rotating angle between passes of 90 o ) with the number of passes N=12.
This earlier study revealed a high dislocation density (ρ ∼5⋅10 9cm-2) and a number of intermetallic particles (Al3Sc and Al3Zr) with 30-60 nm sizes in the grain volume.
The ECAP processing gives a significant decrease in the average grain size of the alloy to an ultra-fine grained state (d ≈ 1 µm).
The grain boundaries in the 1421 alloy after ECAP through a large number of passes demonstrate a spread contrast that is characteristic of an equilibrium state.
The value of Ug in the initial alloy is close to that for grain boundary self-diffusion of Al [15] or Mg diffusion along grain boundaries of an ultra-fine grained Al-Mg alloy [16].

As far as the production method of fine-grained materials of small dimensions has been fully mastered, the producing of fine-grained materials of considerable dimension is still a matter of challenge for engineers.
They are able to transfer very large plastic deformation, retaining the same positions while cutting through a number of grains.
Thus, there are no reasons for grain growth.
The very large plastic strain of these heat treatable alloys results in fine particles shearing by a number of dislocations, leading to their mechanical supersaturation as well as to channeling of the structure by intersecting shear bands causing its refinement and achievement of a chess-board like form.
The subgrain/grain growth is then retarded due to disperse particle development.

Functional Steel Hardness and Wear Improving on a Basis of Phenomena of Grain Boundaries Phase Transition Y.A.MINAEV Leninsky prt.4, Moscow 119049, Russia e-mail: ymin36@mail.ru Keywords: Steel Alloys; Grain Boundaries Phase Transition; Nitrided Coatings.
Based on the phenomena the grain boundaries first-order phase transition in range 0.55 – 0.86 of metal melting point with formation of two-dimensional liquid was elaborated the technology of coatings by synthesis of nitrides using of gaseous nitrogen.
For a development of a technology we focus on a grain boundaries and description of its fundamental property i.e. first-order grain boundaries phase transition (GBPhT) with formation of two-dimensional liquid[1-3].
In referred works it has been shown that an interval of temperature near melting point TS0 the surface layers or grain boundaries of solid metal can be considered as quasi-liquid, i.e. as a separate phase.
The contents of small-sized nitrides (less 300 nm) makes till 65-75% from a total number.

It was found that the number of Nd element in the AZ91 magnesium alloy has effect on the grain refining efficiency, the coarse β-Mg17Al12 phase distributed along the grain boundaries transformed into granular, and the granular or acicular Al3Nd phase precipitated in matrix.
When the chemical composition and corrosion medium have relatively fixed, the grain size of α-Mg phase, the number of β-Mg17Al12 phase and its distribution have greatly impact on the corrosion of magnesium alloys, particularly AZ91 alloy [6,7].
So, the number of Nd element in the AZ91 magnesium alloy has effect on the grain refining efficiency.
Hence, the general corrosion is composed of numbers of micro-couple corrosion.
（a） （b） Fig.3 Relationship between corrosion rate-time, weight loss rate-time of AZ91/AZ91-0.4%Nd alloy immersed in 3.5 wt.% NaCl solution(a)corrosion rate;(b)weight loss rate Conclusions The number of Nd element in the AZ91 magnesium alloy has effect on the grain refining efficiency, the coarse β-Mg17Al12 phase distributed along the grain boundaries transformed into granular, and the granular or acicular Al3Nd phase precipitated in matrix.

Fig. 3 - Distribution of number of pixels in Fig. 4 - Pattern quality averaged in bcc grains bcc and fcc grains as a function of square root of number of pixels in each grain.
Figure 3 shows the distribution of number of pixels in bcc and fcc grains.
In case of bcc, number of small grains composed of less than 10 pixels is large enough to show the highest portion.
The distribution of bcc grains shows a long tail toward large number of pixels, which reflects the large polygonal ferrite grains.
Square root value of number of pixels in a grain that is proportional to grain size is used.

The base metal microstructure consists of large elongated grains (Fig. 3 (A)), with an average grain thickness of 25 µm.
As it can be seen, by increasing the number of FSP passes the cluster size is reduced and the distribution of the Cu particles is improved.
The average grain size of the composites is listed in Table 1.
Results indicate that the average grain size is reduced by increasing FSP passes and the specimens produced by SQ tool have smaller grain size compared to the specimens produced by the SC tool.
Increasing the number of FSP passes decreases the clustered Cu particles, improves the distribution of Cu particles and creates a homogenous SZ with smaller grain size.