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pravirpolly@gmail.com a, sekhar@nitt.edu b, brs@nitt.edu c, kumara@nitt.edu d *Corresponding author: brs@nitt.edu, Mobile number: +91-9442505336.
Ultra-fine grained structures are polycrystals having average grain size less than 1µm will have equiaxed microstructures having high fraction of grain boundaries with high misorientations angles and high dislocation density [4].
The relative density increases with increasing number of passes.
The hardness increases is due to the dispersion and grain refinement.
Sintering improves the bonding between the particles, with less effect on grain coarsening.

Breakdown voltage is controlled by two parameters which are the barrier voltage and the number of grains between electrodes.
As the barrier voltage and the number of grains increase, breakdown voltage increases [1].
ZnO grains are in a high conductivity and the segregation layers (grain boundary) in a high resistivity [1,7].
On the other hand, Bi2O3 addition to ZnO enhances the densification process and the grain growth (average grain size: 9.9 µm).
The ZB system exhibits abnormally grown grains.

Effect of cooling rate on grain size.
Cooling rate, in addition to grain size, affects grain morphology.
On the other hand, decreasing the grain size reduces hot tear susceptibility using some different mechanisms; i) when grains are small the rearrangement of grains maybe happen due to strain accommodation during solidification [7]. ii) Onset of interdendritic feeding is delayed by grain refinement [8]. iii) Decreasing in grain size increases the fracture strain. iv) Decreasing grain size increases grain boundaries and therefore decreases stress concentrated due to increasing concentration zone fraction. v) Decreasing grain size decreases the thickness of interdendritic eutectic phase, so, increases the impingmentation of grains.
Zhou et al. [10] explained that since the remaining liquid channel remains longer at the grain boundaries and the liquid flow enriches the grain boundary channel with solute, the concentration of the minor elements is higher in the grain boundary liquid than in the interdendritic liquid within one grain.
It can be concluded that increasing cooling rate decreases the amount of concentrated stress due to increasing the stress concentration zone, since increasing cooling rate decreases the size of porosities and increasing the number of them.

Without FSP (Figure 3a), the grain size of the as-cast-T4 A206 is ~100µm, but subsequent to FSP (Figure 3c and d), the grain size is decreased to less than 10µm.
No grains are observed when FSPed at 500 rpm-25.4mm/min; blurred grain boundaries are seen when FSPed at 1000 rpm50.8 mm/min.
In contrast, clear grains are seen when FSPed at 1000 rpm-25.4 mm/min.
Grain size is refined from about 100µm to less than 10µm. 2.
References [1] Thomas, W.M. et al. (1991) International Patent Application Number PCT/GB92/02203 and GB Application Number 9125978.8

As could be seen in Fig. 3, when magnetic flux density was controlled at 80mT in the processing of spiral polishing with magnetic force, the steel grits were not effectively attracted and squeezed the workpiece, so the polishing effects was limited and a number of irregular s lines were found.
Impact of SiC Grains Size on Surface Roughness.
When polishing with smaller SiC grains, the surface roughness was hardly improved in that smaller size of grains would cause finer scratches, failing to remove the dents on the workpiece surface.
Impact of SiC Grains Weight on Surface Roughness.
This could be explained by the fact that when SiC grains weight increased, the number of the grains also increased.

The number of diamond grains per square centimeter is [4]: 3 S dn 100 K 441.0N ���= . ˄10˅ where K is diamond concentration in the matrix (%), n is the number of diamond grains per carat and d3 is the weighted mean cubic size of diamond grains.
The pressure which single grain diamond exerts on the stone can be obtained from Eq.9 and Eq.10: dS SSNP f � = . ˄11˅ where Sd is the contact area between the diamond grain and the stone machined.
Namely, drilling will not come about until the pressure which single grain diamond exerts on the rock is greater than the compressive pressure of the rock being drilled.
The protrusive height and protrusive number of diamond in the beads are measured in the different sawing series (see Table 3).
Table 3 The protrusive height and protrusive number of diamond Area cut [cm2] Sawing series Bead-No Mable Granite Protrusive height [µm] Protrusive number 1 115 4135 / 15 263 2 112 5169 518 26 343 3 2 5169 1360 30 334 4 110 5169 1944 29 338 The particle size distribution of cutting debris is measured during the cutting of marble and granite, whose results are listed in Table 4.

The aim of equal channel angular extrusion is to get ultrafine grained bulk materials.
Introduction Equal channel angular extrusion is an attractive method in fabricating bulk materials with ultrafine grain size [1].
One of the factors that affect grain refinement in ECAE is the temperature of the billet in which deformation and recrystallization take place [2].
Thus temperature rise due to heat generated during ECAE affect the grain size formed [3].That maintaining a low pressing temperature ensures the potential for achieving both the smallest possible equilibrium grain size and the highest fraction of high-angle boundaries.
The faster rates of recovery occur at the higher temperatures, which led to an increasing annihilation of dislocations within the grains and a consequent decrease in the numbers of dislocations absorbed into the subgrain walls [2, 4-5].

It should be noted that the recrystallized grains in the magnetically annealed specimen are smaller in size and fewer in number as compared with its counterpart.
(The Black lines correspond to the grain boundaries with misorientation higher than 15°, and thin grey lines correspond to low- angle grain boundaries with misorientation between 0°and 15°.)
Their grain size distributions are shown in Fig. 4.
It is seen that more numbers of small grains (< 10µm) and were found for the magnetically annealed specimen as compared with the non-field annealed one.
The mean grain size for the magnetically annealed specimen is about 11.8µm.

Table 2 Crushing strength of vanadium billet Number Pressing pressure MPa Fpc0.2 N Fmc N Crushing strength MPa 2# 80 95.17 151.89 0.4 3# 120 326.63 446.09 1.5 4# 160 943.21 1400.89 3.4 5# 200 2228.84 2656.66 6.0 6# 240 2107.62 2433.57 7.0 7# 280 2265.3 2740.35 7.4 Fig. 3 Crushing force curve Fig. 4 Pressing pressure and crushing strength Sintering of vanadium billet.
Table 3 Sintered density of vanadium billet Number Pressing pressure MPa Isostatic pressing and sintering density g/cm3 relative density hot pressed sintering density g/cm3 Relative density 1# 160 4.87 81.75% 5.12 85.91% 2# 200 4.98 83.98% 5.30 88.93% 3# 240 5.23 87.75% 5.38 90.26% 4# 280 5.28 88.59% 5.51 92.91% Sintering microstructure.
From Fig.5 (c) and (d), vanadium billet was sintered by hot pressing, as sintering temperature had reached recrystallization temperature, recrystallization and grain structure were changed during sintering, because sintering temperature have reached recrystallization temperature.
The interface of sintered particles formed grain boundaries through recrystallization and flowed into particles on both sides, the particles formed grain aggregation and recrystallization, and finally the chain structure of grain was obtained.
Due to the movement of grain boundaries, large grains swallowed up small grains, made grain size more uniform, the density of sintered body was further increased, and recrystallization of some grains was more completed, and new equiaxed grains were transformed.

Fracture surface morphologies of TiC-TiB2 composite ceramics presented a mixed mode of transcrystalline fracture in irregular TiC grains and intercrystalline fracture of TiB2 platelets.
Lots of intercrystalline fractures of the TiC-TiB2 ceramic material rooted deeply in the fine TiB2 grains and some regular grooves were left after some TiB2 grains were pulled out, as shown in Fig.3(a).
On the other hand, Fig.3(b) shown that some large pores were observed on the fracture surface of the TiC-TiB2 ceramics, and many larger TiB2 grains were also cleaved on the fracture surface of the TiC-TiB2 ceramics.
However, the crack path in coarse TiB2 grains and irregular TiC grains present a linear crack-propagation path, as shown in Fig.5.
Because of the homogenous microstructure with refined TiB2 grains, the fracture toughness was improved by the intercrystalline failure of finer TiB2 grains.