HAGBHigh-Angle Grain Boundary
HAGBHello And Good Bye
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There is really a hot debate between researchers within the recrystallization field on whether CDRX or GDRX should be responsible for the grain modification of aluminium [16], the center issue is whether the HAGBs experiential after big deformation are altered from LAGBs, microshear/ deformation bands, or original HAGBs.
The mechanism of CDRX is not also understood as for DDRX, the main matter is how the LAGBs increase their misorientation and convert into HAGBs. At low deformation temperatures, it appears that LAGBs can increase their misorientation consistently by the misorientation saturates at relatively low values, and it is the formation of microshear bands or kink bands that ultimately leads to the formation of HAGBs.
At high deformation temperatures, microshear bands or kink bands become less significant, the transformation of LAGBs to HAGBs is done moreover by the equal increase of misorientation or progressive lattice rotation coarsening of dispersoids (reduced pinning force) and HAGBs also depends on the grain orientation, and there exist a number of stable orientations, in which the increase in misorientation is not enough to form new HAGBs even at large deformations.
It is concluded that GDRX takes place once the thickness of the HAGBs reaches 1-2 subgrain size distance.
In previous studies, it was found that UFG Fe-Cr alloys with 20% Cr processed by ECAP, with a large fraction of nonequilibrium high-angle grain boundaries (HAGBs), exhibited higher corrosion resistance than coarse-grained (CG) material, and this was attributed to enhanced passivation [7-11].
After one and two passes, low-angle grain boundaries (LAGBs) with misorientations of 2-15[degrees] are common, with only a few HAGBs with higher misorientations.
However, after eight passes, the pattern becomes ring-like, indicating a UFG structure with a large fraction of HAGBs. This is consistent with the TEM observation results, which indicated that a higher fraction of HAGBs was present following eight ECAP passes, whereas a smaller number of passes produced mainly dislocations.
Following eight passes, the specimen exhibited a high fraction of HAGBs instead of mainly dislocations, as can be seen in Figure 5.
3 shows the estimators of the cross K-functions [[??].sub.3,3](r), [[??].sub.9,3](r), [[??].sub.L,3](r) and [[??].sub.H,3](r), which help to interpret the occurrence of [summation]3 boundaries in neighbourhoods of [summation]3, [summation]9, LAGB's or random HAGB's.
specimen LAGB's [summation]3 [summation]9 HAGB's no annealing 57.92 2.57 0.25 39.26 373 K 2.50 35.83 6.75 54.92 423 K 2.10 54.30 7.42 36.18 473 K 2.89 41.43 6.15 49.53 573 K 3.08 40.13 5.09 51.70