Limits...
Grain rotation mediated by grain boundary dislocations in nanocrystalline platinum.

Wang L, Teng J, Liu P, Hirata A, Ma E, Zhang Z, Chen M, Han X - Nat Commun (2014)

Bottom Line: Grain rotation is a well-known phenomenon during high (homologous) temperature deformation and recrystallization of polycrystalline materials.In recent years, grain rotation has also been proposed as a plasticity mechanism at low temperatures (for example, room temperature for metals), especially for nanocrystalline grains with diameter d less than ~15 nm.Our atomic-scale images demonstrate directly that the evolution of the misorientation angle between neighbouring grains can be quantitatively accounted for by the change of the Frank-Bilby dislocation content in the grain boundary.

View Article: PubMed Central - PubMed

Affiliation: 1] Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, China [2].

ABSTRACT
Grain rotation is a well-known phenomenon during high (homologous) temperature deformation and recrystallization of polycrystalline materials. In recent years, grain rotation has also been proposed as a plasticity mechanism at low temperatures (for example, room temperature for metals), especially for nanocrystalline grains with diameter d less than ~15 nm. Here, in tensile-loaded Pt thin films under a high-resolution transmission electron microscope, we show that the plasticity mechanism transitions from cross-grain dislocation glide in larger grains (d>6 nm) to a mode of coordinated rotation of multiple grains for grains with d<6 nm. The mechanism underlying the grain rotation is dislocation climb at the grain boundary, rather than grain boundary sliding or diffusional creep. Our atomic-scale images demonstrate directly that the evolution of the misorientation angle between neighbouring grains can be quantitatively accounted for by the change of the Frank-Bilby dislocation content in the grain boundary.

No MeSH data available.


Related in: MedlinePlus

Atomic-scale in situ observation of grain rotation in smaller grains.(a) An array of dislocations (as marked with ‘T’) forms a wedge-shaped disclination at the GB. (b–e) During straining, the number of the dislocations decreases, leading to decreasing GB angle G1–3. (e,f) The dislocation number decreases, caused by the GB dislocations climbing into the TJs or other GBs. Meanwhile, the GB angle at G1–2 increased from 12.1° to 21.6°. Scale bars, 2 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4109021&req=5

f3: Atomic-scale in situ observation of grain rotation in smaller grains.(a) An array of dislocations (as marked with ‘T’) forms a wedge-shaped disclination at the GB. (b–e) During straining, the number of the dislocations decreases, leading to decreasing GB angle G1–3. (e,f) The dislocation number decreases, caused by the GB dislocations climbing into the TJs or other GBs. Meanwhile, the GB angle at G1–2 increased from 12.1° to 21.6°. Scale bars, 2 nm.

Mentions: As the grain size decreases, the grain rotation is more obvious. Figure 3 is a series of HRTEM images, showing the GB dislocation-mediated grain rotation process at the atomic scale. Figure 3a highlights five grains (d~5 nm, marked as ‘1’ to ‘5’), separated by high-angle GBs. During the straining, G3 and G4 exhibit no obvious fringe change, indicating that there is no global tilt and shift of the specimen during deformation. The double-ended arrow in Fig. 3b indicates the loading axis. The GB angles are 12.1°, 15.1° and 35.1° for G1–2, G1–3 and G3–4, respectively. As noted with ‘T’ in Fig. 3a, an array of GB dislocations can be seen at G1–3. As shown in Fig. 3b–f, during the in situ straining, the number of the dislocations at G1–3 decreases and the average spacing of the GB dislocations increases from 0.9 to 1.7 nm. This corresponds to a reduced misorientation angle at G1–3, decreasing from 15.1° to 1.9°. Figre. 3e,h show that the GB dislocations have climbed towards the triple junction (TJ) points, and the dislocation annihilation/absorption in the TJ (or other GBs) is the reason of the decrease in the number of GB dislocations. For G1–3, the GB angle increased from 12.1° to 21.6°, while that for the G3–4 decreased from 35.1° to 27.7°. Here again, no intra-grain dislocations were observed inside these tiny grains throughout the deformation process. It is noted that G1 rotates about 9.5° with respect to G2 but 13.2° with respect to G3. If only G1 underwent the rotation process, the rotation angle of G1 with respect to others would be the same. We also measured the rotation angles of G1–4, G2–4 and G2–3. The rotation angles all exhibited different values. This indicates that these grains underwent a simultaneous rotation process, that is, they all participated in rotations relative to one another in a coordinated manner during straining. A number of in situ observations consistently show that for those grains surrounded by small grains, they underwent collective rotation together (Supplementary Tables 1 and 2, and Supplementary Fig. 2). For the grains surrounded by large grains, only the smaller grains underwent rotation (see next paragraph, Supplementary Table 3 and Supplementary Figs 3 and 4).


Grain rotation mediated by grain boundary dislocations in nanocrystalline platinum.

Wang L, Teng J, Liu P, Hirata A, Ma E, Zhang Z, Chen M, Han X - Nat Commun (2014)

Atomic-scale in situ observation of grain rotation in smaller grains.(a) An array of dislocations (as marked with ‘T’) forms a wedge-shaped disclination at the GB. (b–e) During straining, the number of the dislocations decreases, leading to decreasing GB angle G1–3. (e,f) The dislocation number decreases, caused by the GB dislocations climbing into the TJs or other GBs. Meanwhile, the GB angle at G1–2 increased from 12.1° to 21.6°. Scale bars, 2 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4109021&req=5

f3: Atomic-scale in situ observation of grain rotation in smaller grains.(a) An array of dislocations (as marked with ‘T’) forms a wedge-shaped disclination at the GB. (b–e) During straining, the number of the dislocations decreases, leading to decreasing GB angle G1–3. (e,f) The dislocation number decreases, caused by the GB dislocations climbing into the TJs or other GBs. Meanwhile, the GB angle at G1–2 increased from 12.1° to 21.6°. Scale bars, 2 nm.
Mentions: As the grain size decreases, the grain rotation is more obvious. Figure 3 is a series of HRTEM images, showing the GB dislocation-mediated grain rotation process at the atomic scale. Figure 3a highlights five grains (d~5 nm, marked as ‘1’ to ‘5’), separated by high-angle GBs. During the straining, G3 and G4 exhibit no obvious fringe change, indicating that there is no global tilt and shift of the specimen during deformation. The double-ended arrow in Fig. 3b indicates the loading axis. The GB angles are 12.1°, 15.1° and 35.1° for G1–2, G1–3 and G3–4, respectively. As noted with ‘T’ in Fig. 3a, an array of GB dislocations can be seen at G1–3. As shown in Fig. 3b–f, during the in situ straining, the number of the dislocations at G1–3 decreases and the average spacing of the GB dislocations increases from 0.9 to 1.7 nm. This corresponds to a reduced misorientation angle at G1–3, decreasing from 15.1° to 1.9°. Figre. 3e,h show that the GB dislocations have climbed towards the triple junction (TJ) points, and the dislocation annihilation/absorption in the TJ (or other GBs) is the reason of the decrease in the number of GB dislocations. For G1–3, the GB angle increased from 12.1° to 21.6°, while that for the G3–4 decreased from 35.1° to 27.7°. Here again, no intra-grain dislocations were observed inside these tiny grains throughout the deformation process. It is noted that G1 rotates about 9.5° with respect to G2 but 13.2° with respect to G3. If only G1 underwent the rotation process, the rotation angle of G1 with respect to others would be the same. We also measured the rotation angles of G1–4, G2–4 and G2–3. The rotation angles all exhibited different values. This indicates that these grains underwent a simultaneous rotation process, that is, they all participated in rotations relative to one another in a coordinated manner during straining. A number of in situ observations consistently show that for those grains surrounded by small grains, they underwent collective rotation together (Supplementary Tables 1 and 2, and Supplementary Fig. 2). For the grains surrounded by large grains, only the smaller grains underwent rotation (see next paragraph, Supplementary Table 3 and Supplementary Figs 3 and 4).

Bottom Line: Grain rotation is a well-known phenomenon during high (homologous) temperature deformation and recrystallization of polycrystalline materials.In recent years, grain rotation has also been proposed as a plasticity mechanism at low temperatures (for example, room temperature for metals), especially for nanocrystalline grains with diameter d less than ~15 nm.Our atomic-scale images demonstrate directly that the evolution of the misorientation angle between neighbouring grains can be quantitatively accounted for by the change of the Frank-Bilby dislocation content in the grain boundary.

View Article: PubMed Central - PubMed

Affiliation: 1] Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, China [2].

ABSTRACT
Grain rotation is a well-known phenomenon during high (homologous) temperature deformation and recrystallization of polycrystalline materials. In recent years, grain rotation has also been proposed as a plasticity mechanism at low temperatures (for example, room temperature for metals), especially for nanocrystalline grains with diameter d less than ~15 nm. Here, in tensile-loaded Pt thin films under a high-resolution transmission electron microscope, we show that the plasticity mechanism transitions from cross-grain dislocation glide in larger grains (d>6 nm) to a mode of coordinated rotation of multiple grains for grains with d<6 nm. The mechanism underlying the grain rotation is dislocation climb at the grain boundary, rather than grain boundary sliding or diffusional creep. Our atomic-scale images demonstrate directly that the evolution of the misorientation angle between neighbouring grains can be quantitatively accounted for by the change of the Frank-Bilby dislocation content in the grain boundary.

No MeSH data available.


Related in: MedlinePlus