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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

In situ observation of the grain rotation for a 4.8 nm × 6.8 nm sized grain.The white arrow in b indicates the loading axis. In a, G1 is surrounded by high-angle GBs. (b,c) With straining, G1 exhibits the [110] lattice, and the GB G1–3 and G1–4 have changed into small-angle GBs. (d,e) With further straining, the [110] axial-lattice in G1 changed into fringes. (f) The GB G1–2 disappeared and G1–4 extended to become a long GB. (g–j) Schematics corresponding to a–f to illustrate the rotation process of G1 under tensile stress. Scale bars, 2 nm.
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f4: In situ observation of the grain rotation for a 4.8 nm × 6.8 nm sized grain.The white arrow in b indicates the loading axis. In a, G1 is surrounded by high-angle GBs. (b,c) With straining, G1 exhibits the [110] lattice, and the GB G1–3 and G1–4 have changed into small-angle GBs. (d,e) With further straining, the [110] axial-lattice in G1 changed into fringes. (f) The GB G1–2 disappeared and G1–4 extended to become a long GB. (g–j) Schematics corresponding to a–f to illustrate the rotation process of G1 under tensile stress. Scale bars, 2 nm.

Mentions: Figure 4 presents a series of HRTEM images, showing another scenario for the rotation of a small grain, which underwent not only rotation but also tilt. As described above, the small grain G1 (d=4.8 nm) clearly exhibits GB dislocation-mediated grain rotation. In a larger grain, G4 (d>10 nm), full dislocations were frequently observed2829, in lieu of grain rotation. During straining, no change was observed for the lattice/fringe in grains G3 and G4 (Supplementary Figs 3 and 4), indicating that there was no rotation/tilt between G3 and G4. In comparison, the contrast of the G1 grain, and the GB structure of G1–3 and G1–4, changed quickly. From Fig. 4a, G1 is surrounded by high-angle GBs: no fringes were observed in G1, while G3 and G4 exhibit obvious fringes. The TJs are indicated by arrows. With increasing straining, we observed a clear [110] (axis) lattice (Fig. 4b) in G1, and the GB angles are 12.6° and 7.5° for G1–3 and G1–4 (they changed into small-angle GBs), respectively. This is obviously caused by the tilting of G1, together with GB structural changes. On further loading, the lattice in G1 changed into fringes and then became the same as the grain G3 (Fig. 4c–f), involving both in-plane and out-of-plane grain re-orientation. The G1–3 changed from a high-angle GB to a small-angle GB and then disappeared, and the TJ changed into a GB. For G1–4, it changed from a small-angle GB (7.5° in Fig. 4b) to a high-angle GB (15.3° in Fig. 4c). Although the angle of G1–3 cannot be measured after Fig. 4c, it continued to increase because of the rotation and tilt of the grain G1. In Fig. 4d,e, G1–4 shows the features of a typical high-angle GB. Figure 4g–j presents the schematic view corresponding to Fig. 4a–f, for illustrating the rotation process of G1. As illustrated in Fig. 4g,h, the rotation and tilting of G1 led to the appearance of lattice fringe in the image. The G1–3 and G1–4 consist of arrays of GB dislocations (as marked in green lines). With extensive deformation, G1 rotated via GB dislocation motion and annihilation. The GB angles decreased (or increased) as the dislocation number changed, as shown in Fig. 4h,i. The continued rotation and tilting of G1 eventually led to the merge of the grain as the GB in between and the TJ disappear (Fig. 4j). Here, G1 was surrounded by relatively large grains (such as G2, G3 and G4). We can see that the rotation angle of G1 with respect to G3 and G4 is the same, while the rotation angle of G3 with respect to G4 is nearly zero. Thus, only G1 underwent the rotation process during deformation (Supplementary Table 3).


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)

In situ observation of the grain rotation for a 4.8 nm × 6.8 nm sized grain.The white arrow in b indicates the loading axis. In a, G1 is surrounded by high-angle GBs. (b,c) With straining, G1 exhibits the [110] lattice, and the GB G1–3 and G1–4 have changed into small-angle GBs. (d,e) With further straining, the [110] axial-lattice in G1 changed into fringes. (f) The GB G1–2 disappeared and G1–4 extended to become a long GB. (g–j) Schematics corresponding to a–f to illustrate the rotation process of G1 under tensile stress. Scale bars, 2 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: In situ observation of the grain rotation for a 4.8 nm × 6.8 nm sized grain.The white arrow in b indicates the loading axis. In a, G1 is surrounded by high-angle GBs. (b,c) With straining, G1 exhibits the [110] lattice, and the GB G1–3 and G1–4 have changed into small-angle GBs. (d,e) With further straining, the [110] axial-lattice in G1 changed into fringes. (f) The GB G1–2 disappeared and G1–4 extended to become a long GB. (g–j) Schematics corresponding to a–f to illustrate the rotation process of G1 under tensile stress. Scale bars, 2 nm.
Mentions: Figure 4 presents a series of HRTEM images, showing another scenario for the rotation of a small grain, which underwent not only rotation but also tilt. As described above, the small grain G1 (d=4.8 nm) clearly exhibits GB dislocation-mediated grain rotation. In a larger grain, G4 (d>10 nm), full dislocations were frequently observed2829, in lieu of grain rotation. During straining, no change was observed for the lattice/fringe in grains G3 and G4 (Supplementary Figs 3 and 4), indicating that there was no rotation/tilt between G3 and G4. In comparison, the contrast of the G1 grain, and the GB structure of G1–3 and G1–4, changed quickly. From Fig. 4a, G1 is surrounded by high-angle GBs: no fringes were observed in G1, while G3 and G4 exhibit obvious fringes. The TJs are indicated by arrows. With increasing straining, we observed a clear [110] (axis) lattice (Fig. 4b) in G1, and the GB angles are 12.6° and 7.5° for G1–3 and G1–4 (they changed into small-angle GBs), respectively. This is obviously caused by the tilting of G1, together with GB structural changes. On further loading, the lattice in G1 changed into fringes and then became the same as the grain G3 (Fig. 4c–f), involving both in-plane and out-of-plane grain re-orientation. The G1–3 changed from a high-angle GB to a small-angle GB and then disappeared, and the TJ changed into a GB. For G1–4, it changed from a small-angle GB (7.5° in Fig. 4b) to a high-angle GB (15.3° in Fig. 4c). Although the angle of G1–3 cannot be measured after Fig. 4c, it continued to increase because of the rotation and tilt of the grain G1. In Fig. 4d,e, G1–4 shows the features of a typical high-angle GB. Figure 4g–j presents the schematic view corresponding to Fig. 4a–f, for illustrating the rotation process of G1. As illustrated in Fig. 4g,h, the rotation and tilting of G1 led to the appearance of lattice fringe in the image. The G1–3 and G1–4 consist of arrays of GB dislocations (as marked in green lines). With extensive deformation, G1 rotated via GB dislocation motion and annihilation. The GB angles decreased (or increased) as the dislocation number changed, as shown in Fig. 4h,i. The continued rotation and tilting of G1 eventually led to the merge of the grain as the GB in between and the TJ disappear (Fig. 4j). Here, G1 was surrounded by relatively large grains (such as G2, G3 and G4). We can see that the rotation angle of G1 with respect to G3 and G4 is the same, while the rotation angle of G3 with respect to G4 is nearly zero. Thus, only G1 underwent the rotation process during deformation (Supplementary Table 3).

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