Helium bubble growth in tungsten
nanotendril
Yingzhao He, Zhangcan Yang*
Department of Nuclear Engineering and Technology,
Huazhong University of Science and Technology, Wuhan 430074, China

12I.

Background of tungsten fuzz formationand nano-tendril structures

II.

Computational modelIII.

Results and discussions

III.

Summary

3I.

Background of tungsten fuzz formationand nano-tendril structures

II.

Computational modelIII.

Results and discussions

III.

SummaryExtremely harsh environmentl H/He plasma (0-100 eV)
l Neutron: 14 MeVl High heat flux (10 MW/m2)

Issuesl Surface damage: hydrogen

induced blistering; helium
induced fuzz formationl Degradation of mechanicalpropertiesl Melting
4ITER divertor casse0es

Formation conditionsl Temperature: 900 – 2000 K
l He energy threshold: ~ 30 eVl Fluence > 1024 m-2 when EHe=50-80eVl Thickness: up to several micrometersl Fuzz layer growth dynamics: ∝t

De Temmerman, et al.
J. Nucl. Mater. 438: S78–S83 (2013).

Kajita, Nucl. Fusion, 49 (2009) 095005

5M. J. Baldwin, et al. J. Nucl. Mater. 390–391: 886 (2009)

Take a look at the nanotendrils:
Ø Grain boundaries
Ø Faceted and rounded bubbles

Wang et al. Scientific reports 7 (2017): 42315.

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Take a closer look :Qestions:Ø Branch structure near the GB

Ø How He bubble grows near GB?

Ø He bubbles near GB

Ø How surface morphology changes?
C.M. Parish, Scr. Mater. 127 (2017) 132–135.

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8I.

Background of tungsten fuzz formationand nano-tendril structures

II.

Computational modelIII.

Results and discussions

III.

Summary

Create tilt grain boundary structure:
1. Rotate two grains by the oppositeangles;
2. Optimize the GB structure and use theminimum energy structure as themodel.
3. Four common types of GB are studied:Σ3, Σ5, Σ7 and Σ17;

Σ17<100>{410} symmetric tilt grain boundary (STGB).

9Create nanotendrils:
1. Delete the atoms to create a cylindricalnano-column
2. Radius:~8nm, Height:~20nm
3. Total atoms: 250000~300000
4. The grain boundary is located in the
middle of the tendril.
Σ3 GB structure

10GBFree to move

3nm

Nose-Hoover thermostat

T = 1000 K

Several layers of atoms atbottom are fixed11Initial He bubble: radius = 3a0(about 1 nm);He atom is introduced into thecenter of the bubble every 10 ps;
He bubbles rupture after around6000 He insertion.

12I.

Background of tungsten fuzz formationand nano-tendril structures

II.

Computational modelIII.

Results and discussions

III.

Summary• Protruding ≈ 7nm• Most of dislocations are [11-1]
dislocations, leading to the interstitials
stacking in the [11-1] direction• A few <100> dislocations<100>
Dislocations
X

5.1%(2)

<112>

≈7nm

• Σ7 GB structure shows similar
behaviors.

[111]
dislocations[-1-1-1]
dislocations[-111]
dislocations[1-1-1]
dislocations[1-11]
dislocations[-11-1]
dislocations[11-1]
dislocations[-1-11]
dislocations

5.1%(2)0%(0)2.6%(1)

12.8%(5)

0%(0)

0%(0)

41.1%(16)

33.3%(13)13• Much fewer dislocations before bubble
rupture, resulting in fewer adatoms at thesurface and smaller protruding part.
• Adatoms spread on the surface rather thanstacking• No <100> dislocations
X

<310>

[111]
disloca6ons[-1-1-1]
disloca6ons[-111]
disloca6ons[1-1-1]
disloca6ons[1-11]
disloca6ons[-11-1]
disloca6ons[11-1]
disloca6ons[-1-11]
disloca6ons

0%(0)11.8%(2)

17.6%(3)

17.6%(3)5.9%(1)

5.9%(1)

35.3%(6)5.9%(1)14

Bubble bursSng• There are much more adatoms in Σ3and Σ7 structures than the other twostructures when bubbles rupture.
• The rupture time of the four structures
is similar, which means that these fourstructure have equivalent ability toaccommodate He atoms.

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Σ3<111>{112}Σ7<111>{123}Σ5<100>{310}Σ17<100>{410}• Σ3 and Σ7 structures have a more obvious
protruding part caused by the interstitialatoms stacking.
• In the Σ5 and Σ17 structures, the interstitialatoms spread on the surface so that smaller
protruding part forms.
• The number and direction of dislocations
determine the shape and size of theprotruding part.

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• Most of the dislocations in the simulations are hybrid
dislocations. Very few integrated prismatic dislocation
loops exist.
• There are two type of hybrid dislocations.
GBBlue: edge dislocaSon
Pink: screw dislocaSon
Other color: hybrid

GBScrew part

BubbleType (a)Exists in Σ3 and Σ7 structures

17BubbleType (b)Exists in Σ5 and Σ17 structuresΣ7 structures (type a)18• Above: the snapshots of the type (a) dislocation in a Σ3 structure.
• The color from blue to red designates the distance from the present atomic site to thecenter of the He bubble.
• As can be seen from the figure, the grain boundary is an important medium for thesliding of dislocations.
19Σ5 structures20• Above: the snapshots of the type (b) dislocation in a Σ5 structure.
• The edge part dislocation plays a key role in the movement of hybrid dislocations.

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Below:• The shear stress-field nephogram around the He bubble.
• The edge part dislocation and screw part dislocation
have opposite shear stress direction.

Σ3 GB

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• A sketch of the movement process of the type (a) hybrid dislocation under shear stress.
•Firstly, the edge part glides toward the surface and then annihilates at surface leaving apure screw dislocation;• Subsequently the remaining screw part moves toward the GB driven by shear stress andthen annihilates at the GB
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24I.

Background of tungsten fuzz formationand nano-tendril structures

II.

Computational modelIII.

Results and discussions

III.

Summary• He bubble growth in the vicinity of 4 types of GBs in W nanotendril
has distinct features compared to bubble growth in the bulk or nearsurface.
Ø He bubbles are attracted by GBs;
Ø Generation of hybrid dislocations;Ø Rapid evolution of surface morphology.
• The formation of large protruding part in Σ3 and Σ7 structures could
probably explain the formation of branch structures in the fuzz tendrils.
• For both two types of hybrid dislocations, the edge part dislocation
moves first to drive the motion of the entire dislocation and thenannihilates at the surface leaving the screw dislocation part. The
remaining screw dislocation will either move to GBs or move to thesurface.
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Thanks for your attention!

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