Helium trapping and release mechanisms in vanadium alloy

1
Helium trapping and release mechanisms in
vanadium alloy

• Liu Xiang
• Center for Fusion Science, SWIP

2
Outline

• Motivation
• Our investigation---high ion dose
• A.van Veens results---low ion dose
• A complete understanding---helium trapping and
  release mechanisms

3
Motivation

• Vanadium alloy is considered as one of the
  promising first wall and structural materials for
  fusion energy system.
• The interaction of helium with vanadium alloy,
  particularly helium retention and thermal
  desorption properties, is a crucial issue, which
  is not only related to the stability of a burning
  plasma but also the feasibility of its
  applications in fusion devices.
• Helium trapping mechanisms at low ion dose region
  (around 1014 He/cm2) have been studied by A van
  Veen and A.V. Fedorov group. However, helium
  trapping and release mechanisms at high ion dose
  region are not clear up to now.
• In the present presentation, firstly, helium
  retention in V-4Ti alloy supplied by Dr. Chen
  will be investigated by thermal desorption
  spectroscopy of helium pre-implanted with 5 keV
  energy in an ECR ion irradiation apparatus of
  Hokkaido University. Helium trapping mechanisms
  at high ion dose (over 1017 He/cm2) will be
  stressed. Combining with the results of A van
  Veens, a relatively complete understanding of
  helium trapping and thermal release in vanadium
  alloy can be expected.

4
Sample (V-4Ti alloy made at SWIP)
Manufacture method inductive heating
Chemical composition of V-4Ti alloy
Machining procedure
Sample size20?5?0.25 mm, mechanically polished
by 0.05 ?m Al2O3 powder
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Experimental procedure
No pre-heating (as received sample) 973 K, 1223
K, 1373 K for 1 h
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Heirradiation (RT) 5 keV, Flux (0.9-1)?1014
He/cm2.s Fluence (1-8)?1017 He/cm2 Thermal
desorption RT-1600K, heating rate 1K/s
ECR ion irradiation apparatus
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Thermal desorption spectra Pre-heating treatment
1223 K annealing for 1h to make recrystallized
structure
Thermal helium desorption spectra of V-4Ti alloy
after 5 keV He ion implantation to different
doses at RT. Here the annealing temperature is
from RT to 1600 K and heating rate is 1K/s.
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Surface morphology of V-4Ti alloy after TDS
After TDS at a fluence of 2 ? 1017 He/cm2
After TDS at a fluence of 8 ? 1017 He/cm2
SEM images of V-4Ti alloy after TDS. Here, the
annealing temperature is from RT to 1600K.
9
Surface morphology of V-4Ti alloy after He ion
implantation at RT
60?
60?
8E17 He/cm2
2E17 He/cm2
0.1?m
0.1?m
0.2?m
0.2?m
dense blisters, some pinholes
Few blisters
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Weight loss 2E17---No detected 8E17---5.5?g Sputt
ering yield ?0.08
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AES analysis for the case of pre-heating at 1223
K for 1 h
No segregation of Ti was found
12
Thermal desorption spectra Comparison of
different annealing processes
For the as-received sample, the desorption peak
at low temperature region became obvious at low
ion dose and the central temperature move to
higher temperature by about 50 K.
Thermal desorption spectra of helium at 1?1017
(left) and 5?1017 (right) He/cm2 fluence by
different anneal processes as indicated in the
figures.
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The influence of background noise
TDS of helium for no pre-heated treatment sample
TDS of He on the case of exposure to ECR
resource but no He ion was extracted
It indicates that the contribution from
background noise of helium is on the high
temperature range and its influence is negligible.
14
Surface morphology after TDS for V-4Ti alloy
pre-heated at 973 K
Normal view
View from 60? direction
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The amount of helium retained in V-4Ti alloy
1, The saturation took place at about 3 ?1017
He/cm2 and the saturation level is about 2.5
?1017 He/cm2. 2, The total retention in
un- saturated zone does not show any obvious
difference for different annealing processes.
A relationship of the amount of helium retained
in V-4Ti alloy with helium implantation fluence.
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Comparison of data for different annealing
processes
Total helium retention and peak contents do not
show the similar relationship with that of A.
Veens results. Oppositely, the total retention
and peak contents for the case of 1223 K
annealing at 5 ? 1017 He/ cm2 ion fluence
(saturated region) seem to be the largest
values and the minimal values for as received
sample. However, generally speaking, no
significant difference were found.
Peak I (273-773 K) Peak II (1000-1500 K)
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TDS was repeated for the sample irradiated with
1x1017He/cm2
First TDS
Second TDS
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Summary for our study

• Two separate desorption peaks were found in the
  TDS of helium. Combining with the surface
  morphology (surface blisters), they were
  considered as the results of helium release from
  surface blisters and inner gas bubbles,
  respectively.
• Helium saturation will take place at an
  approximately dose of 3?1017 He/cm2 and the
  saturated retention amount is about 2.5?1017
  He/cm2. Which is comparable with carbon based
  materials, such as graphite, B4C and SiC, and a
  little higher than tungsten.
• AS for as-received sample, the desorption peak at
  low temperature region become more obviously and
  its central temperature moves towards higher
  temperature by 50 K. A probability is due to high
  density dislocation defects which restrain the
  growth of surface blisters, in this way induce
  the early formation of surface blisters. On the
  other hand, higher yield strength of cold-worked
  samples compared with annealed ones could be the
  reason of peak move.
• Generally speaking, no significant influence of
  pre-heated treatments on helium trapping and
  thermal release was observed. It is quite
  different from the situation of low ion doses.

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Low ion doses---A.van Veens results
Fig.2 Contribution of helium release from
different temperature intervals versus the
pre-annealing temperature.
Fig.3 Helium release spectra after 1 keV
irradiation with dose of 1014 cm-2 for pure V,
V-5Ti and V-3Ti-1Si alloys.
Fig.1 Thermal desorption spectra for V-4Cr-4Ti
after 1 keV helium implantation at a dose of 1014
cm-2. Before the implantation the samples were
annealed for 1h at the indicated temperature.
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Conclusions from A.van Veen

• Two group peaks of helium release are found, one
  is at 500-1000 K, the other is at 1200-1400 K.
  However, for pure vanadium no peaks at high
  temperature region are observed.
• The first group peaks correspond to HenVX
  clusters, n ? 1,
• V---vacancy, X---impurities, such as O, C
  or N.
• The second peak at high temperature region is
  related to helium trapping into the inherent
  defect sites, such as fine-size precipitates.
• Preheat treatments have obvious influence on
  helium trapping in vanadium alloys since the
  formation of fine-size precipitates is closed
  relevant to annealing temperature.

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Helium trapping mechanisms in V-alloy
Intermediate ion dose
high ion dose
Low ion dose
Dpa?30
Dpa0.1-1
Dpa0.05

• Gas filled bubbles become dominate.
• Swelling .

• Surface blisters and dense inner bubbles.
• Ion-induced defects play a predominate role.

• Low helium and defect concentration.
• No interactions.

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Conclusions

• At very low ion dose(1013-1014 He/cm2), HenVX
  clusters is the maim trap mechanism of helium in
  vanadium alloy Another small desorption peak at
  the higher temperature region is ascribed to the
  release from small helium bubbles formed at the
  pre-existing traps, such as fine-size
  precipitates. There are not interactions between
  implanted helium ions or defects, inherent
  defects play a important role.
• At intermediate ion dose(1014-1016 He/cm2), point
  defects (interstitials and vacancies) produced by
  incident ions agglomerate in dislocation loops
  and may lead to the formation of gas filled
  bubbles in the solid within the ion range. So
  that the desorption peak at high temperature
  regime will become dominate and bulk swelling
  might take place.
• When ion fluence is at high level (1017-1019
  He/cm2), helium saturation will occur and high
  implantation concentration and high ion-induced
  vacancy loops will induce the formation of
  surface blisters. Therefore, the release of
  helium from blisters and inner gas bubbles will
  become comparably important. On the other hand,
  since the local defect concentrations created by
  implanted ions are much larger than the inherent
  ones in vanadium alloy, helium trapping into the
  vacancy clusters produced by energetic He
  implantation plays a dominant role and the
  contribution from the pre-existing defects in
  vanadium alloy can be neglected.