1. Iuliia Ipatova; Iuliia.ipatova@manchester.ac.uk
Evolution of the lattice defects in Tantalum-Tungsten alloys under irradiation
Iuliia Ipatova;
14th of September 2016

2. Outline Introduction: ISIS neutron spallation source application
Experimental results before irradiation
Irradiation experiments in Dalton Cumbrian Facility
Study of irradiated Tantalum-Tungsten samples
Conclusions

3. Ta-W alloys are of prime importance:
to build state-of-the-art ion beam lines in irradiation facility at increasing beam powers (within the framework of the National Nuclear Users Facility)
to understand the stability of target materials which are used at neutron spallation sources (i.e. ISIS).
FYI:
The ISIS target station uses the high energy protons produced by the ISIS accelerator to generate neutrons by the spallation process and to modify their characteristics to make them useful for neutron scattering experiments. 

4. Spallation process and the most desirable properties for a spallation target material:
High atomic number
High density
High melting point
High thermal conductivity
Chemically inert, low corrosion
Resistance to radiation damage
Low resonance integral for neutron absorption

5. ISIS neutron spallation source
TS1
TS2
Consists of 12 solid W plates (105 × 80 mm) of different thicknesses (from 11 to 46 mm)
Each W plate is cladded in a 2 mm thick Ta layer.
Tungsten core is sealed in tantalum «can»
Hot Isostatic Press is used to join all components together

6. Different thermal expansion coefficient of W and Ta
Main Challenges
Different thermal expansion coefficient of W and Ta
=> Doping Ta with controlled amounts of W (up to 10wt.%) would improve the thermal stability of the compound target
Fatigue failure of tantalum cladding
Radiation embrittlement can make the fatigue situation worse and irradiation creep and stress relaxation may reduce the average stress.
=> Addition of W to Ta can potentially increase the yield strength and the rate of work hardening of the final material

7. Transmission Electron Microscopy data received on Philips CM kV TEM: Non-irradiated Ta-W alloys
Left – Dislocation entangles along different directions
and its intersaections in Ta2.5W;
right – Corresponding DP, zone axis [001]
Left - Stage of dislocations birth in pure Ta;
right – Corresponding DP, zone axis [111]
Left – Dislocation nets forming in Ta10W alloy;
right – Corresponding diffraction pattern, zone axis [001]
½ a <111> type Burgers vector (are energetically favoured) on {011} slip plane

8. Ta10wt.%W after 10 hours of annealing
a <100> type Burgers vector of prismatic dislocation loops

9. Conclusions on non-irradiated materials
1. TEM analysis reveals that with increasing W concentration in Ta, dislocation density is increasing and dislocation lines arrange more in cellular structure which can effect hardening behavior.
2. Detailed examination of thin foils indicates that dislocations presented in Ta10W alloy are screw dislocations with ½ a <111> type Burgers vector on {011} slip plane. After long annealing dislocation type is changing to a<001> Burgers vector and they appear as prismatic loops.
3. EBSD analysis indicates significant <111> texture in pure Ta and reveals that W-containing alloys are oriented and dominated by <100> cube texture.

10. 3 MeV proton irradiation experiment in Dalton Cumbrian Facility using 5MV Tandem Pelletron ion accelerator (NEC model 15SDH-4) and high current TORVIS source

11. Target stage assembly Target station with 4 samples mounted
on the top and covered
with tantalum window from the top
Top left –vacuum chamber; top right – target stage with
samples mounted and inserted inside the vacuum chamber (side view);
Bottom – target station (side view) in the process of inserting samples

12. Parameters of implemented 3 MeV proton irradiation experiments
Conditions
Samples
Techniques being used
16th -17th September 2015
Temperature 350° C
Current 9 µA
1.2x1.2 cm2 irr. area
Up to 0.8 dpa*
36 hours in total
Charge –
4 samples:
Ta;
Ta2.5wt%W; Ta5wt.%W; Ta10wt.%W
1 hour of annealing before
irradiation
Nanohardness test;
SEM;
TEM.
29th February –
1st March 2016
0.7x1 cm2 irr. area
Up to 1.6 dpa*
42 hours in total
Charge
2 samples:
Ta10wt.%W
10 hours of annealing before

13. Simulations of the damage profile
Bragg peak
16-17 September 2015
29th February-1st March
Bragg peak

14. Bragg Peak position determination by experimental methods
Scanning electron microscopy (back scattered electron detector)
Nano hardness test
Transmission electron microscopy Bright imaging

15. Scanning Electron Microscopy Back Scattered Electron Imaging
FEI Quanta 250 FEG-SEM
36 µm
35 µm
Ta 0.8 dpa
Ta5W 0.8 dpa
37 µm
The electrons channelling through the compromised crystal structure is hided more and chance of back scattering increases.
Ta10W 0.8 dpa

16. Nano hardness test data
1 h annealing before irradiation
10 h annealing before irradiation

17. Irradiated samples preparation Laser Scanning Microscope
Ta 1.5 dpa
Irradiated samples preparation
for TEM analysis
using
Keyence VK-X200K 3D
Laser Scanning Microscope
Ta10W 1.5 dpa
60% of Bragg/dpa peak
Bragg /dpapeak
26 µm of material are removed

18. Irradiation-induced voids in Tantalum after proton irradiation using FEI Tecnai G2 20
dpa
dpa
[111]
[111]

19. Irradiation-induced voids in Ta and Ta10wt.%W
Pure Tantalum dpa
Ta10wt.%W dpa
[111]

20. Irradiation-induced dislocation loops occurred at 0.8-0.9 dpa
Ta10wt.%W
Pure Tantalum

21. Nucleation and development of irradiation-induced
dislocation loops in Ta10wt.%W
Ta10wt.%W;
dpa
Ta10wt.%W;
dpa

22. First obvious conclusions on proton irradiated Ta-W samples
Data of SRIM calculations, hardness test and SEM comparably coincide and Bragg peak position with the highest density of irradiation-induced defects might be determined at around µm
Voids arrays occur in pure irradiated Tantalum at dpa level. At dpa these void arrays are distributed denser and the distance between void walls is smaller
Ta10wt.%W alloy (at dpa) contains voids which are smaller and do not arrange in perfect arrays. They are located more randomly. At and dpa loops take place and they are increasing in diameter with higher dpa
Addition of W into Ta may slow down the growth of irradiation-induced voids and loops which may cause hardening of the material by pinning dislocations and preventing their glide during deformation which will affect life time of the final material component?

23. Any comments/questions? Thank you! Iuliia.ipatova@manchester.ac.uk