Two Methods for Measuring Residual Strain in ISIS Targets

1. Two Methods for Measuring Residual Strain in ISIS Targets Dan Wilcox High Power Targets Group, RAL, UK ICANS XXIII, Chattanooga, October 16th 2019

2. Outline • Background • Asymmetric-clad strip method • Neutron diffraction method • Results and analysis • Implications for target design

3. Acknowledgements • Sample manufacture – Leslie Jones, Jeremy Moor, Max Rowland, Peter Webb • Asymmetric-clad concept – Peter Loveridge • Instrument scientists – Saurabh Kabra, Tung Lik Lee • ISIS management – Ste Gallimore, David Jenkins

4. Background • ISIS operates two spallation targets; TS1 and TS2 • Both have tungsten cores clad in tantalum for corrosion resistance • Any significant cladding breach means we have to replace the target • Causes activation of the cooling plant, which needs hands-on maintenance • Cladding is attached by Hot Isostatic Press (HIP) • Produces a strong bond with good thermal contact • High residual stress predicted from HIP – maybe more than beam heating • Exact amount of residual stress is a major unknown in current simulations* TS2 TS1 *D. Wilcox, P. Loveridge, T. Davenne, L. Jones and D. Jenkins, "Stress levels and failure modes of tantalum-clad tungsten targets at ISIS," Journal of Nuclear Materials, vol. 506, pp. 76-82, 2018.

5. ISIS Target Manufacture • Tantalum can welded shut with tungsten core inside • Hot Isostatic Press (HIP) process applies high temperature and pressure, bonding cladding and core • Final machining to achieve dimensional accuracy Components of TS2 target HIP assembly

6. ISIS Target Manufacture • HIP cycle in detail: • Cladding deforms plastically under high pressure (140MPa) • Peak temperature is 1200°C, held for 2 hours; this relieves stress, but does not cause grain growth • Tantalum and tungsten are bonded together • As the clad target cools below the annealing temperature, residual stress builds up due to different coefficients of thermal expansion: • Tungsten 4.5E-06/K • Tantalum 6.3E-06/K A typical ISIS HIP cycle

7. Simplifying Assumptions Simplifying assumptions for simulating residual stress: • All stress from welding and HIP pressure is relieved during HIPing • Stress due to final machining and plate welding is small compared to stress from HIPing, and can be ignored • Stress from HIP cooldown is relieved at first, but below a certain ‘lock-in’ temperature stress starts to builds up • Industry uses a stress relieving temperature of around 850°C • Lock-in temperature estimated to be 500°C, but this needs validation

8. Asymmetric-Clad Strip Method • Provides a simple, mechanical method of measuring residual stress after HIPing • HIP a long strip of tungsten with asymmetric tantalum cladding • The strip should deflect in proportion to the residual stress • Calculations predict a spherical surface, concave on the thick tantalum side • Dimensions optimised to produce a measurable deflection without breaking the sample Nominal dimensions of asymmetric-clad strip

9. Manufacture • Manufactured by Jeremy Moor and his team using the same materials and methods as ISIS targets Photos: Jeremy Moor

10. Measurement • Sample was measured before and after HIP using a Faro Edge arm (laser scanner coordinate measuring machine) • Concave on thick tantalum side and almost spherical, as expected • Spherical surfaces fit the upper and lower faces with r2 = 0.990 Upper surface Cloud of points from Faro arm (above) and spherical fits to upper and lower surfaces (right) Lower surface

11. Comparison with Theory • Compare fit and simulated radii of curvature • Estimates lock-in temperature at 385-450°C • To produce the measured deflection the tantalum must have yielded

12. Neutron Diffraction Method • ENGIN-X is an instrument on ISIS TS1 which uses neutron diffraction to make non-destructive strain measurements “The beamline is optimized for the measurement of strain, and thus stress, deep within a crystalline material, using the atomic lattice planes as an atomic 'strain gauge'.” https://www.isis.stfc.ac.uk/Pages/Engin-X.aspx • Good penetration depth even in Ta/W • 1x1x18mm measurement volume • Cannot measure plastic strain • Strains are calculated relative to an unstressed ‘d0’ measurement The ENGIN-X instrument: measuring residual stress within friction stir welds on an Airbus prototype wing rib

13. Prior Experiment by Yanling Ma et al. • Neutron diffraction experiment on ENGIN-X by Yanling Ma et al. on a TS1 plate* • Demonstrated that ENGIN-X can measure depths of up to 13mm in tantalum/tungsten in a reasonable length of time *Yanling Ma et al., “An Experiment Using Neutron Diffraction to Investigate Residual Strain Distribution in a Hot Isostatic Pressed (HIPPED) Target Plate,” in Joint 3rd UK-China Steel Research Forum & 15th CMA-UK Conference on Materials Science and Engineering, 2014.

14. Prior Experiment by Yanling Ma et al. • Measured elastic strains are broadly consistent with the cladding being at the yield stress as predicted – however the sample was too thick to measure all three directions so this could not be confirmed ANSYS Simulation of Engin-X Sample Engin-X Strain Result (Y direction) 662 Microstrain ≈ 600 Microstrain -749 Microstrain ≈ -300 Microstrain

15. ENGIN-X Measurements • Two experiments: October 2018 (3 days) and March 2019 (2 days) • Aims – measure a detailed through-thickness strain profile – check uniformity of strain profile with position – measure all 3 strain components at every point 3, 9, 3Points 9 Points 10mm 50mm 50mm 10mm 20mm Clamp this end 3 Points X Y Z

16. ENGIN-X Measurements • Sample rotated part way through to measure all 3 strain components • Normal direction measured twice Photos courtesy of Tung Lik Lee

17. ENGIN-X Results – Part 1 • Similar profiles in x and y, as expected • Good repeatability between repeat measurements • Quite good agreement with simulation apart from through-thickness (z) strain in tantalum

18. Changes for Second ENGIN-X Run • Previous ‘d0’ samples may not have been truly unstressed • Tantalum d0 sample was rolled and annealed • Tungsten d0 sample was forged, but probably not annealed • Repeated measurement with new d0 samples • Tungsten sample cut into combs to relieve stress from forging • Previous W sample had higher normal strain – possibly due to forging • PreviousTa sample the same to within the ≈100 microstrain uncertainty • Also re-measured old locations and added more measurement points

19. HIP Effects on Nominally Unstressed Samples • Second run also looked at the stress change after HIPing in pure tantalum and pure tungsten • HIP may change the atomic spacing and therefore d0 measurement • An exact copy of each d0 sample was put through a standard ISIS HIP cycle • HIPing induces a strain which is compressive in W, tensile in Ta • Changes are anisotropic, which was unexpected and makes it difficult to apply d0 corrections to other geometry e.g. the strip

20. ENGIN-X Results – Part 2 • Good repeatability between different locations and times • No choice of d0 measurement improves the fit for all materials and directions

21. Conclusions from Experiments • Presence of large tensile residual stress in cladding confirmed by physical test and neutron measurement • Simulations with lock-in temperature ≈ 400°C predict the shape of the deformed strip, but do not explain all the details seen by ENGIN-X • HIPing produces unexpected changes in neutron diffraction strain measurements of pure tantalum and tungsten samples • Differences in ENGIN-X data vs simulation may be due to: • Large uncertainties relative to measurement (high Young’s mod of Ta/W) • Creep of tantalum during cooldownafter HIPing • Deformed grains or other microstructural effects => anisotropic properties • No stress relieving was observed over time. Residual stress may relieve in-beam, but this would be difficult to measure.

22. Implications for Target Design • Large residual stress confirmed, so this should be taken into account in future target analysis work • High tensile stress will reduce fatigue life of the cladding, although current ISIS targets have a large safety factor even with this included • Limited data on combined radiation damage and fatigue effects • If tantalum ductility is lost this may present a problem • Not currently an issue, but could be for potential ISIS-2 targets • Continuing to develop our understanding of failure modes will enable more optimised targets and higher beam powers in future