Thermomechanical effects on the hydrogen diffusion through HAVAR

Gabriel Zamir ¹ Shlomo Haroush ^1,2 Roni Shneck ² Yaniv Gelbstein ² Ido Silverman ³ Idoia Orduna ⁴ Gorka Alberro ⁴ Fernando Legarda ⁴ Igor Peñalva ⁴

¹Department of Materials, Nuclear Research Center Negev
²Department of Materials Engineering, Ben-Gurion University of the Negev
³SARAF, Nuclear Research Center Soreq
⁴Department of Nuclear Engineering and Fluid Mechanics, University of the Basque Country

HAVAR alloy is a high strength, non-magnetic and corrosion resistant cobalt based alloy, suitable for service up to temperatures of 500° C. One application of HAVAR is as a thin foil "window" in a medical cyclotron for isotopes production as ¹⁸F. During protons irradiation, the HAVAR window may be damaged due to radiation effects, exposure to high temperatures and high thermal stresses. In addition, protons can be captured in the alloy and increased the internal stress due to hydrogen located in radiation flaws.

In the annealed state, HAVAR is a solid solution comprising of an FCC structure, equiaxed grains, very low dislocation density and a small number of annealing twins. The HAVAR microstructure can be changed due to cold working, that increased the mechanical properties owing to a high dislocations density, stacking faults, dual phases FCC&HCP and more. The hydrogen diffusion process is highly affected by these imperfections. Therefore, it is very important to know the hydrogen passage throw the alloy.

The aim of the present study was to investigate the impact of HAVAR microstructure on the hydrogen diffusion through the alloy. The investigated microstructures were annealed and cold rolled.

The hydrogen transport parameters were measured by using the evolution permeation technique at different driving pressures and temperatures.

It was found that microstructure has high influence on hydrogen transport. The permeability observed in the cold-rolled HAVAR was three orders of magnitude smaller than in the annealed HAVAR. At low temperatures, the diffusivity was decreased while the Sievert`s constants were increased. A transition between surface diffusion and diffusive regime was observed between low and high temperatures.