The diffusivity and solubility of H in grain-refined material were measured by using a vacuum hot-extraction method. Four samples, with various grain sizes, were prepared by melting, adding Ti and B, casting, re-melting and solidifying unidirectionally. The grain-refined samples contained small particles within the matrix. It was found that the H diffusivity obeyed an Arrhenius type behavior at temperatures ranging from 573 to 873K. The results for air-melted Al-0.05wt%Ti samples could be described by:

D (m2/s) = 2.04 x 10-4exp[-65.7(kJ/mol)/RT]

and the results for Al-0.05wt%Ti samples which had been cast under flowing N could be described by:

D (m2/s) = 2.22 x 10-6exp[-37.4(kJ/mol)/RT]

The equivalent relationships for Al-0.1wt%Ti samples could be described by:

D (m2/s) = 9.10 x 10-3exp[-95.4(kJ/mol)/RT]

D (m2/s) = 1.39 x 10-4exp[-66.9(kJ/mol)/RT]

respectively. The diffusivity was not affected by the small particles in the matrix, and depended only upon the grain size. A so-called grain-boundary cross effect, which referred to fast diffusion along grain boundaries and suppressed diffusion due to H trapping at nodes or grain boundary junctions, was confirmed. A linear relationship, the so-called compensation effect, was found between the logarithm of the pre-exponential factor and the activation energy for all of the materials which were studied here.

M.Ichimura, Y.Sasajima: Materials Transactions, 1993, 34[5], 404-9