The temperature dependence of the volume diffusion coefficient for hafnium in single-crystal tungsten was studied at 2023 to 2573K using secondary ion mass spectrometry. The results were described by:

D(m2/s) = 2.19 x 10-4exp[-557.8(kJ/mol)/RT]

The activation enthalpies for the bulk diffusion of 5d elements (Hf, Ta, W, Re, Os, Ir) in tungsten depended parabolically upon the difference in the valences of the impurity and host atoms.

Volume Diffusion of Hafnium in Tungsten Single Crystals. S.M.Klotsman, S.V.Osetrov, A.N.Timofeev: Physics of Metals and Metallography, 1996, 81[6], 668-74