The temperature dependence of the volume diffusivity of Hf in monocrystalline W samples was studied, at temperatures ranging from 2023 to 2573K, by means of secondary-ion mass spectrometry. The Arrhenius relationship was estimated to be:

D (m2/s) = 0.000219 exp[-557.8(kJ/mol)/RT]

It was found that the activation enthalpies for the volume diffusion of 5d-elements (Hf, Ta, W, Re, Os, Ir) in W depended parabolically upon the difference in valences of the impurity and host atoms.

S.M.Klotsman, S.V.Osetrov, A.N.Timofeev: Physics of Metals and Metallography, 1996, 81[6], 668-74