The impurity diffusion of Zr was determined (using Ti/Ti-3.06at%Zr couples) at temperatures ranging from 1173 to 1773K (table 252), under pressures of between 0.1MPa and 3.0GPa. It was found that the Arrhenius plots had a marked upward curvature, and that the degree of curvature increased with increasing pressure. The data could be described by the expressions:

0.1MPa:     D (m2/s) = 0.000159 exp[-316(kJ/mol)/RT] exp[136(MJK/mol)/RT2]

1.0GPa:     D (m2/s) = 0.000160 exp[-322(kJ/mol)/RT] exp[140(MJK/mol)/RT2]

2.1GPa:     D (m2/s) = 0.000162 exp[-329(kJ/mol)/RT] exp[145(MJK/mol)/RT2]

3.0GPa:     D (m2/s) = 0.000164 exp[-334(kJ/mol)/RT] exp[148(MJK/mol)/RT2]

The activation volumes which were deduced from the isothermal pressure dependence of the diffusion coefficient were between 22 and 36% of the atomic volume of the host metal. These percentages were very close to that (33%) for self-diffusion in -Ti and to those (28 to 0.41%) for W and Sn diffusion in -Ti. On the other hand, the present values were considerably lower than those which were normally expected for diffusion in body-centered cubic metals via a so-called simple mono-vacancy mechanism. The increase in the degree of curvature of Arrhenius plots with increasing pressure, and the small values of the activation volume, were explained in terms of phonon-assisted jumps via mono-vacancies. When this model was applied to the impurity diffusion of Zr in -Ti, the activation volume was predicted to be 14K/GPa. This estimate was in very good agreement with the present experimental results.

H.Araki, Y.Minamino, T.Yamane, T.Nakatsuka, Y.Miyamoto: Metallurgical and Materials Transactions A, 1996, 27[7], 1807-14

Table 252

Activation Volume for Impurity Diffusion of Zr in -Ti