Bulk and interface boundary diffusion in group IV hexagonal close-packed metals and alloys
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self- and solute-diffusion in group IV hexagonal close-packed (hcp) metals has been studied for many years. The first investigations on nominally pure ␣ -Ti and ␣ -Zr revealed “abnormal” behavior with a pronounced curvature of the Arrhenius plot (Zr[1,2,3]) or an unusually small activation enthalpy (Ti[4,5,6]) (Figure 1). These diffusion data deviate from the common empirical rules derived for closedpacked metals.[8,9] The results obtained on materials with different purity levels deviated notably from each other, indicating that some extrinsic effects may govern self-diffusion in these materials. The group IV hcp metals have some specific properties that affect the diffusion behavior and complicate diffusion measurements. The ␣ →  phase transition temperature, T␣, which limits the temperature interval of diffusion measurements in ␣ -Zr and ␣ -Ti, is relatively low (1138 and 1156 K, respectively). Thus, the diffusivities of the hcp phases are rather small (⬍10⫺17 m2s⫺1), and a series of extrinsic factors (e.g., residual dislocations, impurities, etc.) may enhance self-diffusion and mask the intrinsic self-diffusion behavior. The low T␣ temperature prevents elimination of dislocations in specimens, and an increased dislocation density may remain after ordinary prediffusion heat treatments. Moreover, it is extremely difficult to purify the group IV hcp metals. Materials used in the first experiments[1,2,5,6] had a 99.9 wt pct purity, and only recently high-purity Ti[10] and Fe-free Zr[11] became available to diffusion investigations. C. HERZIG, Professor, and S. DIVINSKI, Senior Scientist, are with the Institute of Material Physics, Mu¨nster University, D-48149 Mu¨nster, Germany. Y. MISHIN, Associate Professor, is with the School of Computational Sciences, George Mason University, Fairfax, VA 22030. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
However, even such pure materials still contain much more residual impurities than, for example, 5N8 Cu. Group IV hcp metals are often described as metals with an open structure, in that the ratio of atomic to ionic radii in these materials is abnormally large. This property is likely to be responsible for the interstitial dissolution of small metallic elements (such as Fe, Ni, and Co), which diffuse even faster than small nonmetallic atoms, such as C or O (Figure 2). At about 1000 K, their diffusivity exceeds selfdiffusion by about 10 orders of magnitude in ␣ -Zr and 6 to 7 orders of magnitude in ␣ -Ti (Figure 2). For simplicity, only the diffusivity perpendicularly to the c axis, D⬜, i
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