Diffusion anisotropy of poor metal solute atoms in hcp-Ti

Abstract

Atom migration mechanisms influence a wide range of phenomena: solidification kinetics, phase equilibria, oxidation kinetics, precipitation of phases, and high-temperature deformation. In particular, solute diffusion mechanisms in α-Ti alloys can help explain their excellent high-temperature behaviour. The purpose of this work is to study self- and solute diffusion in hexagonal close-packed (hcp)-Ti, and its anisotropy, from first-principles using the 8-frequency model. The calculated diffusion coefficients show that diffusion energy barriers depend more on bonding characteristics of the solute rather than the size misfit with the host, while the extreme diffusion anisotropy of some solute elements in hcp-Ti is a result of the bond angle distortion.

Development of an interatomic potential for the simulation of defects, plasticity, and phase transformations in titanium

“…The ab initio calculations made in [45] for the vacancy mechanism of diffusion predicts that the  D / D || ratio should vary from 0.33 to 0.5 depending on temperature. The ab initio calculations made in [44] also lead to the prediction that the diffusion in the direction parallel to the c-axis is slower but the  D / D || ratio was found to be 0.89. Potential Ti1 leads to the prediction that the diffusion by the vacancy mechanism along the c-axis is faster while predictions obtained using potentials Ti2 and Ti3 are in agreement with the ab initio calculations (Fig.…”

“…ratio should vary from 0.33 to 0.5 depending on temperature. The ab initio calculations made in [44] also lead to the prediction that the diffusion in the direction parallel to the c-axis is slower but the…”

“…Therefore, the  D / D || ratio cannot really serve as an indicator of what mechanism actually governs the self-diffusion. Since the anisotropy in diffusivity is much smaller than the difference between the diffusivity from ab initio calculations and experiment (which is about an order of magnitude [44]) in our further analysis we will ignore the anisotropy. Figure 5 shows that the vacancy formation energy increases with increasing temperature.…”

“…Even the range of activation energy obtained from experiment for hcp Ti is rather large (see Table V) which does not allow the use of experimental results to make any closing conclusion. In theoretical considerations, it is widely accepted that the self-diffusion proceeds through the vacancy mechanism [42][43][44][45] although the authors of [42] admitted that the self-interstitial mechanism could "make a significant contribution to self-diffusion". Among the potentials we developed in the present study, potential Ti1 seems to be not suitable for the simulation of the self-diffusion in hcp since it leads to the hcp vacancy formation energy larger than the self-interstitial formation (see Table I).…”

Non-classical interstitial sites and anomalous diffusion mechanisms in hcp-titanium

“…However, the empirical surveys cannot convey details on migration processes. On the other hand, ab initio calculations have been successfully used to predict the diffusivites of different solute in α-Ti [22,23,24,7], as well as fundamentals aspects of the diffusion mechanism, such as the solute transitions and the influence of the Ti-solute bond on the diffusion behaviour. Bernstein et al [9] studied the migration barriers and defect formation energies for various solute atoms in Ti, observing that interstitial formation energies for the fast diffusers are comparatively low.…”