First principles study of normal and fast diffusing metallic impurities in hcp titanium

Abstract

a b s t r a c tThe diffusivities of metallic impurities in hexagonal close packed (hcp) Ti vary widely, with some species showing normal diffusion and others diffusing anomalously fast. Based on a transition-state theory model for impurity diffusion mediated by vacancies, self, and impurity interstitials, we find that the diffusion rate will be limited primarily by the formation energies of the various defects. We use density functional theory to calculate these defect energies, including the formation energies of vacancies, self interstitials, and metallic impurities in substitutional and interstitial positions in hcp Ti. We find that for normal diffusers the vacancy mediated diffusion mechanism dominates, while for fast diffusers the dominant mechanism is hopping directly between interstitial sites. The activation energy for the latter mechanism is significantly lower than that for vacancy mediated diffusion, explaining the large diffusivities and low activation energies observed experimentally for these fast diffusing impurities.Ó 2015 Published by Elsevier B.V.

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

“…This allows elements with relatively small metallic radius to dissolve both intrstitially and substitutionally. Fe, Co and Ni are indeed small relative to Ti, with metallic radii of R Fe =1.26Å, R Co =1.25Å, R Ni =1.24Å [9], which suggests these solute atoms could dissolve interstitially in Ti. Yoshida and co-workers were able to experimentally show that this is the case [10,11].…”

First-principles investigation on diffusion mechanism of alloying elements in dilute Zr alloys

“…Some insight into what we expect for the behavior of impurities in Zr can be gained by considering recent studies quite similar to the present work of impurities in Ti, another hcp metal in the same column of the periodic table as Zr. Specifically, Bernstein et al calculated the formation energies to analyze the normal and fast diffusion of some solutes in HCP Ti [19] and Zhang et al identified the dominant diffusion mechanism for some solutes in HCP Ti [20] based on the calculated defect formation energies and migration energies. Both these studies suggest that smaller solutes are more likely to diffuse by interstitial mechanism, consistent with the Zr case.…”

Direct observation of solute interstitials and their clusters in Mg alloys

“…Interstitials have important influences on various properties of materials [1][2][3][4][5][6]. Octahedral and tetrahedral interstitials are the mostly studied ones in metals and alloys so far [1,4,[6][7][8]. A crowdion is a kind of interstitial with an atom inserted between two neighboring matrix atoms along a close-packed direction, instead of at the center of an interstice; and its center of mass can easily move along that axis, according to simulations [9].…”

“…However, some large impurity atoms could diffuse even faster than small nonmetal atoms in hexagonal close packed (HCP) metals, such as Fe, Co and Ni in Ti and Zr [4]. The anomalously fast mobility of relatively large impurity metal atoms was attributed to an interstitial-mediated diffusion mechanism [1,4], which may have potential applications on synthesis and processing of materials. The addition of a thin Ni foil between two Ti plates to be joined was able to greatly improve the weld quality [13].…”

“…It is thus important to study interstitials, in order to understand mechanisms underlying materials performances, such as strengthening, diffusion and solid-state phase transformations, which are closely associated with interstitials. Numerous theoretical modeling and computer simulations have been carried out about the structure and property of interstitials [1,8,15]. But, direct visualization of individual interstitials can provide useful knowledge about local atomic structures of the defects in materials, which is very helpful for understanding their dynamics and function [16][17][18].…”