Insight into point defects and impurities in titanium from first principles

Sanjeev K Nayak,Avinash M Dongare,Cain J Hung,Rainer J Hebert,William J Brindley,S Pamir Alpay,Vinit Sharma

doi:10.1038/s41524-018-0068-9

Abstract

Titanium alloys find extensive use in the aerospace and biomedical industries due to a unique combination of strength, density, and corrosion resistance. Decades of mostly experimental research has led to a large body of knowledge of the processing-microstructure-properties linkages. But much of the existing understanding of point defects that play a significant role in the mechanical properties of titanium is based on semi-empirical rules. In this work, we present the results of a detailed self-consistent first-principles study that was developed to determine formation energies of intrinsic point defects including vacancies, self-interstitials, and extrinsic point defects, such as, interstitial and substitutional impurities/dopants. We find that most elements, regardless of size, prefer substitutional positions, but highly electronegative elements, such as C, N, O, F, S, and Cl, some of which are common impurities in Ti, occupy interstitial positions.

Highlights

Point defects play a key role in mechanical properties of metallic materials and in diffusive phase transformations.[1]
The c/ a ratio is less than the ideal value of 1.633; but it approaches 1.633 at higher oxygen contents
The resulting strengthening effect is exploited in commercially pure titanium alloys of grades 1–4 as the main strengthening mechanism and the yield strength increases with oxygen content as does the fatigue strength

Summary

INTRODUCTION

Point defects play a key role in mechanical properties of metallic materials and in diffusive phase transformations.[1]. The multifold increase examines the energetically preferred sites for point defects, such in specific surface area of powders over bulk materials exposes titanium and its alloys to interstitial elements in the atmosphere of as, Ti vacancy (VTi), self-interstitials, substitutional and interstitial positions of impurity elements with atomic number 1 (H) through the additive machines, notably oxygen, nitrogen and hydrogen. The discrepancy is due to the lack of density-based exchange correlation to account an accurate electron correlation treatment.[44] There is a way to overcome the limitations of GGA by adding the Hubbard U correction term This was not used in our study because of the ambiguity it introduces in the treatment of chemical potentials which affects the calculation of the formation energies. In order to obtain insight into defect complexes, we have calculated the formation energy of a VTi–interstitial Ti pair as a function of the separation of the individual components, i.e., the titanium vacancy, VTi, and the interstitial titanium atom. This result corroborates the charge transfer analysis where H is observed to acquire electrons from neighboring Ti atoms, which is the discussed

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DISCUSSION

Findings

METHODS