October 30, 2020 feature
Mechanistic basis of oxygen sensitivity in titanium
Titanium is extremely sensitive to small amounts of oxygen, which can lead to markedly decreased ductility of the material. Materials scientists therefore aim to lower the costs of purifying titanium, while avoiding the poisoning effects of oxygen. In a new report now on Science Advances, Yan Chong, and a team of scientists in materials science and engineering at the University of California Berkeley and the Lawrence Berkeley National Laboratory in the U.S., detailed a systematic study on oxygen sensitivity of titanium. The team provided a clear mechanistic view of the effects of oxygen impurities on the mechanical properties of the material. The experimental and computational work provided insights for a rationale to design titanium alloys with increased tolerance to the variations in interstitial content (a position between the regular positions in an array of atoms in a material), with notable implications to facilitate the widespread use of titanium alloys in spacecrafts, naval ships, aircrafts and materials engineering.
Titanium alloys contain highly desirable properties including corrosion resistance and high specific strength making them attractive structural materials across a wide range of commercial applications. Interstitial atoms can be intentionally or naturally incorporated to influence the mechanical properties of titanium. Oxygen is a predominant interstitial impurity, widely adopted in titanium-based alloys to enable a potent strengthening effect for diverse applications. Titanium is also inherently expensive due to the tight control of interstitial impurities during their manufacture. Although researchers have documented the embrittlement effects of interstitial impurities in alpha-titanium alloys, the mechanistic origin of the anomalous oxygen sensitivity on the mechanical properties remain to be understood, thereby limiting alloy design and processing strategies. Materials scientists had documented a "wavy-to-planar" transition of dislocation arrangements with increasing oxygen content in the metal. In the present work, Chong et al. conducted a systematic multiscale investigation of the mechanical properties and deformation microstructures of titanium.
The team aimed to reveal the nature of slip polarity associated with higher oxygen content relative to interstitial concentration, strain rate and deformation temperatures. They credited the marked oxygen sensitivity in titanium to transitions in dislocation behavior and twinning activity of the metal. The scientists discussed the atomic origin of the transitions relative to density functional theory (DFT) and molecular dynamics (MD) simulations to provide deeper insights to design interstitial-tolerant titanium alloys. Chong et al. tested three model alloys including pure titanium (with 0.05 weight percentage or wt %), Ti-0.10 (with 0.10 weight percentage—wt%) and Ti-0.30 (with 0.30 wt%) at high temperature, room temperature and cryogenic temperatures using uniaxial tensile tests. A slight variation of the oxygen content caused marked changes to the mechanical properties of Ti-O alloys at room temperature and cryogenic temperatures. The observed failures of Ti-0.30 alloys at low temperatures highlighted its limitations for applications at cryogenic conditions. The strain hardening potential of the Ti-O alloys decreased with increasing oxygen content. Pure Ti and Ti-0.10 exhibited excellent and almost identical strain-hardening rates at cryogenic temperature.
Chong et al. then investigated the typical dislocation morphologies of Ti-O alloys either in a wavy or planar slip-dominant mode through interrupted tensile deformations at different temperatures and strain rates. They schematically combined the analyses of temperature, strain rate and oxygen content dependence of the dislocation morphology. Using transmission electron microscopy (TEM) the team examined the representative dislocation morphologies relative to strain rate, oxygen concentration and deformation temperature. They noted the susceptibility for a wavy-to-planar slip transition (displacement of one part of the crystallographic plane of the material relative to another plane and direction) to occur when the strain rate or the oxygen rate increased, or with decreasing temperature.
Although the planar slip was frequently reported in Ti-O alloys at cryogenic temperatures, the underlying mechanism remains unknown. The short-range-ordering (SRO) or the regular and predictable arrangement of atoms across a short distance, for oxygen atoms, could be a proposed mechanism; however, researchers have not yet experimentally verified the SRO of oxygen in the Ti-O binary system with a dilute oxygen content. The team therefore calculated the diffuse antiphase boundary (DAPB) energies and confirmed planar slip to be temperature and strain independent for titanium-aluminide (Ti-Al)-based alloys, in marked contrast to Ti-O alloys whose planar slip depended on the temperature and strain. The scientists therefore deduced a different origin to the evolving planar slip in Ti-O alloys.
Interstitial shuffling in the Ti-O system and deformation twinning
Chong et al. conducted DFT (density functional theory) calculations to propose interstitial shuffling mechanisms (ISM) for the temperature and rate dependence of wavy-to-planar slip transitions in Ti-O alloys. Based on generalized stacking fault (GSF) energies obtained via computational calculations, the team provided evidence for the slip plane softening effect associated with the shuffling process in the material at lower temperatures and larger strain rates. The oxygen atoms that displaced within the material during the deformation process remained in their positions, reducing the barrier to further slip. The concept of twinning can also give rise to excellent mechanical properties of titanium alloys observed at cryogenic temperatures where dislocation activities typically become difficult.
Researchers have to date reported four common deformation twining modes in titanium, including two tensions twins (T1 and T2) and two compression twins (C1 and C2). Chong et al. considered an overview of twinning behavior as a function of oxygen content and temperature. With increasing oxygen content, the twin fractions at room temperature were continuously reduced to the point where no appreciable twins could be detected in Ti-0.30 alloys at room temperature. The twinning activity enhanced substantially in pure titanium at cryogenic temperatures. They credited the enhanced feature in pure titanium to a larger internal stress level. To further understand the anomalous behavior of twins, the scientists studied the interactions between oxygen and twin boundaries using atomic simulations.
In this way, Yan Chong and colleagues considered the systematic influence of oxygen on the dislocation morphology and twinning fraction to present a mechanistic view of oxygen sensitivity on the mechanical properties of titanium. They credited the origin of temperature strain rate and oxygen content sensitivity of Ti-O alloy slip planarity to the motion of oxygen atoms instead of short-range ordering of atoms. The model of interstitial shuffling mechanisms (ISM) provided an explanation to the observed temperature and strain sensitivity of planar slip in Ti-O alloys. The simulated alloy design strategies that interrupted the interstitial shuffling process in this work may notably increase the interstitial tolerance of titanium alloys to offer strengthening effects without an accompanying sacrifice in ductility.
Bilby B.A. and Crocker A.G. The theory of the crystallography of deformation twinning, Proceedings of the Royal Society A, doi.org/10.1098/rspa.1965.0216
Qian Yu et al. Origin of dramatic oxygen solute strengthening effect in titanium, Science (2015). DOI: 10.1126/science.1260485
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