Review of effect of oxygen on room temperature ductility of titanium and titanium alloys

Abstract

Room temperature tensile ductility is an important property of titanium (Ti) and titanium alloys for structural applications. This article reviews the dependency of tensile ductility on oxygen for α-Ti, (α+β)-Ti and β-Ti alloys fabricated via traditional ingot metallurgy (IM), powder metallurgy (PM) and additive manufacturing (AM) or three-dimensional printing methods and recent advances in understanding the effect of oxygen on ductility. Seven mechanisms have been discussed based on case studies of individual titanium materials reported in literature. The dependency of ductility on oxygen is determined by both the composition and microstructure of the titanium alloy. For Ti–6Al–4V (wt-%), as sintered Ti–6Al–4V shows a critical oxygen level of about 0·33 wt-% while additively manufactured Ti–6Al–4V exhibits different critical levels ranging from about 0·22% to well above 0·4% depending on microstructure. Rare earth (RE) elements are effective scavengers of oxygen in titanium materials even just with a small addition (e.g. 0·1 wt-%), irrespective of the manufacturing method (IM, PM and AM). High cycle fatigue experiments revealed no initiation of fatigue cracks from the resulting RE oxide particles over the size range from submicrometres to a few micrometres. A small addition of RE elements offers a practical and affordable approach to mitigating the detrimental effect of oxygen on ductility.

Effect of Powder Reuse Times on Additive Manufacturing of Ti-6Al-4V by Selective Electron Beam Melting

“…The influence of oxygen on the microstructure and mechanical properties of Ti-6Al-4V has been reviewed recently. 22,23 The increase in tensile strengths with powder reuse times (Table V) can be attributed to the effect of the increased oxygen content. 22 In terms of the influence of oxygen on tensile ductility, a critical oxygen level exists for Ti-6Al-4V, which is around 0.33 wt.%, beyond which the tensile ductility drops sharply due to several oxygen-induced microstructural changes.…”

“…22,23 The increase in tensile strengths with powder reuse times (Table V) can be attributed to the effect of the increased oxygen content. 22 In terms of the influence of oxygen on tensile ductility, a critical oxygen level exists for Ti-6Al-4V, which is around 0.33 wt.%, beyond which the tensile ductility drops sharply due to several oxygen-induced microstructural changes. 23 However, below the critical level of 0.33 wt.%, the influence of oxygen on ductility is insignificant or negligible.…”

An Overview of Densification, Microstructure and Mechanical Property of Additively Manufactured Ti-6Al-4V — Comparison among Selective Laser Melting, Electron Beam Melting, Laser Metal Deposition and Selective Laser Sintering, and with Conventional Powder

“…From this perspective, heat treatment of AM Ti-6Al-4V will have to be selected according to specific requirement and serve the application purpose of the material. Oxygen is detrimental to ductility of Ti-6Al-4V especially when it exceeds certain level [97][98][99]. Miura et al [26] suggests the critical oxygen level for PM Ti-6Al-4V is 0.33 wt.%, above which ductility drops rapidly and can be much lower than the corresponding ASTM specification (see Fig.…”

A novel Ti–6Al–4V alloy microstructure with very high strength and good ductility

“…Its mechanical properties depend on the contents of interstitial elements (O, N, C and H), microstructure and texture. Increasing the oxygen content increases strength, but decreases the ductility [3]. The ductility and fatigue strength can be improved significantly by transforming the fully lamellar structure to fully equiaxed microstructure, and the strength and fatigue strength can also be improved by formation of a bimodal microstructure consisting of primary a grains and lamellar a + b colonies through solution treatment, quenching and aging (STA) [4], but at the cost of slightly reducing ductility.…”