English

• 1. INTRODUCTION

  Titanium (Ti) and its alloys are widely used in aviation, aerospace, chemical industry, biomaterials and other fields due to the high strength, low density, great corrosion resistance and biosecurity. Titanium has three phases (α, α + β, β) at room temperature with different properties to satisfy the engineering requirement^[1]. α-Ti has strong oxidation resistance, thus it can be used at 500~600 ℃. (α + β)-Ti can work at 400~500 ℃ for a long time, but its thermal stability is inferior to that of α-Ti. β-Ti has high strength at room temperature, up to 1666 MPa, but poor thermal stability.

  Three strategies have been applicated to strengthen the mechanical properties of titanium, which include post-heat treatment, processing technique, and composition design^{[2, 3]}. Different post-heat treatment and processing methods are used to obtain higher strength and ductility for titanium alloys, such as lamellar, equiaxed, and transitional microstructures. In terms of composition design, the creation of composites and alloying is two measures to achieve better mechanical properties. Nonmetallic particles or fibers can be introduced into titanium alloys to fabricate metal-matrix composites (MMCs) with higher performance^[2]. Aluminum (Al) servicing as an α-Ti stabilizer is the main alloy element of titanium, which can reduce the specific gravity and improve elastic modulus and strength. β-Ti stabilizers such as molybdenum (Mo), niobium (Nb) and vanadium (V) reinforce mechanical performance. The strengthening effect of these elements has been well studied, and the corresponding alloys are commercialized. For example, the Ti-6Al-4V alloys have occupied 80% of the market share in titanium alloys. However, the costly elements (V, Nb, Mo, etc) make the alloys more expensive, and the toxicity of some elements can introduce inflammation in organisms. Therefore, the traditional element-strengthened titanium alloys are limited in biomedical applications.

  Ubiquitous light elements such as oxygen (O), hydrogen (H), nitrogen (N) and carbon (C) may substitute the above metal elements^[3]. Oxygen is a standard element in the ASTM mechanical strength level classification of pure titanium and the effect of oxygen on titanium and its alloys has been focused. Generally, oxygen enhances the tensile strength but pays the price of brittleness, thus the amount of oxygen is constrained. The oxygen content of titanium alloys including α-Ti, (α + β)-Ti, and β-Ti cannot exceed 0.5 wt.%^[4], and that of powder metallurgy Ti-6Al-4V alloys is 0.33 wt.%^[5], beyond which the tensile ductility drops dramatically until reaching total brittleness. Recently, Kondoh et al. have found that excess oxygen can keep high ductility together with high strength of titanium^[6]. The oxygen solid solution strengthening may be a new strengthening mechanism for high strength and ductility titanium alloys. This paper summarizes the progress of influence and mechanism of oxygen on the strength and ductility in titanium alloys and reviews new findings on the ductility enhancement by oxygen.

  2. ROLE OF OXYGEN IN TITANIUM ALLOYS

  Oxygen solubility was about 14 wt.% in titanium as shown in Fig. 1a^[7], indicating that the oxygen affected the lattice constant instead of crystal structure when oxygen content was below 14 wt.%. With increasing oxygen within 0.5 wt.%, lattice parameters a, c and c/a ratio increased proportionally^[8]. Fig. 1a also shows the oxygen acted as an α-Ti stabilizer, which increased the β transition temperature of titanium. In Ti-6Al-4V alloy, the β transition temperature was in a function of oxygen as follows: β transition temperature = 937 + 242.7× [O]^[9], where [O] was the oxygen concentration in weight.

  Figure 1

  

  Figure 1.  (a) Ti-O phase diagram^[7]. Copyright of Springer. (b) Tensile elongation of as sintered Ti-6Al-4V vs. oxygen content^[4]. Copyright of Taylor & Francis

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  Generally, oxygen increased the tensile strength and reduced formability by decreasing the uniform elongation and bend ductility^[3]. When the oxygen content was higher than 0.7 wt.%, there would be no plasticity in traditional fabricated titanium alloy. The ductility of powder metallurgy Ti-6Al-4V alloy decreased to less than 10% when the oxygen content was over 0.3 wt.%, as shown in Fig. 1b^{[10, 11]}. Oxygen also hardened the titanium and its alloys according to the relationship^[8] of hardness = 80 + 310[O]^1/2. Besides, oxygen decreased the toughness but increased the strain rate sensitivity to produce yield point phenomena^[12].

  The strengthening effect of solutes in pure metals was conventionally attributed to the elastic interaction between lattice strains of solute atoms and dislocations. However, Yu and colleagues^[13] addressed solution hardening by small oxygen additions to hexagonally close-packed (HCP) α-Ti. They found that as the oxygen content increased, the screw dislocation core became smaller, and the amount of oxygen occupying the octahedral gap near the screw dislocation core was increased by using transmission electron microscopy (TEM) and nanomechanical characterization, as shown in Fig. 2a and 2b. Oxide precipitations were not observed in nanopillars oriented along , which distinguished the effect of solid solution hardening from precipitation hardening. In titanium materials with different oxygen contents (0.1 wt.%, 0.2 wt.%, 0.3 wt.%), the size, orientation, initial microstructure and structure of edge dislocation cores were quite similar. Thus, the strong short-range repulsion between oxygen solutes and the core of screw dislocation increased the strength and hardness of pure α-Ti. However, the collective deformation of the bulk sample was more complex than that of the nanopillar, so they matched the in situ TEM movies to the real-time mechanical response of titanium materials. The dislocation on the top of the Ti-0.3 wt.% O sample slid along the prismatic plane and caused a dislocation "burst", as shown in Fig. 2c, which was eventually repined as the tension progresses. These behaviors were caused by the interaction between the oxygen and the helical dislocation core.

  Figure 2

  

  Figure 2.  Imaging of oxygen interstitials and their effect on the dislocation cores in Ti. (a) High-angle annular dark-field scanning-scanning transmission electron microscopy (HAADF-STEM) image of an edge dislocation core in a Ti-0.1 wt.% O sample. Zone axis is . (b) HAADF-STEM image of a screw dislocation core in Ti-0.1 wt.% O (left) and Ti-0.3 wt.% O (right). (c) Corresponding TEM images of the pillars before and after compression. g vector is along . The Ti-0.1 wt.% O and Ti-0.3 wt.% O samples were tested under bright-field TEM mode, whereas this Ti-0.2 wt.% O sample was tested under dark-field TEM mode^[13]. Copyright of Elsevier.

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  It is recognized that both phase formation and deformation of titanium affect the ductility. Oxygen affected the ductility of titanium and its alloys by changing the phase formation of titanium in five ways: (1) oxygen was an α-Ti stabilizer and its precipitations tended to exist in grain boundary layers or as acicular^{[14, 15]}, which would weaken the strength of grain boundary and create cracks; (2) With increasing oxygen, the martensitic transformation temperature increased and martensitic phases (α', α'') formed easier^{[16, 17]}, and the martensitic phases had a strengthening effect on titanium materials but reduced ductility; (3) The oxygen increase avoided the formation of ω phase which was more brittle than martensite^[18]; (4) Oxygen was a more potent α₂ (Ti₃Al) than Al, and α₂ would significantly reduce the ductility of titanium alloy^[19]; (5) Oxygen clusters existed in titanium and titanium alloys to induce ductility as a plastic deformation obstacle (Fig. 3)^[5].

  Figure 3

  

  Figure 3.  (a) Scanning electron microscopy (SEM) secondary electron image of the focused ion beam (FIB)-prepared atom probe tip sample Ti-6Al-4V-0.49O, (b) Mass-to-charge spectrum obtained from the 3D atom probe tomography (3D-APT) analysis to show the ions/elements detected in the sample which corresponds to a fraction of an α- Ti grain and (c) Isosurface processing to reveal O rich clusters (Ti-O clusters in light blue and O clusters in dark blue) in the sample indicated by the arrow and highlighted by the rectangular frame (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article^[5]. Copyright of Elsevier

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  Oxygen also affected ductility by changing the deformation such as slip and twinning activated in titanium and its alloys^[20-22]. The addition of oxygen promoted the formation of α precipitate in β-Ti alloy to hinder slip, thus reducing the plasticity. Zaefferer et al.^[23] found that ~2000 ppm oxygen depressed the deformation of titanium alloys compared with that of ~1000 ppm oxygen. As for bcc β-Ti alloys, the deformation mode changed from twinning to slip with increasing oxygen which decreased ductility.

  3. NEW FINDINGS ON THE DUCTILITY ENHANCEMENT BY OXYGEN

  As mentioned above, the oxygen was deleterious to the ductility of the titanium alloys and there was an empirical content limit in the alloying industry. However, up to date research found that the titanium alloys fabricated by powder metallurgy kept high strength and ductility when the oxygen content was far beyond the previously recognized limit.

  Sun and coworkers^[24] first found when oxygen content was 0.64 wt.%, titanium had yield strength (YS) of 832.8 MPa, ultimate tensile strength (UTS) of 973.6 MPa, and great elongation to fracture (ETF) of 26%. Its ductility was superior to that of Ti-6Al-4V alloy (UTS: 1047MPa, elongation: 16.6%). Chen et al.^[25] obtained ETF of over 20% and YS over 800 MPa in titanium alloys with 0.8 wt.% O (Fig. 4). After adding into the titanium powders produced by gas atomizing process and hydride-dehydride process labeled as TILOP and TC samples, the TiO₂ particles decomposed during the sintering process, and then the produced oxygen dissolved into titanium lattice. With increasing oxygen content, Bragg's law revealed the lattice parameter of c-axis expanded, leading to the c/a inflation, indicating that the added oxygen dissolved into α-Ti interstice.

  Figure 4

  

  Figure 4.  A collection of the engineering strengths and ETF for all the TILOP and TC samples mixed with rutile versus the oxygen atom percentage^[25]. Copyright of Cambridge Core

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  The mechanism of oxygen solution strengthening in titanium alloys with high oxygen content was studied in detail. Kondoh et al.^[24-26] noted that the titanium diffraction peak in XRD shifted to a lower diffraction angle, which meant a lattice explanation of pure titanium with increasing oxygen. The c-axis lattice parameter proportionally increased while the a-axis lattice constant did not change based on the calculation of XRD results. Oxygen atom preferred to occupy the octahedral sites of α-Ti lattice since oxygen atom radius was close to that of octahedral interstitials in hexagonal closed packed (HCP) titanium lattices. With increasing oxygen, titanium lattice expanded and dragged against dislocation slip, leading to the strengthening of pure titanium. This result was in accordance with the interaction between oxygen and the core of screw dislocations that mainly glided on prismatic planes in section 2. Oxygen solution enhancement was calculated by using the Labusch model assuming the strain exerted by oxygen solid solution on titanium was isotropic. A new model^[26] was proposed based on the Labusch model for calculating oxygen solid solution strengthening, assuming that oxygen exerted an anisotropic strain on HCP structure. Besides, when oxygen increased within a certain range, α-Ti grains refined because oxygen solute atoms at α-Ti grain boundaries caused the solute drag to boundary expansion. Grain refinement played a further strengthening role in titanium, which was calculated by using the Hall Petch equation^[27]. In general, crystal grains induced by plastic strain had multi-color or different shapes. However, inverse pole figure (IPF) results^[26] showed unicolor grains with various oxygen contents, which eliminated the effect of residual strains. Therefore, neither TiO₂ particles nor residual strains increased strength. Oxygen solid solution strengthening was the primary contributor to improving the tensile strength of powder metallurgy titanium materials.

  The influence mechanism of oxygen on the ductility of titanium with high oxygen content has also been discussed. Principal slip plane of hcp metals depends on the c/a ratio, where a basal plane slipping was dominant in long HCP structure (c/a > 1.61) and a prismatic plane slipping occurs in short hcp one (c/a < 1.61)^[28]. Titanium sample with high oxygen content showed that great ductility was associated with c/a and slip system. Kariya et al.^[29] found that the ductility of Ti-0.94 wt.% O materials increased with water quenching temperature. Their primary phases were equiaxed α-Ti grains and some martensite phases at the grain boundaries. After water quenching, Ti-0.94 wt.% O materials formed α΄-Ti originating from pre-β phase and orientation misfit between α-Ti and β-Ti, as shown in Fig. 5. The pre-β phase had a lower oxygen content compared with pre-α phases and a high concentration of dislocation after the tensile test. The lower oxygen content phase formation played a role in improving the ductility. Lei and colleagues^[30] found a form of oxygen called ordered interstitial complexes, which revealed a new strain-hardening mechanism of enhancing strength and ductility in TiZrHfNb high-entropy alloy (HEA). This ordered interstitial complex strengthening mechanism might enlighten the study of great ductility of powder metallurgy titanium materials with high oxygen content.

  Figure 5

  

  Figure 5.  (a) Backscattered electron image of Ti (O)-1000Q. (b) EPMA O-mapping analysis results of Ti (O)-1000Q. (c) IPF maps of Ti (O)-1000Q before the tensile test. (d) Schematic illustrations of the formation of α'phase on grain boundary and orientation misfit area in α grain^[29]. Copyright of J-STAGE

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  In order to confirm the effect of oxygen, different methods for adding oxygen have been studied. The alloys of Ti-0.80O, Ti-2.22Zr-0.96O, Ti-1wt.% Cr₂O₃, and Ti-0.97O-0.11H were synthesized by introducing TiO₂, ZrO₂, Cr₂O₃, and O₂ in air, respectively^{[26, 31, 32]}. All the alloys exhibited high UTS and ductility, which were superior to those of Ti-6Al-4V alloy. The results showed the oxygen element play a role for the performance improvement, no matter how it was introduced. Even though the oxygen solutes in titanium materials were non-uniformly dispersed, the oxygen atoms still had strong strengthening effect on titanium materials^[31].

  4. CONCLUSION AND OUTLOOK

  Traditional views suggested that oxygen content in titanium and its alloys must be strictly limited to 0.5 wt.% because oxygen enhanced the strength but dramatically reduced the ductility. However, recent study results revealed that when the oxygen content was far beyond this limit, the ductility of titanium alloys kept good ductility together with high strength. The calculation results indicated that solid solution strengthening and fine crystal strengthening of oxygen both enhanced the strength of titanium alloy, in which solid solution strengthening played a dominant role. In traditionally manufactured titanium alloys with low oxygen content, the oxygen increased tensile strength due to the strong short-range repulsive force between oxygen atoms and the screw dislocation core, which leads to a uniform dislocation "burst". The ductility reduced rapidly due to the oxygen induced phase formation, such as the formation of brittle martensite phases and the deformation of slip and twinning. In titanium alloys fabricated by new way, oxygen solid solution mainly enhanced the strength, which might be associated with ordered interstitial complexes strengthening mechanism. The high ductility was preserved, which was unclear and needed to be further studied.

  The strength and ductility of metal materials are difficult to be strengthened at the same time, thus it is essential to study how titanium alloys with high oxygen content keep both high strength and excellent ductility. Moreover, oxygen, as a cheap and biosecurity element, can reduce the cost of titanium alloys and expand its application in aviation, aerospace, chemical, medical and other fields.

  
  
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• 

  Figure 1  (a) Ti-O phase diagram^[7]. Copyright of Springer. (b) Tensile elongation of as sintered Ti-6Al-4V vs. oxygen content^[4]. Copyright of Taylor & Francis

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  Figure 2  Imaging of oxygen interstitials and their effect on the dislocation cores in Ti. (a) High-angle annular dark-field scanning-scanning transmission electron microscopy (HAADF-STEM) image of an edge dislocation core in a Ti-0.1 wt.% O sample. Zone axis is . (b) HAADF-STEM image of a screw dislocation core in Ti-0.1 wt.% O (left) and Ti-0.3 wt.% O (right). (c) Corresponding TEM images of the pillars before and after compression. g vector is along . The Ti-0.1 wt.% O and Ti-0.3 wt.% O samples were tested under bright-field TEM mode, whereas this Ti-0.2 wt.% O sample was tested under dark-field TEM mode^[13]. Copyright of Elsevier.

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  Figure 3  (a) Scanning electron microscopy (SEM) secondary electron image of the focused ion beam (FIB)-prepared atom probe tip sample Ti-6Al-4V-0.49O, (b) Mass-to-charge spectrum obtained from the 3D atom probe tomography (3D-APT) analysis to show the ions/elements detected in the sample which corresponds to a fraction of an α- Ti grain and (c) Isosurface processing to reveal O rich clusters (Ti-O clusters in light blue and O clusters in dark blue) in the sample indicated by the arrow and highlighted by the rectangular frame (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article^[5]. Copyright of Elsevier

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  Figure 4  A collection of the engineering strengths and ETF for all the TILOP and TC samples mixed with rutile versus the oxygen atom percentage^[25]. Copyright of Cambridge Core

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  Figure 5  (a) Backscattered electron image of Ti (O)-1000Q. (b) EPMA O-mapping analysis results of Ti (O)-1000Q. (c) IPF maps of Ti (O)-1000Q before the tensile test. (d) Schematic illustrations of the formation of α'phase on grain boundary and orientation misfit area in α grain^[29]. Copyright of J-STAGE

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