Development and characterization of new oxygen-tolerant titanium alloys – TiTol
Development and characterization of new oxygen tolerant titanium alloys
Today, the effect of oxygen remains one of the most critical problems associated with titanium metallurgy. According to a widespread idea, small oxygen additions induce on titanium a strong hardening effect coupled with a drastic drop of the ductility (embrittlement). We show in this project that there are strategies to develop more oxygen tolerant alloys. These alloys have an excellent combination of mechanical properties.
High resolution characterization of Ti-Zr alloys with very high oxygen content
Surprisingly, a thorough mechanical characterization showed that oxygen retained its high hardening capacity in Ti-Zr alloys but without any decrease in the associated ductility, contrary to what has been observed so far in titanium alloys. These alloys therefore have, in fact, excellent mechanical properties (hardly reached by other titanium alloys). This development leads to the need to reconsider the generalized paradigm defining oxygen invariably as an embrittling element for titanium alloys, with strategic significance on the whole design approach of the alloys. However, based on the fundamental questions raised by the properties of these new alloys, there is still a significant lack of knowledge, especially regarding the underlying mechanisms. <br />Two main aspects are of great interest: (1) the determination of the atomic scale structure of Ti-Zr-O alloys as a function of the chemical composition (oxygen content), with a particular focus on possible local ordering or on local structural modifications induced by oxygen, (2) the impact of the oxygen content on the activated deformation mechanisms, at room temperature. <br />These studies must be performed at the nanometric/atomic level because the microstructure of these alloys is, according to the first analyses, single-phase alpha at the mesoscopic scale.
The different axes developed had in common the development of approaches aiming at understanding the evolution of the microstructures of Ti-Zr-O alloys. For this purpose, titanium alloys with varying oxygen contents ranging from 0.15 to 0.8 wt% were prepared. These alloys also contain 4.5 wt% zirconium and have a single phase a microstructure. The alloy preparation and forming steps resulted in samples with well-controlled chemistry. In addition to the complete characterization of their mechanical properties at room temperature (uniaxial tensile tests, mainly), the challenge was to characterize these alloys at different scales: first at the mesoscopic scale to characterize the granular structure of the materials. Then at the nanometric/atomic scale with an approach focused on the aspects of oxygen ordering in the alpha matrix of titanium. We have combined advanced characterization techniques (synchrotron diffraction, high resolution electron microscopy (HRTEM), atomic tomographic probe (APT), small angle scattering (SAXS). This approach allowed us to characterize our samples both on the aspects of crystallography (ordering) and chemistry.
ransmission electron microscopy (TEM) observations on Ti-4.5Zr-0.8O (wt%) show unambiguously two types of diffraction patterns. On some of them, such as in the axes of zones [11-20], [11-26] and [41-56], additional reflections to the alpha hexagonal phase were observed and may be associated with the ordering of oxygen. The dark field images obtained from these reflections unambiguously reveal nanoscale precipitates of about 10 nm in size and dispersed very finally in the matrix. The indexing of a complete set of diffraction patterns shows that local oxygen ordering can occur on the basis of a Ti6O type structure. This structure is trigonal (space group P31c) and very similar to that of the a-matrix, with titanium atoms resting on the same hcp lattice and oxygen atoms ordering in every other layer along the  axis with one third of the octahedral sites occupied in each basal layer. The orientation relationship between the two phases, deduced from the various TEM diffraction patterns can be defined as follows: a//Ti6O and (11 ¯00)a//(12 ¯10)Ti6O, which is consistent with the theoretical description and implies a fully coherent interface between the two phases. A fine HRTEM analysis has confirmed this hypothesis
Specifically regarding the chemical aspects related to these ordered nanoprecipitates, tomographic atom probe (TAP) analyses were performed on the oxygen-rich alloys of the series. No statistically significant deviation from a random distribution of Zr and O could be observed within the resolution limits of the technique. Complementarily, small angle X-ray scattering (SAXS), which is also sensitive to chemical fluctuations at the nanoscale, but probes much larger volumes, showed no measurable signal on the same alloys. This tends to show that these ordered precipitates would present a composition that is almost that of the matrix and that they are, in fact, strongly substoichiometric, leading to a composition (Ti1-xZrx)6Oy with y less than 1 and x close to the Zr/Ti ratio of the matrix. The experimental characterization of these nanodomains is new. Their structure has been previously calculated in the literature by ab-initio methods but this is the 1st time they are formally observed.
Many perspectives are opened by the discovery of these ordered nanodomains in titanium alloys. First, it is important to evaluate the influence of this dense distribution of ordered precipitates on the mechanical behavior of the alloys, from a mechanistic point of view. A campaign of interrupted tensile tests, at room temperature, must therefore be carried out on Ti-4.5Zr-xO alloys. During the first mechanical tests, it was clearly observed that the tensile strength increased strongly with the oxygen content, with a gain of about 100 MPa by addition of 0.1 wt% of oxygen. The strengthening effect due to the addition of oxygen, combined with the solid solution hardening of zirconium, gives these Ti-4.5Zr-xO alloys a much higher tensile strength than commercially pure titanium (CP-Ti) for comparable oxygen concentrations.
It is important to note that the mechanical strength observed for Ti-Zr-O alloys is not due to significant strain hardening, which is rather low for all compositions studied and clearly decreases at high oxygen contents. If we now consider the elongation at break, the most remarkable aspect is the conservation of a high ductility with the addition of oxygen in all Ti-4.5Zr-xO alloys, contrary to most titanium alloys where the ductility drops rapidly while the strength increases.
Post-mortem TEM observations in Ti-Zr-O samples deformed at different strain rates should show the progressive evolution of the dislocation structure with oxygen content. This should allow to understand in the medium term the interaction mechanisms between Ti6O nanodomains and mobile dislocations. It should also provide some relevant answers to a broader question related to the origin of oxygen embrittlement in titanium alloys.
Oxygen ordering in titanium alloys achieving a unique combination of strength and ductility
R. Poulain, S. Delannoy, I. Guillot, F. Amann, Raphaëlle Guillou, JP Couzinié, S. Lartigue-Korinek, Z. Kloenne, L.Lilensten, S. Antonov, B. Gault, F. De Geuser, Dominique Thiaudière, JL Béchade, H. Fraser, E. Clouet, F. Prima
Nature communications (submitted, in review)
New insights into oxygen-rich alpha titanium alloys for structural applications«
TMS 2022 Annual Meeting & Exhibition, February 27–March 3, 2022, Anaheim, California, USA (Invited conference)
This project relies on a coupled experimental and theoretical investigation of a new family of oxygen-tolerant Ti-Zr-0 titanium alloys with extra high content of O (up to 1wt%) and displaying an unprecedented resistance/ductility trade-off. The main objective of this project is to provide a comprehensive map of the mechanisms connecting the alloys chemistry both to the development of atomic scale structure of these materials and to the effective deformation mechanisms at room and high temperature, respectively. A special attention will be given to: (1) The sensitivity of the mechanical behavior to the alloy composition. (2) The determination of the atomic scale structure (with respect to oxygen ordering, particularly). (3) The impact of oxygen content on the operative deformation mechanisms, at room and elevated temperature (up to 800°C). The work will be performed both on theoretical (ab-initio) and experimental sides, using cutting-edge characterization facilities (synchrotron).
Monsieur Frederic Prima (Institut de Recherche de Chimie Paris)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
ICMPE Institut de Chimie et des Matériaux Paris-Est
SOLEIL SYNCHROTRON SOLEIL
IRCP Institut de Recherche de Chimie Paris
DMN Département des Matériaux pour le Nucléaire
Help of the ANR 440,540 euros
Beginning and duration of the scientific project: December 2019 - 48 Months