Multiscale investigation of room-temperature viscoplasticity and sustained load cracking of Titanium and Ti-alloys. Influence of hydrogen and oxygen content – FLUTI

Titanium alloys are used for aircraft or rocket engine parts due to their high strength-to-weight ratio, for naval applications thanks to their resistance to corrosion, for orthopaedic use, due to their bio-compatibility and possibly for fusion reactors, since Ti has a fast rate of residual radioactivity decay. But they are prone to room-temperature creep, creep-fatigue and sub-critical cracking (SLC), partly due to creep at the crack tip. The improvement in their resistance to cold creep and SLC is a major issue for the safety of air transport and nuclear energy production as well as for the reliability of a strategic submarine fleet. The viscoplastic behaviour of Ti is controlled by the mobility of screw dislocations, impeded by a large Peierls stress because of a non-planar core. Oxygen modifies the core structure of dislocations, reduce their mobility and increase the flow stress. In some Ti alloys and yet unclear loading conditions, solute H triggers dislocations emission and mobility, softens the material and enhances cold creep, while in other cases, it hinders dislocation glide, hardens the material and slows down cold creep. The influence of solute H seems to be highly dependent on the oxygen content. When H enhances cold creep, SLC is triggered, while in the conditions for which it slows down creep, it also improves the resistance to SLC. There is an evident but yet unexplained correlation between H and O content, the viscoplastic behaviour and the resistance to SLC. The aim of this project is to clarify the underlying mechanisms so as to determine the optimum hydrogen content as a function of the O content to minimize cold creep and creep-fatigue as well as SLC. At nano and micro-scales, tensile tests in a TEM will be performed on Ti and Ti6242 with various O and H content to investigate their effect on dislocation dynamics. Ab initio atomic scale simulations will be performed to compute interaction energies between dislocations and impurities and investigate the influence of H and O on Peierls stress, stacking fault energies, surface energies and cleavage stresses. Traction-separation curves for cleavage as a function of H content will inspire constitutive equations of cohesive elements to be used in finite elements simulations of solute hydrogen embrittlement at a crack tip. At meso-scale, tensile tests inside the SEM with strain field measurements (image correlation) will be performed to analyse deformation and damage mechanisms, in relation with H and O content (slip systems activated, using EBSD, degree of slip planarity and heterogeneity, damage initiation mode). The microprint technique will show the repartition of H in the microstructure. Nano-indentation will be used to explore the impact of H charging on the hardness of each phase in Ti6242. These data will be used to formulate and identify a polycrystal plasticity model depending on the impurity content, which could used to simulate the behaviour of Ti under uniaxial or multiaxial creep and creep-fatigue, whatever its texture. At a macro-scale, the influence of H and O on toughness and SLC will be investigated. TEM observations of thin foils extracted at the very crack tip by FIB will allow us to search for hydrides. The viscoplastic behaviour will be characterized through tensile tests with loading rate jumps, creep and relaxation tests. Constitutive equations describing dynamic strain aging in relation with H and O contents will be formulated and identified. They will be used in finite element simulations of SLC, taking into account H segregation at the crack tip (assuming that diffusible H is in equilibrium with the hydrostatic stress and that H trapped by dislocations is a function of the strain field) and the impact of H on the viscoplastic behaviour and damage process. H-enhanced or inhibited creep and the impact on ductile crack growth as well as H-assisted decohesion will be envisaged and the pertinence of each scenario evaluated.