Wasserstoff als temporäres Legierungselement zur Erzeugung spezifischer Gefügegradienten in der (α+β)–Ti-Legierung Ti–6Al–4V

Technical components are subject to increasing demands with regard to durability and reliability. To meet these expectations, the development of thermochemical processes for an unerring and reproducible microstructure adjustment is necessary. Titanium alloys allow temporary alloying with hydrogen as part of the heat treatment, thus called thermo-hydrogen treatment (THT).
The present work intends to produce a local microstructural adjustment of Ti–6Al–4V by means of THT by changing the local distribution of strengthening precipitates and the grain size as a function of the distance to the surface (microstructural gradient), which should improve the fatigue properties of the material. For the THT parameter values selection, the hydrogen diffusion coefficient and the hydrogen solubility are determined as material parameters. In addition, the THT parameters are evaluated by determining fatigue crack propagation resistance and fracture toughness as a function of solution heat treatment and evaluating different microstructural gradients after hydrogen loading and degassing by means of numerically simulated hydrogen concentration profiles, hardness curves, metallographic investigations and phase analysis using X-ray diffractometry and TEM.
The results prove that a desired microstructural gradient can be achieved by hydrogen charging at 500°C and degassing at 750°C followed by an aging at 550°C. Modelling of kinetics and thermodynamics of the Ti–H interaction reproduces the experimentally generated microstructural gradients in a good approximation and provides the basis for further optimization of the process. The evaluation of the resulting mechanical properties is carried out by tests on a miniature testing machine, which allows in-situ observation of fatigue crack initiation and propagation on a laser microscope, alternating deformation tests to determine the fatigue life as a function of the stress amplitude as well as tensile tests. The investigation shows that THT-induced microstructural grading extends the fatigue life of conventionally and additively manufactured specimens by a factor of 3 – 6 compared to a homogeneous reference microstructure at HCF, and also offers the prospect of enhanced fatigue strength on a near-component demonstrator by means of hardness gradient existing before THT, which THT increases by 4 – 5 %.