Hydrogen as a temporary alloying element for the formation of specific microstructural gradients in the (alpha+beta) titanium alloy Ti-6Al-4V

• Applicant: Professor Dr.-Ing. Hans Jürgen Christ
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Mechanical Properties of Metallic Materials and their Microstructural Origins ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 470236376

Engineering components are subject to increasing demands with respect to durability and reliability. Simultaneously, their design, production and application should fulfil sustainable, ecological and economic demands. In order to meet these requirements, the development of thermochemical or thermomechanical processes is necessary. Because of the comparatively high gas solubility of titanium alloys, a temporary hydrogen charging as part of a so-called thermo-hydrogen treatment (THT) is possible. Since hydrogen causes elastic or (via hydride formation) plastic lattice deformation and is known to be a strong β-stabilizing element, the establishment of a modified microstructure is possible improving the mechanical properties as compared to conventionally generated microstructures of titanium alloys. Moreover, THT is applicable to complex geometries which cannot be surface-treated via conventional mechanical surface treatment processes. This study aims at a microstructural adjustment via a purposeful local change of the β grain size and the distribution and morphology of strengthening precipitates as a function of the distance to surface (microstructural gradient) causing in particular an improvement of the material’s fatigue properties.In order to be able to design the THT process, data on the phase stabilities, the hydrogen diffusion coefficient and the hydrogen solubility in dependence of the initial microstructure needs to be known. Based on this data, the THT process steps will be designed by means of simulation calculations and checked and adjusted by means of systematic experimental investigations. Subsequently, the improvement of the mechanical properties under monotonic and cyclic loading will be evaluated experimentally. In the final stage of the research project, the transferability of the best of the developed THT processes to complex geometries will be documented using a sample part, which represents a real component.Previous studies on the application of THT to establish a hitherto gradient-free microstructure show that a purposeful adjustment of the THT parameters should allow for an establishment of microstructure gradients, which cause an improvement of the fatigue resistance. In this juncture, two concepts seem to be promising, namely the establishment of a temporary hydrogen concentration gradient and the formation of a hydride precipitation zone. These two concepts shall be investigated in the research project planned.

• DFG Programme: Research Grants