Influence of defects on the energetics and dynamics of hydrogen in manganese steels

Lars Ismer, Tilmann Hickel, and Jörg Neugebauer

Ultra-high manganese steels (typically 15–30 wt.% Mn), in particular so called TWIP and TRIP steels, have recently become important for industrial purposes, e.g. the car industry. They combine high work hardening rates and strength with a good ductility and hence allow to reduce the weight of the automobile body and further provide excellent formability for automotive applications and are at the same time cost-effective.

However, for safety design and life prediction of engineering structures it is also important to understand to what extend that material is affected by failures, such as e.g. hydrogen embrittlement. Nevertheless, despite their technological importance only little is known about the resistance of these materials to hydrogen embrittlement. In particular the influence of the specific chemical composition of this steel for the vulnerability/resistance to HE embrittlement is unknown.

A systematic understanding of the influence of the chemical composition requires to isolate the role of the chemical elements of the steel from each other as well as from the microstructural variables. Such analysis is, though highly desired, difficult/impossible to perform experimentally and alternative approaches to the experiment are required.

We therefore employ density functional theory to study the interaction of hydrogen with the host matrix of austenitic Mn-rich steel (Fig.1). Our first aim is to understand how varying Mn concentrations in the steel matrix affect thermodynamic key parameters describing the Hydrogen-steel interaction, such as the solution enthalpy which determines the solubility of Hydrogen in the host system, and the diffusion barrier, which determines the mobility across the host lattice. Furthermore, we investigate the influence of interstitial carbon on these quantities, since TWIP steels are characterised by a comparatively high carbon content.

A further focus of the project is on microstructural defects such as vacancies, grain boundaries and twin boundaries, since they potentially play an important role for hydrogen embrittlement processes. Vacancies, for example, may act as traps for hydrogen impurities, and thus reduce their mobility. However, they also could act as nucleus for hydrogen accumulation and thus initiate embrittlement. Hydrogen accumulation in grain boundaries could enhance inter-crystalline crack-formation. Furthermore, it is believed that grain boundaries may act as "high-speed freeways" for hydrogen diffusion. To obtain a systematic and quantitative understanding of the microscopic origins behind these processes we determine and investigate the potential energy surface for hydrogen in the area of the defect (Fig. 2).