Abstract
Hydrogen embrittlement (HE) is a persistent mode of failure particularly in high-strength steels containing retained austenite. During the service life of these steels phase transformations occur and are a key element that determines their response to the service loads. Thus, the role of H atoms in the relative stability of the phases present and forming in steels is of great interest. At the same time, the kinetics of hydrogen within such a microstructure requires further investigations.
In this work, we discuss the role of H on the relative stability of the fcc/bcc/hcp phases using the ab initio thermodynamics. The results indicate that at low hydrogen chemical potentials the stability of the fcc phase, considered as a representative of retained austenite in steels, is slightly enhanced by the presence of H atoms. In contrast, at high hydrogen chemical potentials the bcc phase is stabilized by H. Moreover, since the excess volume of the hydrogen-rich bcc phase is significantly larger than that of the fcc phase, the presence of a stress field can change the relative stability of these phases in the coexistence regions of the phase diagram. This feature is particularly important for cyclic loading conditions: during the loading cycles forward and reverse phase transformations occur, and the H released by these transformations can damage the material.
The study of the kinetics of H consists of several step. On the one hand, we demonstrate the ab initio results for the segregation of H and C atoms to the ferrite-austenite interface as well as their interplay. To this end, the concept of a metastable intermediate structure (MIS) at the interface is employed. On the other hand, metadynamic simulations are performed for various microstructures and used as an input for the solution of the H diffusion equations. We are able to resolve the impact of these microstructures on the spectra accessible to thermal desorption spectroscopy (TDS).