Acta Materialia 183 (2020) 313–328 Contents lists available at ScienceDirect Acta Materialia journal homepage: www.elsevier.com/locate/actamat Full length article Dependence of hydrogen embrittlement mechanisms on microstructure-driven hydrogen distribution in medium Mn steels Binhan Sun ∗ , Waldemar Krieger, Michael Rohwerder, Dirk Ponge ∗ , Dierk Raabe Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany a r t i c l e i n f o Article history: Received 2 August 2019 Revised 26 October 2019 Accepted 11 November 2019 Available online 14 November 2019 Keywords: Medium Mn steels Hydrogen embrittlement Hydrogen-enhanced decohesion (HEDE) Hydrogen-enhanced local plasticity (HELP) Hydrogen trapping a b s t r a c t The risk of hydrogen embrittlement (HE) is currently one important factor impeding the use of medium Mn steels. However, knowledge about HE in these materials is sparse. Their multiphase microstructure with highly variable phase conditions (e.g. fraction, percolation and dislocation density) and the feature of deformation-driven phase transformation render systematic studies of HE mechanisms challenging. Here we investigate two austenite-ferrite medium Mn steel samples with very different phase character- istics. The first one has a ferritic matrix (~74 vol.% ferrite) with embedded austenite and a high dislo- cation density (~10 14 m −2 ) in ferrite. The second one has a well recrystallized microstructure consisting of an austenitic matrix (~59 vol.% austenite) and embedded ferrite. We observe that the two types of microstructures show very different response to HE, due to fundamental differences between the HE mi- cromechanisms acting in them. The influence of H in the first type of microstructure is explained by the H-enhanced local plastic flow in ferrite and the resulting increased strain incompatibility between fer- rite and the adjacent phase mixture of austenite and strain-induced α’-martensite. In the second type of microstructure, the dominant role of H lies in its decohesion effect on phase and grain boundaries, due to the initially trapped H at the interfaces and subsequent H migration driven by deformation-induced austenite-to-martensite transformation. The fundamental change in the prevalent HE mechanisms be- tween these two microstructures is related to the spatial distribution of H within them. This observa- tion provides significant insights for future microstructural design towards higher HE resistance of high- strength steels. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction During the last decade, medium Mn steels containing 3 to 12 wt.% Mn have received considerable attention, driven by the increasing demand for strong and compositionally lean materi- als with good formability from the automotive industry. The most widely investigated microstructure of this type of steels consists of ultrafine grained ferrite and austenite [1–4], realized by inter- critical annealing (IA). Different alloy compositions, starting mi- crostructures before heat treatment as well as IA conditions can result in a wide range of microstructural characteristics, such as the austenite fraction (typically ranging from 20 to 70 vol.% [3– 7]), grain size (from below 200 nm up to a few micrometers [3,4]), phase morphology (globular or laminated [2,6]), microstruc- ture percolation and dislocation density [8]. Among these param- eters, the fraction of austenite has a crucial influence on the me- ∗ Corresponding authors. E-mail addresses: b.sun@mpie.de, binhan.sun@mail.mcgill.ca (B. Sun), d.ponge@mpie.de (D. Ponge). chanical properties of such steels. For a given alloy composition, a higher austenite fraction typically corresponds to a lower content of C and Mn partitioning into austenite, an effect which leads to lower austenite stability and an enhanced transformation-induced plasticity (TRIP) effect [4,5,7,9]. Most studies on such steels have focused so far on tailoring austenite conditions, in order to achieve an improved strength-ductility combination [3,5,7,10]. The tensile strength of austenite-ferrite medium Mn steels normally ranges from ~800 to 1400 MPa [2–4,10]. This high strength level fuels concerns about hydrogen embrittlement (HE), impeding the use of these steels. At present, the knowledge of HE in medium Mn steels is very limited, with only a few reports published [1,2,9,11]. Ryu et al. [1] investigated the H desorption behavior and evaluated the HE susceptibility as a function of H concentration in two TRIP-assisted 5Mn (in wt.%) steels containing ~30 vol.% austenite. Han et al. [2] compared the HE behavior of a 0.1C-7Mn-0.5Si steel with two different phase morphologies (globular and laminated), focusing on the difference in the H-induced degradation of the tensile proper- ties, H-assisted damage and the resulting fracture surfaces. Shao https://doi.org/10.1016/j.actamat.2019.11.029 1359-6454/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.