The mechanism of enhanced resistance to the hydrogen delayed fracture in Al-added Fee18Mne0.6C twinning-induced plasticity steels Il-Jeong Park a , Kook-Hyun Jeong a , Jae-Gil Jung a , Chong Soo Lee b , Young-Kook Lee a, * a Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea b Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea article info Article history: Received 1 December 2011 Received in revised form 19 February 2012 Accepted 19 March 2012 Available online xxx Keywords: Twinning-induced plasticity steel Hydrogen delayed fracture Oxidation Brittle fracture abstract High Mn twinning-induced plasticity (TWIP) steels are attractive for high performance applications owing to their extraordinary ductility at a giga-graded tensile strength level. Hydrogen delayed fracture (HDF) came to the fore as a key issue to be solved for the application of these steels. Although it was found that Al addition improved the resistance to HDF, the reason was unclear. Therefore, in this study, the fracture surfaces of annealed and hydrogen-charged TWIP steels with different Al contents were examined after slow strain rate tensile tests. Diffusible hydrogen was measured by thermal desorption analysis. It found that the strong resistance to HDF was due to an a-Al 2 O 3 layer formed below the (Fe 0.8 Mn 0.2 )O layer during the hydrogen charging in an aqueous solution prevented the hydrogen to permeate into specimens from the surface. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction Hydrogen delayed fracture (HDF) is well-known to be a unique failure by the generation and propagation of the cracks caused by hydrogen atoms concentrated at highly stressed regions inside high strength materials with the tensile strength of over about 1.2 GPa [1e6]. Therefore, a slew of researches have been performed to understand the causes of the HDF occur- ring in various structural materials [7e12], such as stainless steel [13,14], Ti alloy [15e17], precipitation-hardened alloy [18,19], carbon steel [20], dual phase (DP) steel [21,22], and transformation-induced plasticity (TRIP) steel [23]. To increase the resistance to the HDF in Ti alloys and precipitation-hardened alloys such as Inconel 718, Incoloy 925, and 18Ni maraging steel, the dispersion of fine precipi- tates such as Ti, V, and Nb carbides has been utilized because a high density of precipitates increases the tensile strength and simultaneously provides a number of strong hydrogen trapping sites [15e19]. Also, the resistance to the HDF in DP and TRIP steels is related to the volume fractions of various phases like austenite, ferrite, and martensite and their inter- phase boundaries acting as hydrogen trapping sites as well as material strength [21e23]. Recently, high Mn austenitic twinning-induced plasticity (TWIP) steel has received much attention as an advanced high strength automotive steel due to its high strength of over 800 MPa and large ductility of over 50% [24e26]. Because the TWIP steel has a fully fcc austenite phase with the large solubility and the slow diffusion rate of hydrogen [27], the resistance to HDF of the TWIP steel is expected to be superior to that of high strength martensitic steels. Nevertheless, HDF was observed after 156 days in Fee18Mne0.6C TWIP steel [27] * Corresponding author. Tel.: þ82 2 2123 2831; fax: þ82 2 312 5375. E-mail address: yklee@yonsei.ac.kr (Y.-K. Lee). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy xxx (2012) 1 e8 Please cite this article in press as: Park I-J, et al., The mechanism of enhanced resistance to the hydrogen delayed fracture in Al- added Fee18Mne0.6C twinning-induced plasticity steels, International Journal of Hydrogen Energy (2012), doi:10.1016/ j.ijhydene.2012.03.100 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.03.100