PSI - Issue 13

Xiaofei Guo et al. / Procedia Structural Integrity 13 (2018) 1453–1459 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

High manganese austenitic steels achieved excellent combination of strength and ductility thanks to the twinning induced plasticity (TWIP) effect [1]. Not limited to high manganese austenitic steels, deformation twinning takes place also in other alloying systems as a primary mechanical strengthening mechanism. This applies to nickel alloys, Titanium, and high entropy alloys [2,3]. Although beneficial for mechanical strengthening, the interaction between deformation twins, grain boundaries and surrounding dislocations leads to stress localization and promotes crack nucleation [4,5]. It was reported that the presence of hydrogen promotes homogeneous nucleation of dislocations [6] and reduces the stacking fault energy of austenite [7]. The latter effect facilitates deformation twinning [8]. As a result, the interactions of hydrogen and these lattice defects were intensified, where the material is locally degraded [9]. These fundamental studies provide valuable information on the hydrogen embrittlement mechanisms in TWIP assisted materials. Although intense microstructure-focused research was carried out on the Fe-Mn-C system, very few studies conducted a comparison between different alloying systems. Since Al-alloyed high manganese steels were reported to achieve a significant improvement in hydrogen embrittlement resistance, the potential reasons have been explored in several research work [9-11]. A close comparison regarding the development of the microstructure features in the presence of hydrogen is still lacking, especially at different strain levels. Therefore, the current work was carried out to reveal the hydrogen effect on the dislocation behaviour, stacking fault formation and twinning behaviour, using electron channelling contrast imaging (ECCI). The investigated materials were produced by ingot casting, hot rolling, cold rolling with 50% thickness reduction, and finally recrystallization annealing at 900 °C for 30 minutes. The chemical composition of the investigated TWIP steels are listed in Table 1. The abbreviation of 22Mn and 17Mn-Al are used for the Al free and 1.46% Al-alloyed material. According to a thermodynamically based model [12], 22Mn and 17Mn-Al have similar stacking fault energies (SFE) of 26 mJ/m 2 , and 27 mJ/m 2 respectively. TWIP grades with SFE above 18 mJ/m 2 are assumed to have stable austenite phase, where deformation induced twins develop at room temperature (RT) [1]. 2. Experimental Procedure The material surfaces were mechanical grinded and then polished on canvas with 6 µm diamond paste before hydrogen charging. Hydrogen was charged electrochemically at -800 mV SCE in 0.05 M H 2 SO 4 solution with 1.4 g/l CH 4 N 2 S as hydrogen permeation promotor. The charging time of 166 hours was applied for both materials. Hydrogen content in the charged specimens was measured after 3 hours of homogenization at RT using high vacuum thermal desorption spectrometry (TDS). Hydrogen desorption curves from room temperature to 800 °C at the constant heating rate of 20 °C/min were documented. Notched tensile specimens with a notch radius of 0.25 mm and a specimen width of 12.5 mm were manufactured where the tensile axis was maintained parallel to the rolling direction. The tensile tests were performed at the strain rate of 10 -3 s -1 . For hydrogen pre-charged specimen, the tests were performed after 3 hours homogenization at RT. Digital image correlation (DIC) was as applied to measure the strain distribution in the vicinity of the notches using a GOM-ARAMIS optical system. The tests were interrupted upon detecting primary cracks, which were traced using direct current potential drop. The deformation microstructure was investigated at different strain levels between the notch tip and the centre of the specimen for both, hydrogen free, and hydrogen charged specimens, using a Zeiss Cross-beam instrument (XB 1540, Carl Zeiss SMT AG, Germany) equipped with TSL OIM electron backscatter diffraction (EBSD) system and a solid-state four-quadrant BSE detector. The EBSD images revealed the local crystallographic orientation, while the ECCI gave detailed information of dislocation substructure and the characteristics of deformation twins. Table 1. Chemical composition of the investigated material as determined by optical emission spectrometer in wt%. Abbrev. C Mn Al P S Fe-22Mn-0.6C 22Mn 0.630 22.60 0.008 0.001 0.001 Fe-17Mn-1.5Al-0.6C 17Mn-Al 0.605 16.96 1.460 0.010 0.009