PSI - Issue 14

Aman Arora et al. / Procedia Structural Integrity 14 (2019) 790–797

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Aman Arora/ Structural Integrity Procedia 00 (2018) 000–000

1. Introduction Fatigue life determination in metals is a complex process as 80% of life of metals in fatigue goes in cracks nucleation only (Kumar et al. 2010; Stinville et al.2015b ) . Presence of hydrogen in material makes it more complex as it drastically affects the fatigue life. Generally, fatigue cracks nucleate from large surface pores and inclusions (Milligan et al. 2004; Li et al. 2004; Texier et al. 2016). However, in absence of these large imperfections fatigue crack nucleation highly depends upon microstructural features ( Cowles 1989; Pollock and Tin 2006 ) . Identification of the microstructural regions for crack nucleation holds the key to estimate fatigue life, design it for application of fatigue loading and to develop new materials under application of cyclic loading. Moreover, as the world is shifting to hydrogen as an alternative energy source, hydrogen storage and transportation are seen as a big challenge. Therefore, it has become increasingly important to gain fundamental understanding about crack initiation in presence of hydrogen during cyclic loading. Cracks generally nucleate from grain boundaries with neighbouring grains having high schmid factor and high local elastic anisotropy (Stinville et al. 2017). Moreover, how crack nucleation changes in presence of hydrogen needs a better understanding and more elaborative study. Hydrogen can degrade the structure of nickel broadly by these primary mechanisms such as (i) Hydrogen Enhanced Decohesion (HEDE) in which hydrogen at internal interfaces like phase and grain boundaries decreases the cohesion strength of metal, (ii) Hydrogen Enhanced Local Plasticity (HELP) where hydrogen decreases the dislocation interaction with each other thus creating instability in local plasticity and (iii) Metal-Hydride formation, e.g. in metals like nickel, nickel hydride precipitation results in low energy paths due to its brittle nature (Birnbaum 2003) . In this work, we have performed a crack nucleation study on uncharged and hydrogen charged polycrystalline nickel under in-situ strain controlled low cycle fatigue (LCF) loading inside the scanning electron microscope (SEM). We have characterized nucleated crack by electron back scattered diffraction (EBSD) methods. Schmid factor and elastic modulus maps have been extracted by doing post analysis of EBSD measurements. We have compared the schmid factors and elastic modulus of crack nucleated neighbouring grains in our uncharged and hydrogen charged polycrystalline nickel. 2. Material, mechanical testing and sample characterization 2.1 Materials Nickel shallow notch dogbone type specimen with gauge length 10 mm, width 2mm and thickness 4 mm having purity 99.95% were used for present study. The dimension detailed drawing (in mm) is shown in Fig.1. Samples were annealed at 900 o C for 2 hours in Nitrogen gas atmosphere. Initially samples were polished with different grades of SiC grit paper from 100 to 2000 grit size. Afterwards samples were polished with 8µm, 6 µm, 3 µm, 0.25 µm diamond paste and then with colloidal silica. Then the samples were electro-polished with 20 % perchloric acid and 80% methanol at 15 V for 15 seconds to remove residual surface stresses and a fine smooth surface was achieved. Hydrogen was charged electrochemically in 1 N H 2 SO 4 with thiourea to prevent recombination of hydrogen at current density 20mA/cm 2 for 2 hours. All the mechanical testing was performed in-situ JEOL KSM 6610 SEM system and EBSD was taken by Quantax EBSD system by Bruker at accelerating voltage of 30 kV with 60 µA emission current. EBSD was obtained with hit rate around 97 %. In-situ fatigue testing was done with tensile/compression module from Kammrath Weiss, Germany. 2.2 Tensile and Fatigue Experiments Tensile tests were done at room temperature of uncharged sample and just after hydrogen charging at strain rate of 10 -5 s -1 . Fractured surfaces were analysed after tensile tests under scanning electron microscope. Fatigue testing was done at room temperature. Strain controlled isothermal low cycle fatigue testing was done at 0.3 Hz. Fatigue tests were done in uniaxial push and pull displacement control mode from maximum displacement 100 µm,