Role of metallic microstructure on hydrogen-assisted crack initiation under monotonic and cyclic loading

Abstract

As the world is shifting to hydrogen as an alternative energy carrier, hydrogen storage and transportation are seen as a big challenge. This is due to the fact that hydrogen is associated with an embrittlement phenomenon that imparts substantial damage to the metallic components. As such, hydrogen embrittlement (HE) is a loss of ductility, strength, fracture toughness, and fatigue life of metals under a hydrogen environment. Most of the life of metallic components goes into the first appearance of crack, and hydrogen accelerates this process drastically. Also, it is found that the topic of crack propagation in metals in the presence of hydrogen is vastly studied, whereas hydrogen-assisted crack initiation itself is not well looked at extensively. For specific metals, multiple HE mechanisms can operate simultaneously depending on their crystal structure and microstructure. Therefore, understanding the role of microstructure on hydrogen-assisted crack initiation mechanisms is of prime importance as it can not only prevent catastrophic hydrogen-induced failure but can help material scientists to design metallic microstructures, which are less prone to cracks and damage in the presence of hydrogen. To this end, in this thesis, micromechanics of HE mechanisms responsible for crack initiation in metals is investigated through small-scale experiments and simulations. The objectives of the proposed thesis work are (i) understanding Fatigue crack initiation (FCI), (ii) crack initiation during monotonic loading in uncharged and hydrogen charged metals, and (iii) to assess the role of vacancy-hydrogen (VaH) complexes in the embrittlement phenomenon. Each of these problems forms a separate objective of this thesis, the summary of work done to achieve each of these objectives is given below. For achieving the first objective, FCI is studied in a model material nickel in the uncharged and hydrogen charged state during in-situ strain controlled low cycle fatigue (LCF) testing under a scanning electron microscope (SEM). Crack initiation sites are characterized by investigating the elastic modulus in the loading direction as well as the maximum Schmid factor of the crack neighbouring grains extracted through the electron backscattered diffraction (EBSD) data. The crack frequency for the uncharged and hydrogen charged specimens is then analyzed using the difference in the elastic modulus (Δ𝐸), the difference in the maximum Schmid factor (Δ𝑚), and Δ𝐸/Δ𝑚 ratio between the crack neighbouring grains. The comparison shows that for the hydrogen charged specimens, intergranular FCI sites show high values of Δ𝐸/Δ𝑚 compared to the uncharged specimens. These findings provide a predictive model for hydrogen linked FCI in metals. In addition, the synergistic role of the hydrogen enhanced localized plasticity (HELP) mediated hydrogen enhanced decohesion (HEDE) mechanism responsible for FCI is also demonstrated. For achieving the second objective, crack initiation is studied for the uncharged and hydrogen charged nickel specimens during in-situ tensile loading under the scanning electron microscope. By assuming the material to be elastic at low strains, a novel approach is implemented for generating the microstructural stress maps through strain and stiffness tensor extracted at each point in the region of interest on the specimen surface using high-resolution digital image correlation (HR-DIC) and Euler angles (given by electron backscattered diffraction data), respectively. Based on this analysis at low strain, the crack initiation sites for uncharged and hydrogen charged nickel specimens are correlated with microstructural maps of maximum Schmid factor, elastic modulus in the loading direction, hydrostatic stress, von Mises stress, and triaxiality factor. The analysis highlighted two independent factors responsible for HEDE based intergranular failure observed only at the random grain boundaries, (i) strain localization due to HELP mechanism of hydrogen embrittlement and (ii) hydrostatic stress-based hydrogen diffusion to the crack initiation sites. These critical insights thus can help to design hydrogen embrittlement-resistant metals. In addition, the novel experimental approach can be used to calibrate advance micromechanical models while providing a quantitative estimate of the hydrogen distribution in realistic metallic microstructure responsible for hydrogen-assisted crack initiation with deformation. For achieving the last objective, atomistic studies are performed to assess role of hydrogen atoms and VaH complex on dislocation emission and propagation in nickel singe crystal with five different crystal orientations:Or1[1̅1̅2] [111] [1̅10]; Or2:[100] [010] [001]; Or3:[101] [010] [1̅01]; Or4:[1̅10] [001] [110] Or5: [101][101̅][010]) and at different overall concentrations (𝑐𝐻= 0.01, 0.05, and 0.1) of these species. Various models (simple solution model, random distribution model and slip plane distribution model) are developed to generate variety of configurations of Ni with H atoms and VaH complexes. At first, the effect of concentrations of H atom and VaH complexes on properties such unstable stacking fault energy (𝛾𝑈𝑆𝐹(𝑐)), and stable stacking fault energy (𝛾𝑆𝐹(𝑐)), to characterize the dislocation emission, are determined by studying the relative slip on the {111} 〈112〉 FCC slip system with shear deformation. Next, fracture energy (Γ(𝑐)) values of {110}, {111} and {100} planes are evaluated to quantify the effect of H atom and VaH complex concentrations on the embrittlement ratio, 𝛾𝑈𝑆𝐹(𝑐)/Γ(𝑐). It is shown that with the increase in the concentration of H atoms and VaH complexes, there is a significant increase in unstable stacking fault energy (𝛾𝑈𝑆𝐹(𝑐)), and stable stacking fault energy (𝛾𝑆𝐹(𝑐)) and a decrease in Γ(𝑐). Due to this, there is an increase in embrittlement ratio, and the embrittlement ratio is higher in every orientation in the case of VaH complexes compared to H atoms. Thus, on theoretical basis, it is found that VaH complexes leads to more DBT than H atoms alone. Based on Griffith and Rice theory, the theoretical values of critical 𝐾𝐼 for dislocation emission (𝐾𝐼𝑒𝑡ℎ) and cleavage (𝐾𝐼𝐶𝑡ℎ) are calculated for different concentrations of H atoms and VaH complexes. If for any orientation, 𝐾𝐼𝑒𝑡ℎ< 𝐾𝐼𝐶𝑡ℎ, a ductile failure is expected compared to brittle failure. Further, deformation simulation of pre-cracked configurations of single crystal of Ni with different crystal orientations are performed under Mode-I loading for different concentrations of H atom and VaH complex at the crack front. For pure Ni samples, partial dislocation or twin emission is observed at the crack front for all orientations. This is found to be in contradiction to the Rice theory, the reason of which is found to be the prevailing stress triaxiality at the crack front. For configurations generated using simple solution model, formation of nickel-hydride (NiH) and nickel-hydride with vacancies (NiVaH) phases at the crack front is observed. Using atomistic simulation of pre-cracked configurations of Ni with NiH and NiVaH phases at the crack front, simulation based critical 𝐾𝐼 for dislocation emission is determined. For these configurations, there is a notable difference in the simulated behavior of crack tip deformation with concentrations of H atoms or VaH complexes compared to pure Ni single crystal. Two kind of critical 𝐾𝐼 values are observed for these configurations i.e., 𝐾𝐼𝑒𝑠𝑖𝑚 that corresponds to 𝐾𝐼 when the first distinctive event is observed at the crack front that emerges from the crack tip and 𝐾𝐼𝑒−𝑖𝑠𝑖𝑚 that corresponds to 𝐾𝐼 when the first distinctive event was observed at the crack front that emerges from the interface of Ni and NiH or NiVaH phases. Simulations show that both the concentration of H atoms and VaH complexes in NiH and NiVaH phases and the crystal orientation, controls the kind of behavior exhibited in these configurations i.e, emission of partial dislocations or twin from the crack tip or from the interface of Ni and NiH or NiVaH phases. In general, the value of 𝐾𝐼𝑒−𝑖𝑠𝑖𝑚 is observed to be much lower compared to 𝐾𝐼𝑒𝑡ℎ for all crystal orientations and at different concentration of H atoms and VaH complexes in NiH or NiVaH phases, except of 𝑐𝐻= 0.01 for Or1, Or2 and Or3. This behavior is caused by the softening of the interface of Ni and NiH or NiVaH phases which is shown to be due to the combination of high strain incompatibility, positive hydrostatic stress, and stress triaxiality prevalent at the interface that leads to earlier emission of dislocations from the interface. Lastly, when dislocation propagation is studied using random distribution model and slip plane distribution model, VaH complexes is shown to have a profound effect on dislocation propagation (compared to H atoms alone). It is found that VaH complexes at first create obstacles to the dislocation motion (due to increase in shear stress) and then lead to disintegration of partial dislocation loop when it interacts with VaH complexes. The softening during emission and hardening during propagation and disintegration in continuity of partial dislocation loops due to VaH complexes fit the experimental observations of various dislocation structures on fractured surfaces in presence of hydrogen. This study shows that how HE caused by hydrogen atoms alone and with combination of VaH complexes is a complex phenomenon as compared to understanding gained through just DBT information. The insights provided here can be extended to various non-hydride forming FCC and BCC metals. Also, ab-initio studies, such as the type of VaH complexes depending upon the different number of vacancies and hydrogen atoms, can provide more relevant mechanics about the embrittlement phenomenon.