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This dissertation is composed of two parts. The first part focuses on the mechanical behavior study on pipelines steels. More specifically, the effect of hydrogen on the deformation behavior of the crack tip of an X52 pipeline steel specimen, and the overload effects on the fatigue crack growth of an x70 pipeline weld metal are discussed. For the second part of the dissertation, fatigue behavior of an Al0.1CoCrFeNi high-entropy alloy (HEA) is studied, and the influence of heat treatment is discussed. The objectives of the first part of this proposed work are to (1) detect the influence of hydrogen on the crack-tip deformation of compact-tension (CT) pipeline steel specimens; (2) understand effects of residual stress and plastic zone during crack growth, and obtain a comprehensive understanding of overload effects on the fatigue crack growth of the pipeline steels. In order to address those objectives, fatigue-crack-growth experiments have been performed to precrack the pipeline steel samples. In-situ neutron experiments were performed on both as-received and hydrogen-charged samples to detect the influence of hydrogen. In-situ synchrotron experiments have been performed to characterize the strain/stress field evolution during loading-unloading and the plastic-zone evolution during deformation. An overload has been applied during the synchrotron measurements and the overload effects on the strain/stress field around the crack tip have been investigated. The objectives of the second part of this proposed work are to (1) investigate the fatigue behavior of the Al0.1CoCrFeNi HEA (2) study the influence of heat treatment on the fatigue behavior of the Al0.1CoCrFeNi HEA. To address these objectives, tension-tension fatigue tests have been performed on as-Hiped and heat-treated samples. Failed samples were characterized by OM, SEM, EBSD, and TEM to identify deformation mechanisms and the influence of heat treatment. In conclusion, with the completion of the two parts of study, we now have a better understanding of fatigue and fracture mechanisms of both pipeline steels and HEAs, which can provide insights into the future design of steel pipelines, as well as the possible engineering applications for the HEAs.

Two types of pipeline steels, Alloy B [Fe-0.05C-1.52Mn-0.12Si-0.092Nb, weight percent (wt.%)] and Alloy C [Fe-0.04C-1.61Mn-0.14Si-0.096Nb, wt.%)], were tested. Vickers hardness and nanoindentation tests were used to obtain the hardness and elastic modulus. Compact-tension (CT) specimens were employed for fatigue experiments. Different frequencies (10 Hz, 1 Hz, and 0.1 Hz) and different stress ratios [0.1 and 0.5, stress ratio (R) is the ratio between Pmin. [minimum applied load] and Pmax.[maximum applied load]. R = Pmin./Pmax.] were used, and the tests were done in air, at room temperature. The effects of frequencies and different R ratios on crack-growth rates were compared. It is concluded that a higher R ratio leads to a greater fatigue-crack-growth rate (FCGR), while frequency does not have much influence. Moreover, Alloy B tends to have a better fatigue resistance than Alloy C under various test conditions. The microstructures of two alloys were investigated by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Fracture surfaces show transgranular patterns, and fatigue striation was observed under the scanning electron microscope (SEM). Another type of pipeline steel, X70 [Fe-0.053C-1.52Mn-0.25Cr-0.19Si-0.089Nb, weight percent (wt.%)], was also studied. Fatigue tests were performed at different load levels, and comparisons were made between different parts of weld and base metals. Fracture surfaces were observed by SEM to identify fatigue and fracture mechanisms. Neutron-scattering-diffraction experiments were performed to study the deformation behavior around the crack tip of X52 [Fe-0.071C-1.06Mn-0.24Si-0.026Nb, weight percent (wt.%)] and X70 pipeline steel. Both the hydrogen-charged sample and as-received sample were used to detect the influence of hydrogen. Preliminary results are presented in this thesis.