Effect of crystal orientation on plastic flow of hydrogen-precharged micropillars of high-Mn steel

Abstract

This dissertation aims to clarify the effect of crystal orientation on plastic flow of hydrogen-precharged micropillars of high-Mn steel for the fundamental understanding of relationship between crystallographic texture and hydrogen embrittlement. The dissertation consists of 2 parts. First part is Orientation dependence on plastic flow behavior of hydrogen-charged micropillar of high-Mn steel. Single crystal micropillars were fabricated by focused ion beam (FIB) and electron backscatter diffraction (EBSD) in the grain matrix which orientation is the major texture component of fcc; Goss, Brass, Copper, Cube, and S. Hydrogen was charged into micropillar by cathodic charging method. Compression tests using nanoindenter were followed for uncharged- and hydrogen-charged micropillar. Scanning electron microscope (SEM) and transmission electron microscope (TEM) analysis were conducted for microstructure evolution characterization. The flow behavior of micropillar varies with hydrogen charging and grain orientation. Corresponding to hydrogen enhanced localized plasticity (HELP) and defect acting agent (DEFACTANT) mechanisms, all grains exhibited flow hardening by hydrogen charging. Twinning and slip activity differ with grain orientation following Schmid’s law, which leads to anisotropic behavior in hydrogen-charged condition. Brass and Goss showed a similar flow curve regardless of the texture component. These grains exhibited multiple slip activation after hydrogen charging due to relative high Schmid factor value of secondary slip system. Copper and Cube orientations exhibited the highest flow stress among the investigated samples prior to the hydrogen charging with mechanical twinning. Despite the effective mechanical strengthening under the hydrogen-uncharged condition, Group II showed a notable deterioration in mechanical properties after the hydrogen charging. S orientation showed the activation of the single slip system. Hydrogen atoms trapped at these boundaries reduce the bonding energy of interface. This resulted in the boundary decohesion and the acceleration of intergranular fracture. This orientation possessed the highest Schmid factor for slip among the investigated orientations, leading to the suppressed twinning despite Schmid factor for twinning similar to that of the copper orientation. Second part is Deformation behavior of hydrogen-charged bi-crystal micropillar of high-Mn steel. Bi-crystal micropillars were fabricated on grain boundaries which neighbor grains are component investigated in first part by same process with single crystal micropillar. Cross section of compressed bi-crystal was analyzed by Transmission Kikuchi Diffraction. Results showed that the flow behavior of micropillars was greatly varied with hydrogen charging and grain orientation. Twinning activation led to early failure in H-charged condition. The anisotropic deformation characteristics of various bi-crystals were analyzed in the contexts of grain boundary characteristics, crack resistance factor, and hydrogen transportation. Hydrogen-assisted fracture did not occur when both grains were not favored to twinning. Among twinning-occurred bi-crystal micropillars, bi-crystal containing S grain exhibited the highest hydrogen embrittlement resistance in unidirectional compression test.