Hydrogen-modified dislocation structures in a cyclically deformed ferritic-pearlitic low carbon steel

The fatigue-crack growth rate of a ferritic-pearlitic low carbon steel was faster when the tests were conducted in high-pressure H-2 gas environments than in air. The predominant fracture feature changed from ductile fatigue striations with some "quasi-cleavage-like" regions when the test was conducted in air to mixed "quasi-cleavage" and "flat" facets when tested in a H-2 gas environment. The microstructure beneath the fracture surfaces produced in air was sub-grains, and over a distance of 15 mu m from the fracture surface, the dimensions of the sub-grains increased. With hydrogen, dense dislocation bands and refined dislocation cells existed beneath the "quasi-cleavage" and "flat" fracture surfaces. The cell size increased with distance from the fracture surface. The decrease in the dimensions of the key micro-structural features as the fracture surface is approached is attributed to the propagation of the crack through an already deformed matrix. The differences in evolved dislocation structure are explained in terms of the hydrogen-enhanced localized plasticity mechanism, and the hydrogen-modified dislocation structure establishes the local conditions that promote the fracture mode transition from ductile fatigue striations to a mixture of "quasi-cleavage" and "flat" features, which directly leads to enhanced fatigue-crack growth. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.