Electron spectroscopic study of phosphorus segregated to α-iron surfaces

Fatigue crack initiation in engineering materials has been the subject of considerable research. Most of these investigations focused on gaseous environment effects, and extensive review articles have appeared in recent times discussing the role of gaseous environments on crack initiation. Because of experimental difficulties, the effect of aqueous environments on mechanisms of fatigue crack initiation has received little attention, despite their unquestionable importance from an engineering standpoint. In this review, several of the fatigue crack initiation models are examined in detail and their anomalies discussed. The physics and micromechanisms of crack initiation during cyclic deformation in aqueous environments which are highly corrosive in nature are examined. The characteristics of the crack initiation process in aqueous environments are critically reviewed in the light of the specific role of several concurrent factors involving the nature of the aqueous medium, corrosion interactions, alloy chemistry, processing treatments, intrinsic microstructural effects and test variables.[/p]

Mechanisms of fatigue crack initiation in metals: role of aqueous environments

Gas-solid interfacial reaction is critical to many technological applications from heterogeneous catalysis to stress corrosion cracking. A prominent question that remains unclear is how gas and solid interact beyond chemisorption to form a stable interphase for bridging subsequent gas-solid reactions. Here, we report real-time atomic-scale observations of Ni-Al alloy oxidation reaction from initial surface adsorption to interfacial reaction into the bulk. We found distinct atomistic mechanisms for oxide growth in O2 and H2O vapor, featuring a “step-edge” mechanism with severe interfacial strain in O2, and a “subsurface” one in H2O. Ab initio density functional theory simulations rationalize the H2O dissociation to favor the formation of a disordered oxide, which promotes ion diffusion to the oxide-metal interface and leads to an eased interfacial strain, therefore enhancing inward oxidation. Our findings depict a complete pathway for the Ni-Al surface oxidation reaction and delineate the delicate coupling of chemomechanical effect on gas-solid interactions.

https://advances.sciencemag.org/content/advances/6/17/eaay8491.full.pdf

Deciphering atomistic mechanisms of the gas-solid interfacial reaction during alloy oxidation.

Since the composition at grain boundaries generally differs considerably from that in the grain interior, all considerations of intergranular fracture must take this into account. In this survey we consider the historical development of the theories of low temperature intergranular fracture with particular reference to temper brittleness. The theory is followed through Rice and Thompson’ s analysis of the energy balance between ductility and brittle fracture with Rice’s analysis of the effects of segregant on the ideal work of fracture term. As developed by Mason, Hirth and Rice, and Seah the atomistic term involved in this process is the bond energy of the individual atoms across the fracture plane These terms can be calculated simply, to a first approximation, from tabulated thermodynamic data.

Atomistic Mechanisms of Intergranular Embrittlement

Dissolution at metal surfaces may be structurally or non-structurally dependent. The effects of grain boundaries, interfaces between discrete phases, non-metallic inclusions, and slip lines in promoting structurally dependent attack are considered in the context of a variety of metal-environment systems. Pitting and fissuring, as examples of non-structurally dependent attack, but as precursors to cracking, are also considered, as well as hydrogen derived from corrosion reactions. The effects of geometrical discontinuities, such as pits, fissures, or cracks, may be important as sites for electrochemical change, quite apart from their influences on stress intensification. Finally, consideration is given to the question of why stress-corrosion cracking environments are so specific, in comparison with those that promote corrosion fatigue.

Electrochemical influences in fracture initiation

A common cause of failure of large metallic components is stress corrosion, often in subtle combination with fatigue. In the case of carbon steel as used in pipelines, the problem of understanding the cause of variations in susceptibility to intergranular stress corrosion between steel batches of apparently similar origin, history and operating conditions has been researched for many years. We report here the results of investigations including Auger electron spectroscopy which show that the source of variable behaviour is probably the differences in the micro-chemical content of grain boundary paths, differences which may easily exist between one batch or another of apparently similar steel.

C. Lea

Papers

Grain boundary microchemistry and stress-corrosion failure of mild steel