Light interstitials in iron under extreme mechanical conditions

Abstract

Addition of small amounts of light interstitial elements to iron can alter its physio-chemical characteristics to a great degree. The most crucial of these elements being carbon, overcomes many of iron deficiencies such as lack of hardenability, tensile strength and so on. It is owing to this element that iron in form of steel has become the most commonly used material in modern industry. However, not all interstitial elements have a positive impact on iron's performance, nor their presence is desirable. Due to its high diffusivity, hydrogen can travel inside iron with relative ease and interact with already formed, or forming defects such as dislocations and vacancies. It is believed that this interaction impacts the formation and evolution process of defects significantly. From macroscopic perspective, this is manifested in form of embrittlement of iron, usually referred to as hydrogen embrittlement (HE). Super-ferrite is a newly discovered phase of iron supersaturated in carbon.

It is usually formed under extreme mechanical conditions like severe plastic deformation, from iron and a commonly found form of carbide in steel, namely cementite. The first part of this document delves into many aspects of super-ferrite using atomistic simulations and density functional theory. Of the crucial findings of said chapter, one is the process of super-ferrite formation, which involves a secondary intermediate phase. Another is careful analysis of its structure and its comparison with the more common supersaturated phase, martensite. The second part is devoted to careful examination of a newly proposed HE mechanism in iron. Using the concrete framework of thermodynamics and statistical mechanics, complemented by numerical methods such as molecular dynamics, grand canonical Monte Carlo, and density functional theory, many aspects of this theory are scrutinized.

It is concluded although viable for iron under extremely high hydrogen pressure, this mechanism is not applicable to HE that is commonly observed in industry. As a by product of this part, the iron hydrogen phase diagram is extended to temperatures as low as 100 Kelvin.