FraMoS · Multi-resolution Fracture Models for High-strength Steels: Fully Ductile Fracture to Quasi-cleavage Failure in Hydrogen Environment

Multi-resolution Fracture Models for High-strength Steels: Fully Ductile Fracture to Quasi-cleavage Failure in Hydrogen Environment

Recent advances in Computational Mechanics are towards the development of predictive tools that can accelerate the 'Materials Development Cycle' by unraveling the linkage between macroscopic properties and microstructure. The availability of 3D tomographic tools and the era of Exascale computing have initiated the quest to develop stronger, tougher and more durable alloys by employing 'virtual predictions' in lieu of expensive destructive testing. However, our lack of understanding of the 'structure-toughness’ relations is one of the main bottlenecks in this pursuit. Moreover, the uptake of some of the new high strength alloys (TRIP, TWIP etc) is hampered by the concerns of hydrogen (H) induced cracking. Thus, the main objective of this research is to develop a predictive model for the transition from ductile to brittle behaviour of metals in the presence of hydrogen. The focus of initial research is on modelling of hydrogen induced embrittlement in zirconium alloys where the mechanisms responsible for embrittlement are more established. This will be followed by extending the research work to non-hydride forming metals, particularly, to the high strength steels which are of immediate interest to the automotive industry. Zirconium based alloys used in nuclear reactors are susceptible to hydrogen pickup. When hydrogen concentration in the solid solution exceeds the threshold limit (terminal solubility), a solid state phase transformation reaction occurs in these alloys. The zirconium hydride phase so precipitated is brittle and may influence the integrity of structural components by various mechanisms like hydride blistering, delayed hydride cracking (DHC) etc. Hydride precipitation in zirconium alloys usually occurs by nucleation, growth and coarsening, and is typically influenced by the elastic coherent stresses (resulting from phase transformation) as well by the external stress field. In this work, detailed micromechanics based (Eshelby-type) analyses are carried out to understand the mechanics and energetics associated with precipitation of zirconium hydrides. The proposed models for nucleation and growth of hydride precipitates will be combined with a suitable microscopic criterion for damage development to cover the entire spectrum fully ductile fracture to brittle-type failure in hydrogen environment.

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