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

Periodic Reporting for period 1 - FraMoS (Multi-resolution Fracture Models for High-strength Steels: Fully Ductile Fracture to Quasi-cleavage Failure in Hydrogen Environment)

Reporting period: 2016-12-01 to 2018-11-30

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.
In the present research work, detailed micromechanics based (Eshelby-type) analyses are carried out to examine the influence of morphology and orientation of a hydride precipitate on the thermodynamic driving force (Gibbs free energy change). Special emphasis is laid on the understanding of mechanistic aspects associated with stress-induced hydride reorientation. Unlike the previously reported studies, full account is made of the elastic anisotropy of the zirconium matrix, the mismatch in elastic moduli of the matrix and the precipitate, the distortional eigen strains and the external stress. The present study brings new insights to the mechanics of hydride precipitation. Our elastic calculations indicate that under the influence of an external stress, a hydride precipitate has a tendency to reorient along a certain crystallographic plane, other than the basal plane. The assumption of linear elasticity, however, leads to a large overestimation of the external stresses required for hydride reorientation. Since plastic deformations, associated with hydride precipitation, are path-dependent, we first make an assessment of loading paths on the Gibbs free energy of an elastic-plastic precipitate lying in an elastic matrix. This is followed by the development of an approximate scheme that incorporates the effect of matrix plasticity on the stress field inside an elastic precipitate. Such an assessment lead to hydride reorientation at realistic magnitudes of applied stresses and the results seem to be consistent with the experimental observations.
The research work carried out under the Marie Curie fellowship (MSCA-IF action of the Horizon-2020 program) has led to an improved understanding of the mechanics of hydride precipitation in Zirconium alloys. To date, significant progress has been made and computational codes were developed to analyse the internal stress states generated within the material, as result of phase transformation, and how they influence the way in which the microstructure (shape and orientation of the hydride precipitates) evolve. The role of elastic and plastic deformations, on the energetics of hydride precipitation was examined.
The research work is currently being pursued to combine the developed models for nucleation and growth of hydride precipitates with a suitable microscopic criteria for damage development. The resulting models for hydride embrittlement will cover the entire spectrum from fully ductile fracture to brittle-type failure of Zirconium alloys in hydrogen environment. The understanding gained from modelling of hydrogen embrittlement in zirconium alloys will be combined with on-going work at Oxford on steels to describe the ductile-brittle transition in non-hydride forming materials. The development of such predictive models will be highly useful to the nuclear industry where hydride embrittlement of zirconium alloys is a serious concern for integrity of pressure retaining components like pressure tube in CANDU-type reactors and fuel clads of light water reactors. The unravelling of linkage between microstructure and toughness particularly for non-hydride forming metals like high strength steels is expected to play an important role in the development of stronger, tougher and more durable alloys.
Effect of external stress on hydride orientation, corresponding to minimum Gibbs free energy.