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
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.
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.