CDT Student Jassel Majevadia, supervised by Mark Wenman, Daniel Balint and Adrian Sutton, carried out a critical review of the field of Hydrogen Enhanced Localized Plasticity. This work has been published in the Nuclear Future Journal in June this year.

Figure 1: A crack that has propagated through zirconium fuel cladding, surrounded by hydride. Reproduced from [6]
Figure 1: A crack that has propagated through zirconium fuel cladding, surrounded by hydride. Reproduced from [6]

The degradation of structural materials by hydrogen-related mechanisms has serious implications for many areas of industry, the most expensive of which is the failure of fuel cladding in pressurised water reactors. The process by which zirconium-based fuel cladding fails is the topic of Jassel's academic research, focussed on the multi scale modelling of precipitates and Delayed Hydride Cracking in zirconium alloys. 

Hydrogen solute atoms have a deleterious effect on the integrity of many transition metals. The consequences of hydrogen related embrittlement range from fuel cladding failure [1], to everyday items such as fasteners and fixings. Hydrogen embrittlement arises in two ways – environmentally and in the manufacture process. The former is related to failure owing to hydrogen supplied from the environment while the material is in operation, such as corrosion. The latter arises from pre-existing hydrogen within the material that is left over from the manufacturing process.

There are a number of proposed mechanisms by which hydrogen embrittles a material, which have varying degrees of experimental support. Researchers in the field of hydrogen embrittlement are currently focusing on three well-known categories:

  • delayed-hydride cracking (DHC) [2-4];
  •  hydrogen-induced decohesion (HED) [5]; and
  • hydrogen-enhanced localised plasticity (HELP) [6].

A commonly observed mechanism for hydrogen embrittlement is hydrogen-enhanced localised plasticity (HELP). The mechanism for HELP was first proposed by C. Beachem [7] and is based on the enhanced mobility of dislocations in the presence of solute atoms, leading to lower stresses required to move the dislocations. ‘Enhanced mobility’ means that the speed of the dislocation is raised for a given applied stress, or that a lower applied shear stress is required to move the dislocation.

The purpose of the article is to review experimental observations and discuss the postulated theoretical models to explain HELP. In addition, further avenues for study that can shed light on both HELP and other observed mechanisms are also outlined.

The nature of hydrogen embrittlement is intrinsically multi-scale, and therefore a fitting topic for a research project within the CDT for the Theory and Simulation of Materials. In particular, Jassel's research aims to incorporate the atomic effects of hydrogen atoms within pure zirconium into continuum models of hydrogen related embrittlement with a view to modelling the accumulation of hydrogen to stress concentrations. 

Download Jassel's articl in PDF format.

References:

1. C.E. Coleman (2003) Cracking of hydride-forming metals and alloys, in Comprehensive Structural Integrity, volume 8, pp. 103–161, Elsevier 7. H.K. Birnbaum (1989) Mechanisms of hydrogen related fracture of metals, Office of Naval Research report
2. G. McRae, C.E. Coleman and B. Leitch (2010) The first step for delayed hydride cracking in zirconium alloys. J. Nucl. Mat. 396, 130–143
3. C.E. Coleman (2004) Delayed hydride cracking in zirconium alloys in pressure tube nuclear reactors. IAEAreport, September 2004, 1–92
4. M.P. Puls (2009) Review of the thermodynamic basis for models of delayed hydride cracking rate in zirconiumalloys. J. Nucl. Mat. 393(2), 350–367
5. R. Oriani (1978) Hydrogen embrittlement of steels. Annu. Rev. Mat. Sci. 8, 327–357
6. P. Sofronis and H. Birnbaum (1995) Mechanics of the hydrogen-dislocation-impurity interactions. I - increasingshear modulus. J. Mech. Phys. Solids 43, 49–90
7. C. Beachem (1972) A new model for hydrogen-assisted cracking (hydrogen “embrittlement”). Met. Trans. 3(2),437–451