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HELP

HELP: Hydrogen Enhanced Local Plasticity
Definition:A mechanism that describes the decrease of dislocation motion barriers due to hydrogen.
Explanation:Hydrogen induced failure of metals is a long standing, unresolved problem in materials science. The effect has been observed in face-centered cubic (→fcc), body-centered cubic (→bcc), as well as in hexagonal close-packed (hcp) metals, but is particularly pronounced in high strength steels (→AHSS).

The HELP mechanism represents a model for hydrogen induced material degradation. Both experimental observations and theoretical calculations support that hydrogen decreases the barriers to dislocation motion over a range of temperatures and strain rates. Thus, the amount of plastic deformation increases within regions of hydrogen accumulation and the fracture event becomes a highly localized process of ductile rupture.
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Diagram:

(a) Structure of the prescribed hydride seed in the tensile strain field of an edge dislocation in Ni. (b) H distribution obtained by equilibrating the dislocation-seed system using an EAM potential.
SFB-Link:Affected Materials are generally speaking, Ni and Fe alloys, and specifically speaking, Inconel 718, 304 stainless, A553b stainless, 316 stainless steels.
References:S. M. Myers et al., Rev. mod. Phys. 64, pp 559 (1992)
J. von Pezold, L. Lymperakis, Acta Mat. 59 pp 2969 (2011)