atom

micro

macro

A

B

C
HEDE

HEDE: Hydrogen Enhanced Decohesion
Definition:Mechanism for hydrogen embrittlement based on the weakening of metal-metal bonds by hydrogen.
Explanation:The HEDE is one of the models describing the hydrogen induced embrittlement in metals (see also hydrogen enhanced local plasticity; HELP). In the HEDE mechanism, the hydrogen atoms accumulate within certain regions of the crystal lattice and reduce the cohesive energy of the metal matrix or at specific interfaces (see the figures below). This energy reduction is required to cleave the crystal along certain crystallographic planes, grain boundaries, or phase boundaries.

For well defined (i.e. coherent) interfaces, the relevant energies are also accessible from ab initio calculations. For heterogonous interfaces as displayed in the figure, the distribution of the H atoms on the free surfaces depends on the adsorption energy and is best captured in a grand canonical ensemble. One should also note that a pure thermodynamic approach may be insufficient for describing realistic conditions. The latter are often determined by the kinetics of the separation process when the free surfaces are formed.
Picture /
Figure /
Diagram:

(a) The interface between a precipitate phase and the matrix can be subject to the HEDE mechanism. (b) Schematic respresentation of the HEDE mechanism showing that the presence of H reduces the work for separating an interface into two free surfaces.
SFB-Link:Mechanisms of hydrogen embrittlement are investigated in the projects A7, A9, C6 and T4. In particular, in the transfer project T4 the hydrogen induced reduction of decohesion energies of the interface between Cr carbides and the Fe matrix is determined.
References:J. Morlet, H. Johnson, A. Troiano, Journal of the Iron and Steel Institute 189, 37 (1958)
J. Rice and J. Wang, Mater. Sci. Eng. A 107, 23 (1989).
Y.A. Du, L. Ismer, J. Rogal, T. Hickel, J. Neugebauer, and R. Drautz, Phys. Rev. B 84, 144121 (2011).