First principles investigation of hydrogen embrittlement in FeAl
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The mechanism underlying the hydrogen-induced embrittlement effect in FeAl has been investigated using a local density functional total-energy approach. The bonding characteristics, the bond and cleavage strength between iron and aluminum layers, and the surface energy with and without interstitially absorbed H are calculated from first-principles band-structure and atomic-cluster methods. Our unique combination of techniques permits the simultaneous study of the metallic and localized bonding effects on an equal footing. Results from this study show that FeAl (in the absence of H) is intrinsically highly resistant to cleavage fracture in terms of the high theoretical cleavage strength. Hydrogen locally dilates the Fe-Al lattice, and this is accompanied by a sizable decrease in Fe-Al cleavage (or cohesive) strength. Our results suggest that the underlying mechanism of H-embrittlement in aluminides is a depletion of J-bonding charge on the Fe site resulting from the charge transfer from Fe to H. Results also indicate that the H-embrittlement effect is greater for H adsorbed in Fe-rich sites.
I. INTRODUCTION
Many ordered intermetallic compounds have attractive properties for potential high-temperature structural applications. However, their brittle cleavage fracture behavior and low ductility at ambient temperatures limit their application as structural materials. Traditionally, brittleness has been attributed to intrinsic factors such as weak interlayer bonding, grain boundary fragility, or insufficient numbers of deformation modes. While these factors are important—in many cases dominant—in limiting ductility, recent work by Liu et al.U4 has shown that extrinsic factors are the major cause of low ductility in some systems and that in fact the materials are intrinsically ductile. Such is the case for the prototypical example of the Fe-based aluminides, which have been known for decades to be brittle. Surprisingly, despite extensive studies, the major cause for their brittle behavior had not been identified until the work of Liu et al. Their investigation clearly shows that Fe-Al is in fact intrinsically ductile, and the poor ductility commonly observed in air tests is an extrinsic effect due to the conditions of the tests. This environmental embrittlement (EE) effect involves a mechanism of dissociation of H2O to generate atomic hydrogen, which diffuses into the region of the crack tip and promotes brittle crack propagation. In this work we describe a first-principles study of the atomic-level interactions of H with a host FeAl lattice within the context of the local density functional theory.5 We carry out calculations using a combination of first-principles full-potential linearized augmented plane-wave (FLAPW)6 and atomic cluster (augmented
Gaussian)7 methods in order to study the electronic mechanism underlying the H-induced embrittlement effect as well as the bonding mechanism of FeAl. This combined theoretical approach is unique in the sense that the complementary band structure-cluster treatment provides a
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