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SFE unstable

SFE unstable: Stacking Fault Energy, unstable
Definition:Unstable stacking fault energy
Explanation:Dislocation nucleation is characterized by the unstable SFE, γus, which is the maximum energy barrier encountered in block-like sliding along the slip plane, of one-half of the crystal relative to the other, along a slip plane in the direction of shear which forms a lattice dislocation [1]. It is determined by a model of dislocation nucleation from a crack tip created by Rice [2]. A comparison of surface energy γs and γus was made to determine if a given material will exhibit brittle or ductile behavior [3]. Fig. 1 shows the various states of shear for a simple cubic lattice [2]. The relative shear displacement of the central pair of planes is denoted Δ. Lattice configurations starting at the lower left and going clockwise, correspond to points (a)-(d) forming a shear process. Position (d) corresponds to the unstressed but unstable equilibrium state for which the central pair of lattice planes is displaced by b/2. This is the unstable stacking configuration and the work to create such configuration is called as unstable SFE.
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Fig. 1. Various states of shear for a simple cubic lattice [2].
SFB-Link:The unstable SFE has an influence on the twinning ability. Twinning ability increases with unstable SFE and decreases with increasing SFE. It helps in analyzing the deformation mechanisms.
References:[1] D.Farkas. Embedded atom calculations of unstable stacking fault energies and surface energies in intermetallics[J]. Jouranl of materials research, 1997, 12: 93-99.
[2]J.R.Rice. Dislocation nucleation from a crack tip: an analysis based on the peierls concept[J]. Journal of the mechanics and physics of solids, 1992, 40: 239-271.
[3]J.A.Zimmerman, H.Gao, F.F.Abraham. Generalized stacking fault energies for embedded atom FCC metals[J]. Modelling simulation in materials science and engineering, 2000, 8: 103-115.