Stacking fault
A stacking fault is a planar defect in the atomic structure of a crystalline material, characterized by an interruption in the regular sequence of closely packed atomic planes.[1][2] These defects commonly occur in materials with close-packed lattices, such as face-centered cubic (FCC) crystals where the ideal stacking sequence is ABCABC... along the {111} planes, or hexagonal close-packed (HCP) structures with an ABAB... arrangement.[1][3] Stacking faults are classified into two primary types: intrinsic and extrinsic. An intrinsic stacking fault arises from the removal or collapse of a single atomic plane, effectively shifting the sequence (e.g., from ABCABC... to ABCABABC...), and is often associated with vacancy clustering or quenching processes.[1][3] In contrast, an extrinsic stacking fault involves the insertion of an additional plane, creating a sequence like ABCABACABC..., typically due to interstitial atoms or irradiation effects.[1][3] Both types are bounded by partial dislocations, such as Frank partials with a Burgers vector of a/3<111> or Shockley partials with a/6<112>, which can dissociate from perfect dislocations in FCC metals.[1][3] The presence of stacking faults influences key material properties, including mechanical behavior and deformation mechanisms. The stacking fault energy (SFE), typically on the order of a few hundred mJ/m² (e.g., 51.74 mJ/m² in nickel), quantifies the energy cost of the fault and determines the width of the fault ribbon between partial dislocations; lower SFE values promote wider ribbons, enhanced twinning, and reduced dislocation mobility, affecting ductility, hardening, and thermal conductivity in alloys.[1][2] In nanoscale structures, stacking faults act as barriers to dislocation motion, playing a critical role in strengthening and texture development.[2]Fundamentals
Definition and Types
A stacking fault is a planar defect in crystalline materials in which the regular stacking sequence of atomic planes is locally interrupted, resulting in a disruption of the periodic order in close-packed structures such as face-centered cubic (FCC) or hexagonal close-packed (HCP) lattices.[4][5] This interruption typically spans a two-dimensional plane, distinguishing stacking faults from point defects (zero-dimensional, affecting single atomic sites), line defects like dislocations (one-dimensional, extending linearly), and volume defects (three-dimensional, involving larger regions of misfit).[6] The defect arises in modular or layered crystal structures, where atoms are arranged in repeating close-packed layers, and it often bounds regions of altered symmetry without altering the overall lattice orientation beyond the fault plane.[7] Stacking faults are broadly classified into intrinsic and extrinsic types, depending on whether the fault corresponds to the removal or insertion of atomic layers relative to the ideal sequence. Intrinsic stacking faults form by the removal (or effective absence) of a single atomic plane in the stacking sequence, creating a local shift that maintains order on either side of the fault but repeats a layer position. In FCC crystals, with the normal stacking sequence ABCABC..., an intrinsic fault disrupts this to ABCBCABC..., where the layer following C is B instead of the expected A, effectively introducing a hexagonal-like segment (BCB) amid the cubic order.[8][9] This type of fault is commonly bounded by Shockley partial dislocations and carries relatively low energy, influencing local atomic rearrangements.[10] Extrinsic stacking faults, in contrast, arise from the insertion of an extra atomic plane or, equivalently, a combination of an intrinsic fault and a sessile dislocation loop, resulting in a more complex double-layer disruption. In FCC crystals, this produces a sequence such as ABCACBA..., where two consecutive deviations occur (e.g., after ABC, the next layers are ACB instead of ABC or CBA), often forming a thin region of twinned or hcp-like stacking.[4][11] Extrinsic faults are less common than intrinsic ones due to their higher associated energy and are typically observed in materials under specific deformation conditions.[10] The recognition of stacking faults as distinct defects emerged in the 1940s, alongside foundational work on dislocations during investigations of plastic deformation in metals, providing key insights into crystal plasticity mechanisms.[12]Stacking Sequences in Common Crystal Structures
In close-packed crystal structures, atoms are arranged in layers of hexagonal packing, where each layer occupies one of three possible positions labeled A, B, or C, depending on the lateral shift relative to the layer below. These positions ensure maximal atomic density, with atoms touching their nearest neighbors. The overall three-dimensional structure arises from the repeating sequence of these layers, which determines the crystal's symmetry and properties.[13] The face-centered cubic (FCC) structure features a stacking sequence of ABCABC..., where each successive close-packed plane is shifted to occupy the depressions not used by the previous two layers. This repeating pattern every three layers results in a cubic unit cell with high symmetry, equivalent to the cubic close-packed (CCP) arrangement. Atoms in the A position align directly above those in the first layer, B in the second, and C in the third, before restarting the cycle.[14] In contrast, the hexagonal close-packed (HCP) structure follows an ABABAB... stacking sequence, where the third layer returns to the A position, aligning directly above the first layer's atoms. This alternation produces a hexagonal unit cell with lower symmetry than FCC, though both structures achieve the same packing efficiency of 74%. The HCP configuration corresponds to a local energy minimum for many metals, differing from FCC primarily in the periodic repetition that breaks the cubic symmetry.[14][15] Body-centered cubic (BCC) structures lack distinct close-packed planes, as atoms are coordinated in a less dense arrangement with eight nearest neighbors at varying distances. Stacking faults in BCC are thus less prevalent than in close-packed lattices and typically manifest on {112} planes rather than basal planes. Other non-close-packed structures, such as simple cubic or diamond cubic, exhibit even more irregular layer arrangements, making traditional stacking fault concepts less applicable.[16] These ideal stacking sequences provide the foundational patterns against which disruptions—such as those leading to intrinsic or extrinsic faults—can be understood in subsequent discussions of crystal defects. To visualize them, standard notation uses A, B, and C to denote the threefold positions in the hexagonal layer net:- FCC: Planes stacked as A (layer 1), B (layer 2), C (layer 3), A (layer 4), etc.
- HCP: Planes stacked as A (layer 1), B (layer 2), A (layer 3), B (layer 4), etc.