Stacking faults and twins

The crystalline structure can be thought of as a regular stack of planes following a set pattern that repeat indefinitely: we can characterise this as a stacking sequence.

For example in the face-centred cubic structure, the stacking sequence of \(\left[ 111 \right]\) planes is of the type \(\left[...\ce{ABCABCABC}...\right]\), as shown in the diagram below.

Stack of compact planes {111} in the FCC structure, (cross-section perpendicular to (111) dense planes and (110) dense directions). Dark atoms are in the (100) plane of the image, light atoms are in the planes immediately above or below the image plane | Philippe Lours, École des mines d'Albi-Carmaux, 2014. | Additional information...Information
Stack of compact planes {111} in the FCC structure, (cross-section perpendicular to (111) dense planes and (110) dense directions). Dark atoms are in the (100) plane of the image, light atoms are in the planes immediately above or below the image planeInformation[2]

A modification to the stacking sequence is called a stacking fault. These faults can be:

  • Intrinsic (see diagram): this corresponds to the withdrawal of a plane and is reflected in a stacking sequence such as \(\left[...\ce{ABCACABC}...\right]\)

    Intrinsic stacking faults can also occur if the “upper” part of the crystal is moved in relation to the “lower” part of the crystal by \(a/6 [112]\) so as to bring plane \(B\) to position \(C\).

  • Extrinsic: this corresponds to the addition of a plane and is reflected in a stacking sequence such as \(\left[...\ce{ABCACBABC}...\right]\)

    Extrinsic stacking faults can also appear if the “upper” part of the crystal is moved in relation to the “lower” part of crystal by \(a/6 [112]\) so as to bring plane \(C\) to position \(A\) and the “lower” part in relation to the “upper” part by \(a/6 [112]\) to bring plane \(A\) to position \(C\).

    If the stacking fault does not cross the entire crystal, it is then restricted by one (or two) dislocation(s) whose Burgers vector is not a translation of the lattice but is of the type \(a/6 [112]\). Such partial dislocation is called a Shockley dislocation (see diagram).

Superlattice intrinsic stacking fault in a FCC structure: sketch and transmission electron microscograph of a defect | photographie de Arnaud Coujou et Philippe Lours, Laboratoire de microscopie et structure des matériaux, Université Paul Sabatier. | Additional information...Information
Superlattice intrinsic stacking fault in a FCC structure: sketch and transmission electron microscograph of a defectInformation[4]
Shockley dislocation | Philippe Lours, École des mines d'Albi-Carmaux, 2014. | Additional information...Information
Shockley dislocationInformation[6]

It can happen that the sequence \(\left[...\ce{ABCABCABC}...\right]\) locally transforms to a symmetrical structure \(\left[...\ce{CBACBACBA}...\right]\) (during solidification or recrystallization for example). The resulting sequence is \(\left[... \ce{ABCABCACBACBA}...\right]\). The stacking fault resulting from this alteration to the sequence is called twinning and plane \(A\) is a twin plane (see diagram).

Schematic representation of a twin in the FCC structure and optical micrograph of a stainless steel (twinning is shown as bands with different shades of grey) | photographie de Serge Tovar, ICA Albi - École des mines d'Albi-Carmaux, 2014. | Additional information...Information
Schematic representation of a twin in the FCC structure and optical micrograph of a stainless steel (twinning is shown as bands with different shades of grey)Information[8]