Inmaterials science,slip is the large displacement of one part of acrystal relative to another part alongcrystallographic planes and directions.^[1] Slip occurs by the passage ofdislocations on close/packed planes, which are planes containing the greatest number ofatoms per area and inclose-packed directions (most atoms per length). Close-packed planes are known asslip orglide planes. Aslip system describes the set of symmetrically identicalslip planes and associated family ofslip directions for which dislocation motion can easily occur and lead toplastic deformation. The magnitude and direction of slip are represented by theBurgers vector,b.

An external force makes parts of thecrystal lattice glide along each other, changing the material's geometry. Acritical resolved shear stress is required to initiate a slip.^[2]

Slip inface centered cubic (fcc) crystals occurs along theclose packed plane. Specifically, the slip plane is of type{111}, and the direction is of type <110>. In the diagram on the right, the specific plane and direction are (111) and [110], respectively.

Given the permutations of the slip plane types and direction types, fcc crystals have 12 slip systems.^[3] In the fcc lattice, thenorm of the Burgers vector, b, can be calculated using the following equation:^[4]

${\displaystyle |b|=\frac{a}{2}|⟨110⟩|=\frac{a\sqrt{2}}{2}}$^[4]

Where a is the lattice constant of the unit cell.

Slip inbody-centered cubic (bcc) crystals occurs along the plane of shortestBurgers vector as well; however, unlike fcc, there are no truly close-packed planes in the bcc crystal structure.Thus, a slip system in bcc requires heat to activate.

Some bcc materials (e.g. α-Fe) can contain up to 48 slip systems.There are six slip planes of type {110}, each with two <111> directions (12 systems). There are 24 {123} and 12 {112} planes each with one <111> direction (36 systems, for a total of 48). Although the number of possible slip systems is much higher in bcc crystals than fcc crystals, the ductility is not necessarily higher due to increasedlattice friction stresses.^[3]While the {123} and {112} planes are not exactly identical in activation energy to {110}, they are so close in energy that for all intents and purposes they can be treated as identical.In the diagram on the right the specific slip plane and direction are (110) and [111], respectively.^[4]

${\displaystyle |b|=\frac{a}{2}|⟨111⟩|=\frac{\sqrt{3}a}{2}}$^[4]

Slip inhexagonal close packed (hcp) metals is much more limited than in bcc and fcc crystal structures. Usually, hcp crystal structures allow slip on the densely packed basal {0001} planes along the <1120> directions.The activation of other slip planes depends on various parameters, e.g. the c/a ratio.Since there are only 2 independent slip systems on the basal planes, for arbitrary plastic deformation additional slip or twin systems needs to be activated. This typically requires a much higher resolvedshear stress and can result in the brittle behavior of some hcp polycrystals. However, other hcp materials such as pure titanium show large amounts of ductility.^[6]

Cadmium,zinc,magnesium,titanium, andberyllium have a slip plane at {0001} and a slip direction of <1120>. This creates a total of three slip systems, depending on orientation. Other combinations are also possible.^[7]

There are two types of dislocations in crystals that can induce slip - edge dislocations and screw dislocations. Edge dislocations have the direction of the Burgers vector perpendicular to the dislocation line, while screw dislocations have the direction of the Burgers vector parallel to the dislocation line. The type of dislocations generated largely depends on the direction of the applied stress, temperature, and other factors. Screw dislocations can easilycross slip from one plane to another if the other slip plane contains the direction of the Burgers vector.^[2]

Formation of slip bands indicates a concentrated unidirectional slip on certain planes causing a stress concentration. Typically, slip bands induce surface steps (i.e. roughness duepersistent slip bands duringfatigue) and a stress concentration which can be a crack nucleation site. Slip bands extend until impinged by a boundary, and the generated stress from dislocation pile-up against that boundary will either stop or transmit the operating slip.^[9][10]

Formation of slip bands under cyclic conditions is addressed aspersistent slip bands (PSBs) where formation under monotonic condition is addressed as dislocation planar arrays (or simply slip-bands).^[11] Slip-bands can be simply viewed as boundary sliding due to dislocation glide that lacks (the complexity of ) PSBs high plastic deformation localisation manifested by tongue- and ribbon-like extrusion. And, wherePSBs normally studied with (effective) Burger's vector aligned with extrusion plane because PSB extends across the grain and exacerbate during fatigue;^[12] monotonic slip-band has a Burger's vector for propagation and another for plane extrusions both controlled by the conditions at the tip.

The main methods to identify the active slip system involve eitherslip trace analysis of single crystals^[13][14] orpolycrystals,^[15][8] using diffraction techniques such as neutron diffraction^[16] and high angular resolutionelectron backscatter diffraction elastic strain analysis,^[17] orTransmission electron microscopy diffraction imaging ofdislocations.^[18]

In slip trace analysis, only the slip plane is measured, and the slip direction is inferred. In zirconium, for example, this enables the identification of slip activity on a basal, prism, or 1st/2nd order pyramidal plane. In the case of a 1st-order pyramidal plane trace, the slip could be in either ⟨𝑎⟩ or ⟨𝑐 + 𝑎⟩ directions; slip trace analysis cannot discriminate between these.^[5]

Diffraction-based studies measure the residual dislocation content instead of the slipped dislocations, which is only a good approximation for systems that accumulate networks ofgeometrically necessary dislocations, such asFace-centred cubic polycrystals.^[19] In low-symmetry crystals such ashexagonalzirconium, there could be regions of the predominantly single slip where geometrically necessary dislocations may not necessarily accumulate.^[20] Residual dislocation content does not distinguish between glissile and sessile dislocations. Glissile dislocations contribute to slip andhardening, but sessile dislocations contribute only to latent hardening.^[5]

Diffraction methods cannot generally resolve the slip plane of a residual dislocation. For example, in Zr, the screw components of ⟨𝑎⟩ dislocations could slip on prismatic, basal, or 1st-order pyramidal planes. Similarly, ⟨𝑐 + 𝑎⟩ screw dislocations could slip on either 1st or 2nd order pyramidal planes.^[5]

• Miller indices
• Persistent slip bands

• An online tutorial on slip, explained onDoITPoMS

• Materials science

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