A defect in which the arrangement of atoms in a crystal is disturbed along a line. Metals deform plastically as dislocations move on slip planes. There are edge, screw and mixed dislocations.
Japanese name: 転位
Dislocation appears in 19 practice questions across 8 topics, including High-temperature materials and creep, Heat treatment and metallic materials, Crystal defects and diffusion.
The theoretical shear strength needed to shift an entire plane of atoms at once is estimated at about one tenth of the shear modulus, yet real metals yield at stresses a thousandth of that or less. Dislocations explain this large gap. With a dislocation present, there is no need to break atomic bonds all at once; the crystal slips by rearranging bonds a few at a time near the dislocation line, much as moving a ruck in a carpet shifts the whole carpet. How easily a metal deforms plastically is therefore determined by how easily its dislocations move.
The type of dislocation is determined by the relation between the Burgers vector b, which gives the magnitude and direction of slip, and the direction of the dislocation line. An edge dislocation has the form of an inserted extra half-plane of atoms, with b perpendicular to the line. In a screw dislocation the atomic planes connect like a spiral staircase, with b parallel to the line. Most dislocations are mixed dislocations combining the two. An edge dislocation has a unique slip plane and, at high temperature, can climb up or down by absorbing or emitting vacancies. A screw dislocation can cross-slip onto another slip plane.
Most strengthening methods for metals work by obstructing dislocation motion. Solid-solution strengthening stops dislocations with the strain fields of solute atoms, precipitation hardening with fine precipitates, grain refinement with grain boundaries, and work hardening with entanglement of dislocations among themselves. Cold working raises the dislocation density from about 10¹⁰ m⁻² to about 10¹⁵ m⁻², increasing strength while lowering ductility. Conversely, when recovery and recrystallization occur during annealing, the dislocation density falls and softness and ductility return.
Dislocations can be observed directly by TEM as lines of diffraction contrast caused by strain. At a surface, etching forms etch pits where dislocations meet the surface, and counting their density gives the dislocation density. Silicon wafers are made essentially dislocation-free, but if dislocations are introduced during processing, for example by thermal stress, they increase leakage current and become sites for precipitation of metal impurities. When a film with a different lattice parameter is grown, misfit dislocations form at the interface to relax the strain, and threading dislocations may remain in the film.
Answer: A. They are perpendicular to each other
An edge dislocation has the form of an extra half-plane of atoms inserted into the crystal, and its Burgers vector b is perpendicular to the dislocation line. An edge dislocation glides in the direction of b, and its slip plane is uniquely defined as the plane containing both b and the line. b is parallel to the line in a screw dislocation; because a screw dislocation has no unique slip plane, it can cross-slip onto another slip plane. Most real dislocations are mixed dislocations in between, with the proportions of edge and screw character changing continuously along the line, so there is no fixed 45°.
Answer: C. About 0.286 nm
The magnitude of (a/2)<110> is (a/2) × √(1² + 1² + 0²) = a/√2, so 0.405/1.414 ≈ 0.286 nm. This equals the nearest-neighbor distance in FCC; dislocations take the shortest lattice translation vector along the close-packed direction as their Burgers vector. About 0.405 nm is the lattice parameter itself (a translation along <100>), which is unfavorable because the dislocation energy is proportional to b². About 0.351 nm applies the BCC (a/2)<111>, that is, √3a/2. The elastic energy per unit length of a dislocation is about Gb²/2, so dislocations with smaller b are more stable.
During plastic deformation they multiply by mechanisms such as the Frank–Read source, in which a dislocation pinned at both ends bows out and generates loop after loop. The dislocation density therefore increases as working proceeds, causing work hardening.
If you make a closed circuit around a dislocation, a closure failure remains compared with a perfect crystal. This is the Burgers vector, which gives the amount and direction by which the crystal is displaced when the dislocation passes.
In ionic bonding, slip must avoid bringing ions of the same sign together, and in covalent bonding the bonds are strongly directional, so slip systems are limited and the Peierls stress is large. At room temperature ceramics therefore fracture in a brittle manner before deforming plastically.
An edge dislocation has the form of an extra half-plane of atoms inserted into the crystal, and its Burgers vector b is perpendicular to the dislocation line.
Which correctly describes the relation between the Burgers …This equals the nearest-neighbor distance in FCC; dislocations take the shortest lattice translation vector along the close-packed direction as their Burgers vector.
The Burgers vector of a perfect dislocation in an FCC metal…Yield strength and hardness are structure-sensitive properties governed by how easily dislocations move, and they change greatly with heat treatment and working.
Which statement about the Young's modulus of steel is corre…In BCC metals, the stress needed to move dislocations (the Peierls stress) rises sharply as temperature falls, so at low temperatures cleavage fracture occurs before yielding and the absorbed energy in the Charpy impact test drops abruptly over a certain temperature range.
Which material shows a distinct ductile-to-brittle transiti…Holding it at room temperature (natural aging) or at about 100–200 °C (artificial aging) then forms fine precipitates that are coherent or semicoherent with the matrix, such as GP zones, θ″ and θ′, which obstruct dislocation motion and raise the strength.
Which is the correct sequence for precipitation hardening a…Young's modulus Recrystallization Recovery Creep Burgers vector Metallic glass Work hardening Diffusion creep Power-law creep Nickel-based superalloy