Dislocations Linear Defects

Size: px

Start display at page:

Download "Dislocations Linear Defects"

Claude Carpenter
5 years ago
Views:

1 Dislocations Linear Defects Dislocations are abrupt changes in the regular ordering of atoms, along a line (dislocation line) in the solid. They occur in high density and are very important in mechanical properties of material. They are characterized by the Burgers vector, found by doing a loop around the dislocation line and noticing the extra interatomic spacing needed to close the loop. The Burgers vector in metals points in a close packed direction. Edge dislocations occur when an extra plane is inserted. The dislocation line is at the end of the plane. In an edge dislocation, the Burgers vector is perpendicular to the dislocation line. Screw dislocations result when displacing planes relative to each other through shear. In this case, the Burgers vector is parallel to the dislocation line.

3 Burgers vector b. It help us to describe the size and the direction of the main lattice distortion caused by a dislocation. Dislocations shown above have Burgers vector directed perpendicular to the dislocation line. These dislocations are called edge dislocations. There is a second basic type of dislocation, called screw dislocation. The screw dislocation is parallel to the direction in which the crystal is being displaced (Burgers vector is parallel to the dislocation line).

4 Dislocations Screw dislocation Edge dislocation Burgers vector b b//ζ b Dislocation line ζ b ζ

5 Mixed/partial dislocations: The exact structure of dislocations in real crystals is usually more complicated than the ones shown. Edge and screw dislocations are just extreme forms of the possible dislocation structures. Most dislocations have mixed edge/screw character. To add to the complexity of real defect structures, dislocation are often split in "partial dislocations that have their cores spread out over a larger area.

6 External Surfaces : Interfacial Defects The environment of an atom at a surface differs from that of an atom in the bulk, in that the number of neighbors (coordination) decreases. This introduces unbalanced forces which result in relaxation (the lattice spacing is decreased) or reconstruction (the crystal structure changes). Surface atoms have unsatisfied atomic bonds, and higher energies than the bulk atoms Surface energy, γ (J/m 2 ) Surface areas tend to minimize (e.g. liquid drop) Solid surfaces can reconstruct to satisfy atomic bonds at surfaces.

7 External Surface Free surface can be modeled as a simple termination of the bulk crystal on low-index planes (those having the lowest energy). The resultant picture is the so-called terrace-ledge-kink (TLK) model.

8 Surfaces and interfaces are very reactive and it is usual that impurities segregate there. Since energy is required to form a surface, grains tend to grow in size at the expense of smaller grains to minimize energy. This occurs by diffusion, which is accelerated at high temperatures.

9 Grain Boundaries : Polycrystalline material comprised of many small crystals or grains. The grains have different crystallographic orientation. There exist atomic mismatch within the regions where grains meet. These regions are called grain boundaries. Surfaces and interfaces are reactive and impurities tend to segregate there. Since energy is associated with interfaces, grains tend to grow in size at the expense of smaller grains to minimize energy. This is accelerated at high temperatures. The density of atoms in the region including the grain boundary is smaller than the bulk value, since void space occurs in the interface.

10 Interfacial defects Interfacial defects form either between different phases, or between different crystals. Interfaces between crystals are grain boundaries, as discussed earlier.

11 Microscopy Visible light: 0.4~0.7m Resolution limit ~ 0.5λ ~ 0.3µ Examined by the optical properties of the surface: need etching Depth of field is important: needs a flat surface (polishing) Up to 2000x Brass (annealing twins) FCC - Cu/Zn alloy Bronze (Cu/Sn alloy)

13 Grain Size ASTM (American Society for Testing And Materials) grain size number n With 100x N = 2 n 1 N = no. of grains/in 2 46 grains + 22 on circumference = /2 =57 grains N=57 / π (3.5/2) 2 = 6 grains/in =2 n-1, n=3.6 n N

14 Scanning Electron Microscope 2000x Condenser lens Electron gun High depth of field!!! Scan generator Scan coils Objective lens Specimen Detector Display & storage 10~50,000x Modes: Emissive, reflective, absorptive, X-ray, etc..

$Magnifying lenses Camera Diffraction, Microscopy, Spectroscopy are all possible in the same column.$

17 Transmission Electron Microscope - TEM In TEM, sample thickness < 1000A for electron transmission Electron gun Condenser lens Objective lens Specimen Up to 1,000,000x Magnifying lenses Camera Diffraction, Microscopy, Spectroscopy are all possible in the same column. Applications: determine dislocation, defects, micro-precipitates, etc..

Imperfections in atomic arrangements

MME131: Lecture 9 Imperfections in atomic arrangements Part 2: 1D 3D Defects A. K. M. B. Rashid Professor, Department of MME BUET, Dhaka Today s Topics Classifications and characteristics of 1D 3D defects