Dislocation Mobility

Dislocations of a new slip system, {001} <110>, have been found around indentations on the {001} surface of n-type GaAs. This mode of slip was studied by polish/etch techniques and by TEM, and was found to have a polar character, analogous to that of the normal {001} <110> slip. In GaAs as in most fcc semiconductors, the primary slip system is {001} <110>. Dislocations of this type have been intensively studied, and much is known about their structure and mobility. In particular, As and Ga dislocations have very different mobilities, which depend strongly on doping. To date, however, there has been no evidence for the existence of {001} <110> slip in fcc semiconductors. indentation on {001} planes of GaAs produces extensive slip patterns along <110> directions, corresponding to the {111} <110> primary slip mode. However, close examination of both low 20 o C and high temperature 400 oC indentations revealed slip lines in <100> directions, which would correspond to slip on {110} or {100} planes. This slip type was emined in detail for n-type GaAs (10 %18 Te cm -3) by etching and by TEM.


Dept. of Materials, Oxford Univ.
http://www-sgrgroup.materials.ox.ac.uk/abstracts/CDQ_SGR_1989_1.pdf






Line Defects

Line defects are mostly due to misalignment of ions or presnece of vacancies along a line. When lines of ions are missing in an otherwise perfect array of ions, an edge dislocation appeared. Edge dislocation is responsible for the ductility and malleability. In fact the hammering and stretching of maaterials often involve the movement of edge dislocation. Movements of dislocations give rise to their plastic behaviour. Line dislocations usually do not end inside the crystal, and they either form loops or end at the surface of a single crystal.

A dislocation is characterized by its Burgers vector: If you imagine going around the dislocation line, and exactly going back as many atoms in each direction as you have gone forward, you will not come back to the same atom where you have started. The Burgers vector points from start atom to the end atom of your journey (This "journey" is called Burgers circuit in dislocation theory).

In this electronmicroscope image of the surface of a crystal, you see point defects and a Burger journey around an edge dislocation. The dislocation line is in the crystal, and the image shows its ending at the surface. A Burger vector is approximately perpendicular to the dislocation line, and the missing line of atoms is somehwere within the block of the Buerger journey.

If the misalignment shifts a block of ions gradually downwards or upwards causing the formation of a screw like deformation, a screw dislocation is formed. The diagram here shows the idealized screw dislocation. This diagram is from the above link, which also shows an electromiscopic images of screw dislocations.

Line defects weakens the structure along a one-dimensional space, and the defects type and density affects the mechanical properties of the solids. Thus, formation and study of dislocations are particularly important for structural materials such as metals. This link gives some impressive images of dislocations. Chemical etching often reveal pits which are visible under small magnifications.

Waterloo univ.
www.science.uwaterloo.ca/~cchieh/cact/applychem/defect.html



Comparison of the number densities of nanosized Cu-rich precipitates in ferritic alloys measured using EELS and EDX mapping, HREM and 3DAP

In this paper we compare various microscopy methods which are used to characterize Cu-rich precipitates in pressure vessel steels. EELS and EDX mapping is found to reveal Cu-rich precipitates of sizes greater than about 1-2 nm, both those with the bcc structure, which are coherent with the ferrite matrix, and incoherent transformed precipitates. This allows a direct comparison with results from 3DAP, which so far has been the only way to image such ultra-fine Cu-rich clusters. In HREM images only incoherent precipitates which have transformed to a twinned 9R structure are seen. A comparison is made of precipitate number densities and sizes as measured by EELS and EDX mapping, 3DAP and HREM.


The composition of the Cu precipitates could also be analysed in the 3DAP. Precipitates were identified in the data using the maximum separation method [10,12], where all solute atoms closer than 0.4nm were taken to belong to the same cluster. Only clusters determined to contain more than 50 solute atoms were treated as Cu-rich precipitates for the purpose of number density or composition calculations. Other atoms lying within 0.3nm of any solute atom within a cluster are considered to be part of the cluster. However, cluster atoms in the outer shell of 0.3nm are ignored, for the purpose of calculating composition, to avoid a contribution from the matrix. Using this method, and taking Cu, Ni and Mn to be the solute atoms, the average composition for the precipitates in sample B was measured to be 20% Cu, 16% Ni, 14% Mn, 2% Si and 48% Fe. Taking only Cu atoms as the solute gives an average composition of 43%Cu, 10%Ni, 8%Mn, 2%Si and 37%Fe.


Jenkins ML, Dept of Materials, Oxford Univ



Dislocations

The presence of dislocations was suggested independently in 1934 by Taylor, Polanyi and Orowan in order to account for the observed strength of crystals - particularly metals. Microscopic investigations showed that when a metal crystal is plastically deformed the deformation occurs by the slip of one plane of atoms over another. Deformation does not occur by the individual atoms being pulled further away from one another. Depending on the material, the slip occurs on a particular crystallographic plane and in a well defined direction, which is usually on the plane of closest packing along the direction of the line of closest packing in that plane. If a set of crystal specimens are grown with the slip plane at various angles to the axis of tension and the tensile force which produces the fist sign of yielding in each of them is measured, then it can be established that the operative stress which produces the deformation is a shear stress – ie. The stress is parallel to the slip plane in the slip direction. This may be shown by calculating the components of the applied force both along the slip direction and also perpendicular to the slip plane. It is the shear stress which is found to be constant for a particular material and it is independent of the orientation of the tensile axis.
A crystal may therefore be thought of as deforming like a pack of cards, and on a microscopic scale we might therefore imagine that in order to initiate slip on a particular plane of atoms we must apply a shear stress which is just sufficient to move all the atoms from their original sites on that plane to the next set of equivalent sites one atomic spacing away.

…slip might occur by consecutive motion rather than by the simultaneous motion of atoms. Dislocations are a necessary consequence of this idea of consecutive motion.
From the definition of a dislocation as the boundary between slipped and unslipped regions of the crystal, it is clear that a dislocation must either end on the free surfaces of the crystal or it must form a closed loop within the materials.

The main purpose of introducing the concept of dislocations is to emphasize how the presence of a defect can completely dominate certain physical properties of crystals. One needs to take account of those mechanisms which can impede the motion of dislocations. In addition, it was noted that during deformation dislocations pass right through a crystal and thereby vanish. For continuing deformation we therefore require a mechanism for the multiplication of dislocations. …how ever carefully we prepare a specimen, using extremely high purity materials and very specially controlled methods of crystal growth, one form of defect will always be present – that due to thermal vibration of the atoms. At any instant in time the atoms are never exactly at their correct lattice sites. At room temperature they are vibrating with approx simple harmonic motion at around 10^13 Hz about an origin which is at the geometrical lattice position. Even at very low temperatures the zero-point motion of the atoms is still present. As the temperature is raised the amplitude of the atomic vibrations increases and this means that virtually every property of a materials whose magnitude is determined by the actual position of the atoms, or by the thermal energy, changes with temperature…….reader will also be aware of the increase in the electrical resistance of metals as the temperature is raised....

Rosenberg, Clarendon Press