A dislocation in Aluminum. The colouring does not highlight different atoms; it is just used in the explanation of the process.
Aluminum is a metallic element, and its structure is very similar to most other metals. It is malleable
, and ductile
due to its polycrystalline structure. Aluminum is made up of grains (or crystals) which interlock when the metal is cooled from molten. Each grain comprises of rows of atoms in an ordered lattice arrangement, giving each grain an isotropic (same in each direction) structure. Although the different grains are somewhat randomly arranged with grain boundaries forming during the cooling process, the atoms within each crystal are normally aligned which makes the whole metal isotropic, like the individual grains.
The blue atoms have moved across (due to the applied stress) to next to the orange ones, the dislocation has moved the other way.
However, despite a regular lattice arrangement gaps inbetween atoms often form, which give rise to dislocations This is shown in the image above where the point of the dislocation is marked with a red line. When stress is applied to the metal the atoms move past each other one by one to move these dislocations to the grain boundaries. This effect is extremely important in fracture mechanics and it gives aluminum so many of its important properties
. The image on the left shows the next positions of the atoms after some stress has been applied. It is clear that the blue atoms have been forced across to next to the orange ones, and that the dislocation (shown with red line) has moved the other way.
The dislocation reaches the grain boundary, a layer has slipped by one atomic spacing.
When the dislocation reaches the grain boundary a layer has slipped by one atomic spacing, see image on the left. These dislocations increase the ductility of a metal as the more dislocations, the more easily the atoms within the metal can move around. In substances that don't have dislocations (or where dislocations are pinned, in alloys
) the shape is much more difficult to change, as to move a layer by one atomic spacing involves moving all the atoms, which can take 1000 times more energy. Aluminum is very ductile due to the presence and movement of dislocations within its structure. Aluminum is also very malleable for the same reason, dislocations mean it can be rolled into layers as the atoms can slip past each other (with relatively low stress) to form thinner layers. The difference between malleability and ductility is that malleability is the ability to deform easily upon the application of a compressive force, and ductility is doing the same with tensile force. Being ductile and malleable broadens the scope for aluminum's uses
Ductility means cracks are broadened and blunted, they do not propagate.
Dislocations also give aluminum, and other metals, their ability to prevent the propagation of cracks. As metallic bonds are non-directional when high stress is applied in the form of a crack the dislocations (and atoms) can move (or "flow") to blunt and broaden the crack and reduce the stress. This makes metals harder to break (or tougher) than materials without dislocations, such as amorphous structures. When high stress is applied to aluminum it will blunt rather than fracture due to this process of moving dislocations, however this can be changed with the addition of other elements to aluminum to "pin" the dislocations in place. The alloys that are formed are less ductile than pure aluminum, but stronger.
Summary: Dislocations, formed by imperfections in aluminum's structure, give rise to characteristic metallic properties, such as ductility and malleability. The dislocations also prevent the propagation of cracks.
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