Metallography

The metallography is the technique consisting in determining the structure of a Métal by observing it with a optical Microscope. One can determine thus, according to the cases:

size and the form of the Crystallite;
the distribution of the phase S;
direction of the lines of slip (intersection of the slip surfaces with surface), in the case of a deformed sample (see the articles Plastic deformation and Dislocation ).

The simple observation of a metal object does not give a particular information except the surface quality (asperities, cracks). To have information on the microstructure, a careful preparation is needed in general:

cut of the object, so that one can set up it on the microscope, but also to observe the structure inside if necessary;
Polishing careful: one seeks to observe details about the Micromètre (the physical limit due to the Diffraction of the light being of approximately 0,5 µm), it is necessary thus that the stripes are smaller; in general, one uses coated papers of ceramic particles (kinds of Sandpaper) while passing towards increasingly fine grains, then one uses a paste containing of the microscopic Diamant S; if the details are even smaller, one can resort to an electrolytic polishing (controlled dissolution of surface);
in general, an attack to reveal the structure, in particular grain boundary: one frequently uses Acide S forts or electrolytic reactions, under conditions such as the reaction is faster or stronger with the grain boundaries (because of the surface Tension).

To reveal the structure, one can also use an anodic oxidation: if the oxide coating grows in a different way according to the crystalline orientation, one can easily distinguish different crystallites. One uses also sometimes a thermal shock to cause cracks between the grains (intergranular rupture) visible with the microscope.

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