Eutectic Alloy

This is an image of a Ni-Al-Nb alloy taken with a Scanning Electron Microscope. It is a simple alloy with only three elements, however you can see that its structure is far from simple. It contains three distinct regions: the grey background, the small dark spots and the large pale needles. Each of these regions is a different atomic arrangement. 

When a liquid solidifies, the atoms clump together in one arrangement. As the solid cools new different arrangements can appear.

However, in this case, on solidification the atoms formed two arrangements at the same time; one creating the grey background and another creating the large pale needles.  At a lower temperature the atoms in the grey background rearranged themselves to form the third region (the dark spots).

In this image brighter areas contain heavier elements. The needles contain a lot of a heavy element and the dark spots have more of a lighter element. If the percentage of each of these elements in the alloy is changed, then the amounts of each region you would see in the image will differ (e.g. More or less of the pale needles).