The thin, pointed crystals are alloys of copper and aluminium. The outer part was cooled quickly and few crystals were formed. An intermetallic compound, such as this example (CuAl2), is a constituent of alloys, formed when atoms of two metals combine in certain proportions to form crystals with a different structure from that of either of the original metals. The crystal structure of alloys affects their physical strength and suitability for various mechanical purposes. The crystal size and shape, and the mechanical and thermal treatment, all help to determine the utility of a particular kind of alloy.
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