Welcome to You Ask Andy

John J. Holland III, age 11, of Santa Maria, Calif., for his question:

WHAT EXACTLY IS AN ALLOY?

An alloy is a mixture, usually of two or more suitable metals, and our man made alloys are far superior to the metals found in nature. For example, natural gold is soft and tends to flake apart. When alloyed with small amounts of silver, it becomes harder and more durable  without losing its golden gloss.

Perhaps the first alloy was made by accident, when bits of copper and tin melted and fused together in a long ago campfire. When the mixture cooled, it formed a new and different metal, stronger than either copper or tin. It was the brownish alloy called bronze. The Bronze Age of human history began when mankind learned to repeat this happy accident to make superior tools and weapons.

Most natural metals are too soft, too brittle or too prone to rust to stand up under hard work.  But when melted with small amounts of other materials, they form alloys with superior qualities. For example, our various steels are basically iron alloyed with carbon and magnesium.

Traces of valadium, titanium and cobalt may be added to create the super tough steel alloys used to make heavy duty machine tools. Small amounts of chromium with perhaps nickel, molybdenum and silicon may be added to the basic iron alloy to create stainless steel.

The list of possible alloys is endless, and each mixture has its own special qualities. All this is possible because each ingredient is made of its own atoms or molecules, which are all alike. These invisibly small fragments are arranged in tiny grains or crystals.

The crystals are arranged in formations, and the same pattern repeats throughout the entire structure. This holds the natural metal together, but under stress it tends to slip and dislocate along the cracks. However, when two or more metals are melted together, their crystals separate. As the alloyed mixture cools, the different crystals interlock and form more durable structures. Hence a good alloy is harder, stronger and less breakable than its separate ingredients.

Most alloys gain strength and rust resistance. However natural aluminum is rust resistant, though too soft for hard work. When alloyed with zinc, copper and magnesium it becomes stronger, though some of its rust resistance is lost.

 

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