For alloying to take place, the diameters of the atoms of the parent metal and alloying agents cannot differ by more than 15 percent. Thus, the number of possible alloys is limited. Alloys are made up of regularly shaped crystals, some of which are so large that they can be seen with the naked eye. To study the crystals, scientists use microscopes, spectroscopes, and X rays. They have discovered that alloy crystals are collections of tiny grains between which there is a boundary material.
Some grains are hard and some are soft, because of the different elements mixed in the alloy.
The hard grains support loads and resist wear. The soft grains, being more pliable, permit the hard grains to move. Thus, if one attempts to bend a piece of metal that has only hard grains, it breaks. But if hard and soft grains are intermixed, the piece can be bent. In large-grained alloys, the boundaries may be continuous.