Alloying

Only a few elements are widely used commercially in their pure form. Generally, other elements are present to produce greater strength, to improve corrosion resistance, or simply as impurities left over from the refining process. The addition of other elements into a metal is called alloying and the resulting metal is called an alloy. Even if the added elements are nonmetals, alloys may still have metallic properties.

Copper alloys were produced very early in our history. Bronze, an alloy of copper and tin, was the first alloy known. It was easy to produce by simply adding tin to molten copper. Tools and weapons made of this alloy were stronger than pure copper ones. The typical alloying elements in some common metals are presented in the table below.

The properties of alloys can be manipulated by varying composition. For example steel formed from iron and carbon can vary substantially in hardness depending on the amount of carbon added and the way in which it was processed.

When a second element is added, two basically different structural changes are possible:

1. Solid solution strengthening occurs when the atoms of the new element form a solid solution with the original element, but there is still only one phase. Recall that the term ‘phase’ refers to that region of space occupied by a physically homogeneous material.
2. The atoms of the new elements form a new second phase. The entire microstructure may change to this new phase or two phases may be present.

Solid Solution Strengthening
Solid solution strengthening involves the addition of other metallic elements that will dissolve in the parent lattice and cause distortions because of the difference in atom size between the parent metal and the solute metal. Recall from the section on crystal point defects that it is possible to have substitutional impurity atoms, and interstitial impurity atoms. A substitutional impurity atom is an atom of a different type than the bulk atoms, which has replaced one of the bulk atoms in the lattice. Substitutional impurity atoms are usually close in size (within approximately 15%) to the bulk atom. Interstitial impurity atoms are much smaller than the atoms in the bulk matrix. Interstitial impurity atoms fit into the open space between the bulk atoms of the lattice structure.

Since the impurity atoms are smaller or larger than the surrounding atoms they introduce tensile or compressive lattice strains. They disrupt the regular arrangement of ions and make it more difficult for the layers to slide over each other. This makes the alloy stronger and less ductile than the pure metal. For example, an alloy of 30% nickel raises the cast tensile strength of copper from 25,000 PSI to 55,000 PSI.

Multiphase Metals
Still another method of strengthening the metal is adding elements that have no or partial solubility in the parent metal. This will result in the appearance of a second phase distributed throughout the crystal or between crystals. These secondary phases can raise or reduce the strength of an alloy. For example, the addition of tin, zinc, or aluminum to copper will result in an alloy with increased strength, but alloying with lead or bismuth with result in a lower strength alloy. The properties of a polyphase (two of more phase) material depend on the nature, amount, size, shape, distribution, and orientation of the phases. Greek letters are commonly used to distinguish the different solid phases in a given alloy.

Phases can be seen on a microscopic scale with an optical microscope after the surface has been properly polished and etched. Below is a micrograph take at 125x of lead-tin alloy composed of two phases. The light colored regions are a tin-rich phase and the dark colored regions are a lead-rich phase.