Published: 1 March, 2006

Easy dislocation motion in gold makes it softer than iron

Why is gold such a soft material compared to something like iron?

Most metals are crystalline. That is, they are made up of atoms that are arranged in a structure that repeats itself in three dimensions. You can think of making a crystal by arranging atoms in a plane, then stacking up identical planes. Of course, in a crystal, every atom is bonded not only with neighboring atoms in the same plane as itself, but also with neighboring atoms in the planes above and below it. As it turns out the way that metals can bend or dent is for the planes to slide over each other. This would be nearly impossible if we had to break every bond between two layers all at once—plus we would end up with a broken piece of metal! Thankfully, we don't have to break all the bonds at once. Instead we can form a small ripple-like defect, called a dislocation, by breaking the bonds of only one row of atoms between two planes instead of all the bonds. To conceptualize this, imagine a large rug laying on a carpet, with the rug representing one layer of atoms and the carpet the other layer. To drag the rug across the carpet takes a lot of force. Instead, if we form a small ripple at one edge of the rug and then push the ripple across the rug we will find that the rug has moved across the carpet a small amount. If we want to move the rug a lot we just have to repeat this process several times. A dislocation works in much the same way, each time a dislocation moves through the metal it moves one atomic layer over another by one row of atoms. Thus, we may conclude that the more difficult it is to form and move dislocations the harder (or stronger) the metal will be.

So back to the question: why is gold softer than iron? Based on our above conclusion, gold is softer because dislocations move more freely in gold than in iron. The motion of dislocations in gold is easier for two reasons. The first is that the atoms in the atomic planes of gold are arranged differently than the iron atoms; this difference makes dislocations easier to move in gold. The second factor is that the bonds between gold atoms are weaker than those between iron atoms—so the dislocation can break gold bonds more easily.

Our discussion so far has only considered the hardness difference between pure metals. Many other microstructural features of metals can be changed to make them harder, all of which reduce the production and/or motion of dislocations. For example, adding carbon to iron makes steel, which is strong because the carbon atoms block dislocations. Rapidly cooling a metal from a very hot temperature makes it strong because it creates internal crystal boundaries, called grain boundaries, which impede the motion of dislocations. Adding two metals together creates regions of different compositions, called phases, the boundaries of which also stop dislocations. So we can see that many things can affect the hardness or strength of a metal but they all have the same result - they stop the dislocations!