What is Malleability in Metallurgy?
Malleability is a physical property of metals that defines the ability to be hammered, pressed, or rolled into thin sheets without breaking. In other words, it is the property of a metal to deform under compression onto a different form.
A metal's malleability can be measured by how much pressure (compressive stress) it can withstand without breaking. Differences in malleability among different metals are due to variances in their crystal structures.
Compression stress forces atoms to roll over each other into new positions without breaking their metallic bond. When a large amount of stress is put on a malleable metal, the atoms roll over each other, permanently staying in their new position.
Examples of malleable metals are:
Examples of products demonstrating malleability include gold leaf, lithium foil, and indium shot.
Malleability and Hardness
The crystal structure of harder metals, such as antimony and bismuth, makes it more difficult to press atoms into new positions without breaking. This is because the rows of atoms in the metal don't line-up. In other words, more grain boundaries exist and metals tend to fracture at grain boundaries. Grain boundaries are areas where atoms are not as strongly connected. Therefore, the more grain boundaries a metal has, the harder, more brittle and, therefore, less malleable it will be.
Malleability Versus Ductility
While malleability is the property of a metal deforming under compression, ductility is the property of a metal allowing it to stretch without damage.
Copper is an example of a metal that has both good ductility (it can be stretched into wires) and good malleability (it can also be rolled into sheets).
While most malleable metals are also ductile, the two properties can be exclusive. Lead and tin, for example, are malleable and ductile when they are cold but become increasingly brittle when temperatures start rising towards their melting points.
Most metals, however, become more malleable when heated. This is due to the effect that temperature has on the crystal grains within metals.
Controlling Crystal Grains Through Temperature
Temperature has a direct effect on the behavior of atoms, and in most metals heat results in atoms having a more regular arrangement. This reduces the number of grain boundaries, thereby, making the metal softer or more malleable.
An example of temperature's effect on metals can be seen with zinc, which is a brittle metal below 300°F (149°C). Yet when heated above this temperature, zinc can become so malleable it can be rolled into sheets.
In contrast to the effect of heat treatment, cold working (a process that involves rolling, drawing, or pressing causing plastic deformation a cold metal) tends to result in smaller grains, making the metal harder.
Beyond temperature, alloying is another common method of controlling grain sizes to make metals more workable. Brass, an alloy of copper and zinc, is harder than both individual metals because its grain structure is more resistant to compression stress attempting to forces the rows of atoms from shifting into new positions.
Chestofbooks.com. Malleability And Ductility Of Alloys.
Differencesbetween.net. Difference Between Ductility and Malleability.
Chemguide.co.uk. Metallic Structures.