While we generally consider metals to be some of the hardest materials on the planet, there are in fact more than a few metals that are soft and easily pliable. What are the most malleable metals?

Difference: Malleability, ductility and hardness

Before we can answer that question, we need to distinguish between the three properties that best describe soft metals:

• Ductility – A material’s ability to change its shape and be drawn into wire without losing strength or breaking. We use special test equipment to measure elongation. To increase metal hardness, we can add certain alloys without decreasing the ductility.
• Malleability – The ability of a material to deform permanently under compression without rupture. Ancient cultures defined it as the ability of a metal to be beaten into thin sheets. This property measures the ability of a metal to be rolled or hammered into a sheet. It is another form of plasticity. Although there is no specific test, we usually test malleability as hardness using either Rockwell or Brinell testing methods.
• Hardness – The ability to withstand localised permanent deformation. This refers primarily to dents in the material. We can also use this term to describe a material’s resistance to deformation via:
  • Abrasion
  • Cutting
  • Penetration
  • Scratching

Types of hardness

Knowing the hardness of a metal is especially key in engineering design applications to ensure the material is suitable and will perform as needed.

There are three types of hardness:

• Indentation – This is a material’s resistance to permanent deformation due to a localised continuous load. The process involves applying a constant load to the material until an impression is formed. The Rockwell hardness scale is commonly used to measure indentation hardness.
• Rebound – This measure accounts for the material’s elastic hardness. The Leeb hardness test measures rebound hardness. A diamond-tipped hammer impacts the material and returns the energy causing the hammer to bounce. The opposite of elastic hardness is plastic (indentation) hardness, where the object or material does return to its original shape after deforming.
• Scratch – This measure is the material’s ability to resist scratches and abrasion on its surface. The Mohs Scale measures scratch hardness on a scale of 1 to 10, with talc being a value of 1 and diamonds sitting at 10.

The importance of temperature

While several factors may impact the malleability of metal, the most significant are:

1. Temperature
2. The strength of the metallic bond

As the temperature increases, the valence electrons of the metal gain the necessary energy to move. This increases the vibration of the atoms, resulting in more collisions with drifting electrons. As the temperature rises, the distance between atoms increases, decreasing the strength of the material.

Yield stress also decreases with increased temperature, which in turn makes the forming processes easier to perform. We advise caution as repeated stress can cause some materials such as brass, copper, iron, silver, and steel to become brittle.

The most malleable metals

Malleability in metals is a tricky topic. As we mentioned earlier, there is no subjective test out there for measuring this property. More often than not, we test it as hardness.

Gold and silver are the most malleable and ductile metals. In their pure states, gold and silver are too soft for making objects that will retain their shape.
Interesting fact – we can draw an ounce of gold into a wire over 40 miles long!

The most malleable metals are as follows from least to most malleable:

• Tin (Sn)
• Copper (Cu)
• Aluminium (Al)
• Silver (Ag)
• Gold (Au) – The most malleable metal. We can roll gold into sheets thin enough to transmit light.