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Ductility – Definition and Materials

Ductility is a mechanical property of a material which measures the deformation under strain before fracture. For instance, some steel types can be deformed to more than 25% before they break. In contrast, untreated glass breaks without any significant deformations, meaning that its ductility is very low.

Exploring the concepts of ductility and malleability

Ductility is commonly understood as a material’s ability to be drawn into thin forms, such as wires. In materials science, it is more formally defined by the extent to which a material can undergo plastic deformation under tensile stress before experiencing failure. This property is pivotal in both engineering and manufacturing, dictating a material’s suitability for processes like cold working and its resilience against mechanical stress. Metals like gold and copper are often cited as ductile, with platinum being the most ductile in its pure form. However, not all metals display ductile behavior; some, like cast iron, are more prone to brittle failure. Polymers are generally categorized as ductile due to their typical allowance for plastic deformation.

Malleability, a property closely related to ductility, focuses on a material’s capacity to deform plastically under compressive stress without breaking. Historically, materials were considered malleable if they could be shaped through methods like hammering or rolling. Lead serves as a unique example, being highly malleable but lacking in ductility.

List of common ductile materials

  • Gold: Gold is known as the most malleable material on Earth. One gram of gold can be stretched to 2.4 km (1 oz to 43 miles). Beatgold, which can be rolled to a thickness of 25 µm, can only be produced because of the high ductility of gold.
  • Copper: Due to its excellent electric conductivity, copper is one of the most widely used metals for wires in the world. Thanks to its ductility, it can be stretched into long and thin threads. If copper didn’t display such high ductility, wires would break much easier from bending.
  • Steel: Depending on the type, steel can be very ductile. Martensitic steels, which display high hardness, can be subjected to tempering to increase ductility.
  • Platinum: Platinum is also a very ductile metal. Its ductility rivals that of gold.

Ductile–brittle transition

The degree of ductility correlates to temperature. The general rule is, the lower the temperature, the less ductile a material becomes. As metals are heated, they become less brittle and more ductile and malleable. They need to reach a critical temperature to be deformed easily, so that they can be drawn into wires or pressed into sheets without breaking.

The ductile brittle transition temperature (DBTT) is the temperature at which metals become brittle to a critical degree. For steel, this value is crossed at 40J for a standard Charpy impact test. The DBBT is used to determine when a material becomes more likely to shatter when put under stress. For some steels, the DBTT is around 0°C, making them unfit for applications such as shipbuilding. The most famous example of this is the RMS Titanic, which was made of steel with low DBBT. Once it collided with hard ice, the steel broke instead of bent, leading to the tragic shipwreck of 1912.

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