Ductility
Ductility is a key property of metals that allows them to deform under tensile stress, meaning they can be stretched or pulled without breaking. This characteristic is crucial in manufacturing processes, such as wire production, where metal bars are elongated by being drawn through progressively smaller dies. Ductility is closely related to malleability, which refers to a material's ability to deform under compressive forces. Both properties fall under the broader concept of plasticity, which describes how materials can change shape without fracturing, influenced by temperature.
The ductility of metals is deeply tied to their electronic structure, particularly the nature of metallic bonding. In metals, atoms share valence electrons in a non-directional manner, creating a "sea" of electrons that allows atoms to slide past one another easily. This shared electron structure contributes to metals' ability to withstand deformation. Interestingly, while most metals exhibit ductility, non-metallic materials like certain polymers can also demonstrate plasticity, albeit through different mechanisms. Understanding ductility is essential for applications in engineering and materials science, as it informs the selection of materials for various structural and manufacturing applications.
Ductility
FIELDS OF STUDY: Metallurgy
ABSTRACT
The property of ductility is discussed, and its nature as an intensive property of metals is described. Ductility of metals results directly from the nature of their electronic structure and the physical size of their atoms.
The Nature of Ductility
Ductility is an extremely important property of certain materials, especially metals. The ductility of a metal permits it to undergo deformation via tensile stress (pulling or stretching) without fracture. A common application of this property is in the production of wires and filaments. In this process, a metal bar is subjected to tensile stress as it is pulled through a die that is slightly smaller in diameter than the bar. This extends the length of the bar as it reduces its diameter. The process is repeated with progressively smaller dies until the original rod has been extended so many times that its diameter is reduced to a mere fraction of its original size.
Ductility is usually exploited by applying tensile stress over a slightly extended period of time. A related property, malleability, refers to the ability of a material to undergo compressive force, such as occurs in stamping and hammering, without fracturing. Malleable metals are generally shaped through the instantaneous application of compressive force.
Ductility vs. Malleability
Both ductility and malleability are aspects of plasticity, which is a temperature-related property of many solid materials, not just of metals. The plasticity of a material increases as temperature increases, until the material reaches its melting point, at which point the material becomes fluid rather than plastic and will not retain its shape as a solid. Ironworkers and steelworkers typically heat metal until it glows with a bright orange color. This increases the plasticity of the metal and makes it easier to shape by the application of force. Plasticity is considered to be an intensive property because neither the ductility nor the malleability of a material depends on how much of it is present.
Nonmetal materials that exhibit plasticity include certain polymers, which were termed "plastics" for that very reason. Polymers that become softer when heated are called "thermoplastic" to indicate that their plasticity increases with heating. Like metals, thermoplastics also become fluid at a certain temperature. Generally, plastics are ductile but not malleable: they can be stretched and bent fairly easily and maintain their structural integrity, but they will fracture into numerous pieces when force is applied too quickly. A temperature range known as the glass-transition temperature marks the dividing line between glass-like behavior and malleability. Metals have a similar temperature range for malleability. For example, it has been demonstrated that the metal used to construct the RMS Titanic failed on impact with an iceberg because exposure to the frigid water of the northern Atlantic Ocean lowered its temperature to the point where it lost its malleability.
Ductility and the Electronic Structure of Metals
According to the modern theory of atomic structure, electrons in atoms occupy specific regions of space about the nucleus called orbitals. While the outer electron orbitals of different atoms in a molecule frequently overlap to some degree, combining to form shared molecular orbitals so that the valence electrons of each atom are no longer solely confined to their "parent" atom, this effect is much more pronounced in metals, especially pure metals.
In a metal sample, the orbitals of the individual atoms are able to effectively overlap each other, allowing the valence electrons to be shared by all atoms in the sample and move relatively freely throughout the material. It is a requirement of modern atomic theory that when a number of atomic orbitals overlap, or combine, an equal number of hybrid or molecular orbitals are produced; in a way, this makes the entire body of the metal sample analogous to a single metal "molecule" across which the valence electrons can range. This phenomenon is known as metallic bonding and is characterized by a strong attractive force between the positively charged atomic cores and the negatively charged "sea" of delocalized valence electrons surrounding them. The electrostatic repulsion between the electrons causes them to distribute more or less uniformly throughout the metal so that each atomic core is surrounded by electrons in all directions, resulting in a generalized nondirectional force holding the atoms together. This is an extremely simplified model of metallic bonding, but it serves as a useful introduction to the concept.
The nature of metallic bonding contributes to the characteristic ductility and malleability of metals. The nondirectional attraction and the lack of any strong localized bonds to break make metallic bonds more resilient to applied force than covalent bonds, while the sharing of valence electrons allows the atoms to be packed more closely together, enabling them to slide against each other more easily to produce deformation instead of a fracture. This is especially true when all atoms are of the same size, which is why pure metals are typically more ductile and malleable than alloys.
PRINCIPAL TERMS
- deformation: any permanent change in the shape of an object as a result of the application of force or a change in temperature.
- fracture: a dislocation in the internal structure of an object that causes it to break into two or more pieces.
- intensive properties: the properties of a substance that do not depend on the amount of the substance present, such as density, hardness, and melting and boiling point.
- malleability: the ability of a solid material to be deformed by the application of compressive (pushing) force, such as hammering, without breaking or fracturing.
- metallic bond: a type of chemical bond formed by the sharing of delocalized electrons between a number of metal atoms.
- plasticity: the ability of a material to undergo deformation without breaking.
- tensile stress: a force that acts to pull or stretch a material.
Bibliography
Askeland, Donald R. The Science and Engineering of Materials. 3rd ed. New York: Chapman, 1996. Print.
Douglas, Bodie Eugene, Darl Hamilton McDaniel, and John J. Alexander. Concepts and Models of Inorganic Chemistry. 3rd ed. New York: Wiley, 1994. Print.
Fenichell, Stephen. Plastic: The Making of a Synthetic Century. New York: Harper, 1996. Print.
Jones, Mark M., et al. Chemistry and Society. 5th ed. Philadelphia: Saunders Coll., 1987. Print.
Miessler, Gary L., Paul J. Fischer, and Donald A. Tarr. Inorganic Chemistry. 5th ed. Boston: Pearson, 2014. Print.