Malleability
Malleability is a critical property of certain materials, particularly metals, that allows them to deform under compressive stress without breaking. This property is fundamentally linked to the electronic structure and atomic arrangement of metals, where overlapping electron orbitals enable valence electrons to move freely. As a result, metals can be shaped through processes like forging, stamping, and pressing, where they undergo significant physical changes without fracturing. Malleability is distinct from ductility, which pertains to a material's ability to withstand tensile stress, or stretching forces, over time.
Both malleability and ductility fall under the broader category of plasticity, a temperature-sensitive characteristic that affects various materials. For metals, plasticity increases with temperature until they reach their melting point, at which point they become fluid. Unlike metals, many polymers, which are often referred to as "plastics," exhibit ductility but lack malleability, typically fracturing under rapid force. The unique nature of metallic bonding—characterized by a delocalized "sea" of electrons—enhances the malleability of pure metals, making them more adaptable compared to alloys. Understanding these principles helps explain the behavior of metals in various applications, from manufacturing to construction.
Malleability
FIELDS OF STUDY: Metallurgy
ABSTRACT
The property of malleability is discussed, and its nature as an intensive property of metals is described. Malleability of metals results directly from the nature of their electronic structure and the physical size of their atoms.
The Nature of Malleability
Malleability is an extremely important property of certain materials, especially metals. The malleability of a metal permits it to undergo deformation via compressive stress without fracture. A prime example of this property is the forging or shaping of a metal object by hammering or stamping in a press. In this process, a metal blank is subjected to compressive stress as it is pounded by a hammer or in a die that produces a pattern such as those on coins. This alters the physical shape of the blank as well as its dimensions. The process is repeated until the original blank has been converted into the desired object. Stamping, pressing, and forging are all methods by which a malleable metal may be shaped by the application of force; the difference is the nature of the force that is applied.
Malleability is usually exploited by the instantaneous application of compressive force, such as occurs in stamping and hammering. A related property, ductility, refers to the ability of a material to undergo tensile stress (pulling or stretching) without fracturing. Tensile stress is generally applied to ductile metals over an extended period of time.
Malleability vs. Ductility
Both malleability and ductility are aspects of plasticity, which is a temperature-related property of many 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. Plasticity is considered to be an intensive property because neither the malleability nor the ductility 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.
Malleability 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 malleability and ductility 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 malleable and ductile than alloys.
PRINCIPAL TERMS
- compressive stress: a force that acts to push on or compress a material.
- deformation: any permanent change in the shape of an object as a result of the application of force or a change in temperature.
- ductility: the ability of a solid material to be deformed by the application of tensile (pulling) force, such as bending, without breaking or fracturing.
- 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.
- 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.
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.