As the crystals form, their structures grow in a uniform pattern in all directions. As the metal cools, these crystals meet newly developing crystals forming grains. The line of intersection between grains is called a grain boundary. These grain boundaries are oriented in a variety of directions since the individual grains all formed independently from one another. These newly formed crystalline structures are held together by the electromagnetic force between the atoms.
If a load is applied to a metal it will cause the metal to deform first by elastic deformation and then, if enough force is applied, by plastic deformation. The strength of the electromagnetic force between atoms determines the yield strength as well as the ultimate tensile strength of the material.
Alloying elements help make metals stronger and more resistant to deformation by strengthening their crystal structures. Adding alloying elements – other metals or non-metallic elements – causes the crystal structure to be rearranged, resulting in increased strength. Iron is a good example since in its unalloyed form it is not as strong as most plastics! By alloying with carbon (C) and manganese (Mn) we make steel, however, which is inherently stronger than iron. And we can heat treat steel to make it even stronger still. This is the secret to making a metal a useful engineering material.
Quick Facts (courtesy of Wikipedia, www.wikipedia.com):
Crystal structure - A unique arrangement of atoms in a metal. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice. Motifs are located upon the points of a lattice, which is an array of points repeating periodically in three dimensions. The points can be thought of as forming identical tiny boxes called unit cells that fill the space of the lattice. The lengths of the edges of a unit cell and the angles between them are called the lattice parameters. A crystal's structure and symmetry play a role in determining many of its properties, such as cleavage, electronic band structure and optical properties.
Elastic deformation - The type of deformation that is reversible. Once the forces are no longer applied, the object returns to its original shape
Electromagnetic force - The force that the electromagnetic field exerts on electrically charged particles. It is the electromagnetic force that holds electrons and protons together in atoms and atoms together to make molecules. The electromagnetic force operates via the exchange of messenger particles called photons and virtual photons. The exchange of messenger particles between bodies acts to create the perceptual force whereby instead of just pushing or pulling particles apart, the exchange changes the character of the particles that swap them.
Grain boundary - The interface between two grains in a polycrystalline material. Grain boundaries disrupt the motion of dislocations through a material, so reducing crystallite size is a common way to improve strength, as described by the Hall-Petch relationship. Since grain boundaries are defects in the crystal structure, they tend to decrease the electrical and thermal conductivity of the material. The high interfacial energy and relatively weak bonding in most grain boundaries often makes them preferred sites for the onset of corrosion and for the precipitation of new phases from the solid. They are also important to many of the mechanisms of creep.
Plastic deformation - The type of deformation that is not reversible. An object in the plastic deformation range will first have undergone elastic deformation.
Ultimate tensile strength - The maximum stress a material can withstand when subjected to tension (as opposed to compression or shearing). It is the maximum value on the stress-strain curve at which a material breaks or permanently deforms. Tensile strength is an intensive property and, consequently, does not depend on the size of the test specimen. However, it is dependent on the preparation of the specimen and the temperature of the test environment and material. Tensile strength, along with elastic modulus and corrosion resistance, is an important parameter of engineering materials that are used in structures and mechanical devices.
Yield strength (or yield point) - The stress at which a material begins to deform plastically. Prior to the yield point, the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible.