Question

Why metals do not fracture?

Answer 1

The fracture is the separation of a body into pieces due to stress, at temperatures below the melting point. Now,the metals breaks. However, metals are characterized by high ductility, which is because the metal atoms are arranged such that it may slide over one another and thus can be stretched without breaking. In other words, the metallic bond modeled on the "sea of electrons", believes that the valence electrons of each metal can be moved freely in the crystal lattice. Thus, the metal reticulum considered consists of a set of positive ions (the nucleus surrounded by the layer of electrons) and electrons (valence), instead of being composed of neutral atoms. In summary, a metallic element is considered which is constituted by metal cations regularly distributed and immersed in an "electronic sea" of valence delocalized, acting as an electrostatic binder that holds the metal cations.

The ductile and malleable nature of the metal is allowed by the fact that the delocalized bond extends in all directions; ie is not limited to a specific orientation, as in the case of covalent network solids. When a metallic crystal deforms, not are broken localized bonds; instead, the electron sea simply adapts to the new distribution of the cations, with the energy of deformed similar to the original structure. Although ductile materials can also reach break under the proper effort, this break only happens after large deformations occur.

The fracture of the metals is classified into ductile (with deformation of the material) and brittle without deformation. All solids exhibiting a elastic limit, if it is exceeded, the material will respond to external stresses through the generation of cracks at the atomic scale and spread on the macroscopic scale, whose speed is controlled by instabilities in microscopic scales. A metal can respond mechanically to external efforts ductile or brittle manner, in a situation where both behaviors have their foundations in the elastic theory: Elasticity is the properties that have the bodies to recover its original shape by eliminating the force that deformed. If the deformation of the body completely disappears, it is said that the body is perfectly elastic and if it retains part of the deformation is said to be partially elastic (or plastic).

At low deformations, metals have an elastic behavior, but to increase the effort happens an atomic slip (dislocation) which enables absorbs energy, exhibiting plastic behavior before breaking occurs.