In this article we will discuss about the atomic, crystal and metallic structure of metals with the help of diagrams.
Atomic Structure of Metals:
Most of the material properties are dependent on its atomic structure. The changes in atomic structure do not influence the surface appearance and as such no judgement about material properties can be made by seeing the surface appearance. For definitive comparison, microscopic and X- ray examination are used to show the relationships between structure and properties of materials.
All materials are made up of atoms. An atom of an element is the smallest particle that retains the physical characteristics of that element. (Iron atom has a diameter of 1.24 x 10-10 m). Matter is a collection or agglomeration of atoms whose position and behaviour determines its properties and characteristics.
In metals the atoms occupy fixed positions and strong bonds exist between them. A degree of order (lattice structure) exists in the atomic arrangement in metals. The atoms are located at sites which are decided by the structure of the atoms and the nature of the bonds.
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The combination of the properties of the atoms and the way in which they are assembled into collections determines the essential characteristics of a material. The mechanical properties of a particular metal are related to the patterns found in its lattice structure. In iron, the atoms are arranged in regular lines, in layers throughout the thickness (long-range order) (Refer Fig. 1.1), called crystalline structure.
Some non-metals like common salt, graphite also have a crystalline structure, but most non-metals are not crystalline. In non-metals, there is no recognisable repeated grouping, but molecules are distributed in random manner as in the case of glass. Materials which do not show long range order are often termed non-crystalline or amorphous.
The ability of an atom to bond with other atom depends on the number of electrons in the outermost shell (valence electrons) which varies from one to eight. If there are less than eight electrons in outermost shell, it is incomplete and contains vacant sites.
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In most non-metallic compounds (as with plastics) atoms are joined together by covalent bonding (sharing of valence electrons). Two or more atoms bonded together form a molecule. More than one atoms can combine to form a molecule having completely different characteristics than the constituent elements.
Some atoms can enter into a different type of bond which relies on their ability to gain or lose electrons. An atom after losing/gaining electron acquires positive/negative charge and is called ion. Ionic bond is established by attraction between electropositive and electronegative ions.
Crystal Structure of Metals:
When a metal is melted, metallic atoms detach themselves from other metallic atoms and vibrate rapidly in random directions. For solidification of metal from liquid stage, all that is required it that a sufficient number of atoms may exist in the proper arrangement so that a crystal may grow.
Various crystals or grains grow and join in a dendritic columnar structure. In fact growth is in a three-dimensional pattern so that atoms exist in straight lines and in planes. Fig. 1.2 shows schematically the way unit cells formed at places grow in all the three directions to produce a crystal, or grain. A boundary, known as a ‘grain boundary’ is formed when development of growth of crystals is stopped by interference with adjacent structures, (Refer Fig. 1.3).
Metallic Structure of Metals:
Metallic bond exists between a large numbers of atoms in close proximity. In a piece of metal, the valence electrons of all the atoms are shared mutually in a complex system of orbitals. Metallic structure can be visualised as comprising metallic ions occupying fixed positions in a cloud of electrons which are moving along a number of limited paths.
The interaction between a large number of atoms leads to the formation of a three-dimensional lattice structure which is a characteristic of all metallic materials. Metal atoms in a lattice may occupy the corners of a cubic space or hexagonal arrangement. In hexagonal arrangement there is less space between the atoms and it is thus also called close packed. These two unit cells (cubic and hexagonal) feature in most of the lattice structures found in metals.
The shape and dimensions of the lattice structure in a metal play an important role in the control of mechanical properties. X-ray diffraction studies show that the lattice parameter of the cubic unit cell (i.e. the length of one of its sides) in case of iron metal is 2.86 x 10-10 m.
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Space Lattices:
(i) Body-Centred Cubic (b.c.c.) Lattice:
There are atoms at each corner of cube and one in the centre of body of cube. This arrangement is found in case of iron.
(ii) Face-Centred Cubic Lattice (f.c.c.):
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There are atoms at each corner of cube and additionally atoms at the centre of each face of the cube. Aluminium, copper, nickel and most ductile metals have f.c.c. structure.
(iii) Closed-Packed Hexagonal (c.p.h.) Lattice:
There are three layers in each of such non-symmetrical cell. Top and bottom layers consist of six atoms in a hexagon with one atom at the centre. The middle layer has three atoms in the form of a triangle. Least ductile materials like zinc and magnesium have this structure.
Imperfection in Lattice Structure of Metals:
Theoretically above lattices should be found uniformly throughout the length, breadth and depth of metal. In practice, however, due to some interruption in the growth of the crystal from molten metal or due to inclusion of atom of another metal, imperfections do occur. Imperfection may be in the form of dislocation, or a vacancy in the crystal lattice.
Dislocation refers to a break in the continuity of the lattice. In edge dislocation, one plane of atoms gets squeezed out. A plane of atoms comes to a stop and at this point the two neighbouring planes move closer until they are at the correct interatomic spacing [Refer Fig. 1.5 (a)].
Dislocations are an essential element in the forming of metals because they allow plastic deformation to take place at achievable stress level. Vacancy is another imperfection which occurs due to an atom not taking up the prescribed site [Refer Fig. 1.5 (b)]. It plays an important role in the diffusion or movement of atoms, through the lattice.
Another crystal-lattice imperfection is interstitials, i.e. extra atoms that fit into the interstices between the normal atom structure. Such a structure exists when carbon atoms (much smaller) are introduced in iron atoms and they squeeze into the structure.
Elastic and Plastic Deformation:
Below elastic limit, the crystal structure yields by a small amount temporarily and recovers when load is removed. But beyond this limit, the atomic structure slips along certain crystal planes called slip planes. Metals having few slip planes and few directions of slip (like HCP) are difficult to form.
It may be noted that slip systems are 3 in HCP, these are 12 and 48 respectively in FCC and BCC metals. However HCP metals experience deformation by twinning by plastic deformation. All the atoms in the twinned region move a given amount and change orientation.
Strain Hardening:
External force causes slip to occur in the atomic structure at the points of imperfection or dislocation and twinning. With increase in external force, more crystals deform, and more dislocations occur along the slip planes. As more and more dislocations are forced to intersect on various slip planes, strong interactions start occurring offering resistance to further deformation (strain hardening) and metal becomes harder and stronger.
Fracture:
Every solid material resists stress up to a point but finally fractures suddenly. Brittle fracture is characterised by the small amount of work absorbed and by a crystalline appearance on the surfaces of fracture. The dislocation defects make the metals vulnerable to crack formation.
On application of high load, if no slip is taking place, then atomic bonds are subjected to great stress. Beyond their design limit, a few grains rupture, forming a tiny crack. Once a crack is started, it propagates at a fast rate leading to rupture.