Different crystal structures of Engineering materials

This article describes the basic crystal structure of common engineering materials which are now basic building blocks of daily life. Engineering materials like Aluminium, copper, Iron, Cobalt, etc are made up of different types of crystal structures like BCC, FCC, and HCP. Each structure has its own unique arrangement and based on atomic radius and atomic distance, the different crystal structure has different density and so on for other material properties.

Importance of crystal structures

Every physical body in this world is made up of a basic entity called atoms. When these atoms arranged in a regular pattern, unique arrangement, they form a lattice structure for a crystalline solid/liquid body. The lattice structure finally determines one's physical properties.

Types of lattice structure or crystal structure

Basic of the crystal structure consists of a space lattice pattern and basis

Crystal structure = space lattice + basis

The Basis may contain one atom per lattice pattern

e.g. -
1) FCC (Space lattice) + 1 Aluminium atom at each lattice point (basis) = FCC Crsystal of Aluminium (Crystal structure)

2) BCC + 1 Iron atom at each lattice point (basis) = BCC Crystal of Iron

  • FCC = Face Centred Cubic
    e.g. Copper, Aluminium, Nickel, Pb, Ag

  • BCC = Body Centred Cubic
    e.g. W, Mo, Cr

  • HCP = Hexagonal close pack
    Mg, Zn, Ti, Cd, Zr

  • Unit Cell: The basic difference between all these patterns lies in the difference between the unit cell which is nothing but the repeatability of each pattern in lattice space.

    Crystal structures of metals FCC BCC HCP

    B.C.C (Body Centred Cubic)

    • 1 atom at the centre of the cube and one atom each at all the corners
    • No. of atoms per unit cell = Nc/8+Nf/2+Ni/2 Where Nc = No. of atoms at the centre, Nf = No. of atoms at the face and Ni = No. of atoms inside.
    • Atomic radius = 1.73*a/4, Where a = distance between two adjacent atoms
    • Atomic packing radius = 0.68
    • v, Mo, Ta, W are having BCC structure at room temperature
    • coordination no is 8
    • The nearest distance between two atoms = 1.73*a/2

    BCC Body Centred Cubic

    F.C.C (Face Centred Cubic)

    • 1 atom at each corner of the cube
    • 1 atom at the intersection of the diagonal of each of six faces of the cube
    • No. of atoms per unit cell = Nc/8 + Nf/2 + Ni/2 = Four atoms
    • Atomic radius = 1.41*a/4
    • Atomic packing factor = 0.74
    • e.g. Cu, Al, Pb, Ni, Co etc
    • Coordination number = 12
    • Nearest distance between two atoms = a/1.41
    • No. of atoms per unit cell shows the density packedness
    • So, FCC is more densely packed than BCC

    H.C.P.(Hexagonal Close Pack)

    • 1 atom at each corner of hexagon
    • 1 atom at each centre of hexagon faces
    • 1 atom at the centre connecting perpendicular s in three rhombuses
    • No. of atoms per unit cell = 6
    • Atomic radius = a/2
    • Atomic packing factor = 0.74
    • Coordination number = 12
    • e.g. Zn, Cd, Mg

    Because of allotropes of iron, it got two different lattice structures at different temperatures.

    • At room temperature crystal structure of Iron is BCC(Body-Centered Cubic)
    • At 912 °C the crystal structure changes(Face Centred Cubic) and
    • At 1,394 °C its crystal structure changes to a Body-centered cubic (BCC)

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    Author: Umesh17 Jan 2022 Member Level: Platinum   Points : 3

    When we see an item with our naked eyes, we see its macroscopic shape and structure. We recognise it from that as well as it's colour and texture. We are able to do all this analysis only in the presence of some light that falls on this item and reflects, refracts or disperses through it. With naked eyes, we can visualise all these external things about it.

    But when we want to know more about the internal structure of these materials at the microscopic level, then we have to use high-efficiency microscopes like electron microscopes to find out the structure at atomic or molecular levels. The arrangement of atoms at that elementary level tells us the crystal structure of that material. The basic crystal structure of the material which is a characteristic property decides its apparent shape, appearance and overall impact by which we say that it is such and such material. So, understanding the crystal structure and arrangement of atoms in its lattice gives us a lot of information about the interatomic distances, packing and other physical parameters related to the physical properties of that material.
    For example if we take a piece of natural limestone rock we will find that it has some crystal structure. Now we break this piece in smaller parts and then it seems to have lost that binding because of the breaking force that we applied on it. But if we take a small part then it would again look like the original material and inside it the same lattice structure would be preserved as we had visualised in the main original piece. So by dividing a material in smaller parts it is possible to reach the basic structure and photograph that for our understanding of the physics behind that. Of course thin film techniques and electron microscopy would be required to undertake such projects. In essence, learning of crystal structures gives us valuable information about the various engineering materials.

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