STRUCTURES, PROPERTIES AND PROCESSING

As discussed in previous week that differences in properties of various materials are due to the differences in the structures of materials. Different materials possess different struc­tures. The structure of a material exhibits its internal and surface details. These details can be examined and expressed in different orders of magni­fication varying from a few times to several thousands. In order of decre­asing magnification, the structure of a solid material can be expressed as follows:

(i) Atomic structure (ii) Crystal structure (iii) Microstructure (iv) Macrostructure

Detail description of these structures started in previous week. This week continues the same.

  MICROSTRUCTURE

 The appearance of the structure of a material under a microscope is called microstructure. Optical microscopes are used for magnifications upto 1000 times while electron microscopes can produce magnifications upto several thousand times. Microstructure of a material consists of phase structure and grain structure.

The phase structure is expressed in terms of various phases present, their relative amounts, distribution and alignment. Depend­ing upon the number of phases present, microstructures are either called single phase or multiphase structures.

The grain structure of a material shows shape and size of the grains (crystals) which form the bulk material. It is characterised by grain boundaries, grain shape and grain size. Typical examples of grain structures are columnar, dendritic and equiaxed grains.

 Phases in Metals: Pure metals consist of identical atoms. These atoms combine together to form crystals. Each crystal has a definite lattice structure and represents a phase. Another crystal having the same lattice structure would have the same phase. Two crystals represent different phases if their lattice struc­tures are different. The lattice structure of a crystal is expressed in terms of lattice parameters and the number of atoms per unit cell.

Metallurgically, a phase is a substance, or a portion of matter, which is homogeneous, physically distinct and mechanically separable. It is homo­geneous in the sense that its two smallest parts cannot be distinguished from one another. Physically distinct and mechanically separable means that the phase will have a definite boundary surface.

The number of phases present in a system is the number of different sub­stances that exist in it. Each substance must be chemically and structurally homogeneous within itself and physically separable by definite boundary surfaces from all dissimilar substances.

 A phase can exist in three different states depending upon the values of a set of quantities, such as, pressure, temperature, etc. These states could be either vapour, liquid or solid.

Since all gases mix with one another in all proportions, there can be only one vapour phase in a system. Two liquids may dissolve in each other to form one phase. On the other hand, if they are essentially insoluble in each other or have limited solubility, they will separate into two distinct liquid phases. In solids, each different type of crystalline substance present forms a separate phase. A solid phase will have a definite arrangement of atoms given by its lattice structure. Each different lattice structure constitutes a different phase. The lattice structure is given by lattice parameters such as a, b, c, α, β and γ  and the arrangement of atoms in the lattice.

When a phase changes its state, it is called a phase change. A phase change is accompanied by a change in pressure or temperature. The phase changes which take place in magnesium metal by changing pressure and temperature are shown in Figure 2.1. The figure shows that solid phase can change directly to vapour phase without going into liquid phase by chang­ing temperature at low pressures.

Allotropy: Many metallic elements change their arrangement of atoms and the lattice structure due to changing external conditions of pressure and temperature. Such a change of phase in solid state is called allotropy or polymorphism. For example, iron at room temperature has body centered cubic (BCC) structure with a lattice parameter of 2.866 A. When the temperature of iron is in­creased and reaches 910⁰ C, rearrangement of atoms 1n the lattice takes place giving rise to lower free-energy to this form of iron. Above this tem­perature iron will have face-centered cubic structure (FCC). Again at 1400°C, iron changes its lattice structure and becomes BCC. At 1539°C iron changes its state and becomes liquid.

Among the non-metallic elements polymorphism is found in phospho­rous (white, black and red forms) and carbon (diamond, graphite, etc.). The diamond structure of carbon is very different from the layer structure of graphite.