ALLOTROPY OF METALS

          The existence of a given metal in two or more stable but different crystal structures is known as allotropy. The essence of allotropic transformations is that the atoms of a crystalline solid are converted from one crystalline form to another, i.e., then form a new crystal lattice. Modifications, stable at lower temperatures, are designated by the Greek letter α (alpha); β (beta) designate second form of the same material that is stable at some higher temperatures: γ (gamma) at still higher temperature. 

          Allotropy of iron is of special importance. Pure iron is relatively soft and ductile and its melting point is 1539°C . Iron exist in two allotropic forms: α-iron with a body centred cubic lattice, stable at temperature up to 910°c and γ-iron, with a face-centred cubic lattice, stable in the range from 910°c to 1400°c.

          In the case of oxygen, there are two forms: 'normal' dioxygen (O2) and ozone, or trioxygen (O3). These two allotropes have different molecular configuations. More commonly, allotropy occurs because of different crystal structures in the solid, and is particularly prevalent in groups 14, 15, and 16 of the periodic table.

          In some cases, the allotropes are stable over a temperature range, with a definite transition point at which one changes into the other. For instance, Tin has two allotropes:  White (metallic) Tin is stable above 13.2°C and grey (non-metallic) Tin is stable below 13.2°C. This form of allotropy is called Enantiotropy.

          Carbon also has two allotropes - Diamond and Graphite - although graphite is the stable form at all temperatures. This form of allotropy, in which there is no transition temperature at which the two are in equilibrium, is called monotropy.

Allotropy of carbon are shown below:

Properties of alloys

(1) Thermal and electrical conductivities :

These properties of a solid solution are less than those of the pure metals.

• According to mathieessens's rule when small quantities of an alloying element are added in solid solution to a metal the increase in resistance does not depend upon the temperature.

For the mixture of insoluble phase, thermal and electrical resistance follow the law of mixture.

(2) Density:

Density is increased by heavier metal in solid solution and is decreased by a lighter metal.

In case of interstitial solid solution, there is little effect on density by the added element.

(3) Specific heat and co-efficient of thermal expansion:

These properties of alloys are governed by the law of mixture.

(4) Melting point:

The greater the difference in valencies between the metal of alloys the wider is the melting range.

(5) Boiling point:

The boiling point (like melting point) is also converted into a range by the addition of alloying element.

Effect of grain size on properties of metals

          A grain is a crystal with almost any external shape, but with an internal atomic structure based on the space lattice with which it was born.

          The mechanical properties of metals that obtain maximum strength depends upon the arrangement of the grains, their shape and especially their size. Grain size directly controls the extent of slip interference by adjacent grains and strength, toughness, ductility and fatigue of metals thus affected. The grain size is controlled by several factors out of which temperature and time of heating are important.

The important effects of grain size are given below:

1. Fine grained structure has higher strength than coarse grained structure.
2. Fine grain provides better resistance to cracking, and better machine finish.
3. Coarse grains make surface rough and make metal less tough.
4. Coarse grained metal is difficult to polish.
5. Coarse grained structure has a better workability than fine-grained structure.
6. Coarse grained materials at high temperature exhibit better creep resistance than grained one.