ALLOYS AND PHASES IN ALLOYS

  Metals in their pure form are seldom used in engineering applications. Most of the useful metallic materials are combinations of metals known as alloys. An alloy is any combination of two or more elements that results in a substance possessing metallic properties.

Elements may combine in different ways to form alloys. The components of the alloys are usually completely soluble in the liquid state. In the solid state the elements may form mechanical mixtures, solid solutions or inter­mediate phases. An alloy may consist of either a single phase, two phases or more than two phases. The element which is present in the largest pro­portion is called the base metal, and all the other elements present are called alloying elements.

 Mechanical Mixtures: Mechanical mixtures are formed when the two elements are completely insoluble in the solid state. For example, lead is essentially insoluble in iron. The alloy of lead and iron is an intimate mechanical mixture of the components where each component retains its own identity, properties and crystal structure.

 Solid Solutions: When two elements mix or dissolve in the solid state, the resulting phase is called a solid solution. If element A dissolves ten per cent (by weight) of element B, then element B is said to have a solid solubility of ten per cent in A. The base metal A is also called a solvent and the alloying element B as solute.

When a solid solution forms, atoms of the solute occupy certain places in the. lattice structure of the solvent. Depending upon the type of the places occupied by the solute atoms, solid solutions could be either substi­tutional or interstitial.

(a) Substitutional Solid Solution: A substitutional solid solution is formed when some of the atoms of solvent are replaced by the solute atoms at their normal lattice points, as shown in Figure 2.2(a). In the formation of substitutional solid solutions, an element A cannot dissolve any amount of element B, its limit (known as solid solubility limit) is determined by certain factors.

The lattice structure of a solid solution is basically that of the solvent with slight changes in lattice parameter. An expansion results, if the solute atom is larger than the solvent atom and a contraction, if the solute atom is smaller.

(b)Interstitial Solid Solution: Interstitial solid solution is formed when atoms of small atomic radii fit into the empty spaces or interstices of the lattice structure of the solvent atoms. Since the empty spaces of the lattice structure are limited in size, only atoms with atomic radii less than I angstrom are likely to form interstitial solid solu­tions.

Interstitial solid solutions normally have limited solid solubility. The well known example of this group is interstitial solid solution of carbon in iron. γ-iron can dissolve upto 2 per cent carbon at 1147°C. This inter­stitial solid solution of carbon in iron is the basis for hardening in steel. Interstitial solid solution of hydrogen in iron formed during acid pickling (cleaning), plating or welding operations with steel causes a sharp decrease in ductility of steel. This harmful phenomenon is known as hydrogen embrittlement.

STRUCTURAL METALS

  Structural metals are those that are used in load bearing metallic frames works or structures.

Knowledge and understanding of the uses, strengths, limitations, and other characteristics of structural metals is vital to properly construct and maintain any equipment, especially airframes. In aircraft maintenance and repair, even a slight deviation from design specification, or the sub­stitution of inferior materials, may result in the loss of both lives and equipment. The use of unsuitable materials can readily erase the finest craftsman­ship. The selection of the correct material for a specific repair job demands familiarity with the most common physical properties of various metals.

  Metals for aircraft structural use: Metals used in the aircraft structures and other application is both ferrous and non-ferrous including their various alloys.

The outstanding structural metals are: (i) Steels and Nickel-base alloys (ii) Aluminium alloy, (iii) Magnesium alloy and (iv) Titanium. Besides, various other non-ferrous metals like copper and its different alloys have widespread applications in aircraft.

In the following sections, ferrous and non-ferrous metals will be discussed.