Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 10M

Year: May 2014

Alloy is a material that's made up of at least two different chemical elements, one of which is a metal. The most important metallic component of an alloy (often representing 90 percent or more of the material) is called the main metal, the parent metal, or the base metal. The other components of an alloy (which are called alloying agents) can be either metals or nonmetals and they're present in much smaller quantities (sometimes less than 1 percent of the total). Although an alloy can sometimes be a compound (the elements it's made from are chemically bonded together), it's usually a solid solution (atoms of the elements are simply intermixed, like salt mixed with water)

Classification of alloys

Steel: - One of the most common categories of alloy is steel, which is extremely malleable. Steel is made when iron, coal and limestone are heated and mixed together and then either refined or mixed with even more metals to create a less brittle product, while retaining malleability. Steel can be categorized into three groups.

• Carbon steel
• Alloy steel
• Stainless steel

Copper Alloys: -Copper is widely used for electricity and thermal conduction, though it is extremely soft in its purest form, so alloys are created to help it retain its shape. Nearly 400 different types of copper alloys are used commercially. One of the oldest types of copper alloys is bronze, which is made by adding tin (in most cases), as well as phosphorus, lead, silicon, nickel, and aluminum in varying concentrations

Aluminum Alloys: -Pure aluminum is soft, resists corrosion, conducts electricity well, so it is great for products like kitchen foil, but for applications requiring a stronger metal, an aluminum alloy needs to be created. Aluminum alloys are classified based on the other type of metal in the mix (zinc, copper, silicon, manganese, lithium or magnesium) and whether the finished product is wrought, which means it was forged and hammered into shape, or cast, which means it, was melted and poured into a shape before cooling

Hume-Rothary conditions of formation of solid solutions:

The Hume-Rothery rules, named after William Hume-Rothery, are a set of basic rules that describe the conditions under which an element could dissolve in a metal, forming a solid solution. There are two sets of rules, one refers to substitutional solid solutions, and the other refers to interstitial solid solutions.

A solid solution is formed when two metals are completely soluble in liquid state and also completely soluble in solid state. In other words, when homogeneous mixtures of two or more kinds of atoms (of metals) occur in the solid state, they are known as solid solutions. The more abundant atomic form is referred as solvent and the less abundant atomic form is referred as solute.

Solid solutions are of two types. They are

(a) Substitutional solid solutions. (b) Interstitial solid solutions.

Hume Rothery rules for the formation of substitutional solid solutions

By studying a number of alloy systems, Hume Rothery formulated certain rules which govern the formation of substitutional solid solutions. These are:

a. Crystal structure factor: For complete solid solubility, the two elements should have the same type of crystal structure i.e., both elements should have FCC, BCC or HCP structure.

b. Relative size factor: As the size (atomic radii) difference between two elements increases, the solid solubility becomes more restricted. For extensive solid solubility the difference in atomic radii of two elements should be less than about 15 percent. If the relative size factor is more than 15 percent, solid solubility is limited. For example, both silver and lead have FCC structure and the relative size factor is about 20 percent. Therefore the solubility of lead in solid silver is about 1.5 percent and the solubility of silver in solid lead is about 0.1 percent.

$$ \% \text{difference} = \bigg(\frac{r_{solute}-r_{solvent}}{r_{solvent}}\bigg) × 100 \% ≤ 15\% $$

c. Chemical affinity factor: Solid solubility is favoured when the two metalshave lesser chemical affinity. If the chemical affinity of the two metals is greater, then greater is the tendency towards compound formation. Generally, if the two metals are separated in the periodic table widely then they possessgreater chemical affinity and are very likely to form some type of compoundinstead of solid solution.

d. Relative valence factor: It is found that a metal of lower valence tends todissolve more of a metal of higher valence than vice versa. For example in aluminium-nickel alloy system, nickel (lower valance) dissolves 5 percent aluminium but aluminium (higher valence) dissolves only 0.04 percent nickel.