What are Alloys?

An alloy is an admixture of two or more metals or a metal and a non-metal. Since ancient times, man has been using alloys. Alloying helps to retain the properties of the parent matter, such as hardness, strength, toughness, ductility and is corrosion-resistant. Copper alloys such as brass have antimicrobial properties that help to fight against (Coronavirus disease 2019) COVID-19 and other hospital-acquired infections. Some common alloys are brass, bronze, solder steel, etc. Most alloys have a lower melting point than their constituents. This is due to the presence of a nonmetal as one of the constituents that behaves as an impurity and restricts the raise of melting point.

Alloys of iron

Iron is a very reactive metal, and it is not found in a free state in nature. In a combined state, iron is mined out from the earth in the following ore:

• Haematite (Fe₂O₃)
• Magnetite (Fe₃O₄)
• Limonite (Fe₂O₃.3H₂O)
• Siderite (FeCO₃)

Cast iron

Pig iron, which is an intermediate product of the iron industry, is refined in a furnace known as a cupola, with carbon content varying 2% to 5%, which is called cast iron. Since pig iron is unsuitable for casting and contains high impurities, refinement is required.

Cast iron alloy has been developed by the addition of different alloying metal elements in cast iron so as to overcome inherent drawbacks such as carbon structure, hardness, poor corrosion resistance, less brittleness, and toughness. The effects of alloying metal with other elements with cast iron are mentioned below:

• Nickel: It is the main alloying metal element in a cast iron. Its addition in cast iron improves the carbon structure of a cast iron and this leads to improvement in the physical properties and mechanical properties, such as machinability and castability of cast iron.
• Silicon: It promotes the formation of free graphite with a decrease in the combined state of carbon in cast iron. Consequently, the addition of silicon leads to softening of cast iron.
• Chromium: It acts as a carbide stabilizer and due to its hardness, strength, and corrosion resistance, it also improves the quality of cast iron.
• Copper: It promotes graphite formation, and improves the toughness, machinability, and castability of cast iron.
• Sulfur: It promotes the formation of cementite and, increases the hardness and brittleness, and decreases its fluidity.

Uses- Cylinder head, piston rings, grinding, and crushing machinery.

Wrought iron

It is the purest form of iron that contains up to 99.8% pure iron. It is unsuitable for casting, as it lacks fluidity, but has high plasticity at a high temperature, and is highly ductile and malleable. It is useful for forging and rolling into desired shapes when hot. It has high resistance to fatigue failure and its ultimate strength can be increased by shaping it below the ambient temperature that is known as cold working.

Uses- Shipbuilding, nuts and bolts, pipes for oil, water, railway couplings.

Steel

Steel is an alloy of iron that has a non-metallic element carbon content of up to 1.5% and is found in a combined form. It is produced either by extracting the carbon from cast iron or adding a small amount of carbon to wrought iron. An increase in the carbon content in steel strengthens the steel. It is an interstitial alloy metal in which a small carbon atom fits into interstices iron matrix in which the carbon content is limited up to 1.5% only.

Classification of carbon steel with the percentage of carbon

a. Dead mild steel (carbon content up to 0.15%): It is highly ductile and malleable. It is the softest among all the steels and can be forged, welded, machined easily to the desired shape.

Uses

• wire and thin sheets
• rods
• chains
• welded and solid drawn tubes

b. Mild steel (carbon content varying from 0.15% to 0.30%): It is soft, ductile, malleable, and also tough but it offers a low wear resistance. It can be forged, welded, and machined easily.

Used for making: 

• screws
• nuts and bolts
• stamping of automobile parts
• rivet and wire

c. Medium carbon steel (carbon content varying from 0.30% to 0.80%): It is harder and less ductile than mild steel. It has good tensile strength and can be welded easily because heat treatment can be given to this type of steel.

Used for making: 

• shafts and axles
• rods and tubes
• hammerhead and fitter’s square

d. High carbon steel (carbon content varying from 0.80% to 1.5%): It is the strongest among the above-mentioned steel and less ductile. It has the highest tensile strength and low machinability.

Used for making: 

• hand tools and woodworking tools
• wheels and springs
• drills and milling cutter

Alloys of steel

It is carbon steel in which one or more elements of metal are added to improve the properties of steel as a tensile strength while maintaining the ductility, toughness, corrosion resistance, and cutting ability of metal. The effects of alloying other elements with steel are:

1. Nickel: It improves the toughness, tensile strength, ductility, and corrosion resistance of steel.
2. Chromium: It improves the tensile strength, thermal resistance, magnetic properties, hardness, and toughness of steel.
3. Cobalt: It improves the tensile strength, thermal resistance, magnetic properties, hardness, and toughness of steel.
4. Manganese: It improves strength, hardness, toughness, and also minimizes the effect of sulfur on steel.
5. Copper: It improves the strength and corrosion resistance of steel.

Alloys of Aluminum

Aluminum is abundant on the earth and is not found in a free state. It is obtained from the processing of bauxite ore (Al₂O₃.H₂0 and Al₂O₃.3H₂0). Alloying of aluminum is done to improve the tensile strength and hardness of the metal. The alloying elements are nickel, copper, zinc, silicon, magnesium, and manganese. Alloys of aluminum with other elements are listed below:

Duralumin

This is an age-hardenable aluminum alloy, which consists of aluminum (93.5%), copper (4.4%), magnesium (1.5%), and manganese (0.6%) as its main constituents.

In this alloy, copper is the main hardening metal, where magnesium improves the hardness and manganese acts as a strengthening element. It is non-magnetic and has a high strength-to-weight ratio and electrical conductivity.

Uses: Piston rods, sparking chisels

Hindalium

This type of alloy is produced by the Hindustan Aluminum Corporation of India. It is manufactured in the form of sheets of 16 gauges. Instead of stainless steel, it is used for the manufacturing of utensils, as the cost of manufacturing is one-third of that of stainless steel. The main constituents of Hindalium are aluminum, silicon, manganese, and magnesium.

Magnelium

This is a lightweight alloy that has the properties of high ductility, good machinability, and can be welded easily. It has the following main constituents:

• Aluminium (95%)
• Nickel (0-1.2%)
• Copper (0-2.5%)
• Magnesium (1-5.5%)
• Manganese (0.2-0.6%)
• Tin (0-3%)

Uses Door handles and fixtures.

Alloys of Nickel

Nickel is a versatile element that combines with most of the metals and forms corrosion-resistant and heat-resistant alloy. Some common nickel-base alloys are:

Nickel-chromium alloy

This alloy has a high resistivity and resistance against corrosion. Consequently, it is used to make heating wires, resistors, electrical cords, and so on.  As the quantity of nickel decreases in an alloy, its heating resistance also decreases.

Nickel-titanium alloy

This alloy has an equal proportion of nickel and titanium and has a Shape-memory effect and super-elasticity property of a metal. This helps to make crack sense wires for civil engineering work, which helps to sense cracks and contract to heal macro cracks.

Context and Applications

The concept holds relevance in various diplomas, undergraduates, and postgraduate courses. It is mainly inculcated in degrees such as:

• Bachelor of Science in Mechanical engineering
• Bachelor of Science in Civil engineering
• Master of Science in Mechanical engineering
• Metal casting training by American foundry society

Practice Problems

Q1) Which of the following category does Mild steel belong to?

a) Medium carbon steel

b) High carbon steel

c) Low carbon steel

d) None of the above

Answer – option c.

Explanation- In mild steel, only 0.15 – 0.30% of carbon is present, which is the lowest among all in the given options.

Q2) What happens to the ultimate tensile strength of mild steel while working at a high strain rate?

a) Decreases

b) Increases

c) Remains constant

d) First increases, then decreases

Answer- option b.

Explanation - Low carbon steel is ductile and the strain rate can increase.

Q3) Why do aluminum alloys find use in the aircraft industry?

a) High strength

b) Low specific gravity

c) Good corrosion resistance

d) Good weldability

Answer- option b.

Explanation- The specific gravity of aluminum alloy is equal to one–third of steel and equal strength.

Q4) Why is copper is used to make electrical conductors?

a) Ductile

b) Resist corrosion

c) Cheap

d) Low resistance

Answer- Option d.

Explanation- Copper is a good conductor of electricity. Consequently, it provides less electrical resistance.

Q5) Brass is an alloy of which of the following?

a) Copper and zinc

b) Copper and tin

c) Copper and aluminum

d) None of the above

Answer- Option a.

Explanation- Brass is made up of copper and zinc (5-45%) metal. It is made by reduction of mixtures including both zinc and copper metal ores.

Related concepts

• Hardness and toughness
• Carbon content
• Strength
• Ore