Alloys (Mixture of metals)
Definition
An alloy is defined as homogeneous mixture of two or more different elements.
Alloys of mercury are called as amalgams.
Properties of alloys:
1. Hard and less malleable.
2. Have lower melting points than the component elements.
3. Have low electrical conductivity.
4. Have corrosion resistance & chemical resistance.
Significance of Alloying (Importance of alloys)
1. Increases the hardness of metals.
Pure metals are soft but their alloys are hard.
(eg) Pure gold is soft. When alloyed with copper, it becomes hard.
2. Lowers the melting point of metals.
By alloying the melting point of metals can be reduced.
(e.g.) Wood’s metal (an alloy of lead, bismuth, tin and cadmium) melts at 60.5˚C. The
melting point of this alloy is much less than its constituent metals.
3. Increases the corrosion resistance of metals.
Pure metals are sensitive to corrosion. But alloying gives corrosion resistance.
(eg) Pure iron rusts easily. But stainless steel (alloy of Fe, Ni, Cr ) is less sensitive to
corrosion.
4. Modifying the chemical activity of metals.
Chemical activity of the metal can be increased or decreased by alloying.
(e.g.) Sodium amalgam is less active than sodium, but Al amalgam is more active than Al.
5. Modifying the colour of metals.
The colour of metals can be improved by alloying.
(e.g.) Brass is yellow in colour Copper (red) and Zinc (silver-white).
Functions and effect of alloying elements
Alloying improves the properties of metals. Some alloying elements in steel and their
functions are,
Metals Properties Uses
Improves- ductility, tensile strength,
Ni
toughness &corrosion resistance. Making balance wheel.
Increases - Tensile strength, corrosion Making surgical instruments,
Cr
resistance, hardness connecting rods etc.
Making grinding wheels rails
Mn Increase - strength, abrasion resistance etc. etc.
.
Increases - tensile strength, ductility and
Making axles, piston rods etc.
V abrasion resistance.
Improves - corrosion and abrasion
resistance. Making high speed tools.
Mo
Becomes strong at high temperature.
Classification of alloys
1. Ferrous alloys
2. Non – ferrous alloys
Ferrous alloys (Alloy steels)
Ferrous alloys mainly contain iron as the major constituent along with C, Al, B, Cr,
Co, Cu and Mn to improve the properties of steel.
(e.g.) Nichrome, Stainless steel and Alnico.
Nichrome
Nichrome is an alloy of Nickel and Chromium.
Composition
Nickel – 60%
Chromium – 12%
Iron – 26%
Manganese – 2%
Properties
• shows good resistance to heat and oxidation.
• Possesses high melting point.
• Possesses high electrical resistance.
• It can withstand heat up to 1000 - 1100˚C.
Uses
• Used for making resistance coils, heating elements in stoves.
• Used for making parts of boiler, gas – turbines, aero – engine valves.
• Used for making electric irons and other household electrical appliances.
Stainless steel
It is otherwise called corrosion resistant steel. It contains chromium, molybdenum
and nickel along with iron. It does not corrode due to the formation of dense film of
chromium oxide.
Types of stainless steel
i) Heat treatable stainless steel
ii) Non – heat treatable stainless steel
i. Heat treatable stainless steel
Composition:
• 1 .2% carbon
• 12 – 16% chromium.
Properties:
• It is water and weather resistant.
• It can be used up to 800 ˚c.
• They are magnetic, tough& can be used in cold condition.
Uses:
• It is used in making blades, scissors and surgical instruments.
ii. Non – heat treatable stainless steel
They possess less strength at high temperatures. They are more resistant to corrosion.
a. Magnetic stainless steel
b. Non – magnetic stainless steel
a. Magnetic stainless steel
Composition:
It contains 0.35% carbon and 12 – 22% chromium.
Properties:
• It is more resistant to corrosion than heat treatable stainless steel.
• It can be forged, rolled and machined.
Uses:
• It is used in chemical equipments and automobile parts.
b. Non – magnetic stainless steel
Composition:
0.15% carbon, 18 – 26% chromium and 8 – 21% nickel.
Properties:
• Resistant to corrosion and is increased by adding a little quantity of Mo.
• Resistance to oxidation.
Uses:
• It is used in making sinks, utensils, dental and surgical instruments.
18/8 STAINLESS STEEL
It is also known as 302 or 304 grade stainless steel. This grade of stainless is generally
regarded as one of the “workhorses” in stainless steel as it is widely available and cheap.
Composition: 18% chromium and 8% nickel and the rest is iron.
Properties
• Provides a great finish when electro polished.
• It is usually soft and prone to surface damage.
• Electro polishing damages its reflectivity and scratch the surface.
• It can be hardened by only cold working and isn’t magnetic.
• Susceptible to corrosion in chloride solutions.
Application
• Used in kitchen and food applications.
• It is also used in buildings, decor, and site furnishings.
