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Properties of Alloys

Alloys around us are mainly classified into two major categories, ferrous and non-ferrous. When we consider the ferrous alloys then we further classify according to the low alloy and high alloy categories. As far the non-ferrous alloys are concerned most of them are combinations of copper, aluminium, magnesium or stannous based.

Each of these alloys whether ferrous or non-ferrous have specific set of properties and we will discuss these properties and their uses in this section.

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Properties of Alloys and Their Uses

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We will segregate the alloys into ferrous and non-ferrous categories and then detail the specific set of properties.

Ferrous alloys:

The ferrous alloys are categorised as cast iron and steels. Steel can be of high alloy grade or low alloy grade. The high alloy grade comprise of stainless steel or tool steel. The low alloy steel can be of low carbon, medium carbon or high carbon grades.

The plain carbon steels usually have low alloying content materials and minimum amount of Mn. The most abundant form of steel is the low carbon steel which cost very cheap in production and huge production levels. These low carbon steel are good in wielding work and works well in machine utility.

Properties and applications of low carbon are as follows:

 Tensile strength   Yield strength 
 Uses low carbon steel 
 325 MPa  180 MPa  28 % (elongation) EL   Panels of automobiles, nails and wires 
 380 MPa  210 MPa  25 % EL  Structural and sheet steel pipes
 400 MPa  220 MPa  23 % EL
 Used mainly in structural parts of buildings and bridges 
 485 MPa  260 MPa  21 % EL  Pressure containers at low temperatures
 435 MPa  290 MPa  21 % EL  Bolted and riveted materials
 655 MPa  552 MPa  15  % EL  Railway carriage and heavy duty vehicles body frames

Medium carbon properties and applications:

In these ferrous alloys the carbon content is about 0.3 to 0.6 %. These alloys can be tempered with, heat treated as well as quenched. These alloys are mostly used in its tempered condition. The medium carbon steel alloys have low hardening characteristics.

The doping with Chromium, nickel, and molybdenum makes these low carbon steel alloys capable of heat treatment which makes them highly ductile and has greater strength.

The use of these medium carbon steel has huge applications in the field of railway wheels, various gears used in vehicles and railway tracks along with the crankshafts of geared vehicles.

The characteristic properties and uses of high carbon steel are as follows:
The carbon content in high carbon steel is around 0.6 to 1.4 %.

The high carbon steel has larger percentage of carbon which makes these quite hard and provides the strength as well. These are considered to be very least ductile and are very hard.

These type of steels are used in hardened condition and moreover, these are used in tempered form more than any other.

Doping with chromium, vanadium, etc are carried out for alloying to form the carbides which eventually gives the specific hard characteristics.

These type of steel are used for making tools and also for making dies as its hardness plays a big role and work against wear and tear process. 

The various effects of providing doping effect of elements on steel are:
  • Manganese provides strength and greater hardness but at the same time decrease the level of ductility and makes it unwieldable
  • Sulphur doping reduces the ductility and provides better impact toughness
  • The enhanced level of phosphorus helps in increasing the strength as well as hardness and reduce the overall ductile property. This element helps in enhancing the impact toughness.
  • The doping with silicon helps in the role as deoxidiser during the steel production but is quite detrimental in overall surface quality of the steel
  • The use of copper as doping material for steel used in hot riveting processes but at the same time helps in resisting corrosion
  • The use of nickel in steel helps in increasing the overall strength of these steel grades and specific hardenability
  • The doping with molybdenum helps in enhancing the hardenability as well as increase the creep resistance

The cast iron properties include low melting point and low shrinkage level. Cast irons have about 2.1 to 4.5 % carbon and 1 to 3 % silicon which makes it a very efficient casting material. This also helps in showing good fluidity.

The various types of cast iron we get to see around us are grey, malleable, graphite compatible and nodular forms.

Non-ferrous alloys:

The other major group of alloys are the non-ferrous types. This group includes alloys of copper, aluminium, noble metals, titanium alloys, and magnesium alloys.

The copper alloys include the brass, bronze, or even copper beryllium precipitates used mainly for landing gears and brushes.

The brass is an alloy of copper and zinc, mainly used for costumed jewellery, coins due to its corrosion resistive characteristics.

The bronze is the alloy of copper doped with stannous, aluminium, silicon and nickel. This is mainly used for making costume decorative items and specific door locks or door knobs.

The aluminium alloys has low density (2.7 g / cc) with copper, Magnesium, silicon, zinc doping makes it ideal for structural parts of aircrafts body parts and for making various packaging units.

The magnesium alloys with again very low density of 1.7 g / cc is used for making ignition materials as it has low ignition temperature.
These are used for making fuses for weapon grade missiles.

Noble metals like gold and silver along platinum are doped with copper to make daily use jewellery items mainly due to their low oxidation limits and high corrosion resistivity.

Titanium alloys with medium density of 4.5 g / cc with high tensile strength are used for space technology and various biomedical application.

Properties of Alloys Compared to Pure Metals

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Pure metals have many superior properties over other types of engineering materials such as plastics and wood due to its strength, durability and various other properties.

Only a few metals are used in practice as pure metals as when a metal is mixed with other doped parts an alloy is made and these alloys often exhibit superior properties of hardness, anti-corrosion, strength which the pure form never showed.

The individual metals have edge as far the purity is concerned and react only with the group which help in enhancing these properties.

Alloys often exhibit superior properties of hardness and rest of good characteristics which a single metal never shows. It is particularly important for recycling metallurgy to understand metal alloys and their properties as it helps in enhancing the doping percentage for even better performance.
Alloys usually exhibit metallic properties with a slight change in specific set of properties compared to the original metals.

Alloys are of four types which are little different from the pure metal types.
  • Solid solution alloy system or type I alloy where the metals are completely soluble with each other. Example: Copper nickel alloy
  • Eutectic alloys are type II alloys where the two components are insoluble in solid state and both of them maintain their individual identities.
  • Partial solubility in solid state or type III alloys where the parent is called solvent and the alloying part as solute. Mostly interstices solutes
  • Intermetallic compounds or type IV where the metallic elements combine to form a compound in definite proportion and are generally used as hardeners. Example: aluminium alloys

These properties are completely different from the pure metals as the durability and re-cyclability of pure metals is less as they undergo corrosion and end up in scrap but with the help of specific type of doping the metals turn into excellent alloys which enhances the life span and also help in recycling process as these alloys mostly work against corrosion.

 Thermo physical properties   Density
 Heat capacity
 Thermo conductivity
 Thermal expansion

 Mechanical properties  Corrosion resistance
 Abrasion resistance
 Fatigue resistance
 Creep resistance
 Chemical properties  Catalytic properties
 Magnetic properties  Electrical conductivity
 Magnetic permeability

Properties of Alloys in Steel

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The properties of alloys in steel depends upon the doping of carbon in ferrous materials:
  • Cast irons: (carbon 2.1 % and Silicon 1%) casting ability, good fluidity, low shrinkage, low melting point. 
  • Grey cast iron: (carbon 3 % and Silicon 1%) wear resistant, very good damping capacity, casting shrinkage is low.
  • Ductile iron: (composite of Magnesium and Cerium) good ductility
  • White cast iron: (carbon 2.5 % and Silicon 0.5 %) very hard and brittle, intermediate to malleable cast iron
  • Malleable cast iron: (carbon 2.3 % and Si 1 %) reasonable strength and good ductility 
  • Compact graphite iron: (carbon 3.1 % and Silicon 1.7 %) high thermal conductivity, low oxidation at high temperatures, resistance to thermal shock.
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