Alloys

1. Gold 

   • Pure: 24K 

   • 22K: in 22 gold atoms and 2 silver+copper 

   • 18K: in 18 gold atoms 6 silver+gold 

   • Nitric acid can be used to extract gold from alloy. 

2. Brass=Cu+Zn 

3. Bronze=Cu+Sn 

4. Solder=Pt+Sn they have higher melting point (>200) compared to solder: 183 

5. Amalgam=Sn(Hg)+Zn(Hg) 

6. Steel=Iron+Carbon+other elements which decide what type of steel it is. 

Importance of Alloys   #

1. To improve hardness 

   1. Eg: gold 

2. To prevent corrosion  

   1. Eg: steel, has better tensile strength compared to pure iron. 

3.  To lower the melting point. 

   1. Eg: Solder, lead and tin have higher melting points and are difficult to work with. Used for making electrical connections.  

4. To improve electrical conductivity 

   1. Eg: Amalgams 

5. To impart luster property. 

   1. Eg: Bronze, more lustrous compared to copper. 

Types of alloys  #

1. Single phase (homogeneous)  

2. Multi phase (coalid) 

   1. Composition varies throughout. 

   2. Eg: brass plated 

3. Also classified as, on the basis of arrangement 

   1. Substitutional  

      1. The atoms of a metal in a crystal structure are replaced with some other metals.  

      2. Eg: Bronze, Brass, 18K gold 

   2. Interstitial alloys  

      1. They are formed insertion of different metals or non metals in the crystal structure of the pure metal.  

      2. Eg: all different types of steel 

Hume Rothery Rules  #

Hume Rothery rules are basic conditions for an element to dissolve in a metal forming solid solution (alloy).  

1. Atomic radii of solute and solvent must be considered. 

   1. Difference must be less than 15%  

   2. Difference must be large 

2. The solute and solvent must have similar electronegativity. 

3. The crystal structure of the metals should be same.  

4. Maximum solubility occurs when the solute and solvent have same valencies. 

Requirement for substitutional alloys 

1. Atomic radii should not be different by more than 15% 

2. Lattice structure be same.  

3. Lower valence metal becomes the solvent.  

Requirement for interstitial alloys  

1. Solute must be smaller than the pores of the solvent. 

2. The metals should have similar electronegativity.  

Some alloys #

Brass  #

1. Pb to make malleable 

2. As to make stable 

3. Sna to make stronger. 

4. Properties of brass 

   1. Brass is highly ductile. This ductility comes from the addition of zinc. Addition of zinc makes the brass porous. (Can start corrosion) 

   2. Brass is more malleable, can be easily beaten into sheets or into any desired shape, without breaking.  

   3. Brass is more resistant to corrosion compared to copper and zinc. 

   4. High tensile strength. 

   5. Anti bacterial properties. 

5. Uses of brass 

   1. Decorative and anti bacterial uses. Door knobs, lamp fittings, dish washer fittings, decorative items such as photo frames, troffees are generally made from brass. 

   2. Mechanical applications. Many parts and instruments which involve low friction can be made up of brass. Eg: plumbing tools, gears, shell casings, bearings, because brass has extended life compared to copper and zinc. 

   3. Musical instruments such as French horns, tambourine, trumpets are made of brass. 

6. Types of brass 

   1. Based on the content, brasses can be classified as 

      1. Red brass 

      2. Cartridge/260 brass/yellow brass 

      3. 330 brass: used for making tubes and poles 

      4. 360 brass: most common. Easily machined. 

      5. Navel brass: which is used in the bottom of the ship. 

7. Corrosion of brass 

   1. Brass is easily attacked by ammonia or amino compounds. 

   2. If the zinc concentration is very high it makes the brass structure more porous, resulting in leaching.  

Steel  #

1. It is a metal-nonmetal alloy made up iron and carbon mainly with minute quantities of chromium, nickel, tungsten, niobium. 

2. In steel the carbon atoms being much smaller occupy the interstitial positions. Other elements like chromium, Nb, Ni distort the regular packing of iron atoms, making the structure more dense and stronger. 

3. Steel can be mainly classified into 

   1. Low carbon steel: the carbon content is less than 0.3% by weight.  

      1. This type of carbon is relatively soft but highly ductile. This kind of steel is easily machinable and ideal for welding. Generally containers for regular domestic, automobile body parts are made up of low carbon steel.  

      2. A special category of low carbon steel is high steel-low alloy where the carbon content varies from 0.05-0.25% by weight,  but it also contains 2% manganese along with nickel and copper for additional strength and hardness.  

      3. HSLA is widely used for materials which undergo high wear and tear or are under immense pressure. Eb: bridges, bridge cables, pressure vessels. 

   2. Medium carbon steel: the carbon content is 0.3-0.6% by weight. 

      1. Relatively stronger but less ductile. Nickel, chromium, molybdenum are added to increase the hardness 

      2. Widely used in railway tracks, wheels, gears, machine parts etc. 

   3. High carbon steel: the carbon content is more than 0.6% by weight 

      1. They are hardest and the strongest type of steel. 

      2. They have no ductility. 

      3. They are highly resistant to wear and tear. 

      4. Knifes, razors, blades are often made up of high carbon steel. 

4. Based on usage steel can be classified as 

   1. Tool steels 

   2. Stainless steel