INTERMETALLIC COMPOUNDS

INTRODUCTION - Most of the alloy system do not show complete solid solubility. When the amount of solute element is more than the limit of solid solubility, a second phase also appears apart from the primary solid solution. The second phase which forms is an intermediate phase. - It is a phase formed at intermediate composition between the two primary components (pure metals). - The crystal structure of the intermediate phase is different from the both primary components. - Some of these intermediate phases have a fixed composition and are called Intermetallic compounds

- Intermetallics are similar to alloys, but the bonding between the different types of atoms is partly ionic, leading to different properties than traditional alloys.  In general, the larger the electro negativity difference between the host atom and the impurity, the greater the tendency to form compounds and the less solubility there is - So, elements with similar electro negativities tend to form alloy, whereas elements with large electro negativity difference tend to have more ionic bonds.

- An intermetallic compound contains two or more metallic elements, producing a new phase with its own composition, crystal structure, and properties. - Intermetallic compounds are almost always very hard and brittle Intermetallics or intermetallic compounds are similar to ceramic materials in terms of their mechanical properties -  Often dispersion-strengthened alloys contain intermetallic compound as the dispersed phase

Classification 1/ Stoichiometric intermetallic - Examples compounds have a fixed composition. They are represented in the phase diagram by a vertical line - Examples Au2Pb in Au-Pb system AlSb in Al-Sb system  MoSi2 in Mo-Si system  Fe3C in Steels, Mg2Pb in Mg-Pb system MgNi2, Mg2Ni in Mg-Ni system

EX1 Aluminum-antimony phase diagram includes a stoichiometric intermetallic compound γ

EX 2 Magnesium - Lead phase diagram includes a stoichiometric intermetallic compound γ. 8

EX3

2/ Nonstoichiometric intermetallic compounds have a range of compositions and are sometimes called intermediate solid solutions - EXAMPLES  γ phase in Mo-Rh system, β’phase in brass, CuAl2 in Al-Cu system, Mg2Al3 in Al-Mg system, TiAl3 in Al-Ti system

EX1

EX 2 The Copper - Zinc Phase diagram, containing more than 30% Zn, a second phase β’ forms because of the limited solubility of zinc in copper

- Properties of some Intermetallic compoundS

- STRENGTH AND DUCTILITY  - Properties of some Intermetallic compoundS - STRENGTH AND DUCTILITY

SOME INTER METALLIC COMPOUND MATERIAL AND THEIR USES 1- Molybdenum disilicide (MoSi2) This material is used for making heating elements for high temperature furnaces  At high temperatures (1000 to 1600°C), MoSi2 shows outstanding oxidation resistance. At low temperatures (500°C and below), MoSi2 is brittle and shows catastrophic oxidation known as pesting

2- Copper Aluminide (CuAl2) Precipitation of the nonstoichiometric intermetallic copper aluminide CuAl2 causes strengthening in a number of important aluminium alloys. Precipitation hardening – by forming θ (CuAl2) phase in α matrix, gives high strength and toughness. - Properties: 1- High strength ( 505 - 520 MPa). 2- Good creep strength at high temp. 3- High toughness at cryogenic temp. 4- Good machinability -APPLICATION 1- Fuel Tanks 2- Pistons, rivets for aircraft constructions : Al2CuMg 22

3- Al-Mg-Si Alloys (Mg2Si) Mg and Si are added in balanced amount to form Mg2Si  Mg + Si (0.8-1.2%) ; Mg + Si (> 1.4%) - Properties 1- Medium-strength structural alloys ( 215 -245 Mpa). 2- Readily extruded 3- Colour anodized -Applications 1 - Car bodies 2- Electric trains 3- Structural Components 4- Satellite dish 5- Large water pipes 6- Aircraft, Automotive

4- TiAl and Ni3Al (Nickel base superalloys) -  Properties 1- TiAl and Ni3Al possess good combinations of high-temperature mechanical properties and oxidation resistance up to approximately 650 - 960°C. 2- Good Toughness and Corrosion resistance. - Applications: 1-Aircrafts, space vehicles, rocket engines 2- Industrial gas turbines 3- Nuclear reactors, submarines 4- Steam power plants, petrochemical equipment 5- Combustion Engine Exhaust Valves 6-Submarines