Metals

Metallic bond Metals are often described as positively charged nuclei in a sea of electrons. The outer electrons of the metal atom nuclei are free and can flow through the crystalline structure. The bonding is caused by attraction between the positively charged metallic atom nuclei and the negatively charged cloud of free electrons. The movement of free electrons makes metal very good electrical and thermal conductors.

Metallic bonds are very flexible, which is why metals can be put into many different shapes. Have you ever crumpled up a piece of aluminium foil? The reason you can easily change the shape of aluminium is because the metallic bonds between the atoms that make up the foil are flexible. You could not do that with a non-metal substance, such as sodium chloride (common table salt) because it forms rigid internal bonds that break instead of bending.

Metals exist as crystals Metals (pure or alloyed) exist as crystals. Crystals are regular arrangements of particles (atoms, ions or molecules). atoms arranged in a cubic close packing pattern

Grain size As the grain size of a metal shrinks, it can become many times stronger, but it also usually loses ductility. Ordinary coarse-grained metals deform when parts of a grain slip past one another as extra planes of atoms, called dislocations, move through the material. The process has been compared to moving a rug by flapping one end of it to create a wave, causing the rug to inch along bit by bit. But the trick won’t work if the rug is too short; likewise, if the dimensions of the crystal grains are too small, dislocations can’t be created or glide through the grain to allow deformation. Here frames from a dark-field TEM video of nanocrystalline nickel under strain show rapid aggregation of a group of grains.

Grain size can be controlled or modified by the rate of cooling of the molten metal, or by heat treatment after solidification. Reheating solid metal or alloy allows material to diffuse between neighbouring grains and the grain structure to change. Changing grain structure through welding.

Slow cooling allows larger grains to form. Rapid cooling produces smaller grains. Directional properties in the structure may be achieved by selectively cooling one area of the solid. By lowering the casting temperature, one changes the microstructure from a highly dendritic structure (Figure 2a) to a rosette-like structure (Figure 2c).

Plastic deformation A permanent deformation of a solid subjected to a stress. Watch the video of brass going through plastic deformation during a tensile test.

Work hardening Work hardening, strain hardening, or cold work is the strengthening of a material by, macroscopically speaking, plastic deformation. In the range of uniform elongation, further plastic deformation is only possible by a continuously increasing load. The dislocation density grows with increasing deformation, making further deformation more difficult due to the interaction between the dislocations. This effect is referred to as strain or work hardening.

As the material becomes increasingly saturated with new dislocations, more dislocations are prevented from nucleating (a resistance to dislocation-formation develops). This resistance to dislocation-formation manifests itself as a resistance to plastic deformation; hence, the observed strengthening.

Alloying Alloying can increase the tensile strength of a material. Alloying one metal with other metal(s) or non metal(s) often enhances its properties. For instance, steel is stronger than iron, its primary element. The physical properties, such as density, reactivity, Young's modulus, and electrical and thermal conductivity, of an alloy may not differ greatly from those of its elements, but engineering properties, such as tensile strength may be substantially different from those of the constituent materials. This is sometimes due to the sizes of the atoms in the alloy, since larger atoms exert a compressive force on neighbouring atoms, and smaller atoms exert a tensile force on their neighbours, helping the alloy resist deformation.

Alloying, malleability & ductility The presence of foreign atoms in the crystalline structure of the metal interferes with the movement of atoms in the structure during plastic deformation.

BEFORE metal is alloyed the atoms look like this:- they can easily slide over each other when a force is applied because of the arrangement of their atoms and because of their free electrons. This is why they can be hammered easily into shapes and can be easily drawn into wires (ductile). ooooooo ooooooo Before alloying

After alloying This is what happens when is it alloyed :- As you can see, the atoms from the other metal has mixed with is. So, it cannot slide easily as it did before. They are much stronger and it is much harder to hammer them into shape and drawn then into wires. This is the reason why alloying change the malleability and ductility of a metal. oooOoOo OoooOOo ooOooooO OOoooOo

Superalloys The strength of most metals decreases as the temperature is increased. Superalloys are metallic alloys that can be used at high temperatures, often in excess of 0.7 of their absolute melting temperature.

Design Criteria There are two design criteria for superalloys. 1.Creep 2.Oxidation A superalloy is an alloy that exhibits excellent mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance.

Creep Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near the melting point. Creep always increases with temperature.

Oxidation When in contact with water and oxygen, or other strong oxidants and/or acids, iron will rust. If salt is present as, for example, in salt water, it tends to rust more quickly, as a result of the reactions. Heavy rust on the links of a chain near the Golden Gate bridge in San Francisco; where it was continuously exposed to moisture and salt-laden air, causing surface breakdown, cracking, and flaking of the metal.

Applications for superalloys Superalloys can be based on iron, cobalt or nickel. Nickel based superalloys are particularly resistant to temperature and are appropriate materials for use in aircraft engines and other applications that require high performance at high temperatures, for example rocket engines, chemical plants