I'm comparing the nature and limitations of strain hardening and precipitation hardening on a material... and I've a feeling I've missed out something here but can't think what it is

They both cause a decrease in the material's ductility. Strain hardening through plastic deformation thus causing dislocation multiplication and ultimately increasing yield and tensile strength... precipitation hardening through the formation of second phase particles (precipitates) which establishes lattice strains at the precipitate-matrix interfaces hence impeding dislocation motions and ultimately increasing yield and tensile strength.

The common limitation that they share is a maximum service temperature. In the strain hardening method, the strengthening effect is lost when recovery occurs (relieve of stored strain energy) and at higher temperatures - recrystallization, as a result of enhanced atomic diffusion. In the precipitation hardening method, overaging occurs as a result of continued particle growth - the rate of which increases sharply with increasing temperatures.

Also, both strengthening methods induce a lost of ductility, increasing the risk of fracture at low stress levels.

That doesn't seem very complete... would someone help point out anything I missed?

If you look at precipitation hardening of steel this is usually done with a quick quenching in cold oil or water. The result of the rapid cooling is a lot of small crystals that will make the material harder but more brittle (knife steel). Slow cooling will have the opposite affect forming fewer but larger crystals making the material softer and more flexible (band steel). If we heat both, they will both head toward the state of the band steel, with larger crystals. This is the lowest energy state for the steel.

The difference between the two is connected to the amount of surface area between crystals, with smaller crystals creating more higher energy surface area within the steel structure. This higher energy surface area between the crystals is frozen in the metal and contains more potential than the iron atoms in the crystals. This surface is trying to go toward lower energy, but is frozen with its extra potential adding hardness but brittleness due to its higher energy state.

In strain hardening, we add work to a piece of metal. This will distort the crystal interface areas, adding potential to the interface areas. This will add hardness similar to quick quenching. An interesting experiment is to take a piece of steel and either hit it will a hammer or drill a hole to add some local strain hardening. This will be the first area to rust due to the higher potential the strain will add to the steel. That is why bends in steel are often the first to rust unless it is heat treated.

hey thanx alot for that MDT... hmm...