Yield point and strain aging

Yield point phenomenon
localized heterogenous type of transition from elastic to plastic deformation produces a yield point in stress-strain curve. The load increases steadily with elastic strain, drops suddenly, fluctuates approximately at constant value of load, and then rises with further strain. The load at which the sudden drop ours is called the upper yield point. The constant load is called the lower yield point.

The deformation occurring throughout the yield-point elongation is heterogenous. At upper yield point a discrete band of deformed metal at a stress concentration such as a fillet, and coincident with the formation of the band the load drops drops to the lower yield point. The band then propagates along the length of the specimen, causing the yield-point elongation.

These bands are generally 45 degrees to the tensile axis. They are usually called Luders bands, hartmann lines or stretcher strains, and this type of deformation is sometimes referred to as the Piobert effect.Yield point was found originally in Low-carbon steel. It is also been observed in Iron, Polycrystalline Mo, Ti, Zn and Al alloys and in single crystal of Fe, Cd, Zn, alpha and beta brass  and Al.

This can be associated with small amounts of Interstitial or Substitutional  impurities. carbon and nitrogen are more effective.

This is reappearance of yield point  in which strength of metal is increases and ductility decreases and a low value of strain rate sensitivity (m) on heating at a relatively low temperature after cold-working.
Plain carbon steel strained plastically through the yield-point elongation to a particular strain X (region A). If it is unloaded and reloaded again without any appreciable delay or any heat treatment yield point doesn't occur since the dislocations have been torn away from the atmosphere of carbon and nitrogen atoms (region B). if it is unloaded and reloaded after aging for several days at room temperature or several hours at an aging temperature like 400 K then yield point reappears and moreover, the yield-point will be increased (region C). This is due to the diffusion of C and N atoms to the dislocations during the aging to form new solute atmospheres anchoring the dislocations.

Nitrogen plays a more important role in the strain-aging of iron that carbon because it has a higher solubility and diffusion coefficient and produces less complete precipitation during slow cooling.

It is important to eliminate strain aging in deep drawing steels because the reappearance of yield point can lead to difficulties with surface markings or stretcher strains due to localized heterogenous deformation.

(1) lower the free amount of carbon and nitrogen by adding strong carbide and nitride formers (Al, V, Ti, Cb, B)
(2) deform the metal by roller leveling or a skin-pass rolling and use it immediately before it can age.

Strain-aging is also associated with the occurrence of serrations in the stress-strain curve (discontinuous or repeated yielding). This dynamic strain-aging behavior is called the portevin-Lechatelier effect. Dislocation arrest and release by solute atoms are reasons for these serrations.Mechanical twinning during deformation or stress-assisted martensitic transformation also produce the same effect.

Plain carbon steels heated in 500 to 650 K shows a decreased tensile ductility and notched impact-resistance and a minimum strain rate sensitivity and maximum strain aging rate due to discontinuous yielding. This region is known as blue brittleness. This is just an accelerated strain aging. Hence plain carbon steels are not worm worked in this temperature range.