User:Khalm555555/dynamic strain ageing

Although sometimes Dynamic Strain Aging is used interchangeably with the Portevin–Le Chatelier effect effect (or serrated yielding), Dynamic Strain Aging refers specifically to the microscopic mechanism that causes the Portevin–Le Chatelier effect. This strengthening mechanism is related to solid solution strengthening and has been observed in a variety of fcc and bcc substitutional and interstitial alloys, non-metallics such as silicon and ordered intermetallic at specific ranges of temperature and strain rates.

Description of Mechanism
In materials, the motion of dislocations is a discontinuous process. When dislocation meets obstacles (like forest dislocations) they are temporary arrested for a certain time. During this time solutes (such as interstitial particles) diffuse around the dislocations further strengthening the obstacles hold on the dislocations. Eventually these dislocations will overcome these obstacles with sufficient stress and will quickly move to the next obstacle where they are stopped and the process can repeat again. This process most well-known manifestations are Lüders Bands and the PLC effect although the mechanism is known to effect materials without these physical observations.

Material Property Effects
Although serrations in the stress-strain curve caused by the Portevin–Le Chatelier (PLC) effect are the most visible effect of dynamic strain aging other effects may be present when this effect is not seen. Often times when serrated flow is not seen dynamic strain aging is marked by a lower strain rate Sensitivity. That becomes negative in the PLC regime. Along dynamic strain aging also causes a plateau in the strength, a peak in flow stress a peak in work hardening, a peak in the Hall Petch constant, and minimum variation of ductility with temperature. Since dynamic strain aging is a hardening phenomenon it increases the strength of the material but it will often effect the ultimate tensile strength of the material more than the yield strength of the material.

Material Specific Example of Dynamic Stain Ageing
Dynamic strain ageing has been shown to be linked to these specific material problems:
 * decrease the fracture resistance of AL-Li alloys
 * Decrease low cycle fatigue life of Austenitic stainless steels and super alloys under test condition which are the service conditions in liquid metal cooled fast breeder reactors which the material is used for.
 * Reduce fracture toughness by 30-40% and shorten the air fatigue live of RPC steels and may worsen the cracking resistance of steels in aggressive environments . The susceptibility of RPC steels to environment assisted creating in high temperature water  coincides with DSA behavior
 * PLC specific problems like blue brittleness in steel, loss of ductility and bad surface finishes for formed Aluminum Magnesium  alloys.