Эффект Портевена — Ле Шателье: механизмы и влияние факторов материала.

The Portevin–Le Chatelier (PLC) effect describes a serrated stress–strain curve or jerky flow, which some materials exhibit as they undergo plastic deformation, specifically inhomogeneous deformation. This effect has been long associated with dynamic strain aging or the competition between diffusing solutes pinning dislocations and dislocations breaking free of this stoppage. The onset of the PLC effect occurs when the strain rate sensitivity becomes negative and inhomogeneous deformation starts.

While the effect is named after Albert Portevin and François Le Chatelier, they were not the first to discover it. Félix Savart made the discovery when he observed non homogeneous deformation during a tensile test of copper strips. He documented the physical serrations in his samples that are currently known as Portevin–Le Chatelier bands. A student of Savart, Antoine Masson, repeated the experiment while controlling for loading rate. Masson observed that under a constant loading rate, the samples would experience sudden large changes in elongation (as large as a few millimeters).

Much of the underlying physics of the Portevin Le Chatelier effect lies in a specific case of solute drag creep. Adding solute atoms to a pure crystal introduces a size misfit into the system. This size misfit leads to restriction of dislocation motion. At low temperature, these solute atoms are immobile within the lattice, but at high temperatures, the solute atoms become mobile and interact in a more complex manner with the dislocations. When solute atoms are mobile and the dislocation velocity is not too high, the solute atoms and dislocation can move together where the solute atom decreases the motion of the dislocation. The Portevin Le Chatelier effect occurs in the specific case where solute drag creep is occurring and there is an applied stress, with a material dependent range, on the sample. The applied stress causes the velocity of the dislocations to increase, allowing the dislocation to move away from the solute. This process is commonly referred to as “breakaway”. Once the dislocation has moved away from the solute, the stress on it decreases which causes its velocity to decrease. This allows the solute atoms to “catch up” with the dislocation. As soon as the solute atom catches up, the stress on the dislocation significantly increases, causing the process to repeat. The presence of precipitates is shown to have an influence on the appearance and disappearance of serrations in the stress strain curve.

The structure of the material, as well, has an effect on the appearance and parameters that describe the PLC effect. For example, the magnitude of the stress drops is larger with a smaller grain size. The critical strain often increases with larger grains, which is linked to the dependence of the dislocation density to grain size. But some findings indicate that the grain size has practically no effect on the band velocity or the band width.

While properties like strain rate sensitivity and critical strain mark the beginning of the PLC effect, people have developed a system to describe the serrations themselves. These types are often dependent on strain rate, temperature, and grain size. Because the type A, B, and C type bands are most found in literature they will be the only ones covered here.

Type A bands are often seen at high strain rate and low temperatures. They are usually described as continuously propagating with small stress drops.

Because the PLC effect is related to a strengthening mechanism, the strength of steel may increase; however, the plasticity and ductility of a material afflicted by the PLC effect decrease drastically. The PLC effect is known to induce blue brittleness in steel; additionally, the loss of ductility may cause rough surfaces to develop during deformation (Al Mg alloys are especially susceptible to this), rendering them useless for autobody or casting applications.