Portevin–Le Chatelier effect

The Portevin–Le Chatelier effect (PLC) 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 dislocation breaking free of this stoppage. The onset of the PLC effect appears when the strain rate sensitivity becomes negative and inhomogeneous deformation starts. This effect also can appear on the specimens surface and bands of plastic deformation. This process starts at a so-called critical strain, which is the minimum strain needed for the onset of the serrations in the stress–strain curve. The critical strain is both temperature and strain rate dependent. The existence of some critical strain are attributed to better solute diffusivity due to the deformation created vacancies and increased mobile dislocation density. Both of which contribute to the instability in the substitutional alloys, while interstitial alloys are only effect by the increase in mobile dislocation densities. While the effect is named after Portevin and Le Chatelier, they were not the first to discover it. Felix 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, Mason, repeated the experiment while controlling for loading rate. Mason 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 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 manor 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 comes for a specific case where solute drag creep is occurring and there is an applied stress, that is 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, the stress on the dislocation significantly increases, causing the process to reoccur.   The cyclic changes described above produce serrations in the plastic region of the stress strain diagram of a tensile test that is undergoing the Portevin -Le Chatelier effect. The variation in stress also causes non-homogeneous deformation to occur throughout the sample which can be visible to the naked eye through observation of a rough finish. Temperature affects both the speed of band propagation through the material and the critical strain. The speed of band propagation is proportional to the temperature (lower temp lower speeds, higher temp higher speeds). Often time the critical strain will first decrease due to temperature.The temperature effect on PLC regime is caused by the increased ability of the solutes to diffuse to the dislocations with increasing temperature. Although the mechanism of diffusion is not entirely defined, it is believed that solute atoms diffuse by either volume (high temperatures), diffusion in stacking fault ribbon between partial dislocations (intermediate) or pipe diffusion(low temperature).