Portevin-Le-Chatelier Effect

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The Portevin-Le-Chatelier effect (PLC effect) is also known as dynamic stretching aging. Its discovery is attributed to Albert Portevin and Henry Le Chatelier . Some alloys show jerky and uneven deformation behavior when subjected to continuous loading. The tensile test shows, for example, a stress-strain curve that is jagged over large sections . The plastic deformation occurs unevenly distributed over the sample. The zones of increased plastic deformation are also referred to as flow figures.

root cause

Materials showing the PLC effect have a non-monotonic stress-strain rate dependence. This can be explained as follows: Plastic deformations of metals are largely caused by the migration of dislocations , resp. Lattice defects , realized in the crystal lattice . If free atoms are dissolved in the crystal , they are located near such a lattice defect. As a result, the lattice defect has to drag a cloud of foreign atoms with it when it migrates, which increases the force or tension required for migration. If the lattice defect should detach itself from the atomic cloud, a lower voltage and, at the same time, faster migration is required. This explains the non-monotonicity of the stress-strain rate curve. If a dislocation detaches itself from its accompanying foreign atoms, it may influence the “atmospheres” of other dislocations and increase the rate of expansion, whereby other dislocations may also tear themselves away. Due to the avalanche-like propagation together with the instability induced by the non-monotonic stress-strain rate dependence , the effect becomes macroscopically visible. The effect is particularly pronounced when the free dislocations and the rearranged dislocations have a similar mobility. This is the case in a limited temperature-strain rate range, the temperature controlling the diffusion rate of the foreign atoms and the expansion rate controlling the speed of the dislocations.

The PLC effect can only be determined within a limited temperature range. In this area, the diffusion speed roughly corresponds to the speed of the dislocation movement. The dislocations then tear themselves away briefly from the Cotrell clouds (decrease in tension) and are then held back by the diffusing cloud (increase in tension). At higher temperatures, however, the PLC effect disappears again.

In Fe-Al alloys, recessed voids can also cause jerky flow.

Similar to dry friction, the PLC effect can be described as a stick-slip phenomenon .

Non-monotonic stress-strain rate dependenceStress-strain curve jagged by strain localizations

Effects

In sheet metal forming, the uneven distribution of plastic strains for z. B. housing, vehicle body or aircraft parts unacceptable surface quality. A typical example are aluminum-magnesium alloys , which are of interest in aircraft construction due to their low density .

Others

The PLC effect was observed as early as the early 19th century, but its discovery is attributed to Portevin and Le Chatelier.

Individual evidence

  1. ↑ Comment on "Portevin-Le Chatelier effect", Kubin, LP, Ananthakrishne, G., Fressengeas, C., Physical Review E, Vol. 65,053501
  2. ↑ tec-science: tensile test. In: tec-science. July 13, 2018, accessed on November 5, 2019 (German).
  3. ↑ K. Yoshimotot et al., In: Interstitial and Substitutional Solute Effects in Intermetallics, TMS Warrendale 1998, pp. 3-65.
  4. ^ F. Savart, Recherches sur les vibrations longitudinales (1837). Annales de Chimie et de Physique, Ed. 2 vol. 65, p. 337-402
  5. ^ AP Masson, Sur l'élasticité des corps solides (1841). Annales de Chimie et de Physique, Ed. 3 vol. 3, p. 461-462
  6. ↑ W. Lüders: About the manifestation of elasticity in steel-like iron rods, and about a molecular movement observed when bending such rods. In: Polytechnisches Journal . 155, 1860, pp. 18-22.
  7. ↑ H. Le Chatelier, Influence du temps et de la temperature sur les essais au choc (1909). Revue de Métallurgie Vol 6, p. 914-917
  8. ↑ A. Portevin, H. Le Chatelier, Sur un phénomène observé lors de l'essai de traction d'alliages en cours de transformation (1923). Comptes Rendus de l'Académie des Sciences, Paris 176, p. 507-510
  9. ^ A. Portevin, H. Le Chatelier, Heat treatment of aluminum-copper alloys (1924). Transactions of the American Society of Steel Treating Vol. 5, p. 457-478