Speaker
Description
Atom Probe Tomography (APT) can be used to analyze the structure of matter at the smallest scales, such as grain boundaries, dislocations or even atomic planes in certain special cases. As such, APT can be used to study dynamic strain-ageing in steels, which relies on the segregation of free interstitials (C and N) on dislocations. Surprisingly, very few studies regarding the segregation on dislocations using APT are available in the literature and there are no reviews concerning carbon segregation to dislocations in steel using APT.
In this study, we present APT results from industrial welds of low-alloy or carbon-manganese steels. The materials were studied in the heat-affected zone (HAZ) or in the weld metal, with or without post weld heat treatment (PWHT). The dislocation density of the studied steels was rather high, about 1014 m-2, as determined by multiple experimental techniques. In some cases, the decoration of dislocations by segregation of solute atoms is easily observed and the dislocation density determined by APT is in agreement with that obtained by the other techniques. However, segregation on dislocations was not observed in some steels, in a total of 200 APT volumes, even though one dislocation is expected every 2 to 3 APT volumes.
Other experiments have been carried out to try and understand the origin of this problem, but none have been able to solve it. A number of hypotheses have been put forward to understand our observations, including the possibility of dislocations escaping from the tip during the APT analysis or the absence of segregation on the dislocations, but none is fully satisfactory to explain the problem encountered.
Keywords: Atom Probe Tomography, Segregation, Dislocations, Steel
