Cu precipitation on dislocation and interface in quench-aged steel
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Research Letters
Cu precipitation on dislocation and interface in quench-aged steel Qingdong Liu* and Shijin Zhao, Key Laboratory of Microstructures and Institute of Materials, Shanghai University, Shanghai 200072, P.R. China *Address all correspondence to Qingdong Liu at [email protected] (Received 17 May 2012; accepted 24 August 2012)
Abstract The Cu precipitation in a quench-aged high-strength low-alloy steel is studied at the atomic scale by atom probe tomography and highresolution transmission electron microscopy. The results indicate that the Cu precipitates greatly correlate with carbides in the aspect of distributional character, i.e., the two phases are prone to coprecipitate on the dislocations and/or interfaces (low angle boundaries of the martensite laths). The crystallographic defects have a significant effect on the sizes, morphology and composition of Cu precipitates with Ni and Mn segregation shell.
Cu precipitation strengthening plays an important role in the fabrication of high-strength low-alloy (HSLA) steels[1–3]. The nature of Cu precipitation and the actual distributional morphology of the Cu precipitates have a significant effect on or directly determine the strength and toughness of HSLA steels.[4–6] Cu precipitation can occur during continuous cooling of austenite by interphase precipitation and occur during isothermal ageing of martensite by dislocation-interacted precipitation.[7] In the former condition, the Cu precipitation greatly correlates with the kinetics of austenite decomposition.[8,9] In the latter condition, the Cu precipitation usually depends on the extent of thermal ageing.[10–13] The martensite lath boundaries and the dislocations favor the nucleation of Cu precipitates as well as the segregation of solute atoms and the precipitation of carbides. The reactions often overlap and occur on the fine scale.[14] The structural, compositional and morphological evolutions of Cu precipitates in Cu precipitation reaction are of fundamental scientific interest. High-resolution transmission electron microscopy (HRTEM) investigations reveal that the structures of Cu precipitates follow a complicated BCC 9R 3R FCC sequence during the precipitation reaction.[15–22] Atom probe tomography (APT) studies suggest that at the initial nucleation stage the developing BCC Cu precipitates contain an equal amount of Fe with Cu as well as a small amount of Ni and Mn. The Ni and Mn are prone to segregate to the precipitate/matrix heterophase interface at the later growth and coarsening stages of Cu precipitation.[23–31] At the same time, the structural and compositional evolutions of Cu precipitates are greatly size-dependent and are in association with the morphological evolutions from spherical to ellipsoidal and then rod-like shape. Solute atom segregation to and Cu precipitation on dislocations and interfaces are the most difficult microstructural
features to quantify.[32–36]. In this paper, we characterize the Cu precipitation on the crystallographic defects of dislocations and martensite lath
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