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Kim B.,University of Cambridge | Boucard E.,CNRS Study of Microstructures, Mechanics and Material Sciences lab | Sourmail T.,metal Research Center Metallurgy | San Martin D.,CSIC - National Center for Metallurgical Research | And 2 more authors.
Acta Materialia | Year: 2014

The strengthening contributions in medium-carbon tempered martensite are unveiled in this work. By using transmission electron microscopy and synchrotron radiation X-ray diffraction, the different microstructural features have been captured; these include precipitation, grain boundary, solid solution and dislocation forest strengthening. The evolution of these features was observed as a function of tempering temperature and silicon content. In trying to elucidate the nature of grain boundary strengthening, three approaches are presented, including a plasticity model based on irreversible thermodynamics, misorientation angle characterization by electron backscatter diffraction, and transmission electron microscopy analysis of failed regions. Based on the findings, it is concluded that silicon inhibits martensite recovery, and that at low tempering temperatures, lath boundaries also appear to contribute to strengthening. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Kim B.,University of Cambridge | Celada C.,CSIC - National Center for Metallurgical Research | San Martin D.,CSIC - National Center for Metallurgical Research | Sourmail T.,metal Research Center Metallurgy | Rivera-Diaz-Del-Castillo P.E.J.,University of Cambridge
Acta Materialia | Year: 2013

The current work presents a comprehensive study that aims at understanding the role of silicon on θ precipitation, as well as on the ε → θ carbide transition in tempered martensite. Cementite nucleation was modelled under paraequilibrium conditions in order to ensure the presence of silicon in the carbide, where both thermodynamic and misfit strain energies were calculated to evaluate the overall free energy change. The growth stage was investigated using in situ synchrotron radiation; three alloys containing 1.4-2.3 wt.% silicon contents have been studied. Silicon appears to play a significant role in carbide growth. It was observed throughout tempering that cementite precipitation was slower in the higher silicon content alloy. Literature reports that cementite growth is accompanied by silicon partitioning, where the silicon content inside the carbide decreases as tempering progresses. Therefore it appears that the limiting factor of the growth kinetics is the rate at which silicon is rejected from the carbide; the silicon piles up at the carbide-matrix interface, acting as a barrier for further growth. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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