Understanding the Behavior of Advanced High-Strength Steels Using Atom Probe Tomography
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INTRODUCTION
THE current trend in the design of new cost-effective steels is aimed to tailor the specific properties toward their applications and, by this, to attain the enhanced performance in service. However, to achieve this, an indepth understanding of the role of alloying additions during various stages of steel manufacturing and service life is required. Atom probe tomography[1–3] (APT) is a technique that is very well suited for this task. It allows three-dimensional characterization at the atomic level of elemental partitioning between phases present in the complex microstructures of steels[4–10] and segregation of solute to the boundaries, interfaces, twins, and dislocations,[8,10–16] as well as following of the evolution of precipitation starting from the early stage of cluster formation to the final stage of the equilibrium precipitates.[15–23] In this work, we use as examples the application of APT to two classes of steels: nanostructured bainitic and transformation-induced plasticity (TRIP) steels. The former ones were first developed by the group of Professor Bhadeshia, Cambridge University,[6,24,25] for applications in defense and construction, which are required to possess extremely high strength and toughness. These steels showed strength of ~2.5 GPa and toughness of 30 to 40 MPa m1/2. These properties were ELENA PERELOMA, Professor of Physical Metallurgy and Director of BlueScope Steel Metallurgy Centre, Faculty of Engineering, is with the School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, NSW 2522, Australia. Contact e-mail: [email protected] HOSSEIN BELADI, Senior Research Academic, and ILANA TIMOKHINA, Senior Research Academic, are with the Centre for Material and Fibre Innovation, Deakin University, Geelong, VIC 3217, Australia. LAICHANG ZHANG, Research Assistant Professor, formerly with the School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, is now with the Department of Mechanical Engineering, University of Western Australia, Crawley, WA, Australia. Manuscript submitted March 31, 2011. Article published online September 17, 2011 3958—VOLUME 43A, NOVEMBER 2012
associated with the formation of 20- to 40-nm-thick nanoscale bainitic ferrite (BF) plates with fine interlayers of the retained austenite (RA) in the microstructure of these steels due to incomplete bainite transformation in the presence of high silicon content. APT was successfully used to study solute redistribution in these steels.[8,9,18,26] However, to achieve this microstructure, heat treatment for 24 to 48 hours or longer is required, which makes these steels economically unviable for many applications outside the heavy section products. In addition, high C content poses a significant problem for welding of this steel. Thus, efforts were made to modify both the composition and processing schedules of this class of steels, especially with the introduction of deformation prior to isothermal heat treatment in order to accelerate the austenite to BF transformation.
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