Characterization and Modeling of Precipitation Kinetics in a Fe-Si-Ti Alloy
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AMONG the precipitation hardening steels, the Fe-Si-Ti system has received some attention for nearly a century,[1–15] particularly before the 1980s and with a more recent revival. This system is capable of forming a large volume fraction of intermetallic phases[9,11] that can substantially increase the material’s strength, with the possibility of a solution treatment in the ferritic domain, so that no allotropic phase transformation occurs during quenching from the solution treatment temperature. Given the low density of the two solute elements, this system is an attractive solution for increasing the strength of steels (e.g., in automotive applications) in conjunction with substantial weight savings. A number of phases can form in this ternary system.[13,14,16] In addition to the main phases of the binary Fe-Si and Fe-Ti systems (Fe3Si and Fe2Ti), Xiong et al.[14] reviewed a large number of stable and metastable phases that are susceptible to form in the ternary system. Several authors agree that at high temperature (above 1070 K (797 °C)), the precipitation in alloys with MALIKA PERRIER, formerly PhD Student with SIMAP, INPGrenoble, St Martin d’He´res Cedex, France, is now Research Engineer with Constellium Research Centre, Voreppe, France. ALEXIS DESCHAMPS and YVES BRECHET, Professors, are with the SIMAP, INP-Grenoble, Contact e-mail: [email protected] OLIVIER BOUAZIZ, Research Scientist, is with the Arcelormittal Research SA, Maizie´res les Metz, France. FRE´DE´RIC DANOIX, Research Scientist, is with the GPM, CNRS, Rouen, France. FRE´DE´RIC DE GEUSER and PATRICIA DONNADIEU, Research Scientists, are with the SIMAP, CNRS, Grenoble, France. KHALID HOUMMADA, Assistant Professor, and PHILLIPPE MAUGIS, Professor, are with the Aix-Marseille Universite´, Marseille, France. Manuscript submitted March 30, 2012. Article published online October 12, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
composition of the order of Fe-3.5 wt pct Si-1.5 wt pct Ti is dominated by the Laves phase Fe2Ti.[3,4,15] At lower temperatures, most authors consider that the main phase that forms is the metastable Heusler phase Fe2SiTi.[8–10,15] However the stability range of this phase is not clear, since it has been observed by some authors to remain for relatively long aging times and by others to be replaced by the Fe2Ti phase progressively. The high volume fractions that can be obtained with this phase and its very small size, associated with its coherency with the matrix, make it difficult to characterize quantitatively its precipitation kinetics by conventional means, such as transmission electron microscopy (TEM). Thus, to this day, very few kinetic studies on this system are available in the literature.[4,11] A full kinetic study requires the use of a combination of complementary tools.[17] Structural information about the precipitate type can be obtained by TEM, while the chemical composition of the nanoscale precipitates can be obtained by atom probe tomography (APT). The evolution of precipitate size and volume
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