Chemically Induced Phase Transformation in Austenite by Focused Ion Beam
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NTRODUCTION
THE faced-centered cubic (FCC) to body-centered cubic (BCC) phase transformation in steel is an important process which can be both dis- or advantageous. For example, in the case of transformation-induced plasticity (TRIP) or TRIP-assisted steels, this phase transformation is used to provide extra strain hardening, as well as deformation capability, to the steels.[1–3] Conversely, in the case of austenitic stainless steels (ASSs), the martensitic phase transformation can drastically influence the corrosion and mechanical properties.[4–6] Abreu et al.[4] have shown that the increase in volume fraction of martensite formed during plastic deformation of ASSs reduces both pitting corrosion and generalized corrosion resistance. Baudry and Pineau[5] reported that the formation of martensite in an Fe-18.25 Cr-6.50 Ni-0.19 C (wt pct) alloy caused earlier initiation and propagation of fatigue cracks. The typical martensitic phase transformation for most commercially available ASSs is below room temperature; even if it is thermodynamically possible, the driving force for the phase transformation is too high, which makes the phase transformation only important in the case of the cryogenic applications.[7–9] There are a few empirical relations available in the literature providing the martensitic transformation temperature (Ms) as a function of the composition of the steel.[7,10–13] However, because the martensitic transformation is a nonADINA BASA, Ph.D. Student, and CHRISTIAN THAULOW, Professor, are with the Department of Engineering Design and Materials, Norwegian University of Science and Technology, 7491 Trondheim, Norway. AFROOZ BARNOUSH, Postdoctoral Fellow, is with the Department of Engineering Design and Materials, Norwegian University of Science and Technology, and also with the Department of Materials Science, Saarland University, 66041 Saarbru¨cken, Germany. Contact e-mail: [email protected] Manuscript submitted March 11, 2013. Article published online November 8, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
diffusional phase transformation accompanied by shear, it can occur if the shear is provided mechanically by external forces. Therefore, cold working of ASSs may trigger the martensite formation well above the Ms. The degree to which this procedure can be accomplished varies with the chemical composition of the steel, and there are empirical equations available for predicting the amount of the phase transformation at a given strain.[14] It is also found that implantation of a wide range of ions triggers the martensitic transformation in the austenite, and it was shown that the main contributing factor is the strain induced by the implantation, which is responsible for the phase transformation.[15–19] Though, a change in the chemical composition of the alloy can play a role as well; for example, it is shown that by implantation of strong austenite-stabilizing ions, such as nickel and nitrogen, it is possible to transform the martensite into austenite.[20] Recently, by the introduction of focus
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