Comprehensive Deformation Analysis of a Newly Designed Ni-Free Duplex Stainless Steel with Enhanced Plasticity by Optimi
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TRODUCTION
DUPLEX stainless steels (DSSs), which consist of a mixture of ferrite (BCC-a) and austenite (FCC-c) phases, combine the advantages of ferritic and austenitic stainless steels. DSSs possess a higher strength and corrosion resistance compared with single-phase stainless steels, particularly when the fractions of ferrite and austenite are optimized. Conventional DSSs are heavy alloyed and classically have elemental compositions ranging from 18 to 26 Cr, 3 to 8 Ni, 1 to 5 Mn, and 1 to 2 Mo (all in wt pct).[1] Similar to other structural materials, the development of advanced DSSs is conventionally accomplished by
MOHAMMAD MOALLEMI and ABBAS ZAREI-HANZAKI are with the The Complex Laboratory of Hot Deformation & Thermomechanical Processing of High Performance Engineering Materials, School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, 11365-4563, Iran. Contact e-mail: [email protected] MOSTAFA ESKANDARI is with the Department of Materials Science & Engineering, Faculty of Engineering, Shahid Chamran University of Ahvaz, Ahvaz, 61357-43337, Iran. ANDREW BURROWS and HOSSEIN ALIMADADI are with the Technical University of Denmark, Center for Electron Nanoscopy, Fysikvej, Building 307, 2800 Kongens Lyngby, Denmark. Manuscript submitted July 20, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
focusing on alloying so that improved mechanical behavior is achieved while considering the economic aspects. Accordingly, there have been many attempts to replace costly elements such as Ni and Mo with N and Mn, without degrading the corrosion and mechanical properties. The focus on a combination of Mn and N stems from the fact that (i) Mn increases the nitrogen (N) solubility and (ii) N and Mn have a synergistic effect on enhancing the mechanical properties and controlling the deformation mechanisms of the austenite constituent.[2–5] The mechanical behavior of DSSs is influenced by the state of both austenite and ferrite phases. The ferrite deformation mechanism mainly involves dislocation slip owing to a high-stacking fault energy (SFE) and the presence of many slip systems.[6] It is worthy of note that in some alloy systems, deformation twins are also formed in ferrite during plastic strain.[7] Austenite deformation is more complex than that of ferrite and the active mechanisms are conventionally characterized by slip, mechanical twinning (resulting in twinning-induced plasticity, TWIP) and a¢-(BCC) martensite formation (resulting in transformation-induced plasticity, TRIP).[8–10] These deformation mechanisms give rise to a strong work hardening effect and potentially to excellent plasticity, particularly where TRIP and/or TWIP is deliberately designed to take place over a wide
range of strain. Decreasing the SFE in the austenite phase stimulates twinning or strain-induced martensite (SIM) formation during deformation. When the SFE is low, perfect {111}h110i lattice dislocations may dissociate into {111}h112i Shockley partial dislocations and stacking faults would be formed in
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