Mechanism of Dynamic Strain Aging in a Niobium-Stabilized Austenitic Stainless Steel
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RODUCTION
HEAT-RESISTANT steels, such as austenitic stainless steel (ASS) and martensitic steel, are widely used in ultra-supercritical power plants.[1] These types of steels always serve at high temperatures and are susceptible to dynamic strain aging (DSA).[2–6] DSA is a phenomenon in which aging occurs synchronously when metals and alloys are deformed at elevated temperatures. Serrated flow, also known as the Portevin-Le Chatelier (PLC) effect, is an important manifestation of DSA on stress–strain curves.[7] DSA is related to the diffusion of solute atoms and locking of movable dislocations. The type of solute atoms depends on deformation temperature, strain rate, and strain, and these conditions result in a different mechanism of DSA in ASS.
HONGWEI ZHOU is with the School of Materials Science and Engineering, Anhui Key Lab of Materials Science and Processing, Anhui University of Technology, Maanshan 243002, Anhui, P.R. China and also with the School of Materials Science and Engineering, Southeast University, Nanjing 211189, Jiangsu, P.R. China. Contact e-mail: [email protected] FENGMEI BAI is with the School of Metallurgical Engineering, Anhui University of Technology, Maanshan 243002, Anhui, P.R. China LEI YANG, HAILIAN WEI, YAN CHEN, GUOSHENG PENG, and YIZHU HE are with the School of Materials Science and Engineering, Anhui Key Lab of Materials Science and Processing, Anhui University of Technology. Manuscript submitted April 27, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Choudhary et al. showed that 316L (N) and 304 ASS exhibited distinct low- and high-temperature DSA regimes, due to diffusion of interstitial carbon (C) or nitrogen (N) atoms and substitutional chromium (Cr) atoms into dislocations.[8] More recently, a new type of ASS based on 18Cr-8Ni[2] has been developed, with the additional alloying elements, such as C, N, titanium (Ti), niobium (Nb), tungsten (W), and vanadium (V). In a Ti-stabilized austenitic stainless steel (Alloy 9)[9] as well as in Nb-stabilized austenitic stainless steels TP347H[10] and SUS347J1TB,[2] these additional elements affected the DSA of these steels. Additional alloy elements shifted the DSA regime to high temperatures in Alloy 9[9] and SUS347J1TB.[2] It is well known that DSA can increase the yield strength and ultimate strength, reducing plasticity.[11–13] Concurrently, DSA can improve the creep resistance of ASS.[2] However, the effect of DSA on fatigue is different from that on creep. Most publications show that DSA reduces low cycle fatigue (LCF) life of ASS.[4] However, the mechanism of DSA in ASS with the additional alloys is not clear. It is necessary to clarify the mechanism of DSA before investigating the effect of DSA on the mechanical properties. Different methodologies including visual techniques and internal friction have been used to characterize the DSA effect. Visual techniques, such as digital image correlation (DIC),[14,15] dynamic digital speckle pattern
interferometry (DSPI),[16,17] and high-sensitivity infrared (IR) thermographic imaging,[18] can obs
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