Understanding the Effect of Grain Boundary Character on Dynamic Recrystallization in Stainless Steel 316L
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ic Recrystallization
DYNAMIC recrystallization (DRX), characterized by the nucleation of new, strain-free grains at existing grain boundaries, occurs in low to medium stacking fault energy materials at temperatures higher than half of the material’s melting temperature, Tm, and at low strain rates.[1] As observed experimentally, the nuclei first form a ‘‘necklace structure’’ around the existing grains.[2,3] The driving force for the nucleation and growth of these new grains is the reduction of stored dislocation and sub-boundary energy which was produced during straining of the material.[1] Previous research has determined that the initial grain size,[4–11] strain rate,[5–9,11–14] and temperature[4–9,11,13,14] all influence DRX behavior. Coarser-grained microstructures have fewer grain boundaries—and thus fewer nucleation sites, so significantly larger grains delay the onset of DRX to higher strains.[4,12] MEGAN BECK, MICHAEL MORSE, CALEB COROLEWSKI, KOYUKI FRITCHMAN, CHRIS STIFTER, CALLUM POOLE, MICHAEL HURLEY, and MEGAN FRARY are with the Materials Science and Engineering Department, Boise State University, Boise, ID. Contact e-mail: [email protected] Manuscript submitted July 25, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
Previous studies have shown that one of the characteristics of DRX is cyclical behavior in the plots of stress vs strain[4,12,15,16] for higher temperature or lower strain rate conditions, as illustrated in Figure 1(a). The decreasing stress after the first yielding peak is due to the nucleation of strain-free grains during the recrystallization process.[3] In addition, the shape of the stress–strain curve is sensitive to the initial grain size; multi-peak behavior is usually associated with finegrained starting microstructures, while single-peak behavior is seen in more coarse-grained microstructures.[5] The critical strain for DRX to occur decreases with either increasing temperature, decreasing strain rate, or decreasing grain size.[5–11,13,14] Because DRX lowers the flow stress of a material, removes dislocations, and refines the grain size, the process can improve ductility and toughness if initialized during deformation.[5,17] DRX has been observed in many slow recovering metals including Cu and its alloys,[5,18–24] Ni and its alloys,[2,8,23,25–30] and stainless steel alloys.[4,27,31–34] B. Effects of Grain Boundaries on DRX Properties of engineering materials are determined largely by their microstructures, which can be described by, among other things, their grain size, grain shape,
Fig. 1—(a) Illustration showing the effects of various temperatures and strain rates on stress–strain behavior. In materials that undergo DRX, the general trend is that higher temperatures and lower strain rates decrease the flow stress and result in a transition from single to multi-peak behavior. (b) Experimentally obtained stress–strain curves displaying some of the effects of increasing temperature and decreasing strain rate (c) Experimental EBSD scan after 25 pct stain showing initial evidence of
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