On the Microstructural Stability of Ultrafine-Grained Interstitial-Free Steel under Cyclic Loading
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ND SIGNIFICANCE
RECENTLY, considerable attention has been paid to the fatigue and conventional deformation response of ultrafine-grained (UFG) materials,[1–7] which are characterized by a grain size of 100 nm to 1 lm.[8,9] To obtain such small grain sizes, several techniques can be employed to refine the microstructure from a coarse-grained (CG) condition. The most commonly used techniques involve heavy plastic deformation that increases the dislocation density in the material, which brings about the formation of low-angle grain boundaries (LAGBs), as well as high-angle grain boundaries (HAGBs), throughout the process.[4,10–13] The techniques that fall into this category are known as severe plastic deformation techniques. High pressure torsion, accumulative roll bonding, and equal channel angular extrusion (ECAE) are widely preferred owing to the lack of problems such as porosity or impurities associated with other techniques.[1,14] Among these, ECAE is the most widely used technique for producing bulk UFG materials because it can provide large samples with uniform microstructures. In this process, the material, originally in the form of a billet, is pressed through an angled die, and thereby undergoes a (nearly) simple shear deformation.[15,16] T. NIENDORF, Graduate Student, D. CANADINC, Postdoctoral Student, and H.J. MAIER, Professor, are with the Lehrstuhl fu¨r Werkstoffkunde (Materials Science), University of Paderborn, 33095, Paderborn, Germany. Contact e-mail: [email protected] I. KARAMAN, Assistant Professor, is with the Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA. This article is based on a presentation made in the symposium entitled ‘‘Ultrafine-Grained Materials: from Basics to Application,’’ which occurred September 25–27, 2006 in Kloster Irsee, Germany. Article published online June 13, 2007. 1946—VOLUME 38A, SEPTEMBER 2007
To date, a considerable amount of research has been conducted on process optimization to obtain materials that display a combination of high yield strength and high ductility.[1,3,4] Materials that possess both of these properties are desired in various industries for the making of parts, such that maximum performance could be achieved in return for a relatively inexpensive investment. Most of the studies undertaken with this motivation focused on the monotonic response and properties of UFG materials,[1,4,14,17–21] while only few investigations reported results on the properties of UFG materials under cyclic loading.[1,5,22–24] However, a solid knowledge of the cyclic stress-strain (CSS) response of the UFG materials in various conditions is essential to their utility as the material of choice in industrial components. In a recent study investigating the low-cycle fatigue (LCF) behavior of UFG interstitial-free (IF) steel,[1] we showed the significant influence of the ECAE processing route on the CSS response. Accordingly, some ECAE routes led to a very stable fatigue behavior, whereas others gave way to softening of the material un
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