Mechanical Behaviors of Ultrafine-Grained 301 Austenitic Stainless Steel Produced by Equal-Channel Angular Pressing
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NANOCRYSTALLINE (NC; grain size d < 100 nm) and ultrafine-grained (UFG; 100 nm < d < 1 lm) materials have been the subjects of widespread research in recent decades, because these materials exhibit unusual mechanical and physical behaviors compared with conventional materials.[1–5] As far as mechanical properties are considered, a great enhancement in strength and hardness is predicted for NC and UFG materials according to the Hall-Petch relationship. It is indeed realized that these new materials show strength several times higher than that of the coarse-grained (CG) counterparts.[1–6] However, an undesirable low tensile elongation at room temperature (RT), particularly poor uniform elongation, is found in most of NC and UFG materials.[6–10] To improve the ductility of these materials,
C.X. HUANG, Humboldt Post-Doctoral Research Fellow, is with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China, and is also with the Institute of Physical Metallurgy and Metal Physics, RWTH Aachen University, 52074 Aachen, Germany. Contact e-mail: [email protected] G. YANG, Professor, and C. WANG, Doctoral Student, are with the Central Iron and Steel Research Institute, Beijing 100081, P.R. China. Z.F. ZHANG and S.D. WU, Professors, are with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences. Manuscript submitted March 26, 2010. Article published online December 15, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
many attempts have been made by researchers and obtained some exciting results in the past decade.[11–13] It has been clarified that the reduced tensile ductility of UFG materials is attributed to their low strainhardening ability.[5–10] During tensile deformation, the onset of plastic instability (or necking) can be expressed by the well-known Conside`re criteria @r r ½1 @e e_ where r and e are the true flow stress and true strain, respectively. The flow stress can be increased significantly by grain refinement to the UFG range. However, the strain-hardening ability of UFG materials is decreased also because of the limited mean free path of dislocation and low efficiency of dislocation storage in the tiny grains. The low strain-hardening ability of UFG materials cannot overcome the geometrical softening of samples under tensile stress, and therefore, necking takes place easily at the early stage of plastic deformation, resulting in poor uniform elongation.[6–10] So far, several interesting mechanical behaviors are reported in the literature, which seem to be the unique characteristics of UFG materials. For instance, an obvious tension-compression asymmetry in strength was reported in UFG Fe,[14] UFG Fe-Cu alloy,[15] and UFG Al.[16] Extensive microshear bands were found in the UFG 6082 Al alloy when the alloy was deformed at a low strain rate.[17] A distinct yielding peak and Lu¨ders VOLUME 42A, JULY 2011—2061
bands were also observed in some UFG face-centered cubic mate
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