Materials Science and Engineering A 410–411 (2005) 413–416 Elevated temperature behavior of SePD materials: Superplasticity or enhanced ductility? A.V. Sergueeva a , N.A. Mara a , R.Z. Valiev b , A.K. Mukherjee a,∗ a University of California, Chemical Engineering and Material Science Department, Davis, CA 95616, USA b Ufa State Aviation Technical University, Institute of Physics of Advanced Materials, 12 Marks Str., Ufa 450000, Russia Received in revised form 1 March 2005 Abstract The generated results on superplastic flow (SPF) in materials produced by severe plastic deformation (SePD) have been analyzed. Formation of sliding surfaces with subsequent cooperative grain boundary sliding (CGBS) has been demonstrated to be a main deformation mechanism of superplasticity under conditions of microstructural stability. A substantial role of dislocation glide in accommodation processes is shown. When grain growth is observed during deformation, continuous grain boundary migration prevents formation of slide surfaces and CGBS is restrained. In this case, deformation of the material is more likely to occur by dislocation or diffusion creep than true SPF despite relatively large strains being reached. © 2005 Elsevier B.V. All rights reserved. Keywords: Severe plastic deformation; Plastic behavior; Superplasticity 1. Introduction Superplasticity as a phenomenon or an effect can be defined as the capacity of materials to undergo abnormally large (hundreds or thousands of a percent) uniform tensile elongations at low flow stresses which are in order of magnitude lower than those under the conditions of ordinary plastic deformation and have a high sensitivity to a change in deformation rate. In general, in order to understand the nature and origin of deformation mechanisms at high temperatures, information related to the dependence of strain rate on stress, temperature and grain size (GS) is essential and constitutive equations are very helpful in this case. In micro- crystalline materials (MCM), superplasticity is well established as a GS dependent phenomenon that can be described by the con- stitutive equation for elevated temperature crystalline plasticity [1]: ˙ ε = A DGb kT  b d  p σ G  n (1) where ˙ ε is the strain rate, D the appropriate diffusivity (grain boundary (GB) in the case of superplasticity), G the shear mod- ∗ Corresponding author. Tel.: +1 530 752 1776; fax: +1 530 752 9554. E-mail address: akmukherjee@ucdavis.edu (A.K. Mukherjee). ulus, b the Burgers vector, k the Boltzmanns constant, T the test temperature, d the GS, p the GS exponent (usually 2 for lattice diffusion and 3 for GB diffusion controlled flow), σ the applied stress and n is stress exponent (n = 1/m, where m is strain rate sensitivity; n = 2 in the case of superplasticity). This equation describes behavior has been observed for metals, intermetallics and ceramics materials. This relationship predicts a shifting of the optimum strain rate to higher values and/or the optimum temperatures to lower values with microstructural refinement. In current investigation, the elevated temperature deforma- tion behavior of metals and intermetallics subjected to severe plastic deformation (SePD) was analyzed to establish possible deformation mechanisms and microstructural effects. 2. Grain size dependence of plasticity Fig. 1 represents a summary of experimental data on GS dependence of maximum elongation observed in different SePD materials tested in tension at optimal conditions. It can be seen that despite a theoretical prediction for advanced superplasticity in nanocrystalline materials (NCM), tensile elongation contin- uously decreases with decreasing GS in microcrystalline range. In addition, Al-alloys have demonstrated a greater propensity for SPF than other metals (solid symbols in Fig. 1). But it was noticed [20] that superplasticity in ultrafine materials is achieved 0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.020