ULTRAFINE-GRAINED MATERIALS The effect of cryogenic deformation on the limiting grain size in an SMG Al-alloy P. B. Prangnell Y. Huang Received: 31 January 2008 / Accepted: 22 April 2008 / Published online: 13 July 2008 Ó Springer Science+Business Media, LLC 2008 Abstract The minimum grain size obtainable in an Al–0.1%Mg submicron grained (SMG) alloy, subjected to cryogenic plane strain deformation, and its subsequent sta- bility during room temperature deformation have been investigated. A decreasing steady state grain size was obtained with reducing deformation temperature. However, a true nanocrystalline grain structure was not obtained even at 77 K with the high angle boundary spacing only approaching the nanoscale in the sample normal direction. The cryogenically deformed material was unstable on sub- sequent deformation at room temperature and underwent rapid dynamic grain growth. Dynamic grain coarsening is shown to limit the minimum grain size achievable in an SPD process, even under cryogenic conditions. Introduction At the ultra-high strains seen in severe plastic deformation (SPD) a steady state grain size is ultimately approached which restricts the level of grain refinement that can be achieved [1–3]. For example, during equal channel angular extrusion (ECAE) of Al-alloys, the high angle boundary (HAB) spacing reaches a minimum when it converges with subgrain size and the fraction of HAB area typically saturates at *70–80% [1]. This limiting grain size has been attributed to dynamic recovery [1, 2, 4]. However, the restoration processes involved are still poorly understood [4]. Similar to steady state subgrain sizes [5], the minimum grain size achievable in an SPD process is thought to be related to the temperature compensated strain rate, or Zener-Holloman parameter (Z)[2, 4]. It is, therefore, pos- sible to achieve a smaller grain size by lowering the deformation temperature, which results in the suppression of thermally activated recovery processes. Indeed, cryo- genic deformation has been claimed to lead to the formation of nanocrystalline structures in Cu and other alloys [6–8] offering the prospect of being able to produce nanograined materials by SPD in a bulk form. Typically, this involves heavily rolling materials at liquid nitrogen temperatures, which have already been deformed by a SPD technique like ECAE to develop a starting submicron grain structure [6, 7]. Because of the fine nanoscale of the crystallite sizes produced in cryogenic SPD processing, much of the infor- mation in the literature is based on TEM evidence (e.g. [6–8]). As a result, reliable data has frequently not been obtained that fully characterizes the misorientations of the boundaries within the materials. It is thus often unclear whether the deformation structures produced are true nanograin structures or still contain a large fraction of low angle boundaries (LABs). In addition, nanograined mate- rials produced by deformation at cryogenic temperatures are likely to be unstable at room temperature, due to their highly non-equilibrium internal state. A potential concern is the rapid dynamic grain coarsening that has been reported to occur during deformation of nanograined materials manu- factured by vapour deposition routes [9–12]. This phenomenon has not been extensively investigated in severely deformed alloys, but is related to the same resto- ration processes that control the limiting grain size during SPD processing. The aim of the work presented is to investigate the effect of low temperatures on the minimum grain size that can be P. B. Prangnell (&)  Y. Huang Manchester Materials Science Centre, The University of Manchester, Grosvenor Street, Manchester M17HS, UK e-mail: philip.prangnell@manchester.ac.uk Y. Huang e-mail: Yan.huang-2@manchester.ac.uk 123 J Mater Sci (2008) 43:7280–7285 DOI 10.1007/s10853-008-2673-3