Acta Materialia 51 (2003) 847–861 www.actamat-journals.com Effect of initial microstructures on grain refinement in a stainless steel by large strain deformation A. Belyakov a,*,1 , K. Tsuzaki a , H. Miura b , T. Sakai b a Steel Research Center, National Institute for Materials Science, Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan b Department of Mechanical Engineering and Intelligent Systems, University of Electro-Communications, Chofu, Tokyo 182-8585, Japan Received 29 July 2002; received in revised form 7 October 2002; accepted 7 October 2002 Abstract Initial grain size effect on submicrocrystalline structure evolution was studied in multiple compressions of a 304 stainless steel at 873 K (0.5T m ). Four sets of specimens with different initial microstructures were used, i.e. annealed samples with grain sizes of D 0 = 15 and 2.2 μm, and dynamically recrystallised ones with D 0 = 3.5 and 1.5 μm. The new ultra-fine-grains (D = 0.25 μm) develop as a result of a continuous increase in the misorientations between the subgrains that evolved during deformation. In the samples with D 0 3.5 μm, the fraction of the strain-induced high- angle boundaries increases rapidly to more than 60% with a straining to about 1.5. On the other hand, their fraction does not exceed 20% at e = 1.5 in the sample with D 0 = 15 μm. The latter needs much more straining to around 6 to obtain 60% of high-angle (sub)grain boundaries.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Severe plastic deformation; Transmission electron microscopy; Stainless steels; Grain refinement; Misorientation 1. Introduction Submicrocrystalline metallic materials with grain sizes of tens to hundreds of nanometers offer improved mechanical properties continuously, which attract the attention of materials scientists [1,2]. Recently, a large strain plastic working at * Corresponding author. Tel.: +81-298-59-2185; fax: +81- 298-59-2101. E-mail address: andrey.belyakov@nims.go.jp (A. Belyakov). 1 On leave from the Institute for Metals Superplasticity Prob- lems, Ufa, Russia. 1359-6454/03/$30.00  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6454(02)00476-7 relatively low temperatures was utilised as a novel processing method to develop ultra-fine-grained structural metals and alloys with a grain size on a submicron scale [3–10]. The evolution of new grain structures occurring under plastic working is frequently discussed as a dynamic recrystallisation (DRX). The main characteristics of DRX during warm to hot deformation have been clarified [11– 17]. Two types of DRX based on the operating structural mechanisms that result in new grain development are commonly discussed in literature. The new grain evolution that takes place in materials with low to medium stacking fault energy (SFE) is associated with the discontinuous DRX.