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and Capital Markets and the Economy workshops, and the Stanford Institute for Theoretical Economics for useful comments. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research. NBER working papers are circulated for discussion and comment purposes. They have not been peerreviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications.

a b s t r a c tWe present an experimental study of the kinematics of twist extrusion (TE) and show that TE has the following properties: (i) as in equal-channel angular pressing (ECAP), the mode of deformation in twist extrusion is simple shear. Unlike in ECAP, there are two shear planes; one of them is perpendicular and the other is parallel to the specimen axis. (ii) The following processes are present during twist extrusion: vortex-like flow with large strain gradient, stretching and mixing of metal particles. We argue that, due to these properties, TE opens possibilities for investigating and forming new microstructures. It has already been successfully used to obtain ultrafine-grained microstructures with good properties in Al, Cu and Ti alloys.

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The present work deals with achieving improvement in the mechanical properties of SUS304L stainless steel through the application of a unique microstructure design termed as 'Harmonic structure', and establishing a co-relationship between various microstructural characteristics and mechanical properties. Harmonic structure essentially means a bimodal grain size distribution with a specific periodic arrangement of coarse-and ultrafine-grain fractions. SUS304L stainless steel samples having such microstructure were fabricated by a powder metallurgy route involving the mechanical milling of pre-alloyed steel powder followed by spark plasma sintering. Due to these peculiar microstructural characteristics, the harmonic-structured SUS304L stainless steels demonstrated a winning combination of high strength, large uniform elongation, and large total elongation to failure, simultaneously. It was also found that the fraction of a shell area (a three-dimensional continuously connected network of ultrafine-grained structure) is an important parameter controlling the balance of the mechanical properties of the harmonic-structured SUS304L steel compacts.

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High purity Al (99.99%) was subjected to severe plastic deformation through twist extrusion at room temperature. Microstructures were examined for 1 pass and 4 passes on the cross section perpendicular to the longitudinal axis of billets using optical microscopy and electron back scatter diffraction analysis. It was shown that a vortex-like material flow was observed on the cross section and this became more intense with increasing number of the pressing. After one pass, subgrain structures with low angle grain boundaries were developed throughout the section but after 4 passes, the microstructure consisted of grains surrounded by high angle boundaries with fraction of $70% in the edge parts. The average grain size at the edge parts is refined to $1:6 mm.