Taku Sakai

Bio: Taku Sakai is an academic researcher from University of Electro-Communications. The author has contributed to research in topics: Dynamic recrystallization & Recrystallization (metallurgy). The author has an hindex of 43, co-authored 189 publications receiving 6691 citations.

Grain refinement in copper under large strain deformation

Abstract: Structure evolution taking place in pure polycrystalline copper was studied in multiple compressions at room temperature. Rectangular samples were compressed with consequent change in the loading direction from pass to pass. The deformation behaviour at high strains of above 2 shows an apparent steadystate flow following a rapid rise in the flow stress at an early stage of deformation. The structural changes are characterized by the evolution of many mutually crossing subboundaries at low to moderate strains, finally followed by the development of very fine grains with medium- to large-angle boundaries at large strains. These new grains are concluded to be evolved by a kind of continuous reaction, that is continuous dynamic recrystallization (DRX). The grains developed under continuous DRX are much finer than expected from the extrapolation of discontinuous DRX data for hot deformation. An average grain size of about 0.2μ evolved at room temperature is roughly similar to that for subgrains develo...

Nucleation and microtexture development under dynamic recrystallization of copper

Abstract: Microstructure and microtexture evolution during dynamic recrystallization (DRX) was investigated in compression of polycrystalline copper in the temperature range from 473 K to 723 K and at strain rates from 10−3s−1 to 10−1s−1. A compression texture of near 〈101〉 direction, evolved by low temperature deformation, is gradually weakened and randomized by the progress of DRX at higher temperature, where 〈101〉 component still exists. New DRX grains are evolved by the operation of bulging of serrated grain boundaries, which is accompanied either by rotation of a bulged portion or twinning at the back of the migrating boundary. The mechanisms of dynamic nucleation and necklace DRX are discussed.

Dynamic recrystallization under warm deformation of a 304 type austenitic stainless steel

Abstract: Warm (and hot) deformation of a 304 type austenitic stainless steel was studied in connection with microstructural developments in compression at temperatures of 873–1223 K (0.5–0.7 Tm) under strain rates of 10−4–10−1s−1. The two deformation domains can be categorized due to their different mechanical and microstructural behaviors. In the region of flow stresses lower than around 400 MPa, the deformation behaviors are typical for hot working accompanied with dynamic recrystallization (DRX). New grains are evolved mainly by dynamic bulging mechanism, which can be accelerated by the development of serrated grain boundaries and strain induced dislocation subboundaries. The relationship between dynamic grain sizes ranged from 2 to 7 μm and peak flow stress can be expressed by a power law function with a grain size exponent of −0.72. In contrast, in the region of flow stresses higher than 400 MPa, the deformation behaviors hardly depend on strain rate and temperature and so can be in the region of athermal deformation. The stress–strain curves under such warm deformation are similar to those affected only by dynamic recovery. The microstructures evolved at high strains are mainly characterized by the dense dislocation walls evolved in pancaked original grains, while grain boundary serration also takes place even at such warm deformation. Mechanisms of this microstructural evolution are discussed in combination with analysis of deformation mechanisms operating under warm deformation.

Recent developments in stainless steels

Abstract: This article presents an overview of the developments in stainless steels made since the 1990s. Some of the new applications that involve the use of stainless steel are also introduced. A brief introduction to the various classes of stainless steels, their precipitate phases and the status quo of their production around the globe is given first. The advances in a variety of subject areas that have been made recently will then be presented. These recent advances include (1) new findings on the various precipitate phases (the new J phase, new orientation relationships, new phase diagram for the Fe–Cr system, etc.); (2) new suggestions for the prevention/mitigation of the different problems and new methods for their detection/measurement and (3) new techniques for surface/bulk property enhancement (such as laser shot peening, grain boundary engineering and grain refinement). Recent developments in topics like phase prediction, stacking fault energy, superplasticity, metadynamic recrystallisation and the calculation of mechanical properties are introduced, too. In the end of this article, several new applications that involve the use of stainless steels are presented. Some of these are the use of austenitic stainless steels for signature authentication (magnetic recording), the utilisation of the cryogenic magnetic transition of the sigma phase for hot spot detection (the Sigmaplugs), the new Pt-enhanced radiopaque stainless steel (PERSS) coronary stents and stainless steel stents that may be used for magnetic drug targeting. Besides recent developments in conventional stainless steels, those in the high-nitrogen, low-Ni (or Ni-free) varieties are also introduced. These recent developments include new methods for attaining very high nitrogen contents, new guidelines for alloy design, the merits/demerits associated with high nitrogen contents, etc.