Tetsuo Sakai

Bio: Tetsuo Sakai is an academic researcher from Osaka University. The author has contributed to research in topics: Recrystallization (metallurgy) & Accumulative roll bonding. The author has an hindex of 21, co-authored 86 publications receiving 4366 citations.

Novel ultra-high straining process for bulk materials—development of the accumulative roll-bonding (ARB) process

Abstract: A novel intense plastic straining process named accumulative roll-bonding (ARB) is proposed. First, a strip is neatly placed on top of another strip. The two layers of material are joined together by rolling like a roll-bonding process. Then, the length of rolled material is sectioned into two halves. The sectioned strips are again stacked and roll-bonded. The whole process is repeated again and again. The process can introduce ultra-high plastic strain without any geometrical change if the reduction in thickness is maintained to 50% every rolling pass. The process has been applied to commercial aluminum (1100), Al–Mg alloy (5083) and interstitial free (IF) steel. Well-bonded bulk materials were successfully obtained. After several cycles of ARB, ultra-fine (sub-micron) grain structure with large misorientations, i.e. polycrystal, was formed and the materials were strengthened dramatically.

Role of shear strain in ultragrain refinement by accumulative roll-bonding (ARB) process

Abstract: The role of shear strain on ultragrain refinement of aluminum during accumulative roll-bonding (ARB) process was studied. The complicated distribution of redundant shear strain through thickness of the ARB processed sheets was quantitatively shown first, and good correspondence between the grain size and the amount of shear strain was found.

Effect of redundant shear strain on microstructure and texture evolution during accumulative roll-bonding in ultralow carbon IF steel

Abstract: Accumulative roll-bonding (ARB) is a severe plastic deformation process that can effectively produce ultrafine grained (UFG) structures in metals and alloys. In previous investigations, the ARB process has often been carried out under high-friction conditions without any lubricant between materials and rolls, which may cause a large amount of redundant shear strain near the sheet surface. Owing to repetition of cutting, stacking and roll-bonding in the ARB, a complicated redundant shear strain distribution is expected through the sheet thickness. The purpose of the present study is to clarify the effect of the redundant shear strain on the microstructure and texture evolution during ARB. A Ti-added ultralow carbon interstitial free steel was deformed by up to seven cycles of ARB (a thickness reduction of 99.2%) at 500 °C, with or without lubrication, in order to investigate the effect of shear strain. Microstructural characterization by electron backscatter diffraction analysis was carried out at various thickness locations of the ARB processed sheets. The sheet processed by one cycle of ARB with good lubrication showed typical deformation microstructures uniformly throughout the thickness. In contrast, the specimen processed by one ARB cycle without lubrication had an inhomogeneous microstructure, and the fraction of deformation-induced high-angle boundaries increased close to the surface. Non-lubricated ARB caused through-thickness microstructural heterogeneity in low numbers of cycles, but repetition of ARB above five cycles finally produced quite uniform UFG structures. It was established that the microstructural parameters of the deformation structures can be basically understood in terms of the total equivalent strain, taking account of the redundant shear strain.

Deformation mechanism and texture and microstructure evolution during high-speed rolling of AZ31B Mg sheets

Abstract: High-speed rolling of AZ31B was carried out under various preheating temperatures from RT to 350 °C. The evolution of texture, grain sizes, and dislocation density distribution (Kernel average misorientation distributions) in the mid-thickness and surface layer were investigated. Computer simulations of deformation textures were also performed in order to understand deformation mechanisms. It is concluded that the temperature increase due to the plastic and frictional working during high-speed rolling makes the slip system more active and, hence, improves the ductility. The surface layer of the specimen has higher temperature and experiences severe shear stress; therefore the texture, microstructure, and dislocation density distribution are different from those of the mid-thickness of the specimen. Both mid-thickness and surface layer are dynamically recrystallized during the high-speed rolling.

Principles of equal-channel angular pressing as a processing tool for grain refinement

Abstract: During the last decade, equal-channel angular pressing (ECAP) has emerged as a widely-known procedure for the fabrication of ultrafine-grained metals and alloys. This review examines recent developments related to the use of ECAP for grain refinement including modifying conventional ECAP to increase the process efficiency and techniques for up-scaling the procedure and for the processing of hard-to-deform materials. Special attention is given to the basic principles of ECAP processing including the strain imposed in ECAP, the slip systems and shearing patterns associated with ECAP and the major experimental factors that influence ECAP including the die geometry and pressing regimes. It is demonstrated that all of these fundamental and experimental parameters play an essential role in microstructural refinement during the pressing operation. Attention is directed to the significant features of the microstructures produced by ECAP in single crystals, polycrystalline materials with both a single phase and multi-phases, and metal–matrix composites. It is shown that the formation of ultrafine grains in metals and alloys underlies a very significant enhancement in their mechanical and functional properties. Nevertheless, it is demonstrated also that, in order to achieve advanced properties after processing by ECAP, it is necessary to control a wide range of microstructural parameters including the grain boundary misorientations, the crystallographic texture and the distributions of any second phases. Significant progress has been made in the development of ECAP in recent years, thereby suggesting there are excellent prospects for the future successful incorporation of the ECAP process into commercial manufacturing operations.

Using high-pressure torsion for metal processing: Fundamentals and applications

Abstract: High-pressure torsion (HPT) refers to the processing of metals whereby samples are subjected to a compressive force and concurrent torsional straining. Although the fundamental principles of this procedure were first proposed more than 60 years ago, processing by HPT became of major importance only within the last 20 years when it was recognized that this metal forming process provides an opportunity for achieving exceptional grain refinement, often to the nanometer level, and exceptionally high strength. This review summarizes the background and basic principles of processing by HPT and then outlines the most significant recent developments reported for materials processed by HPT. It is demonstrated that HPT processing leads to an excellent value for the strength of the material, reasonable microstructural homogeneity if the processing is continued through a sufficient number of torsional revolutions and there is a potential for achieving a capability for various attractive features including superplastic forming and hydrogen storage. The review also describes very recent developments including the application of HPT processing to bulk and ring samples and the use of HPT for the consolidation of powders.

Novel ultra-high straining process for bulk materials—development of the accumulative roll-bonding (ARB) process

Abstract: A novel intense plastic straining process named accumulative roll-bonding (ARB) is proposed. First, a strip is neatly placed on top of another strip. The two layers of material are joined together by rolling like a roll-bonding process. Then, the length of rolled material is sectioned into two halves. The sectioned strips are again stacked and roll-bonded. The whole process is repeated again and again. The process can introduce ultra-high plastic strain without any geometrical change if the reduction in thickness is maintained to 50% every rolling pass. The process has been applied to commercial aluminum (1100), Al–Mg alloy (5083) and interstitial free (IF) steel. Well-bonded bulk materials were successfully obtained. After several cycles of ARB, ultra-fine (sub-micron) grain structure with large misorientations, i.e. polycrystal, was formed and the materials were strengthened dramatically.

Dynamic and post-dynamic recrystallization under hot, cold and severe plastic deformation conditions

Abstract: The evolution of the new microstructures produced by two types of dynamic recrystallization is reviewed, including those brought about by severe plastic deformation (SPD). The microstructural changes taking place under these conditions and the associated mechanical behaviors are described. During the conventional discontinuous dynamic recrystallization (dDRX) that takes place at elevated temperatures, the new grains evolve by nucleation and growth in materials with low to medium stacking fault energies (SFE). On the other hand, new ultrafine grains can be produced in any material irrespective of the SFE by means of SPD at relatively low temperatures. These result from the gradual transformation of the dislocation sub-boundaries produced at low strains into ultrafine grains with high angle boundaries at large strains. This process, termed in situ or continuous dynamic recrystallization (cDRX), is still not perfectly understood. This is because many SPD methods provide data concerning the microstructural changes that take place but little information regarding the flow stress behavior. By contrast, multi-directional forging (MDF) provides both types of data concurrently. Recent studies of the deformation behavior of metals and alloys under SPD conditions, carried out using MDF as well as other SPD methods, are synthesized and the links between the microstructural and mechanical observations are examined carefully. Some models for grain formation under SPD conditions are discussed. Next, the post-dynamic recrystallization behavior, i.e. that of annealing after both dDRX and cDRX, is described. The differing annealing behaviors result from the differences in the natures of the deformed microstructures. Finally, an integrated recrystallization model for these phenomena, i.e. dynamic and static recrystallization of both the continuous and discontinuous types, is presented and discussed.

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.