Yoritoshi Minamino

Strength and ductility of ultrafine grained aluminum and iron produced by ARB and annealing

Abstract Strength and ductility of ultrafine grained (UFG) aluminum and iron fabricated by ARB and annealing were clarified in the grain sizes ranging from 200 nm to 20 μm. Strength held Hall–Petch relationship, while uniform elongation of the UFG materials was limited below a few percents. The limited uniform elongation in the UFG materials could be explained in terms of plastic instability.

Ultragrain refinement of plain low carbon steel by cold- rolling and annealing of martensite

Simple cold-rolling and annealing of martensite starting structure can produce ultrafine grained structure in carbon steel. The microstructural evolution during the process was studied in a 0.13%C steel. The ultrafine lamellar dislocation cells (LDCs) with mean thickness of 60 nm were mainly observed in a 50% cold-rolled specimen as well as the irregularly bent lamellas (IBLs) and the kinked laths (KLs). The LDCs and the IBLs had large local misorientations. The specimens annealed at temperatures from 723 to 773 K showed the multiphased ultrafine structure composed of equiaxed ultrafine ferrite grains with the mean grain size of 180 nm, nano-carbides distributed uniformly and small blocks of tempered martensite. The formation of the ultrafine grained structure was discussed from the viewpoint of characteristics of the martensite starting structure. It was concluded that the fine grained structure of martensite play an important role for ultrafine grain subdivision during plastic deformation.

Microstructural evolution during accumulative roll-bonding of commercial purity aluminum

The microstructure in commercial purity aluminum deformed from medium to high strain (evM=1.6–6.4) by accumulative roll-bonding (ARB) at 473 K was quantitatively examined by transmission electron microscopy. It was found that a sub-micrometer lamellar structure characterizes the microstructure at high strains (evM>1.6), and that the lamellar boundary spacing decreases and the misorientation across the lamellar boundaries increases with increasing rolling strain. This characteristic evolution has also been observed during conventional cold-rolling of commercial purity aluminum. However, a comparison between the two processes shows a significant difference in the evolution of the microstructural parameters. These differences are discussed based on the different processing conditions characterizing ARB and conventional rolling, respectively.

A new and simple process to obtain nano-structured bulk low-carbon steel with superior mechanical property

Abstract A new process to obtain ultrafine grained bulk steel was developed. Plain low-carbon steel sheet with martensite starting microstructure was simply cold-rolled by 50% and annealed. The specimens annealed at intermediate temperatures such as 773 K revealed the multiphased nano-structure and showed superior mechanical properties.

Nanoscale crystallographic analysis of ultrafine grained IF steel fabricated by ARB process

Abstract Nanoscale crystallographic features of ultrafine grained interstitial free steel fabricated by accumulative roll-bonding (ARB) process have been studied by electron back-scattered diffraction in field-emission type scanning electron microscope. This work has clearly indicated that most of the elongated ultrafine grains in the ARB processed sheet are surrounded by high-angle grain boundaries. The characteristic textures in the ARB processed sheet were also clarified.

Formation of nanocrystalline surface layers in various metallic materials by near surface severe plastic deformation

Abstract The surface of the various kinds of metallic materials sheets were severely deformed by wire-brushing at ambient temperature to achieve nanocrystalline surface layer. The surface layers of the metallic materials developed by the near surface severe plastic deformation (NS-SPD) were characterized by means of TEM. Nearly equiaxed nanocrystals with grain sizes ranging from 30 to 200 nm were observed in the near surface regions of all the severely scratched metallic materials, which are Ti-added ultra-low carbon interstitial free steel, austenitic stainless steel (SUS304), 99.99 wt.%Al, commercial purity aluminum (A1050 and A1100), Al–Mg alloy (A5083), Al-4 wt.%Cu alloy, OFHC-Cu (C1020), Cu–Zn alloy (C2600) and Pb-1.5%Sn alloy. In case of the 1050-H24 aluminum, the depth of the surface nanocrystalline layer was about 15 μm. It was clarified that wire-brushing is an effective way of NS-SPD, and surface nanocrystallization can be easily achieved in most of metallic materials.

Microstructure and texture through thickness of ultralow carbon IF steel sheet severely deformed by accumulative roll-bonding

Abstract Ultralow carbon interstitial free (IF) steel was severely deformed up to a strain of 5.6 by the Accumulative Roll-bonding (ARB) process at 773 K. Crystallographic analysis by electron back-scattering diffraction (EBSD) technique in a field-emission type scanning electron microscope (FE-SEM) was carried out for the ARB processed IF steel throughout thickness of the sheet. Microstructural parameters, such as grain size, grain boundary misorientation and crystal orientation, through thickness of the ARB processed specimen were quantitatively clarified by the EBSD analysis. The ARB processed material was homogeneously filled with the lamellar or pancake-shaped ultrafine grains whose mean grain thickness were about 200–300 nm. More than 80% of the boundaries surrounding the ultrafine grains were high-angle grain boundaries. The ARB processed sheet had unique and complex textural distribution through thickness. The region near the thickness center has the conventional but quite weak rolling texture mainly composed of 〈110〉//RD and 〈111〉//ND. On the other hand, the surface region had the sharp shear texture, ND//〈110〉. Such a textural distribution is due to the redundant shear strain induced by high-friction between the sheet and roll during rolling. The correspondence between the textural and microstructural distribution and the shear strain distribution throughout thickness of the sheet was discussed.

Microstructural change of ultrafine-grained aluminum during high-speed plastic deformation

Abstract Effect of strain rate on microstructural change in deformation of the ultrafine grained (UFG) aluminum produced by severe plastic deformation (SPD) was studied. Commercial purity 1100 aluminum sheets were highly strained up to an equivalent strain of 4.8 by the Accumulative Roll-Bonding (ARB) process at ambient temperature. The ARB-processed sheets were found to be filled with pancake-shaped ultrafine grains surrounded by high-angle grain boundaries. The ultrafine grains had a mean grain thickness of 200 nm and a mean grain length of 1100 nm. The ultrafine-grained aluminum sheets were deformed at various strain rates ranging from 2 to 6.0×10 4 s −1 by conventional rolling, ultra-high-speed rolling, and impact compression. High-speed plastic deformation generates a large amount of heat, inducing coarsening of the ultrafine grains during and after deformation. On the other hand, it was also suggested that high-speed plastic deformation is effective for grain-subdivision, in other words, ultra-grain refinement, if the effect of heat generation is extracted.

Effect of rolling reduction on ultrafine grained structure and mechanical properties of low-carbon steel thermomechanically processed from martensite starting structure

Abstract The present authors have invented a novel and simple thermomechanical processing to realize the ultrafine grained microstructure in carbon steels. The key of the process is to start from martensite structure. In the previous study, it has been clarified that conventional cold-rolling to a reduction in thickness of only 50% (equivalent strain of 0.8) and subsequent annealing at warm temperature around 500 °C fabricates the multi-phased ultrafine grained structure composed of the ultrafine ferrite grains with mean grain size of 180 nm, uniformly precipitated nano cementite and tempered martensite. In this study, the effect of the rolling reduction ranging from 25 to 70% (equivalent strains of 0.3–1.5) on the ultrafine grained structure and the mechanical properties of the plain low-carbon steel (Fe–0.13 wt% C) processed from martensite starting structure was studied. In the as-deformed specimen, the area fraction of the region showing the lamellar structure, which is typical for severely rolled metals, increased with increasing the rolling reduction and the strength also increased. After annealing at warm temperature around 500 °C, the multi-phased ultrafine grained microstructures were obtained in all the examined rolling reductions. The area fraction of the region showing the ultrafine ferrite grains increased with increasing the rolling reduction. At higher temperature, conventional recrystallization took place, and the recrystallization temperature became lower with increasing the reduction. Tensile test exhibited that the specimen rolled to the intermediate reduction (50%) performed the best strength-ductility balance (870 MPa of tensile strength and 9% of uniform elongation). The reason for the good strength-ductility balance of the specimen rolled to the intermediate reduction was discussed on the basis of the observed microstructures. q 2003 Elsevier Ltd. All rights reserved.

Microstructures and mechanical properties of bulk nanocrystalline Fe–Al–C alloys made by mechanically alloying with subsequent spark plasma sintering

Abstract The microstructure and superior mechanical properties of bulk nanocrystalline Fe–Al–C alloys made by mechanically alloying (MA) with subsequent spark plasma sintering (SPS) were investigated. Three kinds of nanocrystalline Fe–24 at% Al–X at%C (X = 1, 2, 4) alloy powder were produced by MA from iron and aluminum powder with addition of methanol, and were subsequently consolidated at 1073–1273 K under 64 MPa by SPS. These compacts have the relative densities of 99.97% (1 at%C) to 99.5% (4 at%C). The structure of compacts with 1at%C is composed of grains of Fe3Al of 1.5 μm in diameter and nano κ-carbides (Fe3AlC0.5) precipitates, while those of compacts with 2 and 4 at%C are composed of nanocrystalline Fe3Al of about 80 nm in diameter, nano κ-carbides and small amount of large α-grains of about 1 μm in diameter. These structures maintain the nanostructure even at 973 K, that is, they have the good thermal stability. The mechanical properties of these compacts were measured by compression tests at room temperature (RT) to 973 K in vacuum. The compacts with 1 and 2 at%C of this work perform the superior mechanical properties (e.g. yield strength of 2150 MPa and rupture strain of 0.14 for compact with 2 at%C at R.T.) when compared with the ordinary Fe3Al casting (e.g. the yield strength of 380 MPa and rupture strain of 0.12).