Showing papers by "Kaneaki Tsuzaki published in 2007"

Grain refinement in a commercial Al-Mg-Sc alloy under hot ECAP conditions

Abstract: Grain refinement taking place during equal-channel angular pressing (ECAP) was studied in a commercial Al–6% Mg–0.4% Mn–0.3% Sc alloy at a temperature of 450 °C (∼0.8 T m ) to demonstrate the feasibility of obtaining new fine-grained structure in a hard-to-deform Al–Mg–Sc alloy under hot intense plastic straining (IPS) conditions and investigate the evolution process of new grains. Inhomogeneous deformation occurring during hot ECAP leads to formation of deformation bands. Repeated ECAP results in mutual crossing and increase in number and the misorientation of deformation bands, followed by transformation of the boundaries of deformation bands into high-angle ones. As a result, a new fine-grained microstructure with an average crystallite size of 2.8 μm develops at large strains above 8. It is concluded that grain refinement occurs in accordance with deformation-induced continuous reactions; that is similar to in-situ or continuous dynamic recrystallization. The mechanisms of new grain evolution, as well as factors promoting grain refinement, are discussed in detail.

Plasticity initiation and subsequent deformation behavior in the vicinity of single grain boundary investigated through nanoindentation technique

Abstract: The initiation of plasticity and the subsequent state in the vicinity of a single grain boundary during indentation-induced deformation were investigated to understand an elementary step of a stress-strain behavior of polycrystalline materials. Nanoindentation measurements on several points on a single grain boundary and the grain interior of an interstitial-free steel and an analysis on the pop-in behavior and the plastic nanohardness were carried out. The pop-in load P c that was obtained on the loading curve is different for each measurement. However, the loading curves overlap one another and the unloading curves coincide as well after the pop-in event. The nanohardness Hn has no dependence on the P c in the range of 150–550 μN. The relation between P c and Δh can be expressed as a simple cubic polynomial function based on a geometrically necessary dislocation loop model. The fitted function differed for various grains with different crystallographic orientations.

Comparative study on microstructure evolution upon unidirectional and multidirectional cold working in an Fe–15%Cr ferritic alloy

Abstract: Deformation microstructures of an Fe–15%Cr ferritic stainless steel during cold working to strains above 7 were studied. Two processing methods, unidirectional working consisting of bar rolling followed by swaging and multidirectional working involving multiple forging through three orthogonal axes, were used. The unidirectional deformation resulted in the evolution of a ribbon-like microstructure consisting of highly elongated grains. The mean grain size gradually decreased to about 0.4 μm during rolling/swaging to strains above 7. In contrast, the multiple forging resulted in the development of almost equiaxed fine grains with an average grain size of 0.45 μm at relatively small strains of about 4 followed by a slight grain refinement during further processing. On the other hand, both processing methods were characterised by almost the same strain dependence for the transverse grain and subgrain sizes, which were measured crosswise to the metal flow direction.

Annealing behavior of a ferritic stainless steel subjected to large-strain cold working

Abstract: Mechanisms of microstructure evolution during annealing after cold working were studied in an Fe-15%Cr ferritic stainless steel, which was processed by bar rolling/swaging to various total strains ranging from 1.0 to 7.3 at ambient temperature. Two types of recrystallization behavior were observed depending on the cold strain. An ordinary primary (discontinuous) recrystallization developed in the samples processed to conventional strains of 1.0–2.0. On the other hand, rapid recovery at early annealing resulted in ultrafine-grained microstructures in the larger strained samples that continuously coarsened on further annealing. Such annealing behavior was considered as continuous recrystallization.

Incomplete recrystallization in cold worked steel containing TiC

Abstract: Development of primary recrystallization was studied in a steel containing 0.4 vol.% of fine TiC precipitations with an average size of 12 nm. After sufficiently large cold strains, the recrystallization developed readily upon annealing at temperatures above 600 °C. An increase in the cold strain as well as the annealing temperature resulted in the acceleration of recrystallization kinetics. However, a certain amount of cold worked microstructures of about 15 vol.% remained unrecrystallized even after annealing at a rather high temperature of 700 °C. The unrecrystallized portions were composed of grains with the 〈0 0 1〉 crystallographic direction parallel to the compression axis. Both the low stored energies in these grains and the pinning of recrystallizing grain boundaries by the dispersed carbides were discussed as crucial factors that resulted in the incomplete recrystallization.

Tracing the Goss Orientation during Deformation and Annealing of an FeSi Single Crystal

Abstract: A Goss-oriented single crystal was cold rolled up to 89 % thickness reduction, and subsequently annealed at 550°C or 850°C. During deformation most of the initially Goss-oriented material rotated into the two symmetrical {111} orientations. In addition, Goss regions were observed related to microbands or microshear bands. Goss regions in microshear bands formed during straining, whereas Goss regions between microbands were retained from the initial Goss orientation. The recrystallisation texture for annealing temperatures of both 550°C and 850°C is characterised by a Goss texture. However, the origin of the Goss recrystallisation nuclei appeared to be different for the different annealing conditions. In the material annealed at 550°C, the Goss texture originated from the Goss regions in the microshear bands. In contrast, for an annealing temperature of 850°C, the Goss grains between the microbands are likely to form recrystallisation nuclei.

Application of Local Approach to Hydrogen Embrittlement Fracture Evaluation of High Strength Steels

Abstract: A new method for evaluating the hydrogen embrittlement (HE) susceptibility of ultra high strength steel was studied in order to propose a new method for assessing the delayed fracture property. The material used was 1400MPa tempered martensitic steel with the chemical composition 0.40C-0.24Si-0.81Mn-1.03Cr-0.16Mo(mass%). The local approach originally used for evaluating the brittle fracture property was applied to HE susceptibility assessment after modifying the method to include the effect of hydrogen content. Critical HE data used in the modified local approach was obtained by a stepwise test in which alternating processes of stress increase and stress holding were repeated until the specimen fractured. The specimen used in the stepwise test was 10 mm in diameter and the stress concentration factor was 4.9. Assessment of HE susceptibility for specimens with other dimensions entailed the use of a critical hydrogen content for failure, Hc, representing the maximum hydrogen content among the unfractured specimens in the HE test with constant loading. Matters to be noted for obtaining the material parameters are discussed.

Microstructural evolution in a commercial Al-Mg-Sc Alloy during ECAP at 300°C

Abstract: Microstructural evolution taking place during equal channel angular pressing (ECAP) was studied in a commercial coarse-grained Al-6%Mg-04%Mn-03%Sc alloy at a temperature of 300oC (~06Tm) Samples were pressed using route A to a total strain of 12 and quenched in water after each ECAP pass ECAP at moderate-to-high strains leads to the formation of a bimodal grain structure with grain sizes of around 1 and 8 μm and volume fractions of 03 and 06, respectively The development of new-grained regions has been shown to result from a concurrent operation of continuous dynamic recrystallization that occurs during deformation and static recrystallization that occurs during each ECAP cycle by the exposure of the as-deformed material in the die kept at 300oC for around 15 minutes The microstructural development during warm-to-hot ECAP is discussed in terms of the enhanced driving force for recrystallization, resulting from the evolution of high-density dislocation substructures due to the localization of plastic flow and inhibition of recovery in the present alloy

Corrosion behavior of Al-7wt% Si-1.5wt% Cu severely deformed by equal-channel angular pressing

Abstract: In this study, an Al-7 wt% Si-1.5 wt% Cu alloy was subjected to severe plastic deformation (SPD) by an equal-channel angular pressing (ECAP) technique. The ECAP process was repetitively carried out up to 8 passes using a strain introduction method of route BC, at a temperature of 25 °C and a pressing rate of 0.33 mm s-1. Microstructures of the samples before and after ECAP were observed by a scanning electron microscopy (SEM). Electrochemical properties of the Al-Si-Cu alloy fabricated by ECAP have been investigated in a borate-boric acid buffer solution containing Cl¯ ions at pH 8.3 and 25 °C by potentiodynamic polarization test. Corrosion pits on the sample surface after anodic polarization were investigated by means of SEM. The anodic polarization showed that as-cast Al-Si-Cu alloy with plate-shaped Si particles has poor resistance against pitting corrosion comparing to quenched sample without ECAP. Pitting potentials of ECAPed Al-Si-Cu alloy samples were higher than that of the sample without ECAP. In the Al-Si-Cu alloy, the corrosion pits were found in the region of Si particles and the size of pits formed on the ECAPed samples became smaller than that without ECAP. It is considered that the improvement of the pitting resistance of ECAPed Al-Si-Cu alloy is due to homogenous distribution of spherical Si particles generated during ECAP process.

Texture Invariant Annealing in Severely Deformed Steel

Abstract: This paper showed an example of the phenomena that a strong deformed texture does not change after the annealing process in steels An Fe – 22%Cr – 3%Ni ferritic stainless steel was processed by bar rolling/swaging to a total strain of 44 at an ambient temperature, and its annealing behaviour was studied in a temperature range of 400 ~ 700oC The deformed sample showed a grain size of 200nm, a fraction of high-angle boundary (HAB) of about 06, and a strong fiber texture of {uvw} This texture showed very little change after annealing which was characterised by the development of continuous recrystallization involving recovery processes and followed by a normal grain growth On the other hand, by annealing a sample that was deformed to total strain of 20 containing rather fine grains (270nm) but without a large enough fraction of HAB (03), a discontinuous recrystallization took place, and its deformed texture changed considerably

Microband-To-Microshear Band Transition near Grain Boundaries in BCC Steel

Abstract: Compression tests were performed on Fe-3%Si specimens with few grains. The deformation microstructure and microtexture were investigated by electron backscatter diffraction (EBSD) and related to the initial crystal orientation and grain boundary characteristics. Groups of microbands were found that are characterised by a periodic change in crystal orientation, shear at the grain boundary, and the formation of new grains. It is supposed that these microband groups represent an early stage of microshear band development.

Recovery in 15%Cr ferritic stainless steel after large strain deformation

Abstract: 15%Cr ferritic stainless steel was machined in rectangular samples and then processed by multiple forging to a total cumulative strain of 7.2 at an ambient temperature. The large strain deformation resulted in almost equiaxed submicrocrystalline structure with a mean grain/subgrain size of 230 nm and about 2.2×1014 m-2 dislocation density in grain/subgrain interiors. The annealing at a relatively low temperature of 500oC did not lead to any discontinuous recrystallizations. The grain/subgrain size and the interior dislocation density slightly changed to 240 nm and 2.1×1014 m-2, respectively, after annealing for 30 min, while the Vickers hardness decreased from 3140 MPa in the as-processed state to 2900 MPa. This annealing softening was attributed to remarkable release (by 50%) of internal stresses, which are associated with a non-equilibrium character of strain-induced grain/subgrain boundaries.

Ultra rapid softening of high strength structural steels by thermomechanical treatment

Abstract: This study aims to shorten the softening treatment period as possible in high strength structural steels. The steel used is SCM440 steel. As an initial microstructure, martensite, bainite, pearlite and complicated microstructure consisting of ultrafine polygonal, martensite and equiaxed cementite were extensively examined to understand their softening process on aging at 973K. These initial microstructures were prepared by heat or thermomechanical treatment. Their initial Vickers hardness (Hv(10kgf)) were 634, 281, 219 and 238, respectively. It is noteworthy that within five minutes on aging hardness of the complicated microstructure reached lower than Hv200, while it took more than several hours for other initial microstructures. A quantitative evaluation of microstructures appears to help in understanding the mechanism of the softening kinetics.