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We investigate the effect of the cold reduction rate on ferrite recrystallization behavior during the annealing of low-carbon steel with different initial microstructures. Three types of hot-rolled sheet specimens are prepared: specimens P, B, and M, which consist of ferrite and pearlite, bainite, and martensite, respectively. To evaluate the effect of the cold reduction rate on ferrite recrystallization behavior, hot-rolled sheet specimens are cold-rolled at cold reduction rates of 40% and 67%. The cold-rolled sheet specimens are heated to the target temperature, and then water-quenched to room temperature. Irrespective of the initial microstructures, the ferrite recrystallization is accelerated by increasing the cold reduction rate. In addition, the dislocation densities of specimens P and B increase at the larger cold reduction rate, which accelerates ferrite recrystallization in these specimens. In the case of specimen M, the dislocation arrangement parameter remarkably decreases at the larger cold reduction rate, whereas the dislocation density hardly changes. Thus, we conclude that the accelerated ferrite recrystallization at the larger cold reduction rate for specimen M can be mainly attributed to an increase in the amount of interactions between dislocations in the specimen.

The design of new materials with useful properties is becoming increasingly important. Machine-learning tools Materials Genome Integration System Phase and Property Analysis (MIPHA) and rMIPHA (based on the R programming environment) have been independently developed to accelerate the process of materials discovery via a data-driven materials research approach. In the present work, MIPHA and rMIPHA are applied to steel, where machine-learning-based 2D/3D microstructural analysis, direct analysis of property predictions, and properties-to-microstructure inverse analysis were conducted. The results demonstrate that the prediction models deliver satisfactory performance. The inverse exploration of microstructures related to desired target properties (e.g., stress–strain curve, tensile strength, and total elongation) was realized. MIPHA and rMIPHA are still under improvement. The microstructure-to-processing inverse analysis is expected to be realized in the future., ファイル公開：2021-01-28

In this study, scale-bridging analysis of heterogeneous deformation behavior in SUS310S austenitic stainless steel was conducted using X-ray line profile analysis, digital image correlation (DIC), and transmission electron microscopy (TEM). Specimens were prepared using tensile strain of 3%–40% at 298 ± 2 K. According to the X-ray line profile analysis, increasing strain increased dislocation density and decreased crystallite size. Furthermore, using DIC, low- and high-strain regions were observed in the early stages of deformation. TEM analysis showed that dislocations in the low-strain region were rarely tangled, while in the high-strain region, tangled dislocation and increased dislocation density were observed. TEM analysis also demonstrated that the formation of dislocation cells was promoted in the later stages of deformation, and the size of dislocation cells decreased as the strain increased. Moreover, we showed quantitative relationships between dislocation density, dislocation cell size, and crystallite size based on our findings.

Various industrial applications require materials with both high strength and good ductility. However, strategies for enhancing material performance are usually trapped in a strength–ductility trade-off. In this study, the effect of martensite morphologies (including geometry, connectivity, and distribution direction) on the stress–strain partitioning behavior and mechanical properties of a ferrite-martensite dual phase steel was studied using the secant method and finite element analysis. The results demonstrated that the combination of rhombus and horizontal geometries provided a good balance of strength and ductility. Thus, a combination of 45° and 0° regarding the angle between the directions of martensite distribution and deformation was further determined to be beneficial to the strength–ductility balance. These results are expected to provide a general understanding of strengthening and toughening mechanisms and will promote the development of high-performance steels.

Three-dimensional microstructure observations, macro- to micro-scopic residual stress measurements by three methods and creviced bent beam SCC tests were performed for Inconel/low alloy steel (LAS) weld samples. The possible reasons for the suppression of SCC crack propagation near the weld interface found at a nuclear power plant were estimated to include the crack branching at the grain boundary (GB) parallel to the interface, i.e., Type II GB, compressive residual stresses in the LAS region and crack tip oxidation in the LAS at the interface. The formation mechanism of Type II GB and stress gradient in individual grains in the Inconel are also discussed.

In order to clarify the mechanism of pseudoelasticity in Fe­23.0Al (at%) single crystals with the D03 structure, in situ observation of tensile deformation was performed at room temperature using an optical microscope (OM), an electron back-scatter diffraction (EBSD) device and an X-ray Laue back-reflection (XLBR) machine. In the present study, neither martensite nor twin was observed during the pseudoelasticity. In particular, the whole of the tensile specimen could be indexed with respect to the D03 structure by in situ EBSD measurement. (��), (� 101) and (� 211) slips were confirmed to occur depending on the loading axis by in situ OM observation. When (�) [111] slip was activated, coarse slip bands parallel to (� 101) slip plane were formed during loading. The area fraction of the slip bands increased linearly with increasing plastic strain up to 0.066. During unloading, the slip bands disappeared, resulting in pseudoelasticity. From in situ XLBR tests, the crystal rotation of the loading axis toward [111] slip direction took place during loading. The rotation angle and the plastic strain satisfied the Schmid-Boas relationship at small plastic strains, especially for (��) and (� 211) slips. These results suggest that the reversible motion of 1=4h111i superpartial dislocation dragging an antiphase boundary (APB) was responsible for the pseudoelasticity in Fe­23.0Al single crystals. [doi:10.2320/matertrans.M2013461]

The transformation of austenite to martensite in an Al–Mn–Si transformation-induced plasticity steel was investigated with in situ electron backscatter diffraction (EBSD) measurements under tensile straining. The visualisation of the microstructure upon straining allows for an investigation of the stability of austenite grains against strain-induced transformation, with particular focus on the grain size and the location of the austenite grains. The findings confirm that size and location of austenite grains are significant parameters for their stability. Small austenite grains were observed to be more stable than large grains, while austenite grains located beside bainitic ferrite are the most stable. Moreover, it is demonstrated that austenite grains transform gradually.

Lath martensite is an important structure in steels; however, its quantitative volume fractions have not been measured to date. In this study, we seek to clarify the morphologies and volume fractions of the structural units of lath martensite-packets and blocks—by using three-dimensional observations with local crystallographical analyses. Lath martensite structures in ultra-low carbon and high-carbon steels contain both coarse and small packets, whose threshold of volume fraction in prior austenite grain is 10%. The volume fractions of the coarse packets in prior austenite grain in the specimens are greater than 70%, although the number of packets is fewer than five. In ultra-low carbon steel, the coarse packets have flat packet boundaries and contain three types of blocks with a plate-like morphology. On the other hand, in high carbon steel the morphology of the coarse packets is sponge-like and contains six types of blocks. There are more small packets than coarse packets in the observed specimens, and the volume fraction of the small packets is lower. The number of small packets in high carbon martensite is more than that in the ultra-low carbon martensite. Volume measurements show a distribution of volume fractions of Kurdjumov–Sachs (K–S) variants. In high carbon martensite, all the K–S variants appear continuously. The volume fractions of some K–S variants are relatively low in ultra-low carbon martensite.

Two three-dimensional (3-D) techniques, namely X-ray microtomography and serial sectioning, have been applied for characterization of creep cavitation behavior in tempered martensitic steel. For this purpose samples have been extracted from a series of specimens that were subjected to creep tests over the stress range of 120–180 MPa at 600 °C. The presence of creep voids in the series of samples was un-ambiguously detected in a non-destructive manner using synchrotron X-ray microtomography with a resolution of 1 μm. The 3-D visualization of the datasets provided an assessment of the spatial distribution and morphology of the creep voids as a function of creep stress and high temperature exposure time. The quantitative analyses of the image datasets enabled the development of functional relationships between the macroscopic creep parameters (such as rupture ductility, applied stress, creep life) and cavitation characteristics (such as volume fraction, and number density). The quantitative analyses also provided an evaluation of manifestations of growth and coalescence processes in the respective datasets. A transition of cavitation behavior of the steel has been found to occur in the stress range of 120–150 MPa at 600 °C. The evolution in the pattern of cavitation and its relation to the prior-austenite boundary was explored by combining micro-tomography and serial sectioning techniques, which revealed a new possibility in the progress of cavitation in the long term creep exposed specimens of 9–12% Cr heat resistant steels.

1) Graduate student, Faculty of Engineering, Ibaraki University, Hitachi, 316-8511 Japan. 2) Graduate student, on leave from International Research Institute for Steel Technology, Wuhan University of Science and Technology, Wuhan, 430081 China. 3) Department of Materials Science and Engineering, Ibaraki University, Hitachi, 316-8511 Japan. 4) Department of Mechanical Engineering, Graduate School of Science and Engineering, Kagoshima University, Kagoshima, 890-8580 Japan.

The features present in 3D structure have geometric properties that fall into two broad categories: topological and metric. Metric properties are generally the more familiar; these include volume, surface area, line length and curvature. Equally or even more important in some applications are the topological properties of features. The two principal topological properties are number per unit volume and connectivity. In the present study, a change in morphology of pearlite and dual phase microstructures was examined from differential geometry and topology viewpoint. 3D images of eutectoid pearlite and dual phase steels were obtained by reconstructing serial sectioning images. Their metric and topological features were evaluated using The Euler Poincare formula and The Gauss-Bonnet Theorem. In addition, newly developed fully-automated serial sectioning 3D microscope “Genus_3D” will be also introduced.

In this study, three different microstructures of dual-phase (DP) steel obtained by varying the intercritical annealing temperature were observed three dimensionally by a serial-sectioning method and their metrics and topological properties were quantified. The results demonstrated that martensite in DP steels has various morphologies such as layers, particles, holes, and voids. The fraction of these characteristic morphologies was examined as a function of the intercritical annealing temperature.

1) National Institute for Materials Science, Innovative Materials Engineering Laboratory, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047 Japan. 2) Foremerly at National Institute for Materials Science, Innovative Materials Engineering Laboratory. Nowat Department of Mechanical Engineering, Graduate School of Science and Engineering, Kagoshima University, Korimoto 1-21-24, Kagoshima, 890-8580 Japan. E-mail: adachi@mech.kagoshima-u.ac.jp(

The nucleation of ferrite precipitates at austenite grain faces, edges (triple lines), and corners (quadruple points) was studied in a Co-15Fe alloy in which the matrix phase was retained upon cooling to room temperature by serial sectioning coupled with electron backscatter diffraction analysis. Nearly half of the edges and corners were vacant at an undercooling of 60 K from the γ/(α + γ) boundary where the precipitation occurred significantly at grain faces. A significant proportion of precipitates had Kurdjumov–Sachs (K–S) and to a lesser extent Nishiyama–Wassermann (N–W) orientation relationships with more than one grain at all boundary sites. Vacant edges and corners were readily observed, of which the misorientations of matrix grain boundaries would permit a precipitate to have a specific orientation relationship with multiple grains. Small differences in the nucleation activation energy among the grain faces, edges, and corners may lend support to a view proposed from experiments of nucleation in Fe-C base alloys that ferrite nuclei are more or less surrounded by low-energy facets of α/γ phase boundary.

The effect of ausforming on kinetics, morphology and crystallography of nanobainite steel was examined by electron backscattered diffraction and transmission electron microscopy. Ausforming has been found to accelerate bainite transformation at 573 K. A characteristic microstructure consisting of blocky bainitic laths and retained austenite is observed in the ausformed bainite steel, where strong variant selection takes place due to the operated slip systems.

To understand the reason of high work hardening in high nitrogen steel, a scale-bridging analysis consisting of in situ neutron diffraction, in situ electron backscattering diffraction measurement during tensile deformation and weak beam transmission electron microscopy was performed with particular attention to stress partitioning. This study demonstrates the contribution of stress partitioning, back stress due to dislocation pile-up at grain boundaries and dislocation interaction to work hardening at each strain level.

Differential geometry and toplogy-based three-dimensional (3D) analysis was conducted to understand pearlite spheroidization mechanism in an eutectoid steel. Morphological change during aging below A1 tempearture was examined in terms of Gaussian(K)/mean curvatures(H), genus and Euler characteristics based on 3D images.The holes presentnaturally grown cementite lamella caused shape instability andinduced shape evolution of the lamellar structure during spheroidization. 3D visualization demonstrated that the intrinsic holes played an important role in the initiation and development of pearlitespheroidization. The hole coalescence and expansion causedthe breakup of large cementite lamellae into several long narrow ribbons. H-K plot actually suggested that the number of thses holes decreased with increasing aging period. In addition, small cementite particles and narrow rod cementite decreased during aging. These microstractural evolutions were discussed from the view point of ferrite/cementite interfacial energy.

Acoustic emission (AE) monitoring for corrosion of small chloride droplet was proposed to investigate its mechanism. The corrosion testing in thin plate specimens of SUS304 stainless steel was conducted on an AE waveform measurement system with a high sensitivity AE sensor. Three types of corrosion were observed in the corrosion testing, which were the open pitting, covered pitting and snake-like corrosion, respectively. There were many AE events in the covered pitting and the snake-like corrosion. On the contrary, there was no AE event in the open pitting. Besides, the cross section of the snake-like corrosion was analyzed by EBSD. It shows that there were some cracks at the {111} interface of Σ3 coincidence boundary and annealing twins under the corrosion surface. Therefore, it can be concluded that the detected AE signals of corrosion testing were mainly attributed to the cracking in the covered pitting and the snake-like corrosion.

This study highlites deformation behavior of austenitic and pearlitic steels by in-situ neutron diffraction and 3D/4D EBSD measurement with a particular attention to their hierarchy.In particular stress partitioning in these microstructures is examined from macroscopic as well as microscopic scale length levels, and they are correlated to each other.

With a focus on microstructural hierarchy, work hardening behaviour in high nitrogen-bearing austenitic steel (HNS) was investigated mainly by a combined technique of in situ neutron diffraction and in situ electron backscattering diffraction (EBSD). Stress partitioning due to difference in deformability among grains is enhanced in HNS. The larger stress partitioning among [h k l]-oriented family grains seems to realize high work hardening at a small strain. At a larger strain, dislocation density is higher in HNS than in low nitrogen austenitic steel (LNS), which is a possible reason for high work hardening after straining proceeds, resulting in large uniform elongation.

Electron back-scattered diffraction in conjunction with transmission electron microscopy was employed to investigate the crystallographic nature of bainitic laths formed at relatively low transformation temperatures where a nanostructured bainite forms. It was revealed that the bainitic ferrite laths are close to the Nishiyama–Wasserman orientation relationship with the parent austenite. Furthermore, the temperature showed a significant effect on the retained austenite characteristics and the variant selection of the bainitic ferrite laths. A decrease in temperature generally refined the bainitic structure and weakened the variant selection.

The morphology and crystallography of sub-blocks in lath martensite were studied in an interstitial free steel. In each block the sub-blocks are classified into dominant and minor sub-blocks in terms of the volume fraction. The orientation relationship between the dominant and minor sub-blocks is [011]α′/10.5 degrees. Minor sub-blocks have a plate-like morphology and are connected to each other with the habit plane close to {111}γ, and their growth directions close to 〈10\\bar1〉γ.

Physical meaning of dynamic transformation was re-considered with a particular attention to the effect of post-transformation deformation on crystallography and kinetics of transformed ferrite. An advanced in-situ neutron diffraction experiment was performed to examine the microstructural evolution besides an advanced EBSD measurement. In particular, the deformation behavior in austenite/ferrite two-phase region was investigated as a function of deformation temperature and ferrite volume fraction. Based on these findings, the specific features of dynamic transformation were extensively discussed.

An Fe-3%Si bicrystal was deformed in compression to a strain of 0.9 at ambient temperature. In the interior of deformation bands, characteristic band structures with high orientation gradients and low and high angle boundaries were formed during straining. Furthermore, isolated ultrafine grains were found in the matrix of the deformation bands. The morphology and crystallography of the ultrafine grains indicate that they are caused by slip system partitioning, i.e. local activity of a highly stressed slip system that is different from the active slip system in the surrounding crystal volume.

The microstructures of two high-nickel martensite steels with different carbon contents before and after warm compression were comparably investigated by using a field-emission scanning electron microscope attached with an electron backscattering diffraction equipment (FE-SEM/EBSD) and a transmission electron microscope attached with an energy dispersive X-ray spectroscope (TEM/EDX). The microstructure observations suggest that the carbon addition is beneficial to reduce the critical strain for full recrystallization during warm compression and the increment of carbon-enriched austenite amount accelerates the dynamic recrystallization of ferrite through plastic deformation partitioning in the 17Ni–0.2C martensite steel. Proper pre-tempering promotes the precipitation of the carbon- and nickel-enriched austenite, and then promotes the dynamic recrystallization. On the other hand, long-time tempering leads to the carbon depletion in austenite so as to delay the dynamic recrystallization. The full recrystallization of the 18Ni martensite steel takes place at a higher strain during warm compression, mainly because of no carbon-enriched austenite.

The dynamic recrystallization behavior during warm compression for a low carbon martensite steel was investigated to make clear the effects of initial martensite block size, compression strain and pre-tempering before compression. It is found that the average size of recrystallized ferrite grains is influenced neither by the initial martensite block size (austenitizing temperature) nor by the amount of compression strain. The pre-tempering before compression shows two competitive effects: cementite particles precipitated during pre-tempering at 600°C promotes the occurrence of dynamic recrystallization while the decrease in dislocation density during pre-tempering at a higher temperature delays the dynamic recrystallization. Dispersed cementite particles suppress ferrite grain growth. Hence, there is an optimum tempering condition before warm-compression in order to obtain fine grained microstructure.

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.

Fe–3mass%Si specimens with a columnar grain structure were deformed in compression at ambient temperature. Groups of microbands were found in the vicinity of a grain boundary. These microband groups are characterized by shear and a periodic change of crystal lattice orientation within distances of 1–2 μm. Furthermore, new grains with a size of 200–300 nm were formed in the microband groups during deformation up to 36% strain. A local lattice rotation model is given that explains the experimental observations.

This study aims to analyze the potential of grain boundary corners (GBCs) as nucleation sites of bcc precipitates in an fcc matrix. By combined serial sectioning and electron backscatter diffraction analysis, the crystallography of GBCs and GBC precipitates was analyzed in a Co-Fe alloy.

The warm-compression microstructures of two nickel-containing tempered martensite steels with and without carbon were investigated. The EBSD measurement and TEM observation/Kikuchi pattern analysis confirm that the ultrafine microstructures contain ferriteaustenite dual phases with more than 18% austenite. The difference in the austenite amount before and after warm compression proves that the dynamic austenite precipitation occurs during warm deformation and the carbon addition is beneficial to the austenite precipitation. It is also found that the ultrafine austenite grains as a hard phase promote the dynamic recrystallization of ferrite during warm deformation.

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.

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.

This study aims at examining thermomechanical controlled process to realize ultrafine TRIP-aided multi-phase microstructures in low carbon steels. Heavy deformation at a supercooled austenite region was found to lead the formation of 2 μm ferrite as well as retained austenite with high volume fraction. The morphology of retained austenite was changed from film-like shape to granular shape with lowering finish rolling temperature in austenite field. This ultrafine TRIP-aided multi-phase steel showed good balance of tensile strength with total elongation, ie. 1080MPa and 26.9%. A novel in-situ neutron diffraction measurement demonstrated that the retained granular austenite transformed to martensite at a relatively large strain compared with the retained film austenite. The therein-underlying mechanism of the good mechanical properties was discussed from the view points of the morphological and thermodynamical stabilization of retained austenite.

Microstructural evolution during the ferrite transformation in two low alloy steels was investigated in situ by neutron diffraction. The transformation kinetics determined from the neutron diffraction profiles were well consistent with those measured from the metallographic microstructures, and the carbon enrichment in austenite during the ferrite transformation was confirmed by the changes in (1 1 1) lattice plane spacing of austenite. The deformed and non-deformed austenite microstructures were found to exhibit distinguishable shape difference in their transition curves. The niobium addition and austenite deformation were demonstrated to promote the ferrite transformation.

The defect structure of undoped and Al-doped ZnO films deposited by pulse laser deposition was investigated to understand the charge compensation mechanism in those films. Particularly, the effect of oxidation assist, i.e., O2 gas or oxygen radicals, on the defect structure of the resultant films was examined. The examination indicated that the defect structure of undoped ZnO was not affected by the oxidation assist, whereas the properties of Al-doped ZnO obviously varied with the method of oxidation assist. An analyses of oxygen diffusion in these films revealed that Al-doping enhanced formation of oxygen defects in Al-doped ZnO.

Variant selection of intergranular bcc-Cr precipitates in a Ni–43 mass% Cr alloy was studied by electron backscattering diffraction (EBSD) in a scanning electron microscope equipped with a field emission gun (FESEM) and Kikuchi line analysis in transmission electron microscope (TEM). A single variant was invariably selected at the underlying grain boundaries (GB) when GB-precipitates were KS-related with respect to both the adjacent matrix grains. Meanwhile multiple variants were formed at the GB when GB-precipitates were KS-related to one of the adjacent matrix grains. Theses variant selections were examined with the tilt angle to underlying GB of low-energy interphase boundaries and the orientation relationship with respect to both the adjacent matrix grains. The underlying mechanism was discussed from the viewpoint of activation energy for nucleation of two-dimensional nuclei.

Crystallography of grain boundary (GB) bcc-precipitates formed in a lightly pre-deformed fcc-matrix phase was examined in a Ni-43Cr alloy by EBSD with emphasis on a misorientation from the Kurdjumov-Sachs orientation relationship. The misorientation of GB bcc-precipitates became somewhat larger when the matrix phase was lightly pre-deformed. The GB bcc-precipitates were preferentially formed at the intersection of geometrically necessary dislocation boundaries (GNB) with grain boundaries. In contrast, the pre-deformation did not influence the misorientation of intragranular bcc-precipitates. The preferential precipitation and the increased misorientation at the GNB-GB intersection were discussed.

Effects of light deformations before and after precipitation (pre-deformation and post-deformation) on orientation distribution of intergranular bcc-precipitates were examined in a Ni-43mass%Cr alloy by electron backscattered diffraction. It was found that the deviation angles from both the plane/direction parallel orientation relationships in the Kurdjumov-Sachs orientation relationship of intergranular precipitates were increased by both pre- and post-deformations, but post-deformation was more effective. Based on this result, microstructural evolution of intergranular precipitates formed dynamically was discussed with particular attention to coherent-to-incoherent transition of precipitates.