足立 大樹

Al-12mass%Si binary alloy was fabricated by laser-powder bed fusion (L-PBF). The effect of low temperature annealing (LTA) less than 573 K on electrical and mechanical properties, microstructure, and residual stress of L-PBF Al-12mass%Si was investigated. Due to LTA, electrical resistivity at 77 K and 293 K changed from 34 nΩm down to 10 nΩm and from 68 nΩm down to 45 nΩm, respectively. The lattice constant of α-Al phase increased around 0.0005 nm by LTA according to X-ray diffraction. The change was associated with the decrease in the concentration of solid-solution Si of around 3.4 mass% by electrical resistivity measurement and 3 mass% by X-ray diffraction, when the annealing temperature was 473 K. Whereas, Vickers hardness increases around 145 HV compared with that for the as-built value of 141 HV, and then, decreases down to 126 HV and saturates. TEM/EDS observations confirmed the precipitation of fine Si within the α-Al phase. The decrease in annealing temperature increases the time to reach the Vickers hardness peak. 0.2% proof stress obtained by compression tests shows anisotropy, which can be attributed to the anisotropy on residual stress, which still exists after LTA.

Abstract Adhesive layers were prepared using photoreactive polymer liquid crystals, and their application as dismantled adhesives based on the change in thermal properties associated with photoisomerization was investigated. Anisotropy in adhesive strength was successfully achieved by controlling the alignment of the liquid crystal using a polyimide alignment film. Furthermore, anisotropy in the adhesive strength due to photoalignment was investigated using the axis-selective photoreactivity of polymer liquid crystals to linearly polarized light.

The microstructural factors contributing to the high strength of additive-manufactured Al–Si alloys using laser-beam powder bed fusion (PBF-LB) were identified by in-situ synchrotron X-ray diffraction in tensile deformation and transmission electron microscopy. PBF-LB and heat treatment were employed to manufacture Al–12%Si binary alloy specimens with different microstructures. At an early stage of deformation prior to macroscopic yielding, stress was dominantly partitioned into the α-Al matrix, rather than the Si phase in all specimens. Highly concentrated Si solute (∼3%) in the α-Al matrix promoted the dynamic precipitation of nanoscale Si phase during loading, thereby increasing the yield strength. After macroscopic yielding, the partitioned stress in the Si phase monotonically increased in the strain-hardening regime with an increase in the dislocation density in the α-Al matrix. At a later stage of strain hardening, the flow curves of the partitioned stress in the Si phase yielded stress relaxation owing to plastic deformation. Therefore, Si-phase particles localized along the cell walls in the cellular-solidified microstructure play a significant role in dislocation obstacles for strain hardening. Compared with the results of the heat-treated specimens with different microstructural factors, the dominant strengthening factors of PBF-LB manufactured Al–Si alloys were discussed.

Type-B serrations were observed during room-temperature tensile deformation of Al-2.17mass%Mg alloy with an average grain size of 12 μm. Digital image correlation was used to visualize Portevin-Le Chatelier (PLC) bands, and microstructural changes in these bands were observed by in-situ X-ray diffraction measurements using synchrotron radiation at SPring-8. The results indicated that the overall density of dislocations, including both mobile and pinned dislocations inside the PLC bands, increased substantially as the bands formed. This suggests that serration occurs due to an increase in the mobile dislocation density resulting from the formation of new dislocations from sources inside the PLC bands.

Al-Zn-Mg alloys with different precipitate sizes were investigated to determine the influence of the precipitate size on the flow stress and dislocation density change during tensile deformation. The dislocation density was measured using in-situ X-ray diffraction at the SPring-8 synchrotron radiation facility with a time resolution of about 2 s. In region II with rapid dislocation multiplication, from under-aging to peak aging, the dislocation density increased with increasing aging time. Under over-aging conditions, the amount of dislocation multiplication in region II decreased with increasing aging time. Even in region III, the increase in dislocation density with plastic deformation was the largest for the peak aging conditions. However, the amount of work hardening was small and the contribution of dislocation hardening to the strength of the material was minimal. For over-aging conditions, the increase in dislocation density in region III was smaller than for the other regions, but the amount of work hardening was relatively large. It is considered that the influence of the dislocation density on work hardening is determined by the effectiveness of precipitates as obstacles to dislocation motion.

22Mn-0.6C (wt. %) high-Mn austenitic steel having fully recrystallized microstructure was obtained through 4 cycles of repeated cold-rolling and annealing process. Mechanical properties were evaluated by tensile test at an initial strain rate of 8.3 × 10-4 s-1 at room temperature and the local strain and strain-rate distribution of the specimen were analyzed using digital image correlation (DIC) method. The results revealed that a strain localization behavior characterized by the formation, propagation, and annihilation of strain-localized bands, called Portevin-Le Chatelier (PLC) bands, determines the global mechanical properties including serration behavior on the stress-strain curve. In addition, in-situ synchrotron XRD measurement during tensile test clarified that the dislocation density was significantly accumulated as the PLC band passed through the X-ray beam position, which led to the local strain hardening in the propagating PLC band. Such a local strain hardening was repeated during the whole tensile deformation, and the global strain hardening progressed in the 22Mn-0.6C steel.

Deformation-induced martensitic transformation is a key phenomenon to manage both high strength and large ductility in low alloy multi-phase steels. Significant enhancement of strain hardening ability could be achieved by the phase transformation from soft austenite to hard martensite during deformation, which is known as transformation induced plasticity (TRIP) effect. The occurrence of TRIP effect can be controlled by austenite characteristics like carbon content, grain size and morphology. Additionally, the mechanical interaction with other phases surrounding each austenite grain during deformation would affect the stability of austenite, which has not been clarified. The current study has clarified how surrounding phases affect the mechanical stability of austenite against deformation-induced martensitic transformation. Two types of multi-phase microstructures composed of three different phases, i.e., ferrite (α) + austenite (γ) + martensite (M) and two phases of ferrite (α) + austenite (γ) were fabricated in Fe-1.6Mn-1.4Si-1.0Ni-0.5Al-0.2C, maintaining the characteristics of retained austenite nearly the same. It was found that the α+γ+M specimen exhibited slower rate of deformation-induced martensitic transformation during tensile deformation than the α+γ specimen, indicating higher austenite stability in the α+γ+M specimen. The in-situ synchrotron XRD measurements during tensile deformation revealed that there was significant stress partitioning between austenite and adjacent phases (ferrite, martensite and deformation-induced martensite), which influenced the phase transformation rate of austenite into deformation-induced martensite. Furthermore, it was also clarified by the in-situ synchrotron XRD that the austenite in the α+γ+M specimen always had higher stress than that in the α+γ specimen due to higher dislocation density in austenite introduced by martensitic transformation to form pre-existing martensite, so that higher stress was required for deformation-induced martensitic transformation in the α+γ+M specimen during tensile deformation than the α+γ specimen.

This study investigated the cluster formation process in the early stages of 353 K aging in Al1.04 mass%Si0.55 mass%Mg alloys by means of soft X-ray absorption fine structure (XAFS) measurements and first-principles calculations. XAFS at the Si-K and Mg-K edges was carried out at the BL27SU beamline at SPring-8. To observe the structural changes in detail, an XAFS apparatus able to hold the sample at 353 K in a vacuum chamber and cool it rapidly to suppress the progress of clustering was developed. Density functional theory (DFT) calculations were used to simulate the Si-K and Mg-K edge spectra for various cluster models. Based on the results, the cluster formation process in the early stages of aging at 353 K was qualitatively clarified. Initially, MgVa (Va: vacancy) pairs and SiVa pairs were formed, then 2-MgVa clusters formed by bonding between MgVa pairs along (100); subsequently, L10 clusters were formed by Mg atoms ordered along (100), and then SiVa-py clusters with Va adjacent to the first-nearest-neighbor atom of Si atoms and Si-py without adjacent Va were formed, in which MgVa pairs and SiVa pairs were individually united, respectively. Monolayer and multilayer structures then developed as aging proceeded, involving Mg and Si atoms ordered along (100), in which Mg and Si atoms were bonded.

An accumulative roll bonding up to 8 cycles increases ultimate tensile strength of Al-0.1at%Ni alloy around 2.5 times from around 60 MPa up to around 185 MPa. Whereas, electrical resistivity at 77 K increases less than 20% from 2.8 nΩm up to 3.3 nΩm. The change of the mechanical and electrical properties can be estimated based on the microstructure observations/measurements, such as, the increase in dislocation density from around 2 × 1012 m−2 to around 1 × 1014 m−2, and the decrease in grain thickness from 100 µm down to 0.31 µm. Al-0.1at%Ni alloy gives better balance of mechanical and electrical properties than that of Al-0.1at%Fe alloy as previously reported. It is effective to design new aluminum alloy using the transition metals as alloying element for Al subjected to severe plastic deformation.

Chemically modulated mesoscopic domains in a fcc single phase CrMnFeCoNi equi-atomic high entropy alloy (HEA) are detected by small angle diffraction performed at a synchrotron radiation facility, whereas the mesoscopic domains cannot be detected by conventional X-ray diffraction and 2D mappings of energy dispersive X-ray spectroscopy by scanning electron microscopy and scanning transmission electron microscopy. The mesoscopic domains are deformed and shrieked, and finally destructed by plastic deformation, which is supported by the comprehensive observations/measurements, such as electrical resistivity, Vickers hardness, electron backscattering diffraction, and hard X-ray photoemission spectroscopy. The destruction of the mesoscopic domains causes the decrease in electrical resistivity via plastic deformation, so called K-effect, which is completely opposite to the normal trend of metals. We confirmed that the presence and the size of local chemical ordering or short-range order domains in the single phased HEA, and furthermore, Cr and Mn are related to form the domains.

Face-centered cubic single-phase high-entropy alloys (HEAs) containing multi-principal transition metals have attracted significant attention, exhibiting an unprecedented combination of strength and ductility owing to their low stacking fault energy (SFE) and large misfit parameter that creates severe local lattice distortion. Increasing both strength and ductility further is challenging. In the present study, we demonstrate via meticulous experiments that the CoCrFeNi HEA with the addition of the substitutional metalloid Si can retain a single-phase FCC structure while its yield strength (up to 65%), ultimate strength (up to 34%), and ductility (up to 15%) are simultaneously increased, owing to a synthetical effect of the enhanced solid solution strengthening and a reduced SFE. The dislocation behaviors and plastic deformation mechanisms were tuned by the addition of Si, which improves the strain hardening and tensile ductility. The present study provides new strategies for enhancing HEA performance by targeted metalloid additions.

Hydrogen-bonded cyanostilbene liquid crystalline polymeric composite was prepared and its photoresponsive behavior was evaluated Upon exposure to UV light, the mesophase of the composite destabilized at room temperature due to trans-cis photoisomerization of cyanostilbene moiety. The photoinduced change in mesophase is irreversible due to thermal stability of photogenerated cis moiety. Additionally, the composite bonded two glass substrates together and suspended when the sample was in the LC phase. In addition, upon UV exposure, exfoliation occurred as a result of a photoinduced phase transition.

The liquid crystalline N-benzylideneaniline (NBA) polymer acts as an adhesive, and its bonded pieces detach upon exposure to UV light due to the trans-cis photoisomerization of the NBA moieties and subsequent phase transition of NBA from a liquid crystal polymer to an isotropic polymer. We improved the photoresponsive durability of the polymer and explored the effect of light irradiation on its thermal properties and adhesion stress. We observed that the transition points of the polymer's thermophysical properties (glass transition, liquid crystal-isotropic transition, G" slope, tan δ) shifted to lower temperatures under light irradiation.

Grain refinement is one of the methods applied to strengthen metallic materials, and various peculiar mechanical properties have been reported to be expressed when the grain size is reduced to less than submicron dimensions. This is considered to be due to a change in the behavior of dislocations that are associated with plastic deformation. In situ synchrotron radiation measurements of microstructural changes during deformation in face-centered cubic (fcc) metals with grain sizes of 20 μm to 5 nm were performed to systematically investigate the effects of grain size on dislocation behavior during plastic deformation. In pure aluminum with grain sizes of 20 to 3 μm, the dislocation density during plastic deformation was approximately 1014 m−2, regardless of the grain size. However, when the grain size was less than 3 μm, the dislocation density increased monotonically in proportion to the grain size to the power of -1. Furthermore, in a nickel alloy with a grain size of less than 10 nm, this relationship was no longer satisfied, and the results suggested that deformation progresses due to partial dislocations. In materials with a grain size of less than 1 μm, the dislocation density after unloading became much smaller than that during loading.

The work hardening of aluminum alloys depends on the dislocation substructure in addition to the dislocation density. In this study, in-situ XRD measurements during tensile deformation were conducted on cold-rolled 1200 aluminum and change in the dislocation substructure was evaluated by analysis using Convolutional Multiple Whole Profile method. As a result, the dislocation cell structure was formed before tensile test and when the stress exceeded 32 MPa, the dislocation cell structure decomposed and the dislocation dispersion approached uniformly. It is considered that 32 MPa is a micro-yield stress at which the initial dislocations start to move. As the plastic deformation progressed, dislocation arrangements parameter decreased and finally became 0.5, it is considered that the dislocation cell structure was formed again.

We have investigated microstructural characteristics and solute Si element in the α-Al (fcc) matrix of Al-12%Si binary alloy additive-manufactured by laser powder bed fusion (L-PBF) process in terms of different locations in the melt-pool structure (inside melt-pool or around melt-pool boundaries). Their variations by post heat-treatments at various temperatures were examined. The fraction of Si phase and lattice parameter of α-Al phase were quantified by XRD measurements combined with sample rotation and oscillation systems. In the L-PBF manufactured sample, the columnar α -Al phases contained highly concentrated Si element in solution (above 3 mol%) inside melt-pools, whereas the measured Si content was approximately 2 mol% in the relatively coarsened α-Al phase along melt-pool boundaries. The location dependence of solute Si content would be associated with a continuous change in growth rate of solid phase in solidification by local heating in the L-PBF process. The solute Si content was significantly reduced by heat-treatments and its location dependence appeared less pronounced. The as-manufactured sample exhibited a lower lattice parameter of α-Al phase with concentrated Si in solution in comparison with the heat-treated samples. The fraction of Si phase became higher in the heat-treated samples. These results were rationalized by microstructural change at elevated temperatures.

This study investigated the effect of Sn addition on clustering behavior of natural aging (NA) in Al-0.95mass%Si-0.56mass%Mg-0.04mass%Sn alloys by XAFS measurements, first-principles calculation and Vickers hardness measurements. XAFS measurements at the Sn-K edges were carried out at the BL14B2 beamline, at the Si-K and Mg-K edges were carried out at the BL27SU beamline at SPring-8. It was found that Sn addition brought about the retardation of cluster formations. This retardation was ascribed to the suppression of formations of Mg-vacancy (Va) pairs. By Sn addition, the ratio of Sn-Va pairs was increased from as-quench (AQ) to NA for 36 ks, however Sn atoms were not remarkably included in the clusters. The formation energy of the Sn-Va pair is the smallest in Al matrix, and Sn atoms preferentially bind to vacancies. Since the formation energy of Mg-Va pair is higher than that of the Si-Va pair, the formation of Mg-Va pairs is preferentially delayed. Based on the result of Vickers hardness measurements, negative effect of two-step aging was caused by the participation of Mg atoms in the clusters, not that of Si atoms. It was clarified that Mg atoms have a greater influence on negative effect in two-step aging than Si atoms have.

Electrical resistivity and Vickers hardness of Alloy 625 due to cold rolling were measured, and, discussed with the microstructural change obtained using electron backscattered diffraction and X-ray diffraction. Both increase in dislocation density and grain subdivision due to rolling was observed. Although the electrical resistivity of the normal pure metals increases with increasing the rolling reduction, that of Alloy 625 initially decreased with increasing the rolling reduction of 70%. Then, the electrical resistivity slightly increased with increasing the rolling reduction of 80%. Up to the rolling reduction of 70%, the reduction of electrical resistivity is associated with K-effect, which is the destroy of the short-range ordered domain due to the plastic deformation. On the other hand, Vickers hardness increased with increasing the rolling reduction. It was associated with the contribution of grain refinement, dislocation, solid solution, and sort-range order strengthening.

Tensile mechanical properties of fully recrystallized TWIP steel specimens having various grain sizes (d) ranging from 0.79 μm to 85.6 μm were investigated. It was confirmed that the UFG specimens having the mean grain sizes of 1.5 μm or smaller abnormally showed discontinuous yielding characterized by a clear yield-drop while the specimens having grain sizes larger than 2.4 μm showed normal continuous yielding. In-situ synchrotron radiation XRD showed dislocation density around yield-drop in the UFG specimen quickly increased. ECCI observations revealed the nucleation of deformation twins and stacking faults from grain boundaries in the UFG specimen around yielding. Although it had been conventionally reported that the grain refinement suppresses deformation twinning in FCC metals and alloys, the number density of deformation twins in the 0.79 μm grain-sized specimen was much higher than that in the specimens with grain sizes of 4.5 μm and 15.4 μm. The unusual change of yielding behavior from continuous to discontinuous manner by grain refinement could be understood on the basis of limited number of free dislocations in each ultrafine grain. The results indicated that the scarcity of free dislocations in the recrystallized UFG specimens changed the deformation and twinning mechanisms in the TWIP steel.

Magnetization (M) of binary Al-Mg alloys and intermetallic compounds were measured in the temperature (T) range from 10 to 300 K. The Al-rich alloys Al - 2, 3, 4, 5, and 10 at% Mg) revealed diamagnetic contributions to the magnetization correlating to the Mg concentrations. For the β-phase Al3Mg2 the M vs T curve lay very close to that of pure Mg over the entire measured T region. The magnetization of the γ-phase samples exhibited enhanced paramagnetism for Al44Mg56 and Al12Mg17, but the diamagnetism for Al45Mg55. There were diamagnetic contributions to the magnetization of the Mg-rich alloys (Mg - 5, 10, and 15 at% Al) depending on the Al concentration, possibly due to the γ-phase precipitations. Heat capacity of the γ-phase samples and pure Al were measured to estimate the electronic specific heat coefficients which were found to be almost the same. Density functional theory calculations for the Al-Mg compounds were carried out to aid interpretation of the observed magnetization.

This study investigated cluster formation in the early stages of natural aging in Al-1.04 mass%Si-0.55 mass%Mg alloys by soft X-ray XAFS measurements and first-principles calculation. XAFS measurements at the Mg-K and Si-K edges were carried out at the BL27SU beamline at Spring-8. It was found that the absorption edge energies changed as aging proceeded. Density functional theory (DFT) calculations were used to determine the valence electron densities near Si and Mg atoms and to simulate the Si-K and Mg-K edge spectra for some cluster models. On the basis of the results, it was demonstrated that Si and Mg atoms formed clusters in four stages (IIV) during natural aging. In stage I, Si-vacancy pairs, Mg-vacancy pairs, and a combination of both were formed. In stage II, vacancies were released from the clusters formed in stage I. In stage III, Mg-vacancy pairs were included in the clusters. In stage IV, the clusters coarsened through the release of vacancies. These results indicate that soft X-ray XAFS, which is capable of identifying individual elements, has the ability to provide information on such clusters.

© 2020 Acta Materialia Inc. We have thoroughly clarified the mesoscopic nature of serration behavior in a high-Mn austenitic steel in connection with its characteristic localized deformation. A typical high-Mn steel, Fe-22Mn-0.6C (wt. %), with a face centered cubic (FCC) single-phase structure was used in the present study. After 4 cycles of repeated cold-rolling and annealing process, a specimen with a fully recrystallized microstructure having a mean grain size of 2.0 μm was obtained. The specimen was tensile tested at room temperature at an initial strain rate of 8.3 × 10−4 s−1, during which the digital image correlation (DIC) technique was applied for analyzing local strain and strain-rate distributions in the specimen. Obtained results indicated that a unique strain localization behavior characterized by the formation, propagation and annihilation of deformation localized bands, so-called Portevin–Le Chatelier (PLC) bands, determined the global mechanical response appearing as serration on the stress-strain curve. In addition, the in-situ synchrotron XRD diffraction during the tensile test was utilized to understand what was happening in the material with respect to the PLC banding. Lattice strain of (200) plane nearly perpendicular to the tensile direction dropped when every PLC band passed through the beam position, which indicated a stress relaxation occurred inside the PLC band. At the same time, the dislocation density increased drastically when the PLC band passed the beam position, which described that the material was plastically deformed and work-hardened mostly within the PLC band. All the results obtained consistently explained the serration behavior in a mesoscopic scale. The serration behavior on the stress-strain curve totally corresponded to the formation, propagation and annihilation of the PLC band in the 22Mn-0.6C steel, and the localized deformation, i.e., the PLC banding, governed the characteristic strain hardening of the material.

A pure iron tape with cube orientation was fabricated by cold rolling and annealing. The orientation characteristics of the pure iron tape were evaluated using electron back-scattering diffraction (EBSD) analysis. The secondary recrystallized grains with cube orientation was formed on the tape surface for the pure iron tape. The coarse grains with a grain size of ca. 1mm were observed on the tape surface. The areal fraction of cube orientations with an angular deviation ≤ 20 ̊ amounts to ca. 81%.

Pure nickel was processed by accumulative roll bonding (ARB), and the change in electrical resistivity measured at 77K was about 2.2 nΩm after 8 ARB cycles. The change in electrical resistivity was estimated based on the microstructural parameters, such as, dislocation density of about 3 × 1014m 2 and density of grain boundaries of about 8Mm 1 after 8 ARB cycles. Those values were evaluated using X-ray diffraction and electron backscattering diffraction in a field emission-scanning electron microscope, respectively. The change in the electrical resistivity was associated with the above-mentioned microstructural parameters.

© 2020 The Japan Institute of Light Metals. By conducting in-situ XRD measurement during tensile deformation while oscillating the tensile tester, it was possible to measure the change in dislocation density of a pure aluminum alloy having coarse grains with the grain size of 20 µm. In the coarse-grained material, the dislocation density during tensile deformation changed through four regions, as in the case of the fine-grained material. Since the dislocation multiplication start stress was very low at 22 MPa, the elastic deformation region was very short. Thereafter, the dislocations multiplied rapidly, but when the stress and dislocation density reached 33 MPa and 1.57 © 1014 m12, respectively, the dislocation multiplication rate was greatly reduced. This is considered to be due to the low dislocation density required to progress the deformation by plastic deformation in coarse-grained aluminum.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. Intermetallic compounds are usually considered as deleterious phase in alloy designing and processing since their brittleness leads to poor ductility and premature failure during deformation of the alloys. However, several studies recently found that some alloys containing large amounts of NiAl-type intermetallic particles exhibited not only high strength but also good tensile ductility. To clarify the role of the intermetallic particles in the excellent tensile properties of such alloys, the tensile deformation behavior of an ultrafine-grained Fe-Mn-Al-Ni-C alloy containing austenite matrix and B2 intermetallic particles was investigated by using in situ synchrotron radiation X-ray diffraction in the present study. The elastic stress partitioning behavior of two constituent phases during tensile deformation were quantitively measured, and it was suggested that B2 particles played an important role in the high strength and large tensile ductility of the material.

© 2020 Informa UK Limited, trading as Taylor & Francis Group. Nanocrystalline nickel, having a grain size of about 40 nm, was fabricated by electrolytic deposition and strain-rate jump tests were performed at temperaures between 77 and 473 K to obtain the strain-rate sensitivity (Formula presented.) and the activation volume (Formula presented.). The values of (Formula presented.) changed from about 0.05 to about 0.005 with increasing stress, indicating that the deformation is governed by the motion of glide dislocations. (Formula presented.) increased as the temperature increased from 77 to 200 K, but decreased with further increase in temperature. It is concluded that the rate-controlling deformation mechanism in nanocrystalline nickel is temperature sensitive and changes from forest dislocation cutting to dislocation bowing-out and depinning from grain boundaries as the temperature increases.

© 2020 The Japan Institute of Light Metals. By conducting In-situ XRD measurement during tensile deformation while oscillating the tensile tester, it was possible to measure the change in dislocation density of a pure aluminum alloy having coarse grains with the grain size of 20 μm. In the coarse-grained material, the dislocation density during tensile deformation changed through four regions, as in the case of the fine-grained material. Since the dislocation multiplication start stress was very low at 22MPa, the elastic deformation region was very short. Thereafter, the dislocations multiplied rapidly, but when the stress and dislocation density reached 33 MPa and 1.57×1014 m-2, respectively, the dislocation multiplication rate was greatly reduced. This is considered to be due to the low dislocation density required to progress the deformation by plastic deformation in coarse-grained aluminum.

© 2019 The Japan Institute of Light Metals An accumulative roll bonding process was applied up to 8 cycles on high-purity aluminum, aluminum0.02 mass%iron and aluminum0.2 mass%iron in order to measure electrical properties in addition to the mechanical properties. Ultimate tensile strength increases about 35 times compared with that of coarse grain metals, whereas, the electrical conductivity at room temperature decreases about a few %IACS. The dislocation density and density of grain boundary were evaluated from XRD and SEM/EBSD measurements. The microstructure change in those parameters explains the change in electrical resistivity measured at 77 K.

© 2020 ISIJ. Ultrafine ferrite + austenite steels with the chemical composition of 0.1%C-2%Si-5wt%Mn show excellent strength (TS=1 200 MPa) and high ductility (TEl=25%) balance, compared to conventional TRIP steels. This steel is expected as the third generation advanced high-tensile strength steels (AHSS). This steel can be produced by a simple intercritical annealing, however, longer annealing time is necessary to obtain appropriate ferrite + austenite structure. It is difficult to produce this steel by continuous annealing process. If the annealing time can be drastically reduced, this new TRIP steels can be commercialized. We focused on the effect of the prior microstructures before annealing on the formation of ferrite + austenite structure. The effect of the prior structure is not clear. Therefore, in this study, two kind of prior structures, ultrafine grained ferrite + cementite and martensite were used in 0.1%C-2%Si-5wt%Mn steels. It was found that the prior structure of ferrite + cementite can form large amount (20%) of austenite in a very short time (600 s). This is because cementite finely dispersed in the structure effectively acts as a preferential nucleation site of reverse transformed austenite and C and Mn are concentrated in cementite to enable a short time formation of austenite. Excellent strength-ductility balance (32 000 MPa%) which is superior to conventional TRIP steels is also obtained.

Temperature and time dependences of the magnetization of Al-Zn-Mg alloys with varying Zn to Mg ratios (Zn/Mg = 0.25, 0.5, 1, 2, 5.5, and 9, keeping the total concentration of Zn plus Mg to be 5 at. %) were studied in the range from 10 to 310 K after various periods of natural aging. In particular, for Al1-y(Mg2Zn11)y alloys, the total concentrations of Zn and Mg were also varied from 2 to 20 at. % (y = 0.02, 0.03, 0.04, 0.05, 0.1, and 0.2). The largest time variant enhanced diamagnetism was observed for Al0.95(Mg2Zn11)0.05 as a result of solution heat treatment/quenching and natural aging. Isothermal measurements of magnetization vs time for natural aging temperatures from 260 to 300 K for Al0.95(Mg2Zn11)0.05 provided activation energies for solute clustering: 0.69 ± 0.05 eV (for stages I and II) and 0.78 ± 0.03 eV (for stages II and III). The mechanical hardness vs time at 273 K for Al0.95(Mg2Zn11)0.05 confirmed that the time variation of magnetization was related to the precipitation process of Zn/Mg/vacancy zones. Additionally, temperature dependences of the magnetization of Mg21Zn25, Mg4Zn7, MgZn2, and Mg2Zn11 were examined. The observed magnetization for the Mg-Zn compounds was found to be too small to account for the enhanced diamagnetic contributions to magnetization of Al-Zn-Mg alloys. A possible Zn-Mg-vacancy atomic arrangement responsible for the enhanced diamagnetism is discussed.

<p>Ni polycrystals were processed by accumulative roll-bonding (ARB) of 7 cycles and tensile specimens were cut from the ultra-fine grained (UFG) Ni after ARB. Microstructure and crystallographic orientation of UFG Ni were observed by scanning electron microscopy (SEM) and electron back scattering diffraction (EBSD) method. Three textures component, Copper, Brass and S orientations were found in UFG Ni.</p><p>Three specimens for tensile tests having tensile directions parallel with RD, TD and 45D were cut from UFG Ni. RD and TD indicate rolling and transverse directions of ARB process respectively, and 45D indicates a 45 degree direction between RD and TD. For these specimens, <i>in-situ</i> XRD measurements were performed during tensile deformation. Elastic strains during tensile deformation were calculated from values of peak shift for various diffraction planes obtained by the XRD measurements. For RD specimen, the elastic strains during initial deformation for various diffraction planes were almost the same. However, for 45D specimen, the elastic strains for various diffraction planes were different even at initial loading. TD specimen showed behavior like RD specimen. The difference in the elastic behavior between the specimens are discussed by considering difference in crystallographic directions of grains along tensile directions in the specimens which is caused by the textures generated after ARB.</p>

© 2018 The Japan Institute of Metals and Materials. In this study, the change in electrical resistivity of pure Cu processed by accumulative roll bonding (ARB) was measured. The microstructure observations using electron backscattering diffraction in a field emission-scanning electron microscope were performed in order to obtain the density of grain boundaries. X-ray diffraction was used for evaluating dislocation density. The change in electrical resistivity after 8 ARB cycles was about 0.8 nΩm, which is associated with the change in lattice defects introduced by the ARB; dislocation density of about 5 × 1014 m−2 and density of grain boundaries of about 8 Mm−1 as maximum. The contribution of the density of grain boundary for the electrical resistivity becomes dominant compared with that of dislocation density after ARB 1c. However, the contribution of dislocations for electrical resistivity at high ARB cycle number is about 15% in the case of Cu, whereas that is less than 3% in the case of Al. It was concluded that both dislocation density and the density of grain boundary affect the electrical resistivity of UFG metals fabricated by SPD processes.