永瀬 丈嗣

The phase stability of the σ-CrFe intermetallic compound under fast electron irradiation was studied using high-voltage electron microscopy. Under MeV electron irradiation within the temperature range of 298–473 K, the σ phase does not maintain its original structure, but instead transforms into a body-centered cubic solid solution. No changes in the topological structure are observed at temperatures below 103 K. This temperature dependence of the phase stability in σ-CrFe exhibited the opposite tendency to that of solid-state amorphization. The dominant factor affecting the phase stability was discussed in terms of the Gibbs free energy and the microstructural changes associated with the thermally assisted, radiation-enhanced migration of defects and/or constituent atoms.

The phase stability of the σ-CrFe intermetallic compound under fast electron irradiation was studied using high-voltage electron microscopy. Under MeV electron irradiation within the temperature range of 298–473 K, the σ phase does not maintain its original structure, but instead transforms into a body-centered cubic solid solution. No changes in the topological structure are observed at temperatures below 103 K. This temperature dependence of the phase stability in σ-CrFe exhibited the opposite tendency to that of solid-state amorphization. The dominant factor affecting the phase stability was discussed in terms of the Gibbs free energy and the microstructural changes associated with the thermally assisted, radiation-enhanced migration of defects and/or constituent atoms.

The phase stability of the σ-CrFe intermetallic compound under fast electron irradiation was studied using high-voltage electron microscopy. Under MeV electron irradiation within the temperature range of 298–473 K, the σ phase does not maintain its original structure, but instead transforms into a body-centered cubic solid solution. No changes in the topological structure are observed at temperatures below 103 K. This temperature dependence of the phase stability in σ-CrFe exhibited the opposite tendency to that of solid-state amorphization. The dominant factor affecting the phase stability was discussed in terms of the Gibbs free energy and the microstructural changes associated with the thermally assisted, radiation-enhanced migration of defects and/or constituent atoms.

The effect of pre-straining at and below 823 K prior to hot forging at 973 K on the magnetic properties of hot-deformed Nd-Fe-B magnets was examined. Nd-Fe-B alloys prepared in this study could be hot-forged, even at 723 K. Both the squareness and the coercivity of the magnetization curves at room temperature were improved by pre-straining at 773 K or below, owing to the suppression of the formation of non-aligned coarse Nd2Fe14B grains. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The effect of pre-straining at and below 823 K prior to hot forging at 973 K on the magnetic properties of hot-deformed Nd-Fe-B magnets was examined. Nd-Fe-B alloys prepared in this study could be hot-forged, even at 723 K. Both the squareness and the coercivity of the magnetization curves at room temperature were improved by pre-straining at 773 K or below, owing to the suppression of the formation of non-aligned coarse Nd2Fe14B grains. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Changes in the structure of the rapidly solidified body-centered-cubic (bcc) solid-solution phase in Ti-Cr alloy were investigated in order to determine whether or not the unique heat-induced amorphization (i.e., SV) actually occurs. Annealing-induced amorphization of the single solid-solution phase was not observed during either ex situ isothermal annealing at 873 K or in situ annealing experiments in HVEM. The previously reported SV may not correspond to the formation of an amorphous phase but could actually be due to the formation of a finely grained polycrystalline structure during the decomposition of the bcc solid-solution phase.

A unique entangled duplex structure was formed in rapidly solidified Cu-Fe-Zr-B alloys with Cu/Fe ratio = 1/1 and 6/1. Polycrystalline globules, embedded in a Cu crystalline matrix, were also observed in Cu35Fe35Zr10B20 and Cu60Fe10Zr10B20 alloys; this rapidly solidified structure was drastically different from that of Cu-Fe-Zr-B alloys enriched in Fe. Multi-step liquid phase separation can lead to unique microstructure formation during rapid solidification.

Results are presented of a study of {113}-defect formation in Si nanowires with diameters ranging from 50 to 500 nm. The Si nanowires, used for the processing of tunnel-FET's, are etched into a moderately doped epitaxial Si layer on a heavily doped n-type Si substrate. {113}- defects are created in situ by 2 MeV e-irradiation at temperatures between room temperature and 375 °C in an ultra high voltage electron microscope. The observations are discussed in the frame of intrinsic point defect out-diffusion and interaction with dopant atoms.

Conductive niobium carbide (NbC) nanowires with diameters of 1-3 nm were synthesized within the inner space of carbon nanotubes (CNTs) by exposing the CNTs to niobium (V) chloride vapor through hydrogen reduction. The NbC nanowires were found to have a NaCl-type crystal structure by transmission electron microscopy and transmission electron diffractometry. Results from electronic transport measurements imply that the electrical conductivity of the synthesized product was improved compared with that of empty CNTs. (C) 2014 The Japan Society of Applied Physics

High-entropy alloys (HEAs) are characterized not only by high values of entropy but also by high atomic-level stresses originating from mixing of elements with different atomic sizes. Particle irradiation on solids produces atomic displacements and thermal spikes. The high atomic-level stresses in HEAs facilitate amorphization upon particle irradiation, followed by local melting and re-crystallization due to thermal spikes. We speculate that this process will leave much less defects in HEAs than in conventional alloys. For this reason, they may be excellent candidates as new nuclear materials. We discuss initial results of computer simulation on model binary alloys and an electron microscopy study on Zr-Hf-Nb alloys, which demonstrate extremely high irradiation resistance of these alloys against electron damage to support this speculation. (C) The Minerals, Metals & Materials Society and ASM International 2013

Results are presented of a study of {113}-defect formation in Si nanowires with diameters ranging from 50 to 500 nm. The Si nanowires, used for the processing of tunnel-FET's, are etched into a moderately doped epitaxial Si layer on a heavily doped n-type Si substrate. {113}- defects are created in situ by 2 MeV e-irradiation at temperatures between room temperature and 375 °C in an ultra high voltage electron microscope. The observations are discussed in the frame of intrinsic point defect out-diffusion and interaction with dopant atoms.

High-entropy alloys (HEAs) are characterized not only by high values of entropy but also by high atomic-level stresses originating from mixing of elements with different atomic sizes. Particle irradiation on solids produces atomic displacements and thermal spikes. The high atomic-level stresses in HEAs facilitate amorphization upon particle irradiation, followed by local melting and re-crystallization due to thermal spikes. We speculate that this process will leave much less defects in HEAs than in conventional alloys. For this reason, they may be excellent candidates as new nuclear materials. We discuss initial results of computer simulation on model binary alloys and an electron microscopy study on Zr-Hf-Nb alloys, which demonstrate extremely high irradiation resistance of these alloys against electron damage to support this speculation. (C) The Minerals, Metals & Materials Society and ASM International 2013

Results are presented of a study of {113}-defect formation in Si nanowires with diameters ranging from 50 to 500 nm. The Si nanowires, used for the processing of tunnel-FET's, are etched into a moderately doped epitaxial Si layer on a heavily doped n-type Si substrate. {113}- defects are created in situ by 2 MeV e-irradiation at temperatures between room temperature and 375 °C in an ultra high voltage electron microscope. The observations are discussed in the frame of intrinsic point defect out-diffusion and interaction with dopant atoms.

High-entropy alloys (HEAs) are characterized not only by high values of entropy but also by high atomic-level stresses originating from mixing of elements with different atomic sizes. Particle irradiation on solids produces atomic displacements and thermal spikes. The high atomic-level stresses in HEAs facilitate amorphization upon particle irradiation, followed by local melting and re-crystallization due to thermal spikes. We speculate that this process will leave much less defects in HEAs than in conventional alloys. For this reason, they may be excellent candidates as new nuclear materials. We discuss initial results of computer simulation on model binary alloys and an electron microscopy study on Zr-Hf-Nb alloys, which demonstrate extremely high irradiation resistance of these alloys against electron damage to support this speculation. (C) The Minerals, Metals & Materials Society and ASM International 2013

Conductive niobium carbide (NbC) nanowires with diameters of 1-3 nm were synthesized within the inner space of carbon nanotubes (CNTs) by exposing the CNTs to niobium (V) chloride vapor through hydrogen reduction. The NbC nanowires were found to have a NaCl-type crystal structure by transmission electron microscopy and transmission electron diffractometry. Results from electronic transport measurements imply that the electrical conductivity of the synthesized product was improved compared with that of empty CNTs. (C) 2014 The Japan Society of Applied Physics

MeV electron irradiation-induced structural changes in a C11(b)-Ti2Pd intermetallic compound were investigated by high-voltage electron microscopy to provide evidence for amorphous-phase formation in Ti-based (Ti > 50 at.%) intermetallic compounds. The Ti2Pd compound could not maintain its original structure against the MeV electrons, and a bct solid-solution phase was formed at 22, 103, and 298 K. At temperatures <= 103 K, halo rings appeared in the selected area diffraction patterns after the bct-phase formation under the irradiation. This fact suggests the occurrence of solid-state amorphization; however, an amorphous single phase could not be obtained in the Ti66.7Pd33.3 alloy. (c) 2013 Elsevier B.V. All rights reserved.

Electron-irradiation-induced phase transition in Cr2M (M = Ti and Al) intermetallic compounds was investigated by high-voltage electron microscopy (HVEM). Under MeV electron irradiation, the Cr2Ti intermetallic compound transformed into an amorphous phase, which was followed by the precipitation of a b.c.c. solid solution, indicating the occurrence of a crystalline-to-amorphous-to-crystalline (C-A-C) transition. In contrast, in the Cr2Al intermetallic compound, the formation of the b.c.c. phase occurred directly without an amorphous phase formation through electron-irradiation-induced chemical disordering. These transitions were discussed on the basis of Cr-M binary phase diagrams and a Gibbs free energy model. (C) 2013 Elsevier B.V. All rights reserved.

The microstructure and phase stability of a Zr-Hf-Nb alloy with an approximately equiatomic ratio of Zr, Hf, and Nb was investigated. A body-centered cubic (bcc) solid solution was formed in specimens produced by sputtering. MeV electron-irradiation-induced structural changes were investigated in the bcc phase of the Zr-Hf-Nb alloy using high-voltage electron microscopy (HVEM). The polycrystalline phase with a bcc structure showed high phase stability against irradiation damage, and no structural changes due to irradiation damage were observed at 298 K. © 2013 Elsevier Ltd. All rights reserved.

The structural change in the Zr-Hf-Nb alloy during MeV electron irradiation was investigated using high voltage electron microscopy (HVEM). The nano-crystalline structure, which had a diffraction pattern similar to that of an amorphous phase, could not be maintained under the irradiation. The irradiation-induced structural change was observed after high dpa irradiation (about 10 dpa). The irradiation-induced structural change was sensitive to the irradiation temperature. The difference in the irradiation damage evaluation process between conventional crystalline materials and multi-component alloys was discussed on the basis of the structures of the defects. (C) 2012 Elsevier Ltd. All rights reserved.

MeV electron irradiation via high voltage electron microscopy can lead to amorphous-to-crystal transition (i.e., crystallization) as well as crystal-to-amorphous transition (i.e., solid-state amorphization). Irradiation-induced crystallization can be observed in various alloy systems such as Co-, Fe-, Ni-, Pd-, and Zr-based metallic glasses, indicating that this phenomenon has wide generality in metallic materials. Irradiation-induced crystallization mechanism was discussed based on the following factors; (1) an increase in free energy for an amorphous phase, (2) the formation of crystalline clusters through modification of the atomic configuration near radiation induced defects, and (3) enhanced diffusion. The stability of an amorphous phase against irradiation-induced crystallization can be estimated from the thermal crystallization temperature (T-x), and Ni-Nb based metallic glasses have a tendency for high stability against irradiation because of high T-x. (C) 2011 Elsevier B.V. All rights reserved.

MeV electron irradiation via high voltage electron microscopy can lead to amorphous-to-crystal transition (i.e., crystallization) as well as crystal-to-amorphous transition (i.e., solid-state amorphization). Irradiation-induced crystallization can be observed in various alloy systems such as Co-, Fe-, Ni-, Pd-, and Zr-based metallic glasses, indicating that this phenomenon has wide generality in metallic materials. Irradiation-induced crystallization mechanism was discussed based on the following factors; (1) an increase in free energy for an amorphous phase, (2) the formation of crystalline clusters through modification of the atomic configuration near radiation induced defects, and (3) enhanced diffusion. The stability of an amorphous phase against irradiation-induced crystallization can be estimated from the thermal crystallization temperature (T-x), and Ni-Nb based metallic glasses have a tendency for high stability against irradiation because of high T-x. (C) 2011 Elsevier B.V. All rights reserved.

MeV electron irradiation can stimulate solid-state amorphization in some intermetallic compounds. The irradiation induced amorphization phenomenon facilitates a clearer observation of the composite microstructure of the compounds. MeV electron irradiation is applied to a composite structure containing intermetallic compound "A," which undergoes solid-state amorphization and crystalline phase "B," which does not undergo amorphization. The composite structure transforms into a mixture of amorphous and crystalline phases by the irradiation. The distribution of A and B in the structure can hence be easily determined. High-voltage electron microscopy (HVEM) offers a unique microstructure observation technique that uses the difference between the sensitivities of compounds to undergo solid-state amorphization when MeV electron irradiation is applied to them. © 2010 Elsevier B.V. All rights reserved.

MeV electron irradiation can stimulate solid-state amorphization in some intermetallic compounds. The irradiation induced amorphization phenomenon facilitates a clearer observation of the composite microstructure of the compounds. MeV electron irradiation is applied to a composite structure containing intermetallic compound "A," which undergoes solid-state amorphization and crystalline phase " B," which does not undergo amorphization. The composite structure transforms into a mixture of amorphous and crystalline phases by the irradiation. The distribution of A and B in the structure can hence be easily determined. High-voltage electron microscopy (HVEM) offers a unique microstructure observation technique that uses the difference between the sensitivities of compounds to undergo solid-state amorphization when MeV electron irradiation is applied to them. (C) 2010 Elsevier B. V. All rights reserved.

Electron-irradiation-induced solid-state amorphization (SSA) in supersaturated Ni-Zr solid solution models was investigated by molecular dynamics (MD) simulations, focusing on the threshold composition for the occurrence of SSA. The threshold composition for the MeV electron irradiation-induced SSA can be estimated to be around Ni(82)Zr(18). A Ni(82)Zr(18) solid solution shows SSA, while a Ni(83)Zr(17) solid solution can maintain its original crystalline structure against irradiation. Thermal recovery of the lattice defects introduced during electron irradiation can be seen in both Ni(83)Zr(17) and Ni(82)Zr(18) solid solutions. There is a significant difference between Ni(83)Zr(17) and Ni(82)Zr(18) in terms of changes in the number of 12-faced volonoi polyhedrons, namely, the number of constituent atoms whose coordination number is 12. A change in the combination of various volonoi polyhedrons (a change in coordination number of constituent atoms) during the introduction of lattice defects and thermal recovery precedes SSA in the Ni-Zr solid solution model. (C) 2010 Elsevier Ltd. All rights reserved.

The microstructure of a rapidly solidified melt-spun ribbon of binary Fe-Cu and ternary Fe-Cu-B alloys was investigated The duplex structure composed of Fe-B-rich and Cu-rich alloys was formed by liquid phase separation of Fe-Cu-B alloys regardless of the B content We found that the element Cu shows a strong tendency to segregate on the surface of Fe-Cu-B alloy ribbons resulting in the formation of a macroscopically phase-separated Cu-colored cover layer/core structure (C) 2010 Elsevier B V All rights reserved

The glass-to-liquid transition was evaluated by in situ transmission electron microscope observation of crystalline Cu globule aggregation in Fe-Zr-B amorphous or supercooled liquid matrices in Fe(50)Zr(10)B(20)Cu(20) melt-spun ribbon. Globule aggregation was not observed near the critical glass transition temperature, but was observed near the glass transition temperature evaluated by conventional differential scanning calorimetry measurements. The viscosity of Fe-Zr-B-based metallic glass may decrease during the glass-to-liquid transition, and a polygonal crystalline phase may form by Cu globule aggregation. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

MeV electron irradiation can introduce Frenkel pairs, i.e., a vacancy-interstitial pair, in crystals. In metallic glass, density fluctuation, namely, a pair of free volume and anti-free volume, is introduced, resulting in the devitrification of the amorphous phase in some metallic glasses. In this study, we investigated the temperature dependence of MeV-irradiation-induced crystallization of metallic glasses. Phase selection in crystallization shows a significant temperature dependence in various metallic glasses at and under 298 K. The size of the crystalline precipitates changes with irradiation temperature in some metallic glasses. (C) 2010 Published by Elsevier Ltd.

MeV electron irradiation induced crystallization of supercooled liquid and amorphous phases in Zr66.7Cu33.3 Metallic Glass

A macroscopically phase-separated dual-layer ribbon can be obtained from (Co(75)Si(10)B(15))(70)Cu(30) alloy by using a conventional single-roller melt-spinning method, whereas such a structure cannot be obtained from (Fe(75)Si(10)B(15))(70)Cu(30) alloy. Liquid phase separation and amorphous phase formation take place simultaneously in Co-Si-B-Cu alloy, which leads to the formation of a unique structure during rapid solidification. (C) 2010 Elsevier B.V. All rights reserved.