永瀬 丈嗣

Electron-irradiation induced phase transformation of amorphous, supercooled liquid and crystalline phases in Zr66.7Cu33.3 alloy was investigated. The amorphous phase and supercooled liquid were not stable under 2.0 MV electron-irradiation, and electron-irradiation induced crystallization occurred. The C11(b)-Zr2Cu crystalline phase was precipitated from the supercooled liquid phase and the amorphous phase at and above 552 K, while an f.c.c. solid solution was precipitated from the amorphous phase at and below 298 K. Crystal-to-g I ass or crystal-to-supercooled liquid transition of the C11(b)-Zr2Cu crystalline phase in Zr66.7Cu33.3 metallic glass was not observed at and above 552 K. Phase stability of crystalline phases against electron-irradiation is a dominant factor for phase selection in electron-irradiation induced crystallization of Zr66.7Cu33.3 metallic glass.

The microstructure in rapidly-solidified Fe70-xCuxZr10B20 (v = 0, 10, 20, 30, 35. 60 and 70) alloy ribbons prepared by single-roller melt-spinning method was examined. In spite of the positive heat of mixing in Fe-Cu atom pair, metallic glass was formed in Fe-Cu-Zr-B ribbons. F.c.c.-Cu nano crystalline globules dispersed in Fe-Zr-B based metallic glass was formed in Fe60Cu10Zr10B20 and Fe50Cu20Zr10B20 alloys. Size of the globules in nano-emulsion structure increased with increasing Cu concentration. In Fe35Cu35Zr10B20 and Fe10Cu60Zr10B20 alloys, an entangled marble-like duplex structure composed of Fe-rich and Cu-rich crystalline phases was formed. Zr and B additions in Fe-Cu based alloys cause the formation of metallic glass and unique solidification structures in rapidly solidified melt-spun ribbons.

Change in nanocomposite structure in rapidly solidified Fe86Nd9B5 alloy during electron it-radiation was investigated. Nanocrystalline structure composed of alpha-Fe, Fe3B, Nd2Fe14B and Nd2Fe23B3 crystalline phases was formed by rapid solidification. Electron irradiation can introduce amorphization of intermetallic compounds and crystallization of an amorphous phase. resulting in the formation of it novel nanocomposite structure in which alpha-Fe and Nd2Fe14B nanocrystals are embedded in the amorphous matrix. The mechanism of nanocomposite structure formation was discussed based on the phase stability of amorphous and crystalline phases under electron irradiation.

Solid-state phase transitions in Fe81Zr9B10 alloy were investigated focusing on crystallization and amorphization introduced by thermal process and mechanical process of electron irradiation. Melt-spun amorphous phase can not maintain their original structure under thermal heated state at 828 K and electron irradiation at 103 and 298 K, crystallization of this phase occurred. There was a great difference in crystallization behavior between thermal annealing and electron irradiation. Metastable alpha-Mn type crystalline phase precipitated through thermal crystallization, while nano-crystalline b.c.c. solid solution was formed through electron irradiation induced crystallization. The alpha-Mn type crystalline phase underwent solid-state amorphization by electron irradiation at 103 and 298 K. Both amorphization and crystallization were observed in Fe81Zr9B10 alloy under electron irradiation in spite of same irradiation conditions, resulting in the occurrence of crystal-to-amorphous-to-crystal (C-A-C) transition.

Superplastic viscous deformation and thermal crystallization behavior of supercooled liquid in Zr60.0Al15.0Ni25.0 metallic glass were investigated. The temperature interval of the supercooled liquid region (Delta T-x) was 83 K. The supercooled liquid showed significant viscous plasticity, resulting in large elongation and high strain rate deformation. The stress-strain behavior can be classified into three types: stress overshoot, stable viscous flow with constant flow stress and strain hardening. The strain hardening is due to the precipitation of Zr6Al2Ni crystalline phase with ellipsoidal morphology. Superplastic viscous deformation behavior is very sensitive to thermal crystallization as well as to deformation temperature and strain rate.

Formation of a nanocrystalline structure through rapid solidification, thermal crystallization and electron irradiation induced crystallization was investigated in Fe-Nd-B alloys. A nanocrystalline structure was obtained by rapid quenching of the melt in a Fe86Nd9B5 alloy, while an amorphous single phase was formed in a Fe77Nd4.5B18.5 alloy. In the latter alloy, a nanocrystalline structure was obtained by thermal crystallization and electron irradiation induced crystallization of the amorphous phase. The average grain size of the precipitate obtained by irradiation at 298 K was about 8 nm, which is much smaller than that obtained during thermal crystallization. Results indicate that electron irradiation is effective for obtaining a novel nanocrystalline structure in Fe-Nd-B alloys.

Deformation behavior of a supercooled liquid region in melt-spun Zr60Al15Ni25 and Zr65Al7.5Cu7.5 metallic glass was investigated. The viscous flow behavior is very sensitive to the strain rate and the size of crystalline precipitates, which can be classified into 4 types based on shape of the stress-strain curve: stress overshoot mode, stable viscous flow mode with constant flow stress. strain hardening mode and strain softening mode. The strain hardening and strain softening are due to the crystalline phase distribution in supercooled liquid. The strain hardening mode was observed in Zr60Al15Ni25, while Zr65Al7.5Cu27.5 deformed with strain softening mode rather than strain hardening mode. Tensile deformation enhanced thermal crystallization in a supercooled liquid region was observed in Zu(60)Al(15)Ni(15).

Effect of thermal annealing and electron irradiation on crystallization and phase stability of Fe91-xZr9Bx (x=3, 5, 15, 20 and 25) amorphous alloys was examined. The electron irradiation induced crystallization behavior and crystalline structure depend on irradiation temperature, dose density and B concentration. Nanocrystalline duplex structure composed of alpha-Fe and cubic-Fe2Zr was obtained even in Fe71Zr9B20 alloy by electron irradiation, while no nanoscale precipitates were formed during thermal annealing.

Electron irradiation to metallic glasses is of scientific and practical interest because this process can introduce not only amorphization of crystalline phases but also crystallization of an amorphous phase. Zr 66.7Cu33.3 metallic glass shows simultaneous amorphization and crystallization under electron irradiation in spite of the same irradiation conditions. The thermal equilibrium b.c.t.-Zr2Cu crystalline phase was transformed to the nanocrystalline f.c.c.-Zr2Cu phase through the amorphous state during electron irradiation. The unique solid state amorphization and crystallization behavior under electron irradiation can be explained by the thermodynamical model based on the change in the relative phase stability among amorphous and crystalline phases by electron irradiation.

The electron irradiation effect on microstructure change of the amorphous phase in Zr65.0Al7.5Ni10.0Cu17.5 metallic glass with the largest supercooled liquid region was examined under electron irradiation at 103 and 298 K using an ultra high voltage electron microscope. The nanocrystalline structure composed of f.c.c.-Zr2Cu with a mean grain size on the order of 10 nm order and an amorphous matrix was formed by electron irradiation induced crystallization, while by thermal annealing b.c.t.-Zr2Cu, f.c.c.-Zr2Ni and Zr6Al2Ni phases were formed and the nanocrystalline structure could not be realized. The crystalline structure obtained by electron irradiation induced crystallization was different from that formed by the thermal crystallization of the amorphous phase in terms of the constituent phases and the size of crystalline precipitates. This noble result indicates that electron irradiation to an amorphous phase in metallic glasses is very effective in obtaining the nanocrystalline structure, and encourages future industrial application of amorphous alloys. (C) 2003 Elsevier Ltd. All rights reserved.

Electron irradiation can induce not only amorphization of crystalline phases but also crystallization of an amorphous phase. The electron irradiation to metallic glasses provide an information to clear the origins of high thermal stability of metallic glasses and the relationship of phase stability between crystalline and amorphous phases. A great interest has been focused on Zr-Cu based metallic glasses because of the extremely high thermal stability and superior mechanical properties. Effect of electron irradiation on structural change of Zr66.7Cu33.3 metallic glass was examined. The amorphous phase could not maintain the full glassy structure under electron irradiation. Electron irradiation induced crystallization of metastable f.c.c.-Zr2Cu phase occurred, while b.c.t.-Zr2Cu phase was formed by thermal annealing. Thermal equilibrium b.c.t.-Zr2Cu phase did not appear under electron irradiation, and electron irradiation induced amorphization of this crystalline phase was observed. Electron irradiation changes the relative phase stability among b.c.t.-Zr2Cu, f.c.c.-Zr2Cu and an amorphous phase against thermodynamically equilibrium phases in Zr66.7Cu33.3 metallic glass. As a result,

Effect of electron irradiation on the phase stability and crystallization behavior of Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass which satisfy three empirical rules for high thermal stability of amorphous alloys was examined. Amorphous ribbons were prepared by single roller melt-spinning method. Not only conventional Zr66.7Cu33.3 amorphous alloy but also Zr65.0Al7.5Cu27.5 metallic glass were unstable under electron irradiation and the crystallization of an amorphous phase was accelerated by electron irradiation at an accelerated voltage of 2000 kV at room temperature. Electron irradiation was effective in producing a nanocrystalline structure in Zr-based amorphous alloys, while nanocrystalline structure was not easily obtained by annealing. Effect of a wide supercooled liquid region on crystallization of Zr-based amorphous alloys under electron irradiation was discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

Specimens of Zr66.7Cu33.3, Zr65.0Al7.5Cu27.5 and Zr65.0Al7.5Ni10.0Cu17.5 amorphous alloys with different thermal stability were irradiated by high energy electrons at an accelerated voltage of 2000 kV and 1000 kV. Electron irradiation was performed at 298 K and 103 K. Crystallization of three amorphous alloys was accelerated by this irradiation and nanocrystalline structures were obtained. The critical total dose required for crystallization of the amorphous phase by electron irradiation depends strongly on the irradiation temperature and stability of this amorphous phase. The phase stability and crystallization behavior of the amorphous phase are discussed based on the electron irradiation effect.

The effect of electron irradiation on the microstructural change and phase stability of melt-spun Fe71.0Zr9.0B20.0 metallic glass having a wide supercooled liquid region of 71 K was examined. Crystallization from the amorphous phase was accelerated by electron irradiation, and this irradiation was effective in producing a nanocomposite microstructure.

Effect of electron irradiation on nano crystallization of Fe88Zr9B3 and Fe71Zr9B20 amorphous alloys was examined. The amorphous phase is not stable under electron irradiation at very high acceleration voltage of 2000kV in the temperature range between 103 and 298K. Crystallization of the amorphous phase is accelerated and nanocrystalline structure is formed. The electron irradiation induced crystallization behavior is influenced by irradiation temperature, dose density and thermal stability of amorphous phase.

The β-FeSi2 phase in the well developed eutectoid β-FeSi2 + Si was studied by transmission electron microscopy. Some extra diffraction spots were newly founded in the β-FeSi2 phase. This result suggests that the β-FeSi2 phase has a long period crystal structure. By the diffraction spot analysis, these extra spots were indexed with a six times longer unit cell in the a-direction.

The β-FeSi2 phase in the well developed eutectoid β-FeSi2 + Si was studied by transmission electron microscopy. Some extra diffraction spots were newly founded in the β-FeSi2 phase. This result suggests that the β-FeSi2 phase has a long period crystal structure. By the diffraction spot analysis, these extra spots were indexed with a six times longer unit cell in the a-direction.

Effect of electron irradiation on the crystallization and phase stability of Fe88Zr9B3 and Fe71Zr9B20 amorphous alloys was examined. Electron irradiation at an accelerated voltage of 2000kV was performed at room temperature. The Fe71Zr9B20 alloy showed a wide supercooled liquid region and the ΔTx value was 71K, while no glass transition was observed in Fe88Zr9B3 alloy. The amorphous phase in Fe-Zr-B alloys was not stable under irradiation and crystallization from the amorphous phase was accelerated by the irradiation. Nanocrystalline structure composed of α-Fe and cubic-Fe2Zr was formed in Fe88Zr9B3 alloy by irradiation induced crystallization, while no nanoscale precipitates of intermetallic compounds were formed during annealing. In Fe71Zr9B20 alloy, the formation of nanocrystalline precipitates was also confirmed by irradiation induced crystallization, although the formation of nanocrystalline structure had not been realized in high B concentration Fe-Zr-B alloys by annealing. These new results show that electron irradiation is effective in producing a new nanocrystalline structure.

The addition of a small amount of Cu is effective in accelerating the alpha --> beta + Si eutectoid decomposition. Some elements (Pd, Pt, Ag and Au) that are expected to have similar chemical properties were added to an Fe2Si5 based alloys up to 1.0 at.% to examine whether a similar effect could be revealed. The microstructures, X-ray diffraction and differential thermal analysis (DTA) of slowly solidified or heat treated specimens were investigated in detail. The solubility of all containing elements into the alpha phase was less than 0.2 at.% for the slowly solidified specimen. The excess of the additive solidified as a phase of a eutectic with the Si phase. On the other hand, the solubility of these additives in the beta phase was classified into two groups. The first group had higher solubility in the beta phase than that in the alpha phase. The solubility of the other group was as low as that in the alpha phase. Pd and Au belonged to the former and Pt and Ag belonged to the later. The acceleration of the eutectoid decomposition was only observed in the former group but it was negligible in the later group. The existence of the eutectic melt at the annealing temperature was effective on the coarsening of the eutectoid structure but not essential for the acceleration. The eutectoid decomposition strongly depended on the solubility of these elements into the beta phase. (C) 2002 Kluwer Academic Publishers.

The addition of a small amount of Cu is effective in accelerating the alpha --> beta + Si eutectoid decomposition. Some elements (Pd, Pt, Ag and Au) that are expected to have similar chemical properties were added to an Fe2Si5 based alloys up to 1.0 at.% to examine whether a similar effect could be revealed. The microstructures, X-ray diffraction and differential thermal analysis (DTA) of slowly solidified or heat treated specimens were investigated in detail. The solubility of all containing elements into the alpha phase was less than 0.2 at.% for the slowly solidified specimen. The excess of the additive solidified as a phase of a eutectic with the Si phase. On the other hand, the solubility of these additives in the beta phase was classified into two groups. The first group had higher solubility in the beta phase than that in the alpha phase. The solubility of the other group was as low as that in the alpha phase. Pd and Au belonged to the former and Pt and Ag belonged to the later. The acceleration of the eutectoid decomposition was only observed in the former group but it was negligible in the later group. The existence of the eutectic melt at the annealing temperature was effective on the coarsening of the eutectoid structure but not essential for the acceleration. The eutectoid decomposition strongly depended on the solubility of these elements into the beta phase. (C) 2002 Kluwer Academic Publishers.

Effect of electron irradiation on the crystallization and phase stability of Fe88Zr9B3 and Fe71Zr9B20 amorphous alloys was examined. Electron irradiation at an accelerated voltage of 2000 kV was performed at room temperature. The Fe71Zr9B20 alloy showed a wide supercooled liquid region and the ΔTx value was 71 K, while no glass transition was observed in Fe88Zr9B3 alloy. The amorphous phase in Fe-Zr-B alloys was not stable under irradiation and crystallization from the amorphous phase was accelerated by the irradiation. Nanocrystalline structure composed of α-Fe and cubic-Fe2Zr was formed in Fe88Zr9B3 alloy by irradiation induced crystallization, while no nanoscale precipitates of intermetallic compounds were formed during annealing. In Fe71Zr9B20 alloy, the formation of nanocrystalline precipitates was also confirmed by irradiation induced crystallization, although the formation of nanocrystalline structure had not been realized in high B concentration Fe-Zr-B alloys by annealing. These new results show that electron irradiation is effective in producing a new nanocrystalline structure. © 2002 Elsevier Science Ltd. All rights reserved.

Effect of electron irradiation on the crystallization and phase stability of Fe88Zr9B3 and Fe71Zr9B20 amorphous alloys was examined. Electron irradiation at an accelerated voltage of 2000 kV was performed at room temperature. The Fe71Zr9B20 alloy showed a wide supercooled liquid region and the ΔTx value was 71 K, while no glass transition was observed in Fe88Zr9B3 alloy. The amorphous phase in Fe-Zr-B alloys was not stable under irradiation and crystallization from the amorphous phase was accelerated by the irradiation. Nanocrystalline structure composed of α-Fe and cubic-Fe2Zr was formed in Fe88Zr9B3 alloy by irradiation induced crystallization, while no nanoscale precipitates of intermetallic compounds were formed during annealing. In Fe71Zr9B20 alloy, the formation of nanocrystalline precipitates was also confirmed by irradiation induced crystallization, although the formation of nanocrystalline structure had not been realized in high B concentration Fe-Zr-B alloys by annealing. These new results show that electron irradiation is effective in producing a new nanocrystalline structure. © 2002 Elsevier Science Ltd. All rights reserved.

Effect of electron irradiation on the structural and phase stability of amorphous phase and crystallites in Fe-9Zr-3B alloy was examined. Amorphous ribbons were produced by a single-roller-melt-spinning method. Some specimens were annealed at 873 and 1073 K to obtain α-Fe and the duplex structure composed of α-Fe phase and Fe-Zr intermetallic compounds, respectively. Electron irradiation on the amorphous and crystalline specimens were performed at dose rates of 1.7 × 1024 and 7.5 × 1023 m-2 s-1. The electron irradiation accelerated crystallization from the amorphous phase, while the α-Fe and the intermetallic compounds remained stable in annealed specimens. During electron irradiation, cubic-Fe2Zr crystallized from the amorphous phase prior to α-Fe precipitates in melt-spun specimens without annealing, while the α-Fe precipitation advanced during annealing © 2002 Elsevier Science B.V. All rights reserved.

Effect of electron irradiation on the structural and phase stability of amorphous phase and crystallites in Fe-9Zr-3B alloy was examined. Amorphous ribbons were produced by a single-roller-melt-spinning method. Some specimens were annealed at 873 and 1073 K to obtain α-Fe and the duplex structure composed of α-Fe phase and Fe-Zr intermetallic compounds, respectively. Electron irradiation on the amorphous and crystalline specimens were performed at dose rates of 1.7 × 1024 and 7.5 × 1023 m-2 s-1. The electron irradiation accelerated crystallization from the amorphous phase, while the α-Fe and the intermetallic compounds remained stable in annealed specimens. During electron irradiation, cubic-Fe2Zr crystallized from the amorphous phase prior to α-Fe precipitates in melt-spun specimens without annealing, while the α-Fe precipitation advanced during annealing © 2002 Elsevier Science B.V. All rights reserved.

Effect of electron irradiation on the structural and phase stability of amorphous phase and crystallites in Fe-9Zr-3B alloy was examined. Amorphous ribbons were produced by a single-roller-melt-spinning method. Some specimens were annealed at 873 and 1073 K to obtain α-Fe and the duplex structure composed of α-Fe phase and Fe-Zr intermetallic compounds, respectively. Electron irradiation on the amorphous and crystalline specimens were performed at dose rates of 1.7 × 1024 and 7.5 × 1023 m-2 s-1. The electron irradiation accelerated crystallization from the amorphous phase, while the α-Fe and the intermetallic compounds remained stable in annealed specimens. During electron irradiation, cubic-Fe2Zr crystallized from the amorphous phase prior to α-Fe precipitates in melt-spun specimens without annealing, while the α-Fe precipitation advanced during annealing © 2002 Elsevier Science B.V. All rights reserved.

Effect of B concentration on the thermal stability of amorphous phase and the crystallization process in Fe91-xZr9Bx (x = 0 to 30) alloys was examined. The onset temperature of crystallization (T-x) and glass formation temperature (T-g) monotonously increased with increasing B concentration, while the temperature interval of the supercooled liquid region ( Delta T-x -T-g-T-x) rapidly increased with B concentration up to 20.0 at.%B reaching about 70K. Fe71Zr9B20 alloy having the widest supercooled liquid region in present study showed four stages in crystallization. TTT diagram of Fe71Zr9B20 alloy was determined for the phase transformation from amorphous or supercooled liquid states to alpha-Fe and several compounds and sequential changes of crystalline phases. The crystallization behavior differed from that of Fe88Zr9B3 alloy that forms nanocrystalline structure by first crystallization.

Two rapid solidification process were applied to various Fe2Si5 based alloys. The rapidly solidified structure was examined as a function of Si content by TEM, SEM and XRD. The results were compared with those of slowly solidified alloys. Some metastable phases were formed by rapidly solidification and they were dependent on the composition of alloys. A few α + ε eutectic was newly formed with primary α phase in 70.5 at.% Si alloy where no eutectic was expected from the equilibrium phase diagram. The amount of metastable eutectic decreased with increasing Si content. The lattice constants of the rapidly solidified α phase were different from those of the slowly solidified α phase due to the increase of the solubility of Si in α. A supersaturated α single phase was formed in 71.5-72.0 at.% Si alloys

The time-temperature-transformation diagrams for the eutectoid decomposition (α→ β + Si) in slowly solidified Fe2Si5 alloys with small amount of Mn and Cu were obtained in thetemperature range of 873K and 1173K. The shape of the diagrams was typically described as a character C. The nose temperature in a binary Fe2Si5 alloy was about 1023K and it raised about 50 K for Cu added alloy. The addition of small amount of Cu drastically shifted the diagram to shorter time. The start of the eutectoid decomposition in Cu added alloy was more than 240 times faster than that in Cu free alloy at nose temperature. The shape of the Si dispersoids formed by the eutectoid decomposition changed from lamella in Cu free alloy to granular in Cu added alloy. In FeSi2 alloy, the peritectoid reaction (α+ε→β) was also enhanced by the addition of Cu. These results suggested that the existence of Si was not significant for the acceleration of β formation. The kinetic analysis of β phase transformation based on Johnson-Mehl-Avrami equation suggested that the addition of Cu may not change the transformation mechanism but change the growth kinetics. The size of Si dispersoids was quite fine by h