Transactions of the Japan Institute of Metals, 1978
Mechanical properties of high purity irons were investigated by tensile testing at temperatures f... more Mechanical properties of high purity irons were investigated by tensile testing at temperatures from room temperature down to 4.2 K. Specimens with the residual resistivity ratio (RRRH) of 1800 were brittle and fractured at grain boundaries at 4.2 K. Specimens with RRRH of 3600, of which the grain structure was almost of the bamboo type, were deformed by slip and often showed complete ductile fracture even at 4.2 K. This is the first observation of slip at 4.2 K without pre-straining at high temperatures in a coarse-grained iron. Specimens with RRRH of 5000, of which the grain structure was of a well-developed bamboo type, cleaved at 4.2 K, and never showed grain boundary fracture. Some plastic properties were investigated on specimens with RRRH of 3600 and 6000. Careful comparison of the results with those reported by other investigators shows that the present specimens are pure enough to reveal some of the intrinsic plastic properties of iron. The yield stress as a function of temperature and the strain rate sensitivity of the flow stress at various temperatures obtained with the present specimens may be considered to be characteristic of iron.
Dislocation internal friction observations in iron are reported with "in situ" hydrogen charging ... more Dislocation internal friction observations in iron are reported with "in situ" hydrogen charging to study its influence on the kink pair formation in screw dislocations (Y peak) and its possible relation to earlier observed softening effects. Also some substructures in the hydrogen cold-work peak are revealed.
The manufacturing process of creep specimens and an irradiation technique in a liquid metal envir... more The manufacturing process of creep specimens and an irradiation technique in a liquid metal environment for in-pile and creep measurements of irradiated samples are established for highly purified V-4Cr-4Ti, NIFS-HEAT alloys. Irradiation experiments with sodium-enclosed irradiation capsules in JOYO and lithiumenclosed irradiation capsules in HFIR-17J were conducted using pressurized creep tubes. From thermal creep experiments, the activation energy of creep deformation using pressurized creep tubes was determined to be 210 kJ/mol•K, the creep stress factor was 4.9 for an 800 • C creep test, and its mechanism was determined to be a climb-assisted glide of dislocation motion. It was found that the creep strain rate exhibited a linear relationship with effective stress up to 150 MPa from 425 to 600 • C under JOYO and HFIR irradiation. The activation energy of irradiation creep was estimated to be 46 kJ/mol•K. No significant difference in irradiation creep behavior between the liquid sodium and liquid lithium environments was observed. A set of essential physical data of irradiation creep properties was obtained for V-4Cr-4Ti alloys.
Pinning interaction between a screw dislocation and a void in fcc copper is investigated by means... more Pinning interaction between a screw dislocation and a void in fcc copper is investigated by means of molecular dynamics simulation. A screw dislocation bows out to undergo depinning on the original glide plane at low temperatures, where the behavior of the depinning stress is consistent with that obtained by a continuum model. If the temperature is higher than 300 K, the motion of a screw dislocation is no longer restricted to a single glide plane due to cross slip on the void surface. Several depinning mechanisms that involve multiple glide planes are found. In particular, a depinning mechanism that produces an intrinsic prismatic loop is found. We show that these complex depinning mechanisms significantly increase the depinning stress.
One-dimensional ͑1D͒ migration of small interstitial-type dislocation loops was studied for Fe sp... more One-dimensional ͑1D͒ migration of small interstitial-type dislocation loops was studied for Fe specimens of different purities at room temperature under electron irradiation using a high-voltage electron microscope. Most 1D migration appeared as discrete jumps ͑stepwise positional changes͒ at irregular intervals, and sometimes involved back and forth motion between certain points. The distribution of jump distances extended to over 100 nm in high-purity specimens; it was less than 30 nm in low-purity specimens. Jump frequency was almost proportional to electron beam intensity and was on the same order as the rate of atomic displacement by electron irradiation. Molecular dynamics simulation suggested the suppression of 1D migration of an interstitial cluster ͑7i͒ by an oversized solute Cu atom located in the dilatational strain field of the cluster. We proposed that the 1D jump process occurs in the following sequence: ͑1͒ interstitial clusters are in a stationary state due to trapping effect by impurity atoms, ͑2͒ incident electrons hit and displace impurity atom to cause detrapping, ͑3͒ liberated clusters cause fast 1D migration at low activation energy, and ͑4͒ the cluster is trapped again by another impurity. Experimental results were analyzed and discussed in terms of the proposed model.
Interactions between an edge dislocation and a void in copper are investigated by means of molecu... more Interactions between an edge dislocation and a void in copper are investigated by means of molecular dynamics simulation. The depinning stresses of the leading partial and of the trailing partial show qualitatively different behaviors. The depinning stress of the trailing partial increases logarithmically with the void radius, while that of the leading partial shows a crossover at 1 nm above which two partials are simultaneously trapped by the void. The pinning angle, which characterizes the obstacle strength, approaches zero when the void radius exceeds 3 nm. No temperature dependence is found in the critical stress and the critical angle. This is attributed to an absence of climb motion. The distance between the void center and a glide plane asymmetrically affects the pinning strength.
In fusion reactors, structural materials are expected to experience non-steady histories of irrad... more In fusion reactors, structural materials are expected to experience non-steady histories of irradiation temperature, neutron flux and other parameters during reactor start-up/shut-down, plasma disruptions, etc. The objective of the present study is to clarify the effects of the downward temperature change during irradiation. Vanadium that is the one of candidate materials for the first wall of fusion reactors was irradiated with heavy ions during the downward temperature change. TEM observation, nano-indentation and HVEM observation were carried out. It was reported that the growth of defect clusters including cavities and precipitates occurred under neutron irradiation and heavy ion irradiation during the downward temperature change. In the present paper, the detailed mechanism of growth of defect clusters was investigated by changing the lower temperature as a parameter. The growth of defect clusters occurs just after the downward temperature change followed by re-nucleation of defect clusters at lower temperatures.
The high temperature deformation behavior of a fine-grained and particle-dispersed V-2.3%Y-4%Ti-3... more The high temperature deformation behavior of a fine-grained and particle-dispersed V-2.3%Y-4%Ti-3%Mo (mass%) alloy was investigated. The alloy was fabricated by powder metallurgical methods utilizing mechanical alloying and hot isostatic pressing (HIP), followed by annealing at 1273 K for 3.6 ks. Tensile tests were performed at temperatures from 873 to 1273 K at initial strain rates from 2:5 Â 10 À5 to 1:0 Â 10 À1 s À1 . X-ray diffraction analyses show that the HIPed and annealed specimens contain a significant amount of (Y, Ti) 2 O 3 and small amounts of YN, Ti 2 O 3 and TiO. Transmission electron microscopy observations show that the matrix contains approximately 3%Ti and 3%Mo in solution and that the average diameters of the matrix grains and dispersoids are about 620 and 42 nm, respectively. Tensile test results show that the yield stress depends strongly on test temperature and strain rate. From the dependence of the yield stress measured at 1073 and 1273 K on plastic strain rate, it is found that the dependence is divided into three regions with different deformation controlling mechanisms: a recovery controlling process of a long range internal stress field associated with dispersed particle (the high-strain-rate region), grain boundary sliding (the medium-strain-rate region) and presumably solute atmosphere dragging (the low-strain-rate region). Effects of 4%Ti addition on the microstructures and high temperature deformation behavior are discussed.
The factors controlling irradiation hardening and their contributions to the hardening in electro... more The factors controlling irradiation hardening and their contributions to the hardening in electron irradiated pure-iron and Fe-0.15 mass%Cu alloy were determined by means of post-irradiation annealing experiments, such as hardness measurements, positron annihilation spectroscopy (PAS) measurements, transmission electron microscope (TEM) observations and three dimensional atom probe (3DAP) analyses. In pure-iron, almost complete recovering of the hardness was observed after the annealing to 773 K, which was accompanied by disappearing of the interstitial type dislocation loops (I-loops) that were observed in as-irradiated specimen. In contrast, the hardening of Fe-0.15 mass%Cu alloy recovered in a two-step mode; about a half of the hardening recovered by the 773 K annealing, and a complete recovery was observed after annealing to 973 K. Most of the I-loops observed in as-irradiated specimen again disappeared after the annealing to 773 K. These clearly show that the I-loops are one of the main factors controlling irradiation hardening in iron-copper alloy. The residual hardening in the Fe-0.15 mass%Cu alloy after the annealing to 773 K, which is about a half of the irradiation hardening, was attributed to the copper-rich precipitates (CRP) through the direct observation by 3DAP analysis. PAS measurements revealed the disagreement between the recovery behaviors of the hardness and lifetime parameters. Based on the quantitative data analysis, it was concluded that the factor controlling the irradiation hardening of pure-iron is the Iloops, and those in Fe-0.15 mass%Cu alloy are both the I-loops and CRP of which the contributions to the hardening are almost same.
Analyses have been performed which indicate that an effective method for experimentally investiga... more Analyses have been performed which indicate that an effective method for experimentally investigating helium effects in neutron irradiated vanadium base alloys can be developed. The experimental procedure involves only modest modifications to existing procedures currently used for irradiation testing of vanadium-base alloys in the FFTF reactor. Helium is generated in the vanadium alloy by decay of tritium which is either preinjected or generated within the test capsule. Calculations indicate that nearly constant He/dpa ratios of desired magnitude can be attained by proper selection of experimental parameters. The proposed method could have a major impact on the development of vanadium base alloys for fusion reactor applications.
Reduced activation martensitic steels (RAMSs) are the prime candidate structural material for the... more Reduced activation martensitic steels (RAMSs) are the prime candidate structural material for the DEMO reactor and beyond where the material has been considered to suffer severe embrittlement caused by high-dose neutron irradiation and several thousands appm of transmutated helium. However, recent several works show high resistance to helium embrittlement of RAMS. Good performance of RAMS in the presence of rather high concentration of helium is considered to be due to high trapping capacity for helium atoms in the martensitic structure that consists of dislocations, lath boundaries, grain boundaries and carbide/matrix interfaces. To make clear the role of dislocations in trapping helium, thermal helium desorption spectra were measured for iron specimens annealed at different temperatures after cold work. A desorption peak, which increased its height with increasing dislocation density, was observed at around 550 °C, suggesting that dislocations trap helium atoms. A molecular dynamics simulation study for investigating the helium trapping behavior at helium-vacancy complexes suggests that helium is rather strongly bound to the complexes and increases the binding energy of vacancy to the complex, resulting in increasing stability of the complexes at elevated temperatures by reducing thermal emission of vacancies.
Pressurized thermal creep tubes of highly purified V-4Cr-4Ti, the NIFS-Heat2 alloy have been exam... more Pressurized thermal creep tubes of highly purified V-4Cr-4Ti, the NIFS-Heat2 alloy have been examined following testing in the range 700 to 850°C. It was found that the creep stress exponent of the NIFS-Heat2 alloy is about 5 and that the characteristic creep mechanism was the dislocation creep usually observed in pure metals. The apparent activation energy of creep deformation is about 210kJ/mol in the temperature range 700 to 850°C. Creep deformation was considered to be controlled by climb-controlled dislocation glide at 850°C, where sub-grain boundary structure predominates and consists of dislocation dipole structures and pile-ups of dislocations.
Vanadium alloys, including the highly purified V-4Cr-4Ti alloy called NIFS-Heat2, in the form of ... more Vanadium alloys, including the highly purified V-4Cr-4Ti alloy called NIFS-Heat2, in the form of sodium-enclosed irradiation capsules, were irradiated up to a damage level of 5dpa in the Joyo reactor at temperatures from 395 to 601°C. An increase of the ductile-brittle transition temperature (DBTT) by neutron irradiation and irradiation hardening were observed. No significant loss of ductility was observed even for irradiation at 395 and 450°C. The addition of titanium to V-Cr alloys was effective for irradiation hardening at high temperature. Hydrogen uptake in the cleaning process during dismantling of the irradiation capsules caused ductility loss of the highly purified V-4Cr-4Ti alloys; the alloys recovered their ductility when they were annealed at 400°C in vacuum. The uniform formation of Ti(OCN) precipitate was suppressed in highly purified V-4Cr-4Ti alloys irradiated in Joyo in a 3 liquid-sodium environment.
The intense neutron source for development of fusion materials planned by international collabora... more The intense neutron source for development of fusion materials planned by international collaboration makes a new step to clarify the technical issues for realizing the 40 MeV, 250 mA deuteron beam facility. The baseline concept employs two identical 125 mA linac modules whose beams are combined at the flowing lithium target. Recent work for reducing the cost loading concerns the staged deployment of the full irradiation capability in three steps. The Japanese activity about the design and development study about IFMIF accelerator in this year is presented and the schedule of next several years is overviewed.
The objective of this effort is to improve understanding of the effects of fast neutron irradiati... more The objective of this effort is to improve understanding of the effects of fast neutron irradiation'on microstructural evolution by examining radiation damage response in a I MeV high voltage electron L microscope. Micrnstrtlct_Iral evolution in response to ] MeV irradiation has be,en investigated for three simple ferritic alloys, pure beryllium, pure vanadium, and two simple vanadium alloys over a range of temperatures and doses. MicrostructuraI evolution in Fe-3, -9, and -I8Cr ferritic alloys is found to consist of crenulated, faulted a<]O0> loops and circular, unfalJILed a/2 <Ill> loops at low temperatures, but with only unfaulted loops at high temperatures. The complex dislocation evolution is attributed to sigma phase precipifaults arising from chromium segregation I.o point defect sinks. Beryllium is found to he resistant to electron Hamage: the only effect ohserved was enhanced dislocation mobility. Pure vanadium, 4-5Fe, and V-INi microstructuraI response was complicated l)y precipitation on heating Lo 400°C and above, but dislocation evoltltion was investigated in the range of room temperature to 300°C and at 600°C. The three materi_Is behaved similarly, except that piire vanadium showed more rapid dislocation evolution. This difference does not explain the enhanced swelling ol)served in vanadium alloys.
It is well-known that the dislocation bias should be responsible for the large swelling. In V-Fe ... more It is well-known that the dislocation bias should be responsible for the large swelling. In V-Fe alloys, the extremely large swelling has been observed, which indicates that undersized Fe atoms must play a significant role for the modification of one-dimensional (1D) motion. In the present study, we performed molecular dynamics (MD) simulations with a newly constructed empirical potential based on the Finnis-Sinclair (FS) type embedded atom method (EAM) to investigate the effects of undersized Fe atoms on the static and dynamic properties of the interstitial migration under the strain field of several sizes of interstitial loops and an edge dislocation. The results show that the undersized Fe atoms tend to form a stable mixed-dumbbell configuration and the activation energy of migration towards a dislocation core is much lower than that of SIAs. Moreover, for V-Fe dumbbell the relative probability for intermediate configurations between 111-crowdion and 110-dumbbell below an edge dislocation core becomes significantly higher compared to that of V-V dumbbell. These results indicate that it is very likely that the continuous absorption of interstitial Fe atoms by a dislocation core would be possible and enhances the swelling in V-Fe alloys.
ABSTRACT To examine the mechanism of huge swelling of V–Fe alloys, TEM observation was applied to... more ABSTRACT To examine the mechanism of huge swelling of V–Fe alloys, TEM observation was applied to V–xat.%Fe (x=0, 0.3, 1, 5, 7), after low-dose neutron irradiation of 0.1–1.17dpa at 400–600°C. Concentric Multi-Dislocation Loops (CMDLs) and rafts were observed in V–Fe alloys irradiated to 0.1dpa. Voids were observed only within dislocation loops in both types of configurations. Ashby-Brown (AB) contrasts were always observed around voids, indicating that shell-like iron segregation with substantial elastic strain exists on the inner surface of the voids. For 1.17dpa, swelling V–Fe alloys was always greater than those of un-alloyed vanadium, whatever the dislocation microstructures. Implication of a segregation shell in the swelling mechanism is discussed in addition to a model based on the dislocation bias.
Transactions of the Japan Institute of Metals, 1978
Mechanical properties of high purity irons were investigated by tensile testing at temperatures f... more Mechanical properties of high purity irons were investigated by tensile testing at temperatures from room temperature down to 4.2 K. Specimens with the residual resistivity ratio (RRRH) of 1800 were brittle and fractured at grain boundaries at 4.2 K. Specimens with RRRH of 3600, of which the grain structure was almost of the bamboo type, were deformed by slip and often showed complete ductile fracture even at 4.2 K. This is the first observation of slip at 4.2 K without pre-straining at high temperatures in a coarse-grained iron. Specimens with RRRH of 5000, of which the grain structure was of a well-developed bamboo type, cleaved at 4.2 K, and never showed grain boundary fracture. Some plastic properties were investigated on specimens with RRRH of 3600 and 6000. Careful comparison of the results with those reported by other investigators shows that the present specimens are pure enough to reveal some of the intrinsic plastic properties of iron. The yield stress as a function of temperature and the strain rate sensitivity of the flow stress at various temperatures obtained with the present specimens may be considered to be characteristic of iron.
Dislocation internal friction observations in iron are reported with "in situ" hydrogen charging ... more Dislocation internal friction observations in iron are reported with "in situ" hydrogen charging to study its influence on the kink pair formation in screw dislocations (Y peak) and its possible relation to earlier observed softening effects. Also some substructures in the hydrogen cold-work peak are revealed.
The manufacturing process of creep specimens and an irradiation technique in a liquid metal envir... more The manufacturing process of creep specimens and an irradiation technique in a liquid metal environment for in-pile and creep measurements of irradiated samples are established for highly purified V-4Cr-4Ti, NIFS-HEAT alloys. Irradiation experiments with sodium-enclosed irradiation capsules in JOYO and lithiumenclosed irradiation capsules in HFIR-17J were conducted using pressurized creep tubes. From thermal creep experiments, the activation energy of creep deformation using pressurized creep tubes was determined to be 210 kJ/mol•K, the creep stress factor was 4.9 for an 800 • C creep test, and its mechanism was determined to be a climb-assisted glide of dislocation motion. It was found that the creep strain rate exhibited a linear relationship with effective stress up to 150 MPa from 425 to 600 • C under JOYO and HFIR irradiation. The activation energy of irradiation creep was estimated to be 46 kJ/mol•K. No significant difference in irradiation creep behavior between the liquid sodium and liquid lithium environments was observed. A set of essential physical data of irradiation creep properties was obtained for V-4Cr-4Ti alloys.
Pinning interaction between a screw dislocation and a void in fcc copper is investigated by means... more Pinning interaction between a screw dislocation and a void in fcc copper is investigated by means of molecular dynamics simulation. A screw dislocation bows out to undergo depinning on the original glide plane at low temperatures, where the behavior of the depinning stress is consistent with that obtained by a continuum model. If the temperature is higher than 300 K, the motion of a screw dislocation is no longer restricted to a single glide plane due to cross slip on the void surface. Several depinning mechanisms that involve multiple glide planes are found. In particular, a depinning mechanism that produces an intrinsic prismatic loop is found. We show that these complex depinning mechanisms significantly increase the depinning stress.
One-dimensional ͑1D͒ migration of small interstitial-type dislocation loops was studied for Fe sp... more One-dimensional ͑1D͒ migration of small interstitial-type dislocation loops was studied for Fe specimens of different purities at room temperature under electron irradiation using a high-voltage electron microscope. Most 1D migration appeared as discrete jumps ͑stepwise positional changes͒ at irregular intervals, and sometimes involved back and forth motion between certain points. The distribution of jump distances extended to over 100 nm in high-purity specimens; it was less than 30 nm in low-purity specimens. Jump frequency was almost proportional to electron beam intensity and was on the same order as the rate of atomic displacement by electron irradiation. Molecular dynamics simulation suggested the suppression of 1D migration of an interstitial cluster ͑7i͒ by an oversized solute Cu atom located in the dilatational strain field of the cluster. We proposed that the 1D jump process occurs in the following sequence: ͑1͒ interstitial clusters are in a stationary state due to trapping effect by impurity atoms, ͑2͒ incident electrons hit and displace impurity atom to cause detrapping, ͑3͒ liberated clusters cause fast 1D migration at low activation energy, and ͑4͒ the cluster is trapped again by another impurity. Experimental results were analyzed and discussed in terms of the proposed model.
Interactions between an edge dislocation and a void in copper are investigated by means of molecu... more Interactions between an edge dislocation and a void in copper are investigated by means of molecular dynamics simulation. The depinning stresses of the leading partial and of the trailing partial show qualitatively different behaviors. The depinning stress of the trailing partial increases logarithmically with the void radius, while that of the leading partial shows a crossover at 1 nm above which two partials are simultaneously trapped by the void. The pinning angle, which characterizes the obstacle strength, approaches zero when the void radius exceeds 3 nm. No temperature dependence is found in the critical stress and the critical angle. This is attributed to an absence of climb motion. The distance between the void center and a glide plane asymmetrically affects the pinning strength.
In fusion reactors, structural materials are expected to experience non-steady histories of irrad... more In fusion reactors, structural materials are expected to experience non-steady histories of irradiation temperature, neutron flux and other parameters during reactor start-up/shut-down, plasma disruptions, etc. The objective of the present study is to clarify the effects of the downward temperature change during irradiation. Vanadium that is the one of candidate materials for the first wall of fusion reactors was irradiated with heavy ions during the downward temperature change. TEM observation, nano-indentation and HVEM observation were carried out. It was reported that the growth of defect clusters including cavities and precipitates occurred under neutron irradiation and heavy ion irradiation during the downward temperature change. In the present paper, the detailed mechanism of growth of defect clusters was investigated by changing the lower temperature as a parameter. The growth of defect clusters occurs just after the downward temperature change followed by re-nucleation of defect clusters at lower temperatures.
The high temperature deformation behavior of a fine-grained and particle-dispersed V-2.3%Y-4%Ti-3... more The high temperature deformation behavior of a fine-grained and particle-dispersed V-2.3%Y-4%Ti-3%Mo (mass%) alloy was investigated. The alloy was fabricated by powder metallurgical methods utilizing mechanical alloying and hot isostatic pressing (HIP), followed by annealing at 1273 K for 3.6 ks. Tensile tests were performed at temperatures from 873 to 1273 K at initial strain rates from 2:5 Â 10 À5 to 1:0 Â 10 À1 s À1 . X-ray diffraction analyses show that the HIPed and annealed specimens contain a significant amount of (Y, Ti) 2 O 3 and small amounts of YN, Ti 2 O 3 and TiO. Transmission electron microscopy observations show that the matrix contains approximately 3%Ti and 3%Mo in solution and that the average diameters of the matrix grains and dispersoids are about 620 and 42 nm, respectively. Tensile test results show that the yield stress depends strongly on test temperature and strain rate. From the dependence of the yield stress measured at 1073 and 1273 K on plastic strain rate, it is found that the dependence is divided into three regions with different deformation controlling mechanisms: a recovery controlling process of a long range internal stress field associated with dispersed particle (the high-strain-rate region), grain boundary sliding (the medium-strain-rate region) and presumably solute atmosphere dragging (the low-strain-rate region). Effects of 4%Ti addition on the microstructures and high temperature deformation behavior are discussed.
The factors controlling irradiation hardening and their contributions to the hardening in electro... more The factors controlling irradiation hardening and their contributions to the hardening in electron irradiated pure-iron and Fe-0.15 mass%Cu alloy were determined by means of post-irradiation annealing experiments, such as hardness measurements, positron annihilation spectroscopy (PAS) measurements, transmission electron microscope (TEM) observations and three dimensional atom probe (3DAP) analyses. In pure-iron, almost complete recovering of the hardness was observed after the annealing to 773 K, which was accompanied by disappearing of the interstitial type dislocation loops (I-loops) that were observed in as-irradiated specimen. In contrast, the hardening of Fe-0.15 mass%Cu alloy recovered in a two-step mode; about a half of the hardening recovered by the 773 K annealing, and a complete recovery was observed after annealing to 973 K. Most of the I-loops observed in as-irradiated specimen again disappeared after the annealing to 773 K. These clearly show that the I-loops are one of the main factors controlling irradiation hardening in iron-copper alloy. The residual hardening in the Fe-0.15 mass%Cu alloy after the annealing to 773 K, which is about a half of the irradiation hardening, was attributed to the copper-rich precipitates (CRP) through the direct observation by 3DAP analysis. PAS measurements revealed the disagreement between the recovery behaviors of the hardness and lifetime parameters. Based on the quantitative data analysis, it was concluded that the factor controlling the irradiation hardening of pure-iron is the Iloops, and those in Fe-0.15 mass%Cu alloy are both the I-loops and CRP of which the contributions to the hardening are almost same.
Analyses have been performed which indicate that an effective method for experimentally investiga... more Analyses have been performed which indicate that an effective method for experimentally investigating helium effects in neutron irradiated vanadium base alloys can be developed. The experimental procedure involves only modest modifications to existing procedures currently used for irradiation testing of vanadium-base alloys in the FFTF reactor. Helium is generated in the vanadium alloy by decay of tritium which is either preinjected or generated within the test capsule. Calculations indicate that nearly constant He/dpa ratios of desired magnitude can be attained by proper selection of experimental parameters. The proposed method could have a major impact on the development of vanadium base alloys for fusion reactor applications.
Reduced activation martensitic steels (RAMSs) are the prime candidate structural material for the... more Reduced activation martensitic steels (RAMSs) are the prime candidate structural material for the DEMO reactor and beyond where the material has been considered to suffer severe embrittlement caused by high-dose neutron irradiation and several thousands appm of transmutated helium. However, recent several works show high resistance to helium embrittlement of RAMS. Good performance of RAMS in the presence of rather high concentration of helium is considered to be due to high trapping capacity for helium atoms in the martensitic structure that consists of dislocations, lath boundaries, grain boundaries and carbide/matrix interfaces. To make clear the role of dislocations in trapping helium, thermal helium desorption spectra were measured for iron specimens annealed at different temperatures after cold work. A desorption peak, which increased its height with increasing dislocation density, was observed at around 550 °C, suggesting that dislocations trap helium atoms. A molecular dynamics simulation study for investigating the helium trapping behavior at helium-vacancy complexes suggests that helium is rather strongly bound to the complexes and increases the binding energy of vacancy to the complex, resulting in increasing stability of the complexes at elevated temperatures by reducing thermal emission of vacancies.
Pressurized thermal creep tubes of highly purified V-4Cr-4Ti, the NIFS-Heat2 alloy have been exam... more Pressurized thermal creep tubes of highly purified V-4Cr-4Ti, the NIFS-Heat2 alloy have been examined following testing in the range 700 to 850°C. It was found that the creep stress exponent of the NIFS-Heat2 alloy is about 5 and that the characteristic creep mechanism was the dislocation creep usually observed in pure metals. The apparent activation energy of creep deformation is about 210kJ/mol in the temperature range 700 to 850°C. Creep deformation was considered to be controlled by climb-controlled dislocation glide at 850°C, where sub-grain boundary structure predominates and consists of dislocation dipole structures and pile-ups of dislocations.
Vanadium alloys, including the highly purified V-4Cr-4Ti alloy called NIFS-Heat2, in the form of ... more Vanadium alloys, including the highly purified V-4Cr-4Ti alloy called NIFS-Heat2, in the form of sodium-enclosed irradiation capsules, were irradiated up to a damage level of 5dpa in the Joyo reactor at temperatures from 395 to 601°C. An increase of the ductile-brittle transition temperature (DBTT) by neutron irradiation and irradiation hardening were observed. No significant loss of ductility was observed even for irradiation at 395 and 450°C. The addition of titanium to V-Cr alloys was effective for irradiation hardening at high temperature. Hydrogen uptake in the cleaning process during dismantling of the irradiation capsules caused ductility loss of the highly purified V-4Cr-4Ti alloys; the alloys recovered their ductility when they were annealed at 400°C in vacuum. The uniform formation of Ti(OCN) precipitate was suppressed in highly purified V-4Cr-4Ti alloys irradiated in Joyo in a 3 liquid-sodium environment.
The intense neutron source for development of fusion materials planned by international collabora... more The intense neutron source for development of fusion materials planned by international collaboration makes a new step to clarify the technical issues for realizing the 40 MeV, 250 mA deuteron beam facility. The baseline concept employs two identical 125 mA linac modules whose beams are combined at the flowing lithium target. Recent work for reducing the cost loading concerns the staged deployment of the full irradiation capability in three steps. The Japanese activity about the design and development study about IFMIF accelerator in this year is presented and the schedule of next several years is overviewed.
The objective of this effort is to improve understanding of the effects of fast neutron irradiati... more The objective of this effort is to improve understanding of the effects of fast neutron irradiation'on microstructural evolution by examining radiation damage response in a I MeV high voltage electron L microscope. Micrnstrtlct_Iral evolution in response to ] MeV irradiation has be,en investigated for three simple ferritic alloys, pure beryllium, pure vanadium, and two simple vanadium alloys over a range of temperatures and doses. MicrostructuraI evolution in Fe-3, -9, and -I8Cr ferritic alloys is found to consist of crenulated, faulted a<]O0> loops and circular, unfalJILed a/2 <Ill> loops at low temperatures, but with only unfaulted loops at high temperatures. The complex dislocation evolution is attributed to sigma phase precipifaults arising from chromium segregation I.o point defect sinks. Beryllium is found to he resistant to electron Hamage: the only effect ohserved was enhanced dislocation mobility. Pure vanadium, 4-5Fe, and V-INi microstructuraI response was complicated l)y precipitation on heating Lo 400°C and above, but dislocation evoltltion was investigated in the range of room temperature to 300°C and at 600°C. The three materi_Is behaved similarly, except that piire vanadium showed more rapid dislocation evolution. This difference does not explain the enhanced swelling ol)served in vanadium alloys.
It is well-known that the dislocation bias should be responsible for the large swelling. In V-Fe ... more It is well-known that the dislocation bias should be responsible for the large swelling. In V-Fe alloys, the extremely large swelling has been observed, which indicates that undersized Fe atoms must play a significant role for the modification of one-dimensional (1D) motion. In the present study, we performed molecular dynamics (MD) simulations with a newly constructed empirical potential based on the Finnis-Sinclair (FS) type embedded atom method (EAM) to investigate the effects of undersized Fe atoms on the static and dynamic properties of the interstitial migration under the strain field of several sizes of interstitial loops and an edge dislocation. The results show that the undersized Fe atoms tend to form a stable mixed-dumbbell configuration and the activation energy of migration towards a dislocation core is much lower than that of SIAs. Moreover, for V-Fe dumbbell the relative probability for intermediate configurations between 111-crowdion and 110-dumbbell below an edge dislocation core becomes significantly higher compared to that of V-V dumbbell. These results indicate that it is very likely that the continuous absorption of interstitial Fe atoms by a dislocation core would be possible and enhances the swelling in V-Fe alloys.
ABSTRACT To examine the mechanism of huge swelling of V–Fe alloys, TEM observation was applied to... more ABSTRACT To examine the mechanism of huge swelling of V–Fe alloys, TEM observation was applied to V–xat.%Fe (x=0, 0.3, 1, 5, 7), after low-dose neutron irradiation of 0.1–1.17dpa at 400–600°C. Concentric Multi-Dislocation Loops (CMDLs) and rafts were observed in V–Fe alloys irradiated to 0.1dpa. Voids were observed only within dislocation loops in both types of configurations. Ashby-Brown (AB) contrasts were always observed around voids, indicating that shell-like iron segregation with substantial elastic strain exists on the inner surface of the voids. For 1.17dpa, swelling V–Fe alloys was always greater than those of un-alloyed vanadium, whatever the dislocation microstructures. Implication of a segregation shell in the swelling mechanism is discussed in addition to a model based on the dislocation bias.
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