@misc{oai:ir.soken.ac.jp:00000530,
 author = {CHEN, Jiming and ジミン, チェン and CHEN, Jiming},
 month = {2016-02-17, 2016-02-17},
 note = {Fossil fuels will be depleted if people continue to burn them for energy, and an energy shortfall would appear in less than fifty years. People are looking for innovative energies to meet the demand. The most attractive but challenging one is to make nuclear fusion work on earth. If this is successful, we will have a clean energy source that is inexhaustible owing to the abundant fuel in seawater. One of the promising approaches to fusion is the magnetic confinement concept. A dozen of Tokamak devices have been built around the world. DT plasmas are confined in the torus vessels by magnetic field. When the plasmas are heated up to over 100 million degree with high density and long holding time, fusion reaction will take place. A breakeven condition for energy in and out has been achieved by Tokamak devices and the experiments are approaching an ignition. ITER will be constructed with an expectation of 10 times more energy produced than used for heating the plasma. <br />　It is a great challenge to make fusion a commercial reality. The size, the cost and the complexity of the reactor must be reduced. A safe and efficient tritium handling technique must be developed. Development of fusion materials is also a great concern. The materials must be able to survive high heat flux, retain strength and ductility despite neutron irradiation and have a low activation property. In terms of the applications in a fusion reactor, major fusion materials are classified into plasma facing materials (PFM) and structural materials for vacuum vessel and blanket. Many candidate structural materials have been studied with the high potentiality of ferritic/martensitic (F/M) steel, vanadium alloy and SiC/SiC composite for near, middle and long-term applications. In spite of less experience in large-scale application, vanadium alloy has many advantages over F/M steel, such as better lower activation property, higher thermal load capability and stronger resistance to neutron irradiation. V-4Cr-4Ti is referred as the leading one for fusion application because of its quite low DBTT and acceptable high temperature strength. <br />　There remain a number of critical issues to be resolved for vanadium alloys despite their good properties. One of the issues is the effect of O, C and N interstitials and their precipitates on mechanical properties. Although there were studies showing their effects on recovery, recrystallization, precipitation hardening, high temperature tensile and creep strength, the influence of substitutional solutes were scarcely studied. DSA (dynamic strain aging) caused by the impurities is known to be strongly affected by Ti. But the studies on the effect of Cr were rarely reported. Precipitation is generally not welcome in high temperature service for the purpose of keeping a good thermal stability. But small effort has been made on the feasibility to utilize the precipitation for enhancing the strength of vanadium alloy structures. Effect of alloying elements on precipitation needs to be studied further. Hydrogen in the alloy causes hydrogen embrittlement. Although the embrittlement has been evaluated with tensile tests, the data on the effect of hydrogen on fracture toughness was quite limited and might be more serious. Hydrogen release during annealing or deformation was reported but its effect on mechanical properties was not well understood. Room temperature plastic flow at constant stress is a phenomenon that has never been investigated. <br />　The objective of the present study is to clarify the effects of interstitial and substitutional solutes on mechanical properties of vanadium alloys for fusion reactors. Emphases were placed on the mechanisms of hardening by C, N, O and H, effects of Ti on the role of these interstitial solutes, role of Cr on the interaction of Ti with C, N and O. The role of substitutional solute of W, which is a potential alternative to Cr, was also investigated. The effects of the solutes on hydrogen embrittlement and hydrogen-induced change of mechanical properties were also studied, including the effect of hydrogen release behavior. The study was oriented to supporting to optimize chemical compositions and processing steps of vanadium alloys and the impurity control during the use in the blanket structure. <br />　In the present study, many sorts of V-based alloys, designed according to phase diagram and neutron induced activity, were developed in laboratory including some new alloys with addition of W. The alloys were V-4Ti, V-4Cr-4Ti, V-4Ti-3Al, V-3Ti-1Al-Si, V-8W, V-7W-0.3Al, V-6W-(1-4)Ti and so forth. Alloys were melted in a magnetic floating furnace, forged at ~950-1150℃ and hot rolled at 400-850℃ in air with surface protection, and cold rolled to 0.5-1 mm thick plate finally. 50%CW (defined as thickness reduction by cold rolling) plates were used to study their recovery and recrystallization behavior by isochronal annealing at 200-1100℃ for 1h. The solid solution hardening by the alloying elements was investigated from the hardness data of the 1100℃-annealed samples. Some complete recrystallized alloys were tensile tested at 400-800℃ in vacuum to study DSA and the role of alloying elements. To clarify precipitation hardening, alloy plates in solid solution state were again isochronally annealed at 200-1100℃ for 1h. An aging was then conducted at 600℃ for 1-393 hrs to learn the time dependence of the hardening. Following the aging, samples were further annealed at 200-1100℃ for 1h to study the thermal stability of the hardening. For all cases, hardness test was performed at room temperature (RT). Hydrogen embrittlement was evaluated by tensile, J<SUB>1c</SUB> and impact tests at RT. The specimens were charged with hydrogen at 500-800℃ in a H<SUB>2</SUB> atmosphere. During tensile test, plastic flow at constant stress and hydrogen release were observed and more tests to show their behaviors were performed at various stress levels. TEM and SEM were used to analyze microstructures and fracture features. <br />　Results showed that all alloying elements and interstitial solutes of C, N, O and H are strong solid solution strengtheners. The hardening coefficient of the interstitial solutes is much higher than that by the substitutional solutes of Cr, Ti and W being about 9.55, 8.92 and 7.13Hv/%mass, respectively. Cr contributed more to the solid solution hardening than Ti. Considering the much bigger atomic weight of W, W should be the strongest species per atom to strengthen the alloy at room temperature among the substitutional atoms. <br />　The alloys with many previously formed large precipitates showed weak or no further precipitation in the annealing at 200-1100℃ Precipitation occurred at 600~800℃ for Ti-bearing alloys in solid solution annealing state. The annealing at 600~700℃ produced high number density of fine precipitates of Ti-CON, which hardened the alloy significantly. Peak hardening occurred at 700℃ for V-4Cr-4Ti but 600℃ for V-6W-4Ti in the isochronally annealing for 1 hr. The hardening of V-4Cr-4Ti is more prominent than V-6W-4Ti alloy by aging at 600℃, indicating Cr contributes also to precipitation hardening. The growth of the precipitates is controlled by Ti diffusion, thus Cr is presumed to slow down Ti diffusion due to Cr-Ti interaction. The precipitation hardening is stable at <500℃, since Ti is relatively immobile below the temperature. The aging hardening has a slight effect on ductility of the alloy and at certain aging condition even increases its static fracture toughness, defined as the absorbed energy during tensile test. So the precipitation hardening may be used to attain a high strength vanadium alloy. <br />　Alloying elements studied in the present study showed little effect on the annealing temperature for complete recrystallization. However, the starting temperature for notable hardness recovery of the 50%CW cold-rolled alloys was increased by ~100℃by alloying V with Ti. Besides, unlike V-8W and unalloyed V, the Ti-bearing alloys showed no additional hardening at ~300℃. Interstitial C, N and O impurities in matrix also showed a certain effect on the recovery behavior. More C, N and O in matrix led to less hardness recovery and even additional hardening around 600℃ for the alloys with 4%Ti in mass. All these behaviors seem to be resulted from both the resistance of the interstitials to dislocation motion and the role of Ti reducing the mobility of the interstitials. <br />　During the tensile tests at 400-700℃ in certain strain rate range, interstitial impurities moved to dislocations and caused DSA for V-Ti alloys. As a result, load-displacement serrations occurred and the strength of the alloy increased. The strongest serrations appeared at ~300℃ for unalloyed V. Due to the role of Ti decreasing the mobility of interstitial impurities, the temperature for the serrations shifted to ~600℃ for both V-6W-4Ti and V-4Cr-4Ti. Tensile strength started to increase at ~400℃ with increasing temperature for both alloys, but the V-4Cr-4Ti alloy shows better mechanical performance than V-6W-4Ti at higher than 600℃. The tensile strength of V-6W-4Ti began to decrease above 600℃ while that of the V-4Cr-4Ti continued to increase till 700℃. Their different precipitation-hardening behaviors give evidence that Ti and interstitial impurities in V-4Cr-4Ti are less mobile than those in V-6W-4Ti, probably caused by the Cr-Ti interaction. This should account for the difference in high temperature mechanical performance of V-4Cr-4Ti and V-6W-4Ti. <br />　Though H is mobile at room temperature (RT), DSA didn’t appear in tensile tests of V-4Cr-4Ti containing 119-341 wppm H at RT. Both yield strength and ultimate tensile strength decreased with decreasing tensile strain rate. On the other hand, it was found that the slope of the tensile curve in elastic regime, the young’s modulus, increased with decreasing strain rate. This suggests that hydrogen release took place; since hydrogen in solid solution could reduce the atomic binding force to which young’s modulus is proportional. Further studies by tensile test at fixed load in the elastic regime showed the shrinkage of the tensile specimen with hold time in a decreasing rate. After the fixed-stress loading, tensile tests indicated the decrease of the ultimate tensile strength and Hydrogen concentration measurements showed the decrease of the concentration. So it is suggested that tensile stress could enhance hydrogen release even at RT. <br />　Hydrogen in vanadium alloys causes hardening and embrittlement. In the present study, the embrittlement occurred for all alloys at critical hydrogen concentrations (CHC), below which the uniform elongation hardly changed with hydrogen while the hardening showed an approximately linear increase with the hydrogen concentration. Alloying elements had a strong effect on the behavior. The Ti-bearing alloys showed weaker hardening and had higher CHC in contrast to V-8W. An atomic model was used to explain the behavior and it suggested that alloying elements with bigger difference in atomic size to V could increase the resistance to hydrogen embrittlement. Other results showed that the specimen size had certain effects on the behavior as well. Thicker specimen behaved more sensitive to the embrittlement. The absorbed energy of the Charpy specimen decreased drastically with increasing hydrogen concentration though the fracture seemed to be more ductile as compared to the tensile specimen. It was presumed that in thicker specimens less hydrogen was released and impact loading caused hydrogen embrittlement more pronounced. By comparison, CT (compact tension) specimen in JB1cB test was much more sensitive to the embrittlement. Estimated from the results, CHC evaluated by tensile test for V-4Cr-4Ti was 215-310wppm but less than 130wppm by J<SUB>1c</SUB> test. Additionally, oxygen in vanadium alloys enhanced the embrittlement significantly.<br /> Constant plastic flow was observed at room temperature for V-4Cr-4Ti and V-6W-2.5Ti when they were loaded at a fixed stress above their yield points. The deformation seemed to obey the three-stage behavior with time. The first stage and the second stage were very short, about 2-2.5hrs in total. The 3<SUP>rd</SUP> stage, apparently a steady-state plastic flow, took longer time and had a much bigger deformation rate than the 2<SUP>nd</SUP> stage. For V-6W-2.5Ti, the rate at 300 MPa was about 1.11x10<SUP>-3</SUP>/h in the 3<SUP>rd</SUP> stage. The rate increased at higher stress. It was found that hydrogen in the alloy enhanced the plastic flow due to hydrogen release, namely detrapping of hydrogen from dislocations enhanced dislocation glide. Since the applied stress was higher than yield strength, the plastic flow was considered to be dislocation-glide assisted. <br />　The conclusions of the present study are as follows. <br />(1) The effect of solutes and solute interactions on mechanical properties of vanadium alloys was significant. All of the solutes concerned were solid solution strengtheners. The hardening coefficient of the interstitial solutes was much higher than that of the substitutional solutes. <br />(2) Ti and the interstitial C, N and O were necessary and were responsible for higher high-temperature strength and keeping the cold-rolling hardening at elevated temperature. Additionally they were the cause of precipitation, and the resulting hardening could be utilized for enhancing strength of the vanadium alloy for relatively low temperature application. For these properties, Cr also provided large positive contribution by its effect to reduce the mobility of Ti due to Cr-Ti interaction. <br />(3) A critical new issue of plastic flow at room temperature at constant stress higher than yield strength was found for which more studies should be addressed. <br />(4) As for the hydrogen effect, the hydrogen embrittlement could be a great concern for the fusion application due to the strong sensitivity of the fracture toughness. In addition, the hydrogen release could cause dimensional instability. Oxygen in vanadium alloys enhanced the embrittlement significantly. Alloying V with species that had relatively large difference in atomic size to V could improve the property against hydrogen embrittlement. <br />(5) All these results showed interactive role of interstitial and substitutional solutes on mechanical properties of vanadium alloys through solid solution hardening, mutual binding or trapping, interaction with dislocations, precipitation and precipitate resolution.<br />, application/pdf, 総研大乙第155号},
 title = {Effects of Solute and Solute Interactions on Mechanical Properties of Fusion Candidate Vanadium Alloys},
 year = {}
}