@misc{oai:ir.soken.ac.jp:00000529, author = {LI, Huailin and リ, ハイリン and LI, Huailin}, month = {2016-02-17, 2016-02-17}, note = {One of the most important tasks in the 21^th century is to develop a social system in which the environment and life on a global scale are protected. Central item to this issue is the development of economical energy sources with low emission of carbon dioxide because carbon dioxide leads to global warming and may cause more critical issue of abominable global climate. On the other hand, the supplement of fossil fuels is limited. The development of advanced energy is an important option concerning stable energy source in the century.
　　Among the fission, fusion, and other sources such as wind power, fusion has been actively considered as the 21^st century energy source in major countries to meet the energy requirement of our society in the future, because it has potentially superior characteristics with respect to safety, environment acceptability, and resource availability. The structural integrity of fusion reactor depends on the properties of the structural materials. Thus, the development of reactor materials and power generating blanket system is a critical issue for early realization of fusion energy. Among the three materials for fusion applications-reduced activation ferritic/martensitic steel (RAF/M), V-alloy, and SiC/SiC composites, the RAF/M steel is considered as the leading candidate structural material for the first wall in fusion reactor as there exists a well-developed technology and a broader industrial experience with the ferritic-martensitic steels in fossil and nuclear energy technology.
　　Low cycle fatigue (LCF) behavior of RAF/M steel at elevated temperatures is necessary to design since the cyclic plastic deformation might be expected if the combined mechanical and thermal cyclic loading of the first wall is extreamly high. In addition, mechanical properties of martensitic steels are strongly related to their complex microstructure.
　　Thus, the objective of this research is to study the LCF behavior of a candidate structural material of the first wall, JLF-1 steel, at elevated temperature with an engineering size specimen in vacuum. To avoid the shape, size, and environment effects, the LCF tests were carried out with engineering size (8 mm in diameter) round bar specimens in vacuum. The present research focuseses on;
　　1) temperature effect on the relationship between fatigue life and plastic strain range,
　　2) cyclic softening/hardening and the mechanism,
　　3) cyclic yield stress-strain curves and its application on design.To obtain basic information on RAF/M at elevated temperature, the tensile test was also carried out.
　　Tensile and LCF specimens were polished in the longitudinal direction with #1500 emery paper to remove the circumferential machining layers. The test system consisted of mechanical machine (load capacity of ±98 kN), vacuum system (゛10^-3 Pa), and induction heating system (capacity of up to 973 K). The axial displacement of the specimen was measured by a Shinko 1501-93-20 extensometer (a differential transducer with gage length of 12.5 mm and capacity of ± 1 mm). Based on the temperature profile on the specimen, two thermocouples were welded in the gage length on the specimen for temperature control. One thermocouple was used for the control of the temperature and the other was to monitor the temperature during the test. The temperature difference in the two thermocouples was kept less than 3 K, which was stricter than the requirement of ASTM standard.
　　Tensile tests of JLF-1 steel were carried out from room temperature (RT) to 873 K at strain rate of 0.1%/s and 0.02%/s. The strain rate does not affect yield stress, ultimate tensile strength and reduction in area as far as the tests performed in this study. Strain hardening of JLF-1 steel decreased significantly above 673 K, almost no strain hardening was observed at 873 K. With strain hardening decreased, the aspect of fracture changed from shear type fracture below 673 K to dimple type fracture at 773 K and 873 K. Since a little strain hardening results in lower deformation resistance at 873 K, the deformation becomes larger and larger when the stress should be over the yield point.
　　LCF tests of JLF-1 steel were carried out in vacuum using a fully reversed push-)pull strain controlled triangular wave with strain rate of 0.1 %/s at RT, 673 K, and 873 K. The fatigue life at elevated temperature was almost as same as that at RT when the life was plotted against the total strain range. But when the life was plotted against the plastic strain range, the fatigue life curves for RT, 673K and 873 K were on different lines, which is not in agreement with the Coffin's model. The TEM images showed that dislocation structure is dependent on temperature; dislocation rearrangements keeping high density at RT, dislocation decrease to medium level at 673 K, dislocation density decrease to low level at 873 K. Loss of dislocation pile up will result in reduction of strain hardening at high temperature. So, the loss of strain hardening will be responsible for the increase of fatigue life at high temperature when plotted against the plastic strain range.
　　Cyclic softening was observed during LCF test at elevated temperature in vacuum, which was strongly related to reduction of the dislocation density, formation and loss of dislocation cell structure, and increment of lath width caused by partial annihilation of original lath boundaries. The correlation of Vickers Hardness and microstructure for the present experiment is investigated to obtain mechanistic understanding on the mechanical property change such as fatigue life and cyclic softening.
　　The cyclic stress-strain curve can be obtained from the fatigue stress-strain hysteresis curves around half life. Strain hardening decreased significantly at 873 K. The cyclic yield point was lower than the static one, especially at 873 K, that means cyclic deformation at elevated temperature will reduce the design margin. So, cyclic yield stress-strain curve of JLF-1 steel have to be applied for fatigue design and safety analysis.
　　In summary, this study has demonstrated that the fatigue life was dependent on temperature when plotted against the plastic strain range, which is not in agreement with the Coffin¨s model. The TEM images showed that dislocation structure is dependent on temperature: at RT, dislocation rearrangements keeping high density; at 673 K, dislocation decrease to medium level; at 873 K, dislocation density decrease to low level. Loss of dislocation pile up will result in reduction of strain hardening at high temperature. The loss of strain hardening will be responsible for this phenomenon. It is also indicated that the phenomenon of cyclic softening of JLF-1 steel is a one of important issues for design, which will cause the reduction in design margin significantly. The cyclic yield stress curve is a base for design and safety analysis. The design stress of 1/3 UTS_σUTS is acceptable at 873 K., 総研大甲第985号}, title = {Low Cycle Fatigue Properties of Reduced Activation Ferritic / Martensitic Steel (JLF-1) at Elevated Temperature}, year = {} }