Adequate studies have not been conducted on the low cycle fatigue properties of oxide dispersion strengthened (ODS) steels that are typically used for fusion reactors worldwide. Moreover, the majority of the fatigue properties are examined with a small sample size due to the restricted manufacturing capacity, which is insufficient for determining the comprehensive properties of bulk materials. Research on the fatigue properties of Chinese ODS steels was conducted recently. However, it is quite uncommon to report the fatigue properties of self-produced 9Cr-ODS steel. Consequently, this work is the first to examine the effect of a cyclic strain on the low cycle fatigue behavior of a representative low-activation 9Cr-ODS martensitic steel. Hence, the strain control test method was employed here in a strain amplitude range of 0.3%-0.8% at room temperature. The cyclic stress response curves, hysteresis loops, relationships between strain amplitude and life, stress amplitude and plastic strain were obtained, and the corresponding fatigue parameters were summarized. Furthermore, the microstructural evolution, fatigue fracture morphology, and crack propagation characteristics during the fatigue process were analyzed. The results revealed that the cyclic stress response behavior of the 9Cr-ODS steel was related to the strain amplitude. With an increase in the strain amplitude, the peak stress in the tension zone of 9Cr-ODS steel increased and the fatigue life decreased. The relationship between cyclic strain and life agreed well with the Coffin-Manson model. Additionally, the 9Cr-ODS steel had no obvious cyclic hardening but revealed a cyclic softening under higher strain amplitudes of 0.5%-0.8%. The microstructure analysis showed that for higher cyclic strain amplitudes, the average grain size and the fraction of the large-angle grain boundaries increased gradually with a reduction in the dislocation density, leading to the cyclic softening of the material. The fatigue crack initiated at the surface and propagated inward by the transgranular mode. The fine grain boundaries and subgrain boundaries of the 9Cr-ODS steel could induce crack deflection, reduce crack propagation rate, and increase fatigue crack propagation life. Moreover, under the same strain amplitude, the peak stress in the tension zone of the steel was almost twice that of the China low activation martensitic (CLAM) steel without a reduction in the fatigue life, indicating a superior low cycle fatigue resistance of the 9Cr-ODS steel.

在现有研究中，低活化9Cr-ODS钢虽然具有优异的高温性能，但其低周疲劳性能及微观组织演变规律尚未全面揭示，本工作旨在揭示其在不同应变幅下的循环响应和裂纹扩展特性。采用应变控制测试方法，研究了低活化9Cr-ODS钢在0.3%~0.8%应变幅范围内的室温低周疲劳性能，包括循环应力响应曲线、滞后回线、应变幅与寿命以及应力幅与塑性应变的关系，给出了相应的疲劳参数，并分析了疲劳过程中微观组织演变、疲劳断口形貌及裂纹扩展特性等。结果表明，9Cr-ODS钢的循环应力响应行为与应变幅有关。随应变幅增加，材料的峰值拉应力增加，疲劳寿命降低，循环应变-寿命满足Coffin-Manson关系。9Cr-ODS钢在所有应变幅下未出现明显的循环硬化，但在0.5%~0.8%较高应变幅范围内发生了一定的循环软化。微观组织分析表明，随循环应变幅增加，亚结构发生了回复，位错密度降低，平均晶粒尺寸及大角度晶界占比逐渐增加，从而导致9Cr-ODS钢的循环软化。疲劳裂纹起源于样品表面，但裂纹源附近均未发现明显的夹杂物或析出相。裂纹以穿晶方式扩展，9Cr-ODS钢细的原始奥氏体晶界及亚晶界对裂纹扩展具有强烈的抑制作用。此外，在相同低应变幅且不降低循环疲劳寿命条件下，9Cr-ODS钢所能承受的峰值拉应力达到低活化铁素体/马氏体钢的2倍，显示出较优越的低周疲劳强度。

Oxide dispersion-strengthened (ODS) steels with nano-scale Y2O3 or Y-Ti-O oxides have been considered as potential structural materials used in advanced nuclear systems. In this work, a novel 9Cr-ODS steel, namely, MX-ODS steel, was designed by decreasing carbon content to eliminate conventional M23C6-type carbides and by increasing the content of nitrogen and vanadium to form MX-type precipitates. In addition, the MX-ODS steel was synergistically strengthened by nano-scale MX precipitates and oxides. After fabrication by powder metallurgy, microstructural observation, and mechanical property tests were conducted after different heat treatments. The density of the prepared materials using hot forging instead of hot isostatic pressing was about 98%. Results of the microstructure observation of the MX-ODS steel indicated that after normalizing and tempering, the tempered martensitic structure dominated, and the mean effective grain size was approximately 1 μm. Moreover, the preferential orientation of coarse-grained and fine-grained mixed grains was not detected. By diminishing carbon content, M23C6-type carbides precipitated at the grain and sub-grain boundaries were eliminated. By contrast, MX-type precipitates with a diameter of approximately 30-200 nm were formed in the matrix. Furthermore, nano-scale Y-rich oxides with an average size of approximately 3.0 nm were dispersed in the matrix, and a number density can reach to approximately 1.9 × 1023 m-3. Furthermore, “core-shell” structure precipitates were found, which were spherical in shape with a diameter ranging from 10 to 20 nm. Such precipitates also contained Y, Ta, and O as the core and V as the shell. The mechanical properties indicate that microhardness decreased from 372 to 320 HV with the increase of normalizing temperature from 980oC to 1200oC. In addition, microhardness decreased significantly after tempering but initially increased and then decreased with the increase of tempering temperature from 700oC to 800oC, with a peak microhardness at approximately 750oC. Excellent strength and ductility were obtained after normalizing at 1100oC for 1 h and then tempering at 750oC for 1 h. Yield strength, ultimate tensile strength, and total elongation were 1039 MPa, 1103 MPa, and 20.5% when tested at room temperature and 291 MPa, 333 MPa, and 16% at 700oC, respectively.

以一种极具潜力的先进核能候选氧化物弥散强化(ODS)钢为研究对象，以不控制氧浓度的液态金属Pb-Bi共晶(LBE)为腐蚀介质，研究了静态下高温(600和700 ℃)不同腐蚀时间对ODS钢腐蚀行为的影响及其微观机制。结果表明，600 ℃腐蚀至2000 h，ODS钢表面生成总厚度约为10 μm的典型双层氧化膜，同时在内氧化层下方形成了一层较薄的富Al氧化层。基于致密的尖晶石内氧化层及富Al氧化层的保护作用有效减缓了腐蚀氧化速率，ODS钢显示优异于其它材料的耐LBE腐蚀性能。ODS钢在700 ℃腐蚀所形成的氧化膜结构及厚度与600 ℃明显不同：腐蚀100 h主要形成了厚度约为500 nm的Al₂O₃保护膜，大幅降低了腐蚀速率；腐蚀时间延长至500 h，大部分区域Al₂O₃氧化膜仍然存在，但同时出现的少量“疖状氧化物”破坏了Al₂O₃保护膜的连续性，从而成为了腐蚀的突破口。

Oxide dispersion strengthened (ODS) steel is a promising structural material for advanced nuclear power systems. In this study, an ultra-low carbon 9Cr-ODS steel with a bimodal grain structure was prepared using powder metallurgy, and a superior matching of strength and plasticity was expected by adjusting the soft-hard matching of the coarse-grained and fine-grained regions. The effects of heat treatment on microstructure and mechanical properties of the ultra-low carbon 9Cr-ODS steel were evaluated through OM, SEM, TEM, microhardness, and tensile tests. The results demonstrated that the ultra-low carbon 9Cr-ODS steel exhibited a tempered martensite structure after normalizing at 1050-1200oC, and then tempering at 700 and 750oC. Moreover, it presented the microstructure characteristics of coarse-grained and fine-grained regions, in which the average grain size of fine-grained regions was 1.6 μm and that of coarse-grained regions was 4.3 μm. The dislocation density in the ultra-low carbon 9Cr-ODS steel was very high and the number density of nano-scale oxide particles was up to about 1022 m-3. The microhardness in fine-grained regions was higher than that in coarse-grained regions. As the normalizing temperature increased, the microhardness of the ultra-low carbon 9Cr-ODS steel first increased and then decreased. The microhardness reached the highest after normalizing at 1100oC. When the normalizing temperature increased to 1200oC, the microhardness decreased due to the growth of austenitic grains. Regarding the tempering temperature, the microhardness first decreased and then increased as the tempering temperature increased from 700oC to 800oC. Furthermore, the decrease in microhardness when tempering at 700 and 750oC was because the microstructure was recovered and softened. The higher the tempering temperature, the lower the microhardness. However, when tempering at 800oC, the microhardness increased significantly, mainly due to the partial austenite transformation of martensite. The tensile test results at 25oC showed that the strength of the ultra-low carbon 9Cr-ODS steel first decreased and then increased by increasing the tempering temperature, which was consistent with the microhardness change while the opposite was observed for elongation. The tensile test results at 700oC showed that the strength of the ultra-low carbon 9Cr-ODS steel slightly decreased by increasing the tempering temperature. Moreover, the fracture morphology was dominated by fine dimples and secondary tearing, indicating that the ultra-low carbon 9Cr-ODS steel underwent ductile fracture. Combined with the mechanical property and fracture analysis results, the ultra-low carbon 9Cr-ODS steel exhibited superior matching of strength and plasticity after normalizing at 1150oC for 1 h and tempering at 750oC for 1 h.

采用粉末冶金方法制备了一种具有双峰晶粒结构的超低碳9Cr-ODS (氧化物弥散强化)钢,通过OM、SEM、TEM、显微硬度和拉伸性能测试,研究了热处理工艺对其显微组织和力学性能的影响。结果表明,超低碳9Cr-ODS钢经正火+回火后为回火马氏体组织,具有粗、细晶分明的结构特征。细晶区的平均晶粒尺寸约为1.6 μm,粗晶区的平均晶粒尺寸约为4.3 μm。同时,基体中存在大量的位错结构,且纳米级氧化物数密度可达约10²² m^-3。不同的热处理工艺不会改变超低碳9Cr-ODS钢粗、细晶双峰晶粒结构特征。经热处理后,细晶区比粗晶区具有更高的硬度。随正火温度升高,粗、细晶区的显微硬度先上升后下降,且在1100℃正火时达到最高。正火温度一定,回火温度从700℃升高至800℃时,粗、细晶区的显微硬度先下降后上升。700和750℃回火时,组织得到回复,发生软化,温度越高硬度越低；而在800℃回火时,超低碳9Cr-ODS钢因发生部分奥氏体相变导致硬度提高。25℃拉伸实验结果与硬度的变化趋势一致,随回火温度升高,超低碳9Cr-ODS钢的强度先降低后增加,延伸率则呈现相反趋势。700℃拉伸实验结果表明,超低碳9Cr-ODS钢的强度随回火温度的升高稍有降低。结合力学性能及断口分析结果,分析了双峰晶粒结构超低碳9Cr-ODS钢的断裂机制。经1150℃、1 h正火+ 750℃、1 h回火后,超低碳9Cr-ODS钢具有最优的强塑性匹配。

分析了正回火热处理制度对吨级中国低活化马氏体（CLAM）钢的组织与力学性能的影响。结果表明，正火温度主要影响吨级CLAM钢马氏体组织中原奥氏体晶粒度的大小，回火温度主要影响组织中的亚结构；相比正火温度，回火温度对吨级CLAM钢力学性能的影响较大。吨级CLAM钢在980 ℃正火+760 ℃回火的热处理条件下可获得最佳的组织与综合力学性能。

Compared to austenitic stainless steels, largely employed in the early fission reactors, high chromium Ferritic/Martenstic (FM) steels, developed since the first half of the 20th century for fossil-fuel power-plants, have a number of advantageous properties among which lower thermal expansion, higher thermal conductivity and better void swelling resistance. At the beginning of the 1970s, FM steels found their first nuclear application as wrapper and fuel cladding materials in sodium-cooled fast reactors. They are now the reference materials for in-vessel components of future fusion reactors, and are considered for in-pile and out-of-pile applications in Generation IV reactors as well as for various other nuclear systems. In this paper, after an introductory historical overview, the challenges associated with the use of FM steels in advanced reactors are addressed, including fabrication, joining and codification issues. The long term evolution of mechanical properties such as the creep and creep-fatigue behaviors is discussed and the degradation phenomena occurring in aggressive environments (lead alloys, high temperature gases, super-critical water and CO2, molten salts) are detailed. The paper also provides a brief overview of the radiation effects in FM steels. The influence of the key radiation parameters e.g. temperature, dose and dose rate on the microstructure and mechanical properties are discussed. The need to better understand the synergistic effects of displacement damage and helium produced by transmutation in fusion conditions is highlighted. (C) 2019 Elsevier B.V.