550℃真空下CLAM钢蠕变—疲劳行为及失效机理研究

发布时间：2018-04-30 07:55

本文选题：CLAM钢 + 真空环境　；参考：《中国科学技术大学》2017年博士论文

【摘要】：聚变实验堆包层在稳定运行和服役过程中承受恒定载荷作用的同时,还可能承受脉冲载荷等复杂载荷的综合作用,使结构材料产生蠕变-疲劳交互损伤,因此,针对包层结构材料开展蠕变-疲劳损伤行为及机理研究对聚变堆包层的设计与优化具有重要的科学意义和工程价值。本文以中国抗辐照低活化结构钢(CLAM钢)为研究对象,开展了高温550℃、真空5x10-3 Pa下应变控制的低周疲劳和蠕变-疲劳实验研究,旨在探究CLAM钢的低周疲劳和蠕变-疲劳力学行为和失效机理,获得保载时间、应变幅和保载方式对CLAM钢真空蠕变-疲劳力学行为的影响规律与机制,确保CLAM钢在聚变实验堆、甚至未来聚变堆服役环境下的安全运行。本论文主要研究内容与结论如下:首先,研究了高温真空环境下CLAM钢在不同应变幅控制下的疲劳力学行为。结果表明:循环载荷下,峰值应力持续软化。0.5%应变幅疲劳失效后,CLAM钢位错密度由2.4×1014m-2降为0.2×1014m-2,板条宽度由0.7μm粗化为1.3μm。基于位错强化和亚晶强化理论,位错密度降低和板条粗化造成应力下降。前奥氏体晶粒尺寸和析出物尺寸变化不大,对应力软化的贡献不大。基于疲劳寿命与弹/塑性应变关系分析,建立CLAM钢的Manson-coffin模型,疲劳寿命随应变幅增大而减小,并同其他RAFM钢真空环境中疲劳寿命相当。同时,对比CLAM钢在真空环境和大气环境中的疲劳寿命,真空环境中的疲劳寿命更优。分析裂纹扩展轨迹显示,真空环境存在两种裂纹萌生扩展模型。其次,在研究高温真空环境下低周疲劳行为的基础上,引入峰值载荷保载,分析了压缩保载方式下的蠕变-疲劳交互力学行为。压缩保载方式下,CLAM钢产生循环软化和应力松弛现象,除板条粗化分解、亚晶形成和位错密度下降的影响以外,析出物的粗化和亚晶向晶粒转化同样产生影响,并基于EBSD分析和位错滑移/攀移分析,揭示了大角度晶界的形成机制。保载对裂纹萌生和蠕变空洞形核的促成是蠕变-疲劳较纯疲劳寿命下降的微观原因。另外,保载时间和应变幅的增大,产生更为明显的应力松弛,并造成蠕变-疲劳寿命下降。低应变幅控制下的蠕变-疲劳寿命较纯疲劳寿命下降54%,而高应变幅控制下寿命下降39%,这说明低应变幅控制下的蠕变-疲劳更容易发生交互作用。最后,引入拉伸保载并同压缩保载对比,进一步研究了不同保载方式对CLAM钢蠕变-疲劳力学行为的影响。结果表明:拉伸保载方式下,CLAM钢仍会产生循环软化和应力松弛现象。CLAM钢的蠕变疲劳对拉伸保载更为敏感,这不同于大气环境下的压缩保载敏感性。除去氧化这一外部因素的影响,应力松弛是保载方式敏感性的根本原因,而平均应力并非主导因素。通过断裂行为分析显示,拉伸保载较压缩保载更能促进主裂纹的扩展,且在稳态扩展阶段拉伸保载更容易形成空洞和次级裂纹,促进裂纹的进一步扩展并加速材料的断裂失效。
[Abstract]:In the process of stable operation and service, the cladding of the fusion experimental reactor can withstand the complex load of the complex loads such as pulse load, which can cause the creep fatigue interaction damage of the structural material. Therefore, the design of the creep fatigue damage behavior and mechanism of the cladding material is designed for the design of the fusion reactor cladding. It is of great scientific significance and engineering value. In this paper, the low cycle fatigue and creep fatigue test of strain controlled low activation structural steel (CLAM steel) in China (CLAM steel) at high temperature and vacuum 5x10-3 Pa are studied. The purpose is to explore the low cycle fatigue and creep fatigue mechanical behavior and failure mechanism of CLAM steel. The influence law and mechanism of the loading time, the amplitude and the loading mode on the vacuum creep fatigue mechanical behavior of CLAM steel, and ensure the safe operation of the CLAM steel in the fusion experimental reactor and even the future fusion reactor. The main contents and conclusions of this paper are as follows: first, the different amplitude control of CLAM steel in the high temperature and vacuum environment is studied. The results show that under cyclic loading, the dislocation density of CLAM steel is reduced from 2.4 x 1014m-2 to 0.2 x 1014m-2, and the width of the slats is reduced from 2.4 x to 1014m-2, and the width of the strip is reduced from 0.7 mu m to 1.3 Mu based on the theory of dislocation strengthening and subcrystal strengthening, and the decrease of dislocation density and the roughening of the slats. The size of the grain size and the size of the precipitates have little change, and the contribution to the force softening is little. Based on the analysis of the relationship between the fatigue life and the elastic / plastic strain, the Manson-coffin model of CLAM steel is established. The fatigue life decreases with the increase of the strain amplitude, and is equivalent to the fatigue life of other RAFM steels in vacuum environment. At the same time, the comparison of the CLAM steel in the vacuum environment and the large size of the steel is compared. The fatigue life in the air environment is better than the fatigue life in the vacuum environment. Analysis of the crack propagation path shows that there are two crack initiation and propagation models in the vacuum environment. Secondly, on the basis of the low cycle fatigue behavior under the high temperature vacuum environment, the peak load is introduced and the creep fatigue interactive mechanics under the compression loading method are analyzed. Under compression loading, CLAM steel produces cyclic softening and stress relaxation. In addition to the influence of slats coarsening, subcrystal formation and dislocation density decline, the coarsening of precipitates and subgrains are also affected by grain transformation. Based on EBSD analysis and dislocation slip / climb analysis, the formation mechanism of large angle grain boundary is revealed. The formation of crack initiation and creep cavity nucleation is the microcosmic cause of creep fatigue more than pure fatigue life. In addition, the increase of loading time and strain amplitude, more obvious stress relaxation and creep fatigue life decrease. The creep fatigue life under the control of low strain amplitude is 54% lower than that of pure fatigue life, and high strain amplitude The reduced life decreased by 39%, which indicates that the creep fatigue is easier to interact with the low strain amplitude control. Finally, the influence of the loading and compression loading on the creep fatigue mechanical behavior of CLAM steel is further investigated. The results show that the CLAM steel will still produce cyclic softening and softening under the tensile loading. The creep fatigue of.CLAM steel is more sensitive to tensile loading, which is different from the compression loading sensitivity under the atmospheric environment. The stress relaxation is the root cause of the sensitivity of the loading mode except for the external factors of oxidation. The average stress is not the dominant factor. The tensile loading is more compressed through the fracture behavior analysis. Loading can promote the expansion of the main crack, and it is easier to form cavitation and secondary cracks in the steady state expansion stage, which can promote the further expansion of the crack and accelerate the fracture failure of the material.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TL64

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