【作者】 王瑾；

【导师】 李会军；

【作者基本信息】 天津大学 ， 材料学， 2019， 博士

【摘要】 聚变堆第一壁材料服役环境极其恶劣(高温、高压和高通量的中子辐照),由此产生的级联碰撞和氦(He)辐照损伤对反应堆安全及稳定运行至关重要。He通常不溶于第一壁结构材料且扩散激活能很低,易与材料中其它缺陷(空位、位错和晶界)发生相互作用形成He泡,进而导致材料性能的恶化,如肿胀、硬化和脆化等。目前对这些现象的微观尺度机制还不是十分清晰,通过实验方法深入探究He原子扩散、偏聚行为,以及He泡与材料结构缺陷的交互作用及其影响机制难度较大。分子动力学(Molecular dynamics,MD)方法是一种在原子尺度研究多原子或分子系统的模拟技术,它在揭示材料微观物理机制方面具有不可比拟的优势。本文的研究对象为体心立方结构的Fe-Cr基合金,以点、线、体缺陷和宏观裂纹等辐照缺陷为主线,采用MD方法研究了点缺陷扩散、平衡弛豫演化行为,位错(环)结构的形成与演化行为;在此基础上,探究了 He泡形核与长大机制,He泡与位错的交互作用机制,不同尺度的结构缺陷(包括中间相、位错、孔洞和裂纹)、温度、成分等因素对Fe-Cr合金中He原子扩散、偏聚行为的影响规律,从原子尺度层面深入揭示了 He泡的平衡演化、偏聚过程及其对基体塑性变形机制的影响。主要结论如下:(1)以置入弗兰克尔对点缺陷的方式构建了 Fe-14Cr合金电子辐照模型,深入研究了点缺陷的扩散与弛豫演化行为。研究表明,在存在电子辐照缺陷的Fe-14Cr合金中,Fe及其置换元素(如Cr)原子发生短程扩散,辐照点缺陷演化行为仅促进基体中空位团簇和位错环的生成,不会产生置换元素有序富集进而出现中间相(富Cr的α’相)现象。辐照点缺陷浓度增加,会导致基体Fe和Cr原子扩散加剧,诱发产生空位团簇和位错环。随着温度的升高,辐照点缺陷复合概率增加直至复合完全,空位团簇和位错环密度降低。当温度为1200 K时,点缺陷在时间为2.6 ns时就能完全复合。Fe-14Cr合金中置换Cr原子会与辐照点缺陷诱发产生的间隙原子形成<1 11>构型的Fe-Cr、Cr-Cr挤列子,阻碍辐照点缺陷之间的复合,增加位错环密度。(2)在Fe-Cr合金基体中构建了不同位错密度与空间构型的刃型位错模型,研究了辐照He泡与位错的交互作用与影响机制,阐明了低温抑制He泡长大,高Cr含量促进He泡形核微观机理。研究表明,He原子的扩散偏聚行为可分为两种机制:低He浓度(<0.1 at.%)下的长程扩散机制和高He浓度(>0.1 at.%)下的短程扩散机制。温度升高、置换Cr组元含量减少会增加辐照He原子的扩散速率和位错可动性。He原子向位错的偏聚效应会显著影响He泡的空间分布,一些离散分布的大尺寸He泡在塑性变形过程能够诱发位错大量增殖,使基体呈现出较高的塑性。这表明可以通过控制位错的分布改善材料的辐照脆性。(3)在Fe-Cr合金基体中引入不同数目密度和尺寸的富Cr的α’相、孔洞和不同氦/空位(He/V)比例的He泡三维辐照体缺陷,通过拉伸测试,根据应力-应变响应的变化,研究了三维体缺陷对基体塑性变形机制的影响规律,揭示了富Cr的α’相、孔洞和He泡加速基体断裂进程的原子尺度机制。研究表明,含与不含富Cr的α’相模型的塑性变形过程均包括弹性变形、面心立方(fcc)相变、密排六方(hcp)层错和断裂四个阶段;含孔洞模型的塑性变形过程仅包括弹性变形、fcc相变和断裂三个阶段。孔洞和富Cr的α’相数目密度增加会抑制hcp层错转变。富Cr的α’相尺寸增大会诱发fcc相变、加速hcp层错转变。当He泡中He/V 比例不同时,含He泡模型断裂机制不同:当He/V 比例从0.5升高到5时,断裂机制由fcc相变机制转变为位错强化机制。(4)澄清了不同取向裂纹扩展行为、辐照He原子和置换Cr原子对裂纹扩展行为的影响规律。研究表明,(0 0 1)[0 1 0]裂纹扩展机制为弹性变形-相变-裂纹尖端沿相变区解理断裂;(1 2 1)[1 1 1]裂纹扩展机制为弹性变形-孪晶扩展-孪晶尖端应力集中诱发多空洞合并断裂。He浓度和Cr含量对裂纹扩展行为产生截然相反的影响。He浓度对(00 1)[0 10]和(1 2 1)[11 1]裂纹扩展行为影响包括两个方面:低He浓度(<0.9 at.%)下,两类裂纹扩展机制不改变,He添加仅仅减缓相变或者孪晶转变速率;高He浓度(>0.9 at.%)下,两类裂纹扩展机制改变,该浓度下两类裂纹扩展机制相同,均为He团簇受力演化成空洞,多空洞合并断裂。Cr原子对两类裂纹扩展行为的影响与Cr含量多少无关,Cr原子能够增加相变转变比率和加快孪晶转变速率,延迟(0 01)[010]裂纹断裂发生,诱发(1 21)[11 1]裂纹断裂提前发生。更多还原

【Abstract】 First wall material of the fusion reactor will suffer from harsh service conditions(e.g.,high temperature,high pressure and high flux neutron irradiation),and these will lead to helium(He)cumulative effects and atomic displacements from radiation cascades,which are of great importance to safe and stable operation of the fusion reactor.He atoms can easily migrate and form He bubbles with other defects(such as vacancies,dislocations and grain boundaries)owing to low migration energy,which are responsible for severe degradation of structural materials(e.g.,swelling,hardening,embrittlement etc.).But at present,the microcosmic mechanism of these phenomena is not very clear,it is difficult to deeply further understand He diffusion and segregation behaviors,as well as the interaction between He bubbles and material structure defects through the experimental method.Molecular dynamics(MD)is a simulation technology to study multi-atomic or molecular systems at the atomic scale,and it has incomparable advantages in revealing the microscopic physical mechanism of materials.The research mainly focuses on Fe-Cr alloy with body centered cubic structure,and irradiation defects(including point,line,body defects and macroscopic crack)are main lines in this paper.Molecular dynamics simulation was used to investigate the point defects diffusion and equilibrium relaxation evolution behavior,formation and evolution of dislocation(ring)structures,He bubbles nucleation and growth mechanism,and He bubbles interaction with the dislocations.Effect of material structure defects(precipitated phase,dislocations,voids and cracks),temperature and composition on He atoms diffusion and segregation behavior in Fe-Cr alloy were also investigated.All these results revealed He bubbles equilibrium evolution,segregation behavior and their effects on plastic deformation mechanism at the atomic scale.The main conclusions can be summarized as follows:(1)Fe-14Cr electron irradiation models were established by imbedding Frenkel pairs,and point defects diffusion and evolution behaviors were investigated.The results showed that,during the relaxiation process,Fe and its substitutional element(such as Cr)atoms could diffuse in short distance,and the evolution of point defects only promoted the formation of vacancy clusters and dislocation loops.The ordered enrichment of substitutional element and intermediate phase(Cr-rich a’ phase)could not observed in the entire process.With the increase of Frenkel pairs concentration,Fe and Cr atoms diffusion increased,and the possibility of dislocation loops and vacancy clusters nucleation also increased.With the increase of temperature,the probability of Frenkel pairs recombination increased until the recombination was complete,the number of vacancy clusters and dislocation loops decreased.Especially,at 1200 K,complete recombination of Frenkel pairs occurred at 2.6 ns.Cr interaction with interstitial atoms could form<1 1 1>Fe-Cr,Cr-Cr crowdion,which resisted the recombination of Frenkel pairs and increased dislocation density.(2)Edge dislocation models with different dislocation densities and spatial configurations were built in Fe-Cr alloy,and He bubble interaction with dislocations was investigated.The microcosmic mechanism that low temperature could inhibit He bubble growth,and high Cr contentration could promote He bubble nucleation,were also elucidated.The results showed that,He diffusion behavior could be divided into two mechanisms due to He concentration:one was long-range diffusion mechanism at low concentration of He atoms(<0.1 at.%),and the other was short-range diffusion mechanism at high concentration of He atoms(>0.1 at.%).Higher temperature and lower Cr concentration could increase He atom diffusion rate and dislocation mobility.Furthermore,it was found that He segregation on dislocations could adjust the spatial distribution of He bubbles,and larger He bubbles with discrete distribution could promote dislocations multiply and enhance deformability in the process of tensile deformation.Therefore,material irradiation brittleness could be improved by controlling dislocation distribution.(3)Voids and Cr-rich α’ phase with different number densities and sizes,and He bubbles with different He/V ratios were built in Fe-Cr alloy.Through tensile testing,according to the stress-strain response,plastic deformation mechanism of voids,He bubbles and Cr-rich α’ phase in Fe-Cr alloy were investigated and atomic scale mechanism of voids,He bubbles and Cr-rich α’ phase accelerating matrix fracture process were revealed.The results showed that,plastic deformation of Fe-Cr model with and without Cr-rich α’ phase could be divided into four stages:elastic deformation,fcc phase transition,hcp fault and fracture.Plastic deformation of Fe-Cr model with voids could be divided into three stages:elastic deformation,fcc phase transition,and fracture.The number density increase of voids and Cr-rich α’ phase could inhibite hcp fault generation.The diameter increase of Cr-rich α’ phase could induce fcc phase transition and hcp fault generation.When the He/V ratio of He bubble was different,the fracture mechanism of the He bubble model was different:with the increase of He/V ratio from 0.5 to 5,the fracture mechanism was changed from fcc phase transition mechanism to dislocation strengthening mechanism.(4)The effects of different crack orientations,irradiated He and substitution Cr concentration on crack propagation were clarified.The results showed that,(0 0 1)[0 10]crack growth behavior was suffering from elastic deformation,phase transformation,and cleavage fracture along the phase zone boundary.(1 2 1)[1 1 1]crack growth behavior was suffering from elastic deformation,twin transformation,and multiple cavity coalescence fracture due to stress concentration at the twin tip.He and Cr concentration had opposite influence on the crack growth behavior.The influence of He concentration could be divided into two respects:At low concentration(<0.9 at.%),He addition could only slow down the rate of phase transition and twin transition,and the fracture mechanism of two crack models could not be changed.At high concentration(>0.9 at.%),the fracture mechanism of two crack models were changed,and He clusters could evolved into cavities,multiple cavities coalescence and fracture in both of two crack models.However,the influence of Cr atoms on crack growth behavior in two crack models was independent of Cr concentration,Cr addition could increase the phase transition ratio and accelerate the twin transition rate,which could postpone the fracture of(0 0 1)[0 1 0]crack but induce the fracture of(1 2 1)[1 1 1]crack.更多还原