核结构钢辐照氦效应的原子尺度模拟

发布时间：2018-06-25 21:01

本文选题：结构钢 + 氦效应　；参考：《中国科学技术大学》2017年博士论文

【摘要】：目前在设计或实验阶段的新型反应堆有一个共同的特点:反应堆结构材料服役环境中,中子的能量远高于传统商用热中子反应堆。鉴于目前反应堆建造和设计中,结构材料以合金,主要是铁基合金(钢)为主,在高能中子辐照条件下,中子通过与铁及合金元素的(n,α)嬗变反应,会在结构钢中引入大量的氦原子。由于氦在钢中的溶解度极低,而且迁移势垒也很低,很容易通过间隙位迁移。在迁移过程中容易与中子辐照产生的空位结合,形成氦空位团簇,小的氦空位团簇在高温条件下可以迁移聚集成大的氦泡,大的氦空位团簇倾向于通过间隙原子踢出机制成长。实际的结构钢中都存在一些本征缺陷(位错、晶界、沉淀等),氦泡容易在材料中额外空间大的缺陷处成核生长。结构钢中引入的氦泡会很大程度上降低结构钢的机械性能,这种由氦引起的结构钢机械性能的改变称之为结构钢的辐照氦效应。大量的实验已经证明了氦泡会引起结构钢的硬化,脆化以及肿胀效应,但是目前对这些现象的微观尺度机制还不是十分清晰。很多微观尺度的现象无法通过实验手段进行研究,需要借助原子尺度的模拟技术来进行研究。本论文使用分子动力学,分子静力学等原子尺度模拟技术分别从以下几个方面研究了与辐照氦效应相关的微观尺度机制。(1)研究了氦泡成核和生长中的氦泡平衡问题,阐明了两种不同的平衡氦泡形成机制,回答了关于平衡氦泡的氦/空位比率的争论,为进一步研究氦泡诱导结构钢硬化和脆化做了铺垫,研究结果表明氦泡在成核和生长过程中存在两种不同定义的平衡状态:一种是基于能量最低的平衡状态;一种是基于机械平衡的平衡状态。氦泡在成核和生长中,当氦泡中氦/空位比率达到0.38-0.55时,氦泡率先达到机械平衡状态,这个比率随着氦泡的长大会有一定程度上升,然后当氦泡中氦/空位比率达到1.0-2.6(随氦泡尺寸增加而增加)之间时,氦泡达到能量最低的平衡状态。此外,当氦泡中氦/空位比率达到3.4-5.0(随氦泡尺寸增加而减少)之间时,氦泡会通过间隙型位错环踢出机制获得生长能力的大幅提升。(2)研究了氦泡诱导结构钢硬化的微观机理,通过研究1/2111{110}型刃位错与不同氦/空位比率的氦泡的相互作用,分析了氦泡诱导结构钢屈服强度上升的微观机制,探究了通过控制氦泡中氦/空位比率改善结构钢硬化程度的可行性,研究结果表明氦泡诱导结构钢硬化的微观机制是氦泡阻碍位错滑移,提高位错滑移的临界剪切应力,进而提高材料的屈服强度。1/2111{100}型刃位错的临界剪切应力随着氦泡中氦/空位比率的变化而存在显著变化,在氦/空位比率为2.0时,临界剪切应力值达到最低,继而在更高氦/空位比率条件下大幅提高,这主要是由氦泡与位错作用机制的改变所引起,位错在与高氦/空位比率的氦泡发生作用时,未与氦泡直接作用,而是与氦泡周围踢出的位错环相互作用,进一步提高了位错滑移的临界剪切应力。(3)最后研究了氦泡诱导晶界脆化的机制,通过一系列对称倾斜晶界的拉伸测试,根据应力-应变响应的变化,研究了不同类型晶界的变形机制。通过在晶界中引入不同大小和氦/空位比率的氦泡,研究了晶界在拉伸变形中与氦泡的相互作用,揭示了氦泡诱导晶界强度和延伸率降低的原子尺度机制,研究结果表明:所研究的晶界被分为两类,其一是应力-应变曲线中存在常应力段的晶界,对应的微观机制是拉伸过程中晶界的滑移,该类晶界的错配角一般小于90°,包括所有具有100倾斜轴的晶界和具有110倾斜轴的晶界中错配角小于90°的晶界;其二是应力-应变曲线在晶界断裂前无常应力段存在的晶界,该类晶界的错配角一般大于90°,包括110倾斜对称晶界中错配角大于90°的晶界。Bain path应变相变是诱导晶界滑移的微观尺度机制,晶界附近的原子通过Bain path相变由BCC结构转化为了 FCC结构,进而在整个晶体中形成了由三个晶粒组成的晶界,随着拉伸应变的增加,Bainpath相变的区域变大,引起晶界的滑移。氦泡诱导晶界延伸率降低的根本原因是阻碍了 Bain path相变机制,抑制了晶界的滑移。晶界的强度和延伸率随着氦泡尺寸的增加而降低,同样随着氦泡中氦/空位比率的增加而降低,但是氦/空位比率的影响要小于氦泡尺寸的影响。本论文的所有研究结论有望为如何有效地控制结构钢的硬化和脆化提供有价值的参考。
[Abstract]:A new type of reactor at the present stage of design or experiment has a common feature: the energy of the reactor is much higher than the traditional commercial thermal neutron reactor in the service environment of the reactor structure. In view of the current construction and design of the reactor, the structure material is mainly alloy, mainly iron based gold (steel), and neutrons under high energy neutron irradiation conditions. A large number of helium atoms are introduced into the structural steel by the transmutation reaction with the iron and alloy elements (n, alpha). Because the solubility of the helium in the steel is very low and the migration barrier is very low, it is very easy to migrate through the gap position. The large helium space clusters tend to kick out through the interstitial atoms. There are some intrinsic defects (dislocation, grain boundary, precipitation, etc.) in the actual structural steel. Helium bubbles are easy to nucleate in the large defects in the material. The helium bubbles introduced in structural steel will be greatly reduced. The mechanical properties of low structural steels, such as the changes in mechanical properties of structural steels caused by helium, are called irradiated helium effects of structural steels. A large number of experiments have proved that helium bubbles will cause hardening, embrittlement and swelling effects of structural steels, but the micro scale mechanism of these phenomena is not very clear at present. In this paper, the microscopic scale mechanism related to the irradiated helium effect is studied by atomic scale simulation techniques such as molecular dynamics and molecular statics. (1) the helium bubble level in helium bubble nucleation and growth is studied in this paper. Two different equilibrium helium bubble formation mechanisms are clarified. The argument about the helium / vacancy ratio of the equilibrium helium bubble is answered. It is a paving for further study of the hardening and embrittlement of the helium induced structural steel. The results show that there are two different definitions of equilibrium state in the process of nucleation and growth of helium bubbles: one is based on the most energy. A low equilibrium state; a equilibrium state based on mechanical equilibrium. Helium bubbles are in nucleation and growth. When the helium / vacancy ratio reaches 0.38-0.55 in helium bubbles, the helium bubble first reaches the state of mechanical equilibrium. The ratio increases with the growth of the helium bubble, and then the helium / vacancy ratio reaches 1.0-2.6 in the helium bubble (with the size of the helium bubble increasing. In addition, the helium bubble reaches the lowest energy equilibrium state. In addition, when the helium / vacancy ratio reaches 3.4-5.0 (with the increase in the helium bubble size) in the helium bubble, the helium bubble will be greatly enhanced by kicking out the gap dislocation ring mechanism. (2) the microscopic mechanism of the hardening of the helium bubble induced structural steel has been studied. The interaction between 1/2111{110} type edge dislocations and helium bubbles with different helium / vacancy ratios is used to analyze the micromechanism of the rise of yield strength of helium bubbles induced by helium bubbles. The feasibility of improving the hardening degree of structural steel by controlling helium / vacancy ratio in helium bubbles is explored. The results show that the micromechanism of helium induced structural steel hardening is helium bubble hindering. The critical shear stress of the dislocation slip is increased by dislocation slip, and the critical shear stress of the.1/2111{100} blade dislocation is significantly changed with the helium / vacancy ratio in the helium bubble. When the helium / vacancy ratio is 2, the critical shear stress values are the lowest, and then the higher helium / vacancy ratio is larger. It is mainly caused by the change of the mechanism of the helium bubble and the dislocation mechanism. The dislocation is not directly acted with the helium bubble when it acts with the helium bubble with high helium / vacancy ratio, but is interacted with the dislocation ring kicked around the helium bubble, and further improves the critical shear stress of the dislocation slip. (3) finally, the grain boundary embrittlement induced by helium bubble is studied. The mechanism of the mechanism is to study the deformation mechanism of different types of grain boundary according to the variation of stress strain response. By introducing helium bubbles with different sizes and helium / vacancy ratios in the grain boundary, the interaction between the grain boundary and helium bubble in the tensile deformation is studied, and the intensity and delay of the helium bubble induced grain boundary are revealed. The study results show that the grain boundary is divided into two types, one is the grain boundary in the stress strain curve, and the corresponding micro mechanism is the slip of the grain boundary during the stretching process. The mismatching angle of this kind of grain boundary is generally less than 90 degrees, including all the grain boundary with 100 inclined axes and 110 tilt. The grain boundary in the grain boundary of the axis is less than 90 degrees; the second is the grain boundary of the stress strain curve at the abnormal stress section before the grain boundary fracture. The mismatching angle of the grain boundary is generally greater than 90 degrees, including the grain boundary.Bain path strain phase transition of the grain boundary in the 110 inclined symmetry grain boundary, which is more than 90 degrees, is the micro scale mechanism to induce the grain boundary slip, and the grain boundary is near the grain boundary. The atoms of the Bain path transition from the BCC structure to the FCC structure, and then form a grain boundary composed of three grains in the whole crystal. With the increase of the tensile strain, the region of the Bainpath phase change becomes larger and the grain boundary slips. The root cause of the decrease of the grain boundary elongation at the helium bubble is hindering the Bain path phase transition mechanism and inhibition. The grain boundary slips. The strength and elongation of grain boundary decreases with the increase of helium bubble size, and decreases with the increase of helium / vacancy ratio in helium bubbles, but the effect of helium / vacancy ratio is less than the effect of helium bubble size. All research conclusions of this paper are expected to provide effective control of hardening and embrittlement of structural steel. A reference to value.
【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】：TL341

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