【作者】 王伟；

【导师】 周邦新；

【作者基本信息】 上海大学 ， 材料学， 2011， 博士

【摘要】 提高Cu含量的反应堆压力容器(RPV)模拟钢样品经880℃加热0.5 h水淬,或再经过660℃加热10 h调质处理,随后在340℃至500℃进行不同时间的时效处理,测量了维氏硬度,并采用原子探针层析技术(atom probe tomography, APT)研究了富Cu原子团簇的析出过程,采用了高分辨透射电镜(HRTEM)、能谱分析(EDS)和萃取复型方法研究了富Cu相的析出过程和晶体结构演化,获得以下主要研究成果。(1)时效前不同的热处理制度对富Cu原子团簇析出的影响利用APT对时效样品分析结果显示,RPV模拟钢淬火后经400℃时效100 h的样品中析出了富Cu原子团簇,团簇的数量密度为1.69×1023 /m~3,当时效时间延长至300 h后,团簇的数量密度增加到6.23×1023 /m~3;模拟钢在调质处理后,经400℃时效100 h的样品中并未发现析出富Cu原子团簇,只有经1000 h时效处理后才析出了富Cu原子团簇,团簇的数量密度为6×1022 /m~3,与淬火后直接时效处理的样品相比,团簇的数量密度降低了一个数量级。淬火后马氏体中较高的位错密度,是促使富Cu原子团簇形成的原因。(2)Ni,Mn等合金元素对富Cu原子团簇析出的影响研究表明,富Cu原子团簇除了容易在界面、位错等高能区形核外,也容易在Ni含量较高的位置处形核,并随着富Cu原子团簇中Cu原子聚集程度的增加,原子团簇中心处Cu含量逐渐增加,Ni含量逐渐减少;在原子团簇与α-Fe基体界面处,Ni和Mn含量逐渐增加,最后形成了富Ni和富Mn包裹富Cu原子团簇的“壳层”结构。模拟钢中0.81 at. %的Ni在400℃加热时虽然可以固溶在α-Fe中,但是由于浓度起伏,仍然可以形成富Ni原子团簇,富Ni原子团簇又是富Cu原子团簇析出时成核的地方,因而增加钢中的Ni含量会促使富Cu原子团簇的析出。这是合金元素Ni会增加RPV钢中子辐照脆化敏感性的本质原因。(3)调质处理后样品在370℃时效不同时间析出富Cu相的晶体结构演化HRTEM分析显示,调质处理后的RPV模拟钢样品在370℃经1000,3000,4500和6000 h时效处理的4种试样中,用萃取复型的方法分别得到了26,31,52和56个富Cu析出相。统计结果显示,随着时效时间的延长,用萃取复型得到的富Cu析出相平均直径从11 nm增加到20 nm,析出相略有长大,从萃取复型样品上观察到的富Cu析出相的数量也有所增加。大多数的富Cu相都是球形,在时效6000 h的样品中也观察有短棒状的富Cu相。调质处理后的RPV模拟钢样品在370℃经不同时间时效处理后析出的富Cu相,虽然它们的Cu含量有较大的差别,晶体结构也不相同,有的是9R结构,有的是fcc结构,但是与富Cu析出相的大小之间并无明显的对应的关系。在fcc结构的富Cu相中,常常可以观察到孪晶的存在。(4)调质处理后样品在400℃时效2000 h析出富Cu相的晶体结构和成分变化调质处理后样品在400℃时效2000 h后样品中观察到一个大小为20 nm,成分为65.8 at. %Cu和34.2 at.%Fe的富Cu相,通过FFT和IFFT分析该富Cu相的高分辨晶格条纹像表明,富Cu相为单斜9R结构和2H结构的混合相,这是由于9R结构富Cu相(001)晶面间存在层错的缘故,也说明了富Cu相析出过程中晶体结构转变的复杂性。9R结构富Cu析出相的(001)面并不垂直于(100)面,两面之间的夹角大约为86.9°;2H结构富Cu析出相的(001)面垂直于(100)面,其晶格常数为:a = b = 0.254 nm,c = 0.417 nm,2H富Cu相为六方结构,其轴比c/a = 1.642。(5)富Cu析出相fct结构相变的晶体学研究调质处理后样品在400℃时效2000 h后样品中观察到一个大小为27 nm,成分为80.5 at. % Cu,19.5 at.% Fe的富Cu相,通过HRTEM,FFT和IFFT分析发现,在富Cu相的结构演化过程中观察到目前尚未报道过的ε’’-Cu相以及一种相变过渡态,通过理论分析得出,这种相变过渡态是ε’-Cu向ε’’-Cu的相变过程中的产物。ε’-Cu相和ε’’-Cu相分别为晶体结构相同(同为fct结构),晶格常数不同的两种过渡相。经过理论推导,ε’-Cu的晶格常数为:a = b = 0.410 nm, c = 0.415 nm,轴比c/a =1.01;实验测量得到ε’’-Cu相晶格常数:a = b = 0.441 nm, c = 0.369 nm,轴比c/a = 0.84;即ε’-Cu向ε’’-Cu发生相变时,沿a轴,b轴方向伸长7.6%,c轴方向压缩11.1%;基于实验结果,建立了ε’-Cu向ε’’-Cu相变的晶体学模型;认为从ε’-Cu向ε’’-Cu的相变过程可分为两步进行。第一步,以{110}面为切变平面,[11-2]为切变方向的简单切变,平面内原子位置改变,使得ε’-Cu相的[11-2]和[01-1]之间的夹角由29.9°变成ε’’-Cu相晶胞中的32.9°。第二步,进行线性调整使符合实际的晶胞尺寸。通过相变晶体学的演算过程显示,所建立的相变模型与实验结果吻合度较好。更多还原

【Abstract】 The samples for this study were taken from reactor pressure vessel (RPV) model steel having higher Cu content. The samples were divided into two sets. The first set of samples were heat treatment of 0.5 h at 880℃and quenched into water, and the second set of samples were tempered at 660℃for 10 h followed by air cooling after an initial heat treatment of 0.5 h at 880℃and quenched into water. Two sets of samples were then isothermally aged at 300℃-500℃for different times up to 6000 h. The Vickers microhardness was measured by microhardness tester (HV-10) with a load for 5 kg for 15 s. The precipitation of Cu-rich clusters and crystal structural evolution of Cu-rich precipitates in RPV model steel were investigated by means of atom probe tomography (APT), extraction replica, EDS and HRTEM. The main conclusions are described as follows.(1) Effect of the different heat treatments before aging on the precipitation of Cu-rich clusters in RPV model steels.The analysis of APT revealed that smaller Cu-rich clusters were observed in RPV model steels aged for 100 h at 400℃after quenching. The number density of Cu-rich clusters was estimated to be 1.69×1023 /m~3. The number density of Cu-rich clusters increased to 6.23×1023 /m~3 after quenching and aging at 400℃for 300 h. After quenching and tempering, there were no Cu-rich clusters precipitated in the samples aged for 100 h at 400℃. Cu-rich clusters were observed in the samples aged for 1000 h at 400℃and the number density was estimated to be 6×1022 /m~3. The number density of clusters is approximately an order of magnitude lower in the samples after quenching and tempering compared with the samples after quenching. Higher dislocation density in the martensite after quenching could promote the precipitation of Cu-rich clusters.(2) Effect of Ni and Mn on the precipitation of Cu-rich clusters in RPV model steels.Besides at the interfaces and the dislocations, Ni-rich clusters could also act as the nucleation sites for the precipitation of Cu-rich clusters. The Cu content increases and the Ni content decreases at the central cores with increasing Cu atoms congregation, Ni and Mn atoms segregation on the exterior side of the cluster/matrix interface is also evident. Ni-rich clusters would act as the nucleation sites during the precipitation of Cu-rich clusters. Therefore the increase of Ni content in RPV steels could promote the precipitation of Cu-rich clusters. This is the essential reason that the presence of Ni in RPV steel could increase its sensitivity to neutron irradiation embrittlement.(3) Crystal structural evolution of Cu-rich nano phase in the samples with aging at 370℃for different aging time after quenching and tempering.There are 26, 31, 52 and 56 precipitates on the extraction replicas to be collected for samples aged for 1000, 3000, 4500 and 6000 h at 370℃after quenching and tempering, respectively. It can be seen that the average size of these nano phases was found to the range from 11 to 20 nm, and the number of precipitates is increased by longer aging. Most of these Cu-rich nano phases were found to be roughly spherical, but elongated ribbons have also been observed in the samples aged for 6000 h.The different Cu-rich nano phases have different Cu content and crystal structure, for example, some are 9R and some are fcc, but no distinct correlation with the size of Cu-rich nano phase. Twinned structure can often be observed in fcc Cu-rich nano phase.(4) Crystal structural evolution and composition of Cu-rich nano phase in the samples with aging at 400℃for different aging time after quenching and temperingA Cu-rich nano phase with 20 nm diameter was observed in the sample aged for 2000 h at 400℃by extraction replica analysis and the average composition of the precipitate was 65.8 Cu-34.2 Fe (in at.%). It had been found in the present work that, besides the 9R structure occurring, there exit also 2H variant and stacking faults within a copper precipitate in aged samples (2000 h at 400℃). The IFFT pattern shows that the (001) plane of the 9R structure copper phase is not perpendicular to the (100) plane but rather has a relative orientation of about 86.9°. And the (001) plane of the 2H structure copper phase is perpendicular to the (010) plane. The lattice parameters of the 2H structure are estimated to be a = b = 0.254 nm and c = 0.417 nm. The 2H variant has a hexagonal unit cell with axial ratio c/a = 1.642.(5) Crystallographic study of fct Cu-rich nano phases transformationA Cu-rich nano phase with 27 nm diameter was observed in the sample aged for 2000 h at 400℃by extraction replica analysis and the average composition of the precipitate was 80.5 Cu-19.5 Fe (in at.%). The results by HRTEM, FFT and IFFT analyses revealed that a type of transition phase and a transition state that have not been reported so far occurred during the crystal structural evolution of Cu-rich nano phase. This transition state is a product duringε’-Cu transition toε’’-Cu phase. Andε’-Cu andε’’-Cu phases are two phases that possess the same crystal structure (fct) and the different lattice parameters.Through the theoretical calculation, it can be obtained that the lattice parameters of theε’-Cu nano phase are to be a = b = 0.410 nm and c = 0.415 nm. Theε’-Cu nano phase has an fct unit cell with axial ratio c/a = 1.01. Through the experimental studies, it can be obtained that the lattice parameters of theε’’-Cu nano phase are to be a = b = 0.441 nm and c = 0.369 nm. Theε’’-Cu nano phase has an fct unit cell with axial ratio c/a = 0.84. To obtainε’’-Cu phase, theε’-Cu unit cell is expanded about 7.6% parallel to the a-axes and b-axes, and contracted about 11.1% along the c-axes.A crystallographic model of the phase transformation fromε’-Cu toε’’-Cu phase based on the experimentally observed were established. Two steps of the phase transformation fromε’-Cu toε’’-Cu phase was proposed as follows. The first step: the {110} planes is the invariant planes and simple shear movement of atoms on {110} plane along [11-2] direction occurs at the beginning of transformation, and the angle between [11-2] direction and [01-1] direction inε’-Cu is transformed into the angle ofε’’-Cu from 29.9°to 32.9°. In the next step, an inhomogeneous lattice invariant deformation (lineal adjustment) produces a slipped that matches the observed shape. This model agrees well with the experimentally observed orientation relationship.更多还原