【作者】 夏爽；

【导师】 周邦新；

【作者基本信息】 上海大学 ， 材料学， 2008， 博士

【摘要】 690合金作为压水反应堆的蒸汽发生器传热管材料,已经逐渐代替600合金,虽然取得了较好的效果,但随着核电工业的发展,进一步提高蒸汽发生器可靠性是急待研究的问题。蒸汽发生器传热管道破坏泄漏的主要原因在不同时期有不同的情况,从近几年统计的结果来看,现在与晶界有关的应力腐蚀和晶间腐蚀仍然是蒸汽发生器传热管道破坏的主要原因。包括690合金在内的绝大部分工程材料都是多晶体,材料的性能与其显微组织及晶界特性有着非常紧密的联系。如晶间腐蚀、断裂、合金及杂质元素的偏聚、蠕变等问题,都会受到晶界结构特征的影响。1984年Watanabe提出了晶界设计与控制这种概念,继而在上世纪90年代形成了“晶界工程(grainboundary engineering GBE)”这一研究领域.通过合适的形变及退火工艺,可以明显提高材料中的低∑CSL(CSL是“coincidence site lattice”的缩写,即重位点阵。低∑CSL是指∑≤29)晶界比例,优化其分布,就可以改善材料与晶界有关的多种性能。本工作应用光学显微镜、扫描电子显微镜、电子背散射衍射(EBSD)和取向成像显微技术(OIM)研究了加工和热处理工艺对690合金的晶界特征分布(各种晶界所占的比例)的影响。仔细观察分析了形变后退火过程中690合金的显微组织结构的演化过程,阐明了形变退火工艺与晶界特征分布之间关系及其演化的机理。通过晶间腐蚀试验考验了含有不同比例低∑CSL晶界690合金的耐腐蚀能力,探讨了其中的机理。得出以下主要结论:1)在低层错能面心立方金属690合金中,低∑CSL晶界比例的提高是基于退火孪晶(∑3)的形成,退火前的冷轧压下量在调整晶界特征分布中起关键作用。冷轧5%再在1100℃退火5min可使690合金的低∑CSL晶界比例提高到70%以上(按Palumbo-Aust标准统计),这时形成了数百微米尺寸的“互有∑3~n取向差关系晶粒的团簇”是其显微组织的重要特征(n为正整数,在低∑CSL范畴中只有n=1,2,3)。这种晶粒团簇的尺寸和内含∑3~n晶界的数量随冷轧压下量的增加而下降。初始经过固溶处理或固溶后再时效处理对690合金经过冷轧5%再在1100℃退火后的晶界特征分布影响不大。将冷轧和退火工艺重复使用,和单次处理效果基本一致。2)再结晶形核密度和多重孪晶的发展是控制690合金晶界特征分布的关键因素。“互有∑3~n取向差关系晶粒的团簇”是由再结晶时一个再结晶晶核在长大过程中不断产生退火孪晶,发生多重孪晶形成孪晶链而形成的。形变量小,形核密度低致使再结晶晶核有充分的空间发展,再结晶晶核与形变基体之间的随机晶界有很长的迁移距离,在不断迁移的过程中就能不断产生退火孪晶,发生多重孪晶,从而发展出很长的孪晶链,形成了大尺寸的晶粒团簇,同时也提高了∑3~n晶界的比例。因此,小形变量冷轧后再在高温短时间再结晶退火可以明显提高低∑CSL晶界比例。形变量越大,形核密度就越高,使得晶粒团簇尺寸小,∑3~n晶界比例也就低。3)对于低∑CSL晶界比例高的样品来说,用Brandon标准判定时得到的随机晶界网络的连通性可以明显被一些低∑CSL晶界片段打断,但这些低∑CSL晶界的相对偏差都很大。而用更为严格的Palumbo-Aust标准统计时随机晶界网络的连通性几乎不会被打断。因此,低∑CSL晶界比例提高以后随机晶界网络的连通性可以被打断的说法并不可靠。4)低∑CSL晶界比例高的样品比低∑CSL晶界比例低的样品耐晶间腐蚀性能明显好,其原因是前者晶粒团簇的尺寸明显比后者大,形状也不规则,并不是由于随机晶界网络的连通性被打断的缘故。更多还原

【Abstract】 The nickel-based Alloy 690,with main composition Ni-30Cr-10Fe(wt.%),is currently replacing Alloy 600(Ni-16Cr-9Fe) as a steam generator tube material in pressurized water reactors,because of its superior resistance to intergranular stress corrosion cracking(IGSCC).But with prolonged service lifetime and improved performance being demanded by the nuclear energy industry,the need to improve the resistance to IGSCC and intergranular stress corrosion(IGA) in Alloy 690 should also be considered.Grain boundary structure has long been known to play a critical role in many material properties,such as precipitation,corrosion,and cracking.Grain boundary engineering(GBE),which was under the light of "grain boundaries design and control" proposed by Watanabe,developed a lot and was fruitful during the last two decades.The grain boundaries related properties of materials can be enhanced by exercising control over the population of low∑CSL(∑≤29) grain boundaries,as defined by the coincident site lattice(CSL) model.A large body of works proved that the grain boundary character distribution(GBCD) can be altered via proper thermomechanical treatments.Some face-centered cubic(fcc) metal materials with low to medium stacking fault energy,such as Pb-base alloy,Ni-base alloy,OFE copper,and austenitic stainless steel,were successfully applied this concept to promote∑3~n(n=1,2,3....) boundaries formation,and their properties were greatly enhanced.This may be an approach to further improve the resistance to intergranular corrosion of Alloy 690 which is also a f.c.c material with low stacking fault energy.The effects of thermomechanical treatments on GBCD in Alloy 690 were studied by optical microscope(OM),scanning electron microscope(SEM),electron backscatter diffraction(EBSD) and orientation imaging microscopy(OIM).The microstructure evolution of Alloy 690 during annealing at 1100℃after various cold rolling were observed and the microstructures in the partially recrystallized state were analyzed in detail.Based on these observations and analyzing the mechanism of the GBCD manipulation was elucidated.Finally,the intergraunlur corrosion testing was carried out on the specimens with different GBCD to reveal their intergraunlur corrosion resistance,and the mechanism involved.It is possible to reach the following general conclusions that are given below:(1)In Alloy 690,small strain(5%) and subsequent high temperature(1100℃) annealing for short time(5 min) can produce high proportion of low∑CSL grain boundaries(more than 70%),which mainly are∑3~n.In this case,the large size twin-induced grains-clusters(several hundred microns in dimension) constitute the microstructure.The proportion of Low∑CSL grain boundaries and the size of the grains-cluster decreased with the increase of strain.The starting state,i.e.solution annealed or aged after solution annealed,,didn’t affect the GBCD obviously. Multi-cycle treatment didn’t show different effects on the GBCD apart from single cycle treatment.(2) Based on the observation of partial and full recrystaUization state,it is clear that the nucleation density and multiple twinning are the key factors affecting the GBCD.The grains-cluster is formed by continuous chain of twinning events starting from a single nucleus of recrystallization,and with the associated presence of multiple∑3~n boundaries.As a consequence,all the grains inside the same cluster have close∑3~n misorientations even at a long distance.That the mean size of the grains-clusters and proportion of low∑CSL boundaries decrease with the increasing strain,is caused by the increasing nucleation density of recrystallization with the increase of strain.(3)In the case of high proportion of low∑CSL boundaries,connectivity of random boundaries network is very sensitive to the applied criterion of CSL.The connectivity of random boundary network is not fragmented obviously when the Palumbo-Aust criterion is used,whereas substantially interrupted when the Brandon criterion is applied.Almost all those low∑CSL boundaries(Brandon criterion) appear on the network of the random boundaries have very large deviations.Hence, the marked discontinuity of random boundaries according to Brandon criterion is not reliable.(4) When the proportion of low∑CSL boundaries is enhanced,the high resistance to mass loss during intergranular corrosion testing is attributed to the large size of the grains-cluster and its zigzag shape,rather than the unreliable discontinuity of the random boundary network.更多还原