合金化元素钛对U-0.79wt.%Ti合金氢化行为影响研究

【作者】 史鹏；

【导师】 汪小琳；

【作者基本信息】 中国工程物理研究院 ， 核燃料循环与材料， 2013， 博士

【摘要】 铀及其合金性质活泼,极易与环境气氛相互作用而发生腐蚀,导致材料使用性能降低甚至失效。这些反应中,氢化腐蚀由于具有反应剧烈、伴随有较大体胀、反应产物易燃等特性,对材料的破坏损伤也极为严重,因而备受研究者关注。由于具有非常重要的科学研究和工程应用价值,因此过去的几十年间,铀及其合金的氢化腐蚀行为一直是核材料科学研究的热点前沿。但是,铀及其合金的氢化反应相当复杂,气氛压力、反应温度、杂质气体、氧化层性质和金属的微观组织结构等诸多因素都会产生影响。更为重要的是,氢化反应具有点蚀特性,在某些特殊位置会优先成核生长。目前,研究者普遍接受晶界等处是金属铀氢化反应的优先成核点,但在反应机理方面存在争议,主要有两种观点：以英国AWE和以色列NRC-Negev为代表的一派学者认为表面氧化层差异是决定氢化成核点位的决定因素；另外一派以美国LANL和LLNL为主的研究者认为金属自身的性质特别是化学组成和微观组织结构才是决定氢化成核生长点的根本因素。从工程应用角度来讲,确定氢化反应特别是成核行为的根本机理,可以作为铀及其合金材料工程防护方案设计的出发点和支撑点。此外,这一研究领域公开的文献报道更多集中在金属铀,对于工程上应用更为广泛的铀合金涉及却相对较少。铀钛合金--作为一种重要的铀合金材料,一方面由于较好的机械力学和抗氧化腐蚀行为在核工业中作为结构和功能材料得到广泛应用,因此研究铀钛合金氢化行为具有重要的实际应用价值；另一方面,可以对铀钛合金采取不同的热处理方法得到多样化的微观组织结构和化学组成分布,这也为氢化腐蚀成核机制等未解决的重大基础问题提供了独特的模型体系和研究机遇。所以,本论文的研究工作综合运用多种分析方法,以U-0.79wt.%Ti(后文用U-0.79Ti指代)为模型体系,我们首先确定了合金化元素钛在铀合金中的不同存在形态；制备了具有不同微观组织结构和化学分布的合金试样并研究了不同样品反应初期氢化成核点的形成、初期生长行为的差异性；观测到U-0.79Ti合金氢化成核生长点与金属微观组织结构之间存在显著关联性；实验结果为铀氢反应成核点的学术争论提供了新的实验证据,并探讨了氢化反应初期成核生长行为的反应机理。本论文得到的主要结果如下：(1)系统研究了U-0.79Ti合金微观组织结构的多样性,特别是合金化元素钛的添加、热处理工艺、时效处理等的影响。首次阐述了钛在U-0.79Ti合金中主要以固溶(α’)、金属间化合物(U2Ti)和夹杂物等三种形态存在的观点,并指出了利用U-0.79Ti合金微观组织结构的热处理过程敏感性,可以针对性地设计样品来考察钛的各种存在形态对氢化行为的影响行为；(2)通过原位XPS研究从室温到700℃真空热处理过程中表面化学性质的变化,结合铀钛合金相图确定了适合于U-0.79Ti合金氢化实验研究的200℃真空热处理的实验方案。确认了低温热处理(<200℃)会将铀及U-0.79Ti合金最外层氧化物从U02+x转化成U02,或者生成UOxCy,并讨论了其对缩短铀氢反应孕育期,加速铀氢反应的影响。此外,通过与贫铀样品的对比,本论文第一次观察到高温真空热处理过程(>45℃)中U-0.79Ti合金表面UOxCy的逆分解以及钛向表面的偏析行为,揭示了钛对合金表面物理化学性质的影响,这些发现可以为铀合金中合金元素的扩散、偏析及其与微量元素相互作用的研究提供特殊而有趣的模型。(3)利用在线体视显微镜、激光共聚焦显微镜和扫描电镜研究了具有"β+U2Ti"微观组织结构的铀钛合金氢化成核及初期生长行为,首次在实验上观察到具有某种优先取向、条状生长的铀氢化物,而且氢化物的成核在表面有较厚氧化层的α-U,生长也沿着α--U相并受到U2Ti相的约束。而且,这种结构为我们讨论氢化成核的机理提供了非常独特的模型。’β+U2Ti"铀钛合金微观上由交替的α-U和U2Ti条纹组成。扫描Kelvin探针的实验结果揭示了两种相表面功函数具有较大的差别,反映出两种相与气氛反应的活性存在差异。经分析我们发现α-U反应活性更强。因此,α-U表面生长有更厚的氧化层,如果AWE和NRC-Negev的“氧化层”观点具有普适性的话,后续的氢化成核点应该在U2Ti相,但是我们的实验结果并不支持这样一种推论,因此这一实验结果至少说明氧化层的影响是次要的,而非决定性的因素。与此相反,氢化物的成核与初期生长过程与微观组织结构之间非常好的关联对应表明金属基底的性质——微观组织结构、化学分布、物理化学性质等才是决定氢化成核生长行为的根本因素。(4)利用在线体视显微镜和激光共聚焦显微镜,我们进一步研究了500℃时效2h的U-0.79Ti合金的氢化行为,观察到氢化成核点非常形象地勾勒出了铀钛合金原高温Y相的晶界。结合500℃时效会导致马氏体首先沿着晶界分解为α+U2Ti的实验事实,通过对比相分解区域和未发生分解区域的氢化行为,我们发现固溶钛能够提升铀的抗氢化腐蚀能力。这一实验现象进一步佐证了“金属材料的性质是影响和决定氢化成核点的主要因素”的观点。(5)进一步运用在线体视显微镜我们原位研究了钛的夹杂物在氢化反应初期与成核点之间的关联性,发现夹杂物附近通常毗邻氢化成核点,而且夹杂物附近的氢化反应更加剧烈,大部分扩展为成核生长点,对金属材料的损伤更具侵略性。通过与文献中关于金属铀中夹杂物(主要指UC)氢化行为的对比,我们发现,两者之间存在一定的可类比性。这种缺陷加速氢化反应的机制目前还不是十分清楚,需要开展进一步的研究工作。但是,这个实验发现和展示了化学元素分布的不均匀性、微观结构的不连续性对氢化反应的显著影响。通过本博士论文研究工作的开展,一方面确定了合金化元素钛在合金中的不同存在形态,以及各种形态对氢化腐蚀成核生长的影响；另一方面,铀钛合金微观组织结构多样性的特征为研究氢化成核机制提供了独特的模型,而实验上观察到的氢化成核点和微观组织结构之间的对应、关联性为“金属的性质是决定氢化成核点的主要因素”的观点提供了有力佐证,促进了氢化腐蚀成核机制的研究。此外,研究结果揭示出合金中化学组分分布的不均匀性和微观组织结构的差异性往往会对材料的抗氢化腐蚀带来不利的影响,这也为工程上改善铀及其合金的抗氢化腐蚀能力提供了有益的指导和支撑。更多还原

【Abstract】 Uranium and uranium alloys are very sensitive to the enviroment and readily react with many kinds of gases, which causes corrosion and decrease in material nature and may lead to failure. Hydriding is characterized with fast reaction rate, big volume expansion and pitting corrosion, which could destroy the mother material very seriously and have drawn much research attension in the last few decades. The study on the hydriding behavior of uranium and uranium alloys was one of the frontiers in nuclear materials science due to the great scientifical and engineering value. However, this reaction is very complex and influenced by many factors, including hydrogen pressure, temperature, impurity gases, characters of the oxidation layer, microstructures of the metal and so on. What is the most important, the hydide corrosion of uranium has been observed to be localized, spatially hetorogeneous, and seemingly random, the hydride seems to nucleate at some special locations. Until now, with virtually no reports to the contrary, a strong correlation of hydride corrosion to misorientation boundaries on uranium has been repeatedly observed. Despite the near unanimity in associating hydride corrosion with at least certain types of defects, there is a considerable divergence of opinion on causation. Researchers at AWE and NRC-Negev regard the properties of the superficial oxide layer as the determinants of initiation site. While other institutions such as LANL and LLNL hold the opposite opinion, relegating the oxide layer to a secondary role while implicating the metal properties. A thorough review of uranium hydride corrosion reports reveals that, the dissension on the issue of’essential’causation in the community of uranium hydride research continues, and the chief goal is to identify the root causes and the mechanism for hydride corrosion, especially for hydride nucleation sites. The later could used to define protection strategies of uranium and uranium alloys. Besides, although uranium has been extensively studied, in contrast, the more widely used uranium alloys have been less concerned in the literatures.Firstly, U-0.79wt.%Ti alloy——one kind of the most important uranium alloys, is widely used in the nuclear industry for its good mechanical and anti-corrosion nature. Hence, it is of practical value to study the the hydride corrosion of U-0.79Ti alloy. Secondly, samples of various microstructure and chemical elements distribution could be prepared through different thermal treatment, which provides an idea model system for studies of fundamental problems in the issue of root causes for hydride nucleation. In this thesis, by applying and combining different kinds of analysis method to U-0.79wt.%Ti (U-0.79Ti) as a model system, we studied the reaction behaviror with hydrogen. We firstly identified the three forms of titanium elements in U-0.79Ti alloy, and then studied the hydride nucleation and growth in the early stage of the specially-prepared samples of different microstructures. We have oberved a strong correlation of hydride sites with the microstructures of the underlying metals. The results provided new evidences for the dispute on the issue of root causation for uranium nucleation sites. And finally we discussed the reaction mechanisms of hydride corrosion for U-0.79Ti alloy.The main research work and contents are listed as follows:(1) We have systematically studied the variety in the microstructures of U-0.79Ti alloy and revealed the effect of the introduction of alloying element Ti, thermal aging on the microstructure of U-Ti alloy. For the first time, we identified that titanium exists mainly in three forms, solution (a’), intermetallic (U2Ti) and titanium inclusions. And we pointed out that, by using the U-0.79Ti microstructure’s thermal treatment sensitivity, the effects of the difference existing forms of titanium on hydride corrosion could be studied by specially-prepared samples.(2) The method of pre-treatment for U-0.79Ti was determined by in-situ XPS monitoring the character on the very surface of uranium and U-Ti alloy during vacuum thermal heating from room temperature to700℃. The XPS spectrum suggested that, during the low temperature heat treatment (<200℃), the outmost UO2+x oxidation layer was transformed into UO2or UOxCy, which accouts for the decrease in the induction time. As we know, by comparing with uranium metal, it is the first time in the reports that, a novel decomposition of UOxCy and segeration of titanium on the very surface was obsevered. Which show the influence of titanium on the surface physical characters of U-Ti alloy. The results provide an intesting model for the study of diffusion, segeragation, and interaction with minor elements.(3) We firstly observed the strip-like hydride corrosion in U-0.79Ti alloy with the so-called "p+U2Ti" microstructure. The hydride nucleates in and grows along α-U phase and is constrained by α-U/U2Ti boundary. The samples are composed of alternative α-U and U2Ti planelets. The Scanning Kelvin Probe (SKP) results shew that the work function of these two phases are quite different. The reactivity of a-U is higher. In our experiments, the a-U phase developed a thicker oxide layer. If the view-point of AWE and NRC-Negev is suitable for all kinds of uranium alloys, the hydride should nucleate in the U2Ti phase, the results did not support such a deduction. This means that the oxidation layer is of secondary importance in determing the nucleation sites. In contrast, the correlation between the hydride nucleation and the microstructure indicates that the metal properties are the determinant factor.(4) By using the state-of-the-art hot stage microscopy and laser confocal microscope, we studied the hydride corrosion of500℃/2h aged U-0.79Ti alloy and observed the preferred hydride nucleation in the phase boundary of the mother y phase. It is well known that, the thermal annealing at500℃cause the decompostion of martensite into α+U2Ti along the phase boundary. By composition of the hydride corrosion in the decomposed area and the martensite, it is concluded that the solution titanium could decrease the reactivity toward hydride corrosion. The results supported the viewpoint of "metal properties is the key factor that determine where the hydride corrosion starts".(5) The relationship between titanium inclusions and hydride nucleation in the early stage was studied by in-situ hot stage microscope. We found that inclusions are always connected with hydride nucleation sites, most of them developed into growth centers and are more aggeresive for the metal. The effects of inclusions on hydriding in U-Ti alloy is similar to that of the inclusions (most are UC) in uranium. Until now, the root causation of this behavior is not clear. Which need further researches. However, the results shew the influence of chemical elements’distribution on hydriding corrosion.Through these systematic experimental studies, the effects of the triple forms of titanium on hydride nucleation have been identified. Besides, the variety in the microstructure of U-0.79Ti alloy provides an ideal model for the study of the root causation. The experimentally observed link between the microstructure and chemical elements distribution and the hydriding nucleation sites provided further evidence to support the opinion of "metal properties is the determinants for the hydride nucleation", which put forward the study on the root causation of hydride corrosion. It can help us understand the pitting corrosion character of uranium hydriding, and provide some beneficial guidance in the corrosion protection for uranium and uranium alloys.更多还原