【作者】 张延志；

【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， 2015， 博士

【摘要】 金属铀在核工业中有着重要应用,但其化学性质活泼,容易遭受腐蚀；且延展性差,不利于加工。通过添加少量的Nb能够明显改善金属铀的性能,当Nb的添加量在4.2～6.9wt.%时,通过高温快速淬火(>20。C/s)能够获得一种被称为Ⅸ”相的单斜结构的马氏体组织,此时合金具有良好的抗腐蚀性和延展性。另外,该合金具还有一些独特的力学性能,如：低屈服强度和高塑性、双屈服和形状记忆效应,这也使得变形行为成为该合金最受关注的研究方向之一,但目前对其变形细节仍然缺乏足够的理解。另外,室温下该合金处于亚稳定状态,随着贮存时间的增加会发生老化,虽然研究者对此方面开展了大量的研究,但目前对该结构铀铌合金的低温时效机理的认识还不明确,存在较大的争议。本论文针对这两个科学问题,利用XRD、TEM、SEM.金相、力学拉伸等多种分析技术,对U-5.8wt.%Nb合金的时效和变形开展了较为系统的实验研究。淬火态U-5.8.wt.%Nb合金为马氏体组织结构,内部有大量的孪晶,孪晶的移动和变形是合金对外加应力响应的重要特征。实验中,利用X射线衍射和力学加载装置较为系统地原位研究了铀铌合金宏观变形过程中的微观晶格演化规律,对孪晶重取向等现象进行了分析。实验结果证实了U-5.8wt.%Nb合金弹性变形和滑移变形之间的孪生变形特征,并获得了从外力加载到试样断裂过程中各衍射晶面对外加应变的响应规律。在孪晶变形阶段,不同晶面方向对外加应力的响应具有显著的各向异性特征。拉伸过程中夹杂物演化的原位观察结果显示,孪生变形期间部分夹杂物发生断裂,断口逐渐长大并发展为孔洞,可能对合金的断裂行为有重要影响。提出了α”相铀铌合金低温时效的相变机制,并讨论了相转变的晶体学路径和影响因素,研究结果对深入理解合金的低温时效强化和贮存老化现象有重要意义。实验结果表明,低温时效过程中,α”相U-5.8wt.%Nb合金会发生α”→γ°相变。利用晶体学唯相理论对结果进行分析,提出α”(-111)晶面和γ°(111)晶面的原子堆垛之间的相互转换是两相转变的主要晶体学路径,使转变体现出明显的晶面选择性。系统考查了高温对α”相铀铌合金结构和力学性能的影响。实验结果显示,合金300-C时效发生相变,但元素分布未发现变化；400℃-600℃时效后,铀铌合金元素分布变化显著,相结构分解为低Nb含量的α-U和高Nb含量的类γ相固溶体。高温时效后,铀铌合金试样拉伸过程中均发生脆性断裂,但断裂机制不同,300℃以解理断裂为主,400℃主要为沿晶断裂,500℃和600℃为准解理断裂。初步考查了晶粒尺寸对铀铌合金结构、性能以及时效的影响。结果显示,晶粒尺寸对淬火态铀铌合金的晶体结构和力学性能没有明显影响,但对合金的低温时效有较大影响,大晶粒试样的低温时效相变速率显著高于小晶粒试样。此外,文中还讨论了低温时效对合金形状记忆效应的影响。本论文工作通过系统的实验研究,获得了U-5.8wt.% Nb合金随外加应力的晶格响应规律,加深了对合金变形规律和机制的认识；提出了α”相铀铌合金低温时效的相变机制,该机制的提出为今后合金的热处理工艺、老化动力学和物理建模等方面的研究工作指出了新的方向并奠定了理论基础。另外,在介观尺度上,发现了合金拉伸变形至断裂过程中夹杂物的演化规律；初步研究了晶粒尺寸和微观应变对合金的相稳定性影响。这些研究能够为合金的制备和热处理工艺提供科学参考。更多还原

【Abstract】 Uranium and its alloys are used in various applications due to their high density and nuclear properties. But the uranium metal is easily corrodible due to the high reactive activity. In order to improve the perfonnances, Nb is added to uranium. When the concentration of Nb is between 4.2wt.% and 6.9wt.%, the martensitic structure a" phase can be formed through rapid quenching (>20℃/s). The a"-UNb alloy has excellent erosion resistance and special mechanical properties, e.g. lower yield strength, higher toughness and ductility, and shape-memory effect.But the content of Nb is supersaturated and the U-Nb alloys are metastable phase after rapidly cooling to room temperature by quenching. The performances of a"-UNb alloy will change with time, even at 25℃ (room temperature). On the other hand, the a"-UNb alloy can also be strengthened farther through aging under low temperature (<250℃). Therefore, understanding the low temperature aging mechanism is very important. Unfortunately, the low temperature aging mechanism is yet unclear. There are several opinions about the mechanism of age hardening under low temperature. In order to have an insight into the deformation mechanism, there are also need to do more works on the deformation of a"-UNb alloy in detail. In this work, the aging and deformation of U-5.8wt.% Nb alloy were studied using X-ray diffraction, scanning electron microscope, optical microscope, transmission electron microscope and in situ tensile test.The lattice response and deformation behavior of as-quenched U-5.8wt.% Nb alloy were investigated using in situ XRD and uniaxial tensile. The experiment results disclose that the lattice parameters, diffraction peak intensity and interplanar spacing change regularly with applied stress during tensile deformation. The elastic deformation and twinning deformation (reorientation) coexisted during the first yield platform. The lattice response suggests that the remarkable anisotropy of elastic limit in various directions should be the genesis of double yield curve. The experimental results show the inclusions have a disadvantageous effect on the fracture of alloy during tensile.The phase transformation mechanism of a"-UNb alloy was propounded for the first time, which is very important to understand the hardening and aging mechanism under low temperature. The experimental results disclose that the phase transformation of a"→y° occurs during aging at low temperature (from 40℃ to 200℃). The phase γ° formed during aging can hamper the deformation of twinning, and increases the strength of alloy remarkably. The phase γ° can also reverse to α" under uniaxial tensile because of stress-induced phase transformation. The phase transformation between α" and γ° is reversible martensitic transition. The internal relation between α" (-111) and γ°(111) can illuminate the relationship of crystallography in the forward and reverse phase transformation.The effects of high temperature (300℃-600℃) on the structure and mechanics property were studied. The components analysis shows the distribution of Nb does not change after aging at 300℃, and changes markedly after aging within 400℃-600℃. The crystal structure transforms to γ° (or γ) from a" during aging in 300℃-600℃. Although brittle failures occur during tensile test for all samples after aging at high temperature, the failure mechanisms are dissimilar for different temperatures. The failure mechanism is cleavage fracture for the samples aged at 300℃, intergranular fracture for the samples aged at 400℃, and quasi-cleavage fracture for the samples aged at 500℃ and 600℃.The effects of grain size on structure, mechanics and aging were investigated. The results indicate that the crystal structure and mechanics of as-quenched U-5.8wt.% Nb alloy are the same for two samples with different grain size. But the differents are remarkable between the two samples during low temperature aging. The phase transformation kinetics of α" →γ° in coarse-grained sample is faster than fine-grained sample during aging at 200℃. The effect of aging on shape memory effect was also discussed in the paper.In conclusion, the tensile and aging of as-quenched U-5.8wt.% Nb alloy were studied using tensile test and in situ X-ray diffraction. The changes regulation of lattice parameters, diffraction peak intensity and interplanar spacing with applied stress during deformation were analyzed and discussed, wich will help us to understand the behavior of deformation in α" U-Nb alloy farther. The phase transformation mechanism of low temperature aging is disclosed in a" U-5.8wt.% Nb alloy, which can be used to illuminate the aged strengthening, and to study the heat treatment, aged kinetics and physics model of U-Nb alloy in the future. In addition, inclusions and grain size are also investigated during tensile and aging, and the effect on mechanics properties and phase stability were analyzed in this thesis.更多还原