U-2.5wt%Nb合金的氢蚀及其对力学性能影响

【作者】 李瑞文；

【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， 2009， 博士

【摘要】 金属铀及其合金作为核工程中重要组成材料而备受关注。铀及其合金非常活泼且存在多种相结构,在其贮存和使用过程中存在两个方面的老化问题:与环境气氛作用引起的表面腐蚀和不稳定相分解引起的结构性能变化。因此这两方面就成为铀及其合金研究的热点和难点。本论文以U-2.5wt%Nb合金表面氢腐蚀可能引起合金力学性能老化的问题为研究背景而展开。（1）系统地利用在线显微镜研究了U-2.5wt%Nb合金氢蚀初期氢化物的生长与成核动力学。研究结果表明,在一定温度范围内,氢化物的生长速度与反应温度符合Arrhenius关系,U-2.5wt%Nb合金氢化物生长激活能为24.34kJ/mol。在远离平衡态的实验条件下,压力对氢化物生长速度影响显得非常弱。样品表明氢化物成核数目的试验结果表明氢化物成核速度与温度遵从Arrhenius关系,与压力成正比。对氢蚀成核位置的初步研究发现,对于U-2.5wt%Nb合金,晶界并不是成核的优先选择,样品表面某些尖角处和粗糙处容易发生氢蚀,材料表面越粗糙,氢蚀越易发生。（2）系统地研究了U-2.5wt%Nb合金氢蚀初期孕育期影响因素,并探讨了其内在机制。研究结果表明:U-2.5wt%Nb合金氢蚀存在孕育期,相对于未合金化铀,U-2.5wt%Nb合金更易发生氢蚀。U-2.5wt%Nb与U合金氢蚀成核形貌也有所不同。温度和压力对孕育期实验揭示了孕育期存在机理,即“扩散屏蔽”机制。较低温下（＜125℃）孕育期随反应温度的升高而减小,表现为Arrhenius关系,其表观活化能为22.92kJ/mol。孕育期随反应压力升高而减小,孕育期与反应氢压力成反比关系。孕育期随氧化膜增加而显著增加。超过125℃,孕育期随反应温度的升高而增加,表明孕育期的“扩散屏蔽”机制已经失效。利用现有处理手段,对氢蚀动力学不同影响因素进行了研究,结果表明,预热处理显著影响U-2.5wt%Nb合金氢蚀孕育期;表面等离子注氮能显著提高U-2.5wt%Nb合金的抗氢蚀性能。添加不同浓度CO的氢气非常明显的延长了孕育期。（3）利用加速腐蚀方法,采用标准拉伸力学试样,研究了氢腐蚀对材料力学性能的影响。结果显示,U-2.5wt%Nb合金断后伸长率和断面收缩率下降非常明显,抗拉强度略有下降,屈服强度和弹性模量变化不明显。拉伸样表面腐蚀形貌、断口形貌、剖面形貌试验结果显示,氢蚀引起的表面缺陷如蚀坑、微裂纹、氢扩散区导致了材料力学性能的下降,生成氢化物相是导致材料变脆的主要原因。获得了U-2.5wt%Nb合金准静态下拉伸本构方程,氢蚀后材料的应变硬化能力发生变化。采用应变能密度理论,以表面存在微裂纹的拉伸样来模拟氢腐蚀的影响,计算了表面存在不同长度微裂纹时应变能密度的变化。模拟结果显示,表面腐蚀形成的缺陷如微裂纹确实降低了氢化腐蚀后铀铌合金抗拉强度和延伸率,特别是延伸率下降较多。随着裂纹尺度的增加,力学性能参数下降更多。有限元计算结果与实验结果吻合较好。计算了含有一定环境气氛的铀材料在密闭体系经过长期充分作用后,体系内各组分的含量,尤其是氢分压和氢量。结果表明湿度引起的水含量生成的氢气量,足以使材料发生氢腐蚀,当湿度为80%时最大单位面积腐蚀量可达0.096mol/mm²,但这个腐蚀量不足以引起材料整体力学性能发生较大变化。但是如果这么大的氢气量在某个薄弱点发生腐蚀,不能以平均值计算。（4）利用Hopkinson杆实验研究了U-2.5wt%Nb合金动态力学行为和微结构变化,并研究表面氢蚀对其动态力学行为的影响。获得了材料在10^-3-10³/s应变速率范围内材料的应力应变曲线,利用Johnson-Cook本构模型获得了U-2.5wt%Nb合金高应变速率下的本构方程。高应变速率下材料的屈服强度明显增加说明U-2.5wt%Nb合金是应变率敏感材料。氢蚀样和原始样一样对应变速率敏感,敏感程度相近,氢蚀后在塑性变形阶段的应变强化效应略有不同,应变指数增大,抗应变能力增强,氢蚀样屈服强度提高,屈服应变增大,腐蚀缺陷加快了材料塑性变形。材料在高应变速率下的微结构演化结果显示,在应变率低于1500s^-1时,合金的变形机制仍以滑移为主,晶界结构未见明显变化,尚未观察到明显剪切带。更多还原

【Abstract】 U-Nb alloys have been widely concerned in the field of nuclear engineering as an important material. However, because of its reactiveness to entironment atmosphere and the metastable structure, the aging behavior of U-Nb alloys has been investigated in recent years, and the study of surface corrosion and phase transformation in aging process is hot problem. As a structural material, the mechanical properties of U-Nb alloys are very important. In the present work, surface hydrogen corrosion and its effect on the mechanical properties has been systemically studied.The hydrogenation kinetic of U-2.5wt%Nb has been studied systemically by a in-situ hot-stage microscope（HSM） and a P-V-T method. The nucleation and growth processes of hydride were continuously monitored and recorded on a computer. The results showed that the temperature dependence of the hydride front velocity at sufficently low temperature obeys the Arrhenius law and activation energy is 24.34kJ/mol. At hige temperatures, a maximum velocity is reached beyond which the velocity decreases sharply. At pressure much higher than the equilibrium pressure, the this velocity becomes practically independent of hydrogen pressure . The density of nucleation hydride spots increases with reaction temperature up to a certain value, and then decreases with temperature increase. The density of nucleation hydride spots increases with hydrogen pressure increase. For the bigining of hydriding of U-2.5Nb alloy, grain boundary is not the preferential spot and the nucleation of hydide is apt to happening on harsh surface. Hydiding induction time decreases with the temperature increases, which follow the arrhenius law under a temperature of about 125℃, but beyond 125℃, induction time increases sharply with temperature increasing. Induction time varies as the inverse of the hydrogen pressure, which follow the reciprocal relation. The oxide thickness on U-2.5Nb surface affect the induction time clealy, and induction time increases with the increasing of oxide thickness. U-2.5Nb alloy is susceptible to hydrogen corrosion, the hydriding rate of U-2.5Nb alloy is higher than that of U. The effect of Pre-heat treatment, the presence of CO impurities and Plasma based nitrogen ion implantation on induction time were also studied.To evaluate the effect of hydrogen corrosion on tensile properties of U-2.5Nb, tensile specimens were hydrided quantificationally, and then tested on tensile test machine to measure the tensile properties. The morphologies of hydrogen corrosion and fracture surface were observed by SEM and OM. The results show that, ductility parameter such as elongation and reduction in area decrease distinctly due to hydrogen attack, and ultimate tensile strength decreases slightly with the increasing of hydrogenation corrosion. According to morphologies of hydrogen corrosion and fracture surface, it is concluded that the pits, micro-cracks and diffuse hydrogen in solid induced by hydrogen attack result in the loss of ductility of U-2.5% Nb. According to strain energy density theory, base on the model of a crack on surface, the effect of corrosion crack on mechnical properties was calculated by finite element method. The calculated value accords with the experiment result.Dynamic mechanical properties and microstructure evolvement of U-2.5Nb under different strain rates(10^-3-10³/s) were studied by Split Hopkinson Pressure Bar （SHPB） apparatus, and effect of hydrogen corrosion on dynamic mechanical properties was also studied. The stress-strain curves show a usual strain hardening behavior, flow stress increased with the increasing strain rate. A constitutive relations based on Johnson-Cook model was constructed to describe the stress-strain relationship of U-2.5Nb at the investigated strain rates. The correlations between the simulated curves and the experimental results are in good agreement. Hydrogen corrosion resulted in difference in strain hardening behavior and increase of flow stress. There is no obvious grain microstructure change, and slippage is the main deformation method at the investigated strain rates.更多还原