【作者】 王建伟；

【导师】 王力军；

【作者基本信息】 北京有色金属研究总院 ， 有色金属冶金， 2012， 博士

【摘要】 氢化锆具有高温稳定性、高氢密度、低中子捕获截面、优良的导热性能和负的温度反应因子,是一种理想的固体中子慢化材料。采用氢化锆作为慢化剂的反应堆能在较高温度下工作而无需高压容器,因此氢化锆尤其适用于小型核反应堆(如空间堆)。世界航空航天大国美国和俄罗斯都应用了氢化锆作为空间核反应堆电源中的慢化材料。氢化锆在氢化制备过程中伴随体积膨胀和晶格畸变,而且氢化锆本身是一种脆性相,这使得高氢含量无裂纹氢化锆的制备有很大难度。如何抑制块状锆合金在氢化过程中裂纹的形成,对于提高以氢化锆作为慢化剂的核动力能源的使用寿命和安全性具有重要意义。目前,国内还没有对氢化锆慢化材料的制备和裂纹控制机理问题有系统的研究。本文针对含Nb氢化锆慢化材料的制备,对Nb元素在高氢含量无裂纹氢化锆制备过程中的作用,以及在氢化锆中的存在形式等展开研究。利用以CALPHAD(相图计算)技术为基础的材料计算手段,来指导含Nb氢化锆慢化剂的制备,结合试验现象和结果,揭示氢化锆慢化材料制备过程中的氢含量和裂纹控制机理。采用先进的计算机辅助材料计算手段,可以大幅度提高探索制各工艺方法和工艺参数的效率。根据对锆合金吸氢性能研究的需要,本文初步建立了Zr-H-M多元热力学数据库,添加元素M包括Al、Cu、Fe、La、Mg、Na、Nb、Ni、Pd、Sm、Ti等11种元素。针对含Nb氢化锆的制备问题,收集整理了相关数据,结合本工作中对Zr-Nb合金PCT(Pressure-Composition-Temperature)吸氢性能的试验测试结果,对H-Nb二元系和Zr-H-Nb三元系进行了热力学优化,利用所建立的热力学模型计算得到的相平衡信息与实验数据吻合良好。这完善了Zr-H-M热力学数据库,而且可对Zr-H-Nb体系在富Zr端(wt.%Nb<2.5)的热力学性质进行比较可靠的计算和预测,指导含Nb无裂纹氢化锆的制备。为了得到Zr-Nb合金的吸氢性能,本文对不同Nb含量(1wt.%-30wt.%)Zr-Nb合金在低压下700～900℃温度范围内进行了PCT测试分析。试验结果表明,Nb含量对锆合金的吸氢性能有较大影响。随着Nb含量的增加,锆合金的平衡氢含量明显减少。这是由于在室温以上,高氢含量的δ-NbH2只有在高压条件下才能稳定存在。Nb提高了βZr+fcc两相区的上限温度,使βZr相要在更高的温度下才能完全转变为fcc相(δ相)氢化锆。结合热力学计算结果,还发现了Nb有抑制fct相(ε相)析出的作用。Nb除了对Zr-Nb合金的吸氢性能有一定影响,而且对氢化锆的裂纹产生有抑制作用,这是由Nb在氢化锆中的存在形式决定的。本工作在低压下对纯Zr和不同Nb含量(1wt.%-50wt.%)的Zr-Nb合金进行了氢化试验,借助XRD、SEM、EDS和金相观察等分析测试手段对氢化后产物的组成、结构、形貌和组织等进行研究。结果表明,Nb含量越高,合金氢化产物越不容易产生裂纹。在充分吸氢的情况下,不同Nb含量的Zr-Nb合金氢化产物的主要组成都是ZrH2、ZrH1.950和ZrH1.801的ε相氢化锆混合物。Nb含量较高时,会生成低氢含量的NbH、固溶体,影响锆合金的整体吸氢量。氢化过程中H优先与Zr结合,当H含量较低时,大部分Nb以锆铌固溶体小颗粒的形式弥散分布在氢化锆基体中。Nb改善了氢化锆的多缺陷状态,减少了氢富集的位置,降低了诱发裂纹形成的可能性。虽然Nb对氢化锆裂纹控制有利,但是较多的Nb会影响氢化锆中的氢含量,因此添加量不应过多。氢化锆中氢含量越多,孪晶结构特征越明显,氢化锆产生的裂纹主要分布在晶界处。在上述研究结果的基础上,结合热力学计算方法和氢化锆制备基础试验,系统研究了高氢含量无裂纹氢化锆(含1wt.%Nb)制备工艺中的关键问题,寻找合理的工艺参数,并进行机理分析。结果表明,氢化过程中的上限氢分压不宜过高,否则会加大裂纹形成的可能性,本试验通过程序控制使氢分压不大于105KPa。固溶体的形成对氢化锆制备过程中裂纹的产生与否无明显影响,而在βZr→fcc和fcc→fct两个相转变过程,以慢速降温和慢速通氢的方法降低锆合金／氢化锆的吸氢速度,是控制裂纹产生的关键。塑性下降是和氢化锆的析出密切相关的,它们是裂纹源。根据热力学计算结果预测,氢分压为105KPa时,βZ→fcc的相转变温度为872℃,fcc→fct,的相转变温度为795℃,结合试验制定了多段控制的氢化温度曲线。在βZr→fcc和fcc→fct相变区采取小流量恒流氢化,降低相变阶段氢分压,减慢合金的吸氢速度。同时,通过降低氢分压的方法,使βZr→fcc相变过程有了较大的温度范围,为材料释放内应力提供更多的条件。温度对氢化锆中的氢含量起决定作用。压力一定时,温度和氢化锆中的平衡氢含量呈一一对应关系。通过热力学计算预测,氢分压为105KPa时,Zr-1Nb-H体系在820℃C达到平衡状态时主要组成为δ相氢化锆,氢含量约为1.6H/Zr(at.)；在788℃达到平衡状态时主要组成为为ε相氢化锆,氢含量约为1.8H/Zr,试验结果与计算结果基本一致。另外,氢化过程中应给予样品,尤其是大尺寸样品足够的平衡保温时间,使氢扩散完全,保证氢含量的均匀和饱和。同时保温过程也起到了去应力退火的作用,可以抑制裂纹的产生。更多还原

【Abstract】 Zirconium hydride in solid state with high thermal stability, high hydrogen content, low thermal neutron absorption cross section, good mechanical properties and negative reaction factor of temperature, is considered to be one of the most ideal moderators. The nuclear reactor using zirconium hydride as moderator can run in elevated temperature without high pressure vessels, therefore the zirconium hydride moderator is particularly suitable for the space nuclear power system. The United States and Russia leading the top air space technologies have all used zirconium hydride as moderator in their space nuclear power systems.However, the volume expansion and lattice distortion would accur during the preparation of the zirconium hydride moderator. Addirionally, the zirconium hydride itself is a brittle hydride phase. Those make the preparation of the high hydrogen crackfree zirconium hydride is very difficult. It is meaningful to resolve the cracking problem in the hydriding process of the zirconium hydride for improving the life and safefy of the space nuclear power system, which applies zirconium hydride as moderator. Due to technical blockade, currently the domestic research on the preparation of zirconium hydride and relative crack control mechanism is rather limited.The effect of Nb on the preparation of high hydrogen crackfree zirconium hydride and the existing status of Nb in the zirconium hydride were investigated at present. The thermodynamic properties and equilibrium phase information of zirconium hydride system can be used to guide the preparation of zirconium hydride moderator. Combining CALPHAD and experiment methods, the key problems such as the cracking and hydrogen content control, were tried to resolve in this paper. By means of advanced computer technologies, the efficiency on the investigations of preparation method and process parameters can be greatly improved.The Zr-H-M multielement thermodynamic database was established for the research needs of hydrogen absorption properties of zirconium alloys. The adding elements M in the database including11elements which were Al, Cu, Fe, La, Mg, Na, Nb, Ni, Pd, Sm, Ti. To aim at the preparation of Nb-containing zirconium hydride, the thermodynamic models of H-Nb binary system and Zr-H-Nb ternary system were optimized based on the literature information and the experimental data in this work, and the calculated results agreed well with the experimental data. The equilibrium phase information and thermodynamic properties of Zr-H-Nb system on the Zr-rich side (wt.%Nb≤2.5) could be calculated and predicted availably, to guide the preparation of Nb-containing crackfree zirconium hydride.To obtain the hydrogen absorption properties of Zr-Nb alloys, the PCT (Pressure-Composition-Temperature) measurements for Zr-Nb alloys with different Nb content (1wt.%～30wt.%) were carried out at low pressure in the temperature range of700℃～900℃. The experimental results indicated that the equilibrium hydrogen content of zirconium alloy decreased apparently with the Nb content increasing. As the Nb content of the alloys increased, the equilibrium hydrogen pressure also increased for any fixed concentration of hydrogen and the δ zirconium hydride precipitation from saturated βZr is unfavorable in presence of Nb. And Nb could delay the precipitation of s zirconium hydride.Nb affects the hydrogen content and cracking formation of the zirconium hydride, which is determined by the existing status of Nb in the zirconium hydride. The composition, structure, morphology and metallographic phase of hydriding products of Zr-Nb alloys with different Nb content (1wt.%～50wt.%) were investigated by the methods of XRD, SEM, EDS and metallographic test. In the case of fully absorption, the hydriding products of Zr-Nb alloys with different Nb content were all composed of mixed ε zirconium hydrides of ZrH2, ZrH1.95and ZrH1.801.When the Nb content was high, the products of NbHx solid solution with low hydrogen content downgraded the whole hydrogen content of the alloy, which was because of the δ-NbH2with high hydrogen level only existed steadily in the condition of high pressure above room temperature based on the thermodynamic calculated results. The solubility of Nb in zirconium hydride is limited and the majority of Nb disperes on the surface of zirconium hydride in the form of small white particles of H-contaning Zr-Nb solid solution under the condition of low hydrogen level. The existence of Nb doesn’t significantly change the lattice parameter of zirconium hydride, but improves the organizational structure of zirconium hydride, which reduces the H-concentration defect sites and the possibility of cracking. Although Nb can help control the formation of crack, but Nb also affects the hydrogen content of the zirconium hydride, so the dosage of Nb should not be too much. In addition, more hydrogen content contained in the zirconium hydride with1wt.%Nb, the more obvious features of twin stucture can be observed. The crack forms mainly along grain boundaries.Through the above analysis, combined with thermodynamic calculations and experiments for the preparation of zirconium hydride, the key issues in the preparation of crackfree zirconium hydride with high hydrogen content were investigated systematacially. The reasonable parameters, related mechanisms and phenomenons in the preparation process were tried to obtain and analysis at present. The results showed that the upper limit of hydrogen pressure in the hydriding process should be restricted in a low level. The hydrogen pressure in this work was programmed not to exceed105kPa. The formation of solid solutions during the preparation of zirconium hydride had no effects on cracking. In order to control the cracking, slow cooling rate and slow hydrogen flow during the βZr→δ and8→εe two phase transition processes were the key factors. The precipitation of hydride phases, which were the source of cracking, destroyed the plastic nature of the material. According to thermodynamic calculations,βZr→δ transition temperature was872℃and8δ→ε transition temperature was794℃. A reasonable multi-step hydriding temperature curve was made based on the calculations and experiments. The hydrogen absorption of the alloys was in a slow rate and the cracking was inhibited efficiently.Temperature plays a decisive role on the hydrogen content of zirconium hydride. Keep pressure in a constant level, a rule of correspondence between equilibrium hydrogen concentration in the alloy and temperature was concluded. As the temperature decreased, the hydrogen composition increased and the equilibrium phases of the system also changed. For Zr-1Nb-H system, the zirconium hydride with hydrogen concentration of1.8H/Zr in atom ratio was fct structure in equilibrium at788℃, and the zirconium hydride with1.6H/Zr was fcc structure at820℃. During the hydriding process, enough hydriding time should be given to make hydrogen diffuse completely and ensure the uniform and saturation of hydrogen content in the alloys, especially for the large size samples. This process also played the annealing role to remove the internal stress and inhibit the cracking.更多还原