【作者】 戴雷；

【导师】 刘永长；

【作者基本信息】 天津大学 ， 材料学， 2012， 博士

【摘要】 氧化物弥散强化（Oxide-Dispersion-Strengthened，ODS）钢因其具有优异的抗辐照性能、良好的高温蠕变性能和高温抗拉强度而被认为是未来核能系统最佳备选结构材料。该新型材料的强化思路是在基体中引入弥散分布的纳米级氧化物粒子，从而在本质上获得优异高温性能的材料，而这些氧化物粒子是通过新的工艺如机械合金化和后续的热压烧结获得的。本文系统研究了ODS钢的合成、机制和相变行为，探索了纳米级氧化物颗粒的形成机制与过程，阐明了ODS钢合成过程中的组织演化行为，成功利用机械球磨和热压烧结获得了MgAl₂O₄型Fe-Cr ODS钢，实现低温烧结形成MgAl₂O₄粒子，在JMAK模型基础上澄清了ODS钢的奥氏体相变行为,并以Fe3Al作为Al载体添加Al组元合成Fe-Cr-W-Ti-Y-Al系ODS合金。本工作从理论上和实验方面为ODS钢的开发与应用提供了有力依据。研究了Y₂O₃尺寸和结构在机械球磨和后续退火过程中的演化行为。研究表明：在机械球磨过程中，混合粉末的颗粒尺寸和晶粒尺寸随着球磨时间的延长而减小。但随着球磨时间的进一步延长，球磨造成的冷焊和破碎作用达到平衡，混合粉末的颗粒尺寸和晶粒尺寸不再减小。机械球磨过程中，Y₂O₃晶体逐渐被破碎并形成了不规则外形的纳米晶，在随后的退火过程中，Y₂O₃纳米晶发生长大且其外形趋近球形。其变化过程为：完整晶体不完整晶体（长程有序结构破坏）细小晶粒（纳米晶）。研究了纳米级Ti-Y复合氧化物颗粒的形成过程和机制。研究表明：纳米级Ti-Y复合氧化物主要以三种形式存在：（a）完全结晶型，该类型粒子的晶体结构与标准物质相同；（b）部分结晶型，该类型粒子并未完全结晶，其边缘存在一定区域的非晶相；（c）非晶分布于粒子内部型，该类型粒子内部分布着少量非晶区域。Ti-Y复合氧化物的这三种存在形式为探索其形成机理与过程提供了有力证据。在机械球磨过程中，Y₂O₃纳米晶和非晶相与基体元素（M: Fe, Cr, Ti）的混合相在强烈塑性变形作用下形成，即在随后的热压烧结过程中，Y₂O₃纳米晶和Ti组元的反应以及自身不稳定的[YMO]_amorphous非晶相的晶化行为形成了纳米级Ti-Y复合氧化物颗粒，研究了ODS钢机械球磨和热压烧结过程中的组织形貌演化行为。Fe-Cr-Y系和Fe-Cr-Ti-Y系ODS钢基本组织为微米级等轴铁素体和分布于晶粒中的纳米级氧化物颗粒。两种钢的基体中存在一些未强化区域即无粒子分布区域，这种现象表明，尽管强化相粒子尺寸达到纳米级，但是仍然存在分布不均一的问题。门槛应力值的计算结果（最大值为360MPa）表明细小和密集分布的粒子有利于提高材料强度。不同的区域对应着不同的门槛应力值，这是由于各个区域的纳米级氧化物粒子的尺寸和分布均一性造成的，这可能是门槛应力值不能通过蠕变测试结果获得的原因。成功利用机械球磨和热压烧结获得了MgAl₂O₄型Fe-Cr ODS钢，实现低温烧结形成MgAl₂O₄粒子。研究表明：在机械球磨过程中，强烈的塑性变形没有导致MgO、Al₂O₃发生分解，Mg原子和O原子之间以及Al原子和O原子之间的结合并未被强烈的塑性变形破坏，并且Mg、Al和O原子未溶入Fe-Cr体系。氧化物本身尺寸的减小导致的表面能增大和因Fe晶粒尺寸减小、晶界面积增加导致的晶界能增大是氧化物向非晶转变的驱动力。在后续烧结过程中非晶晶化形成了MgAl₂O₄粒子。MgAl₂O₄强化相粒子形成温度为1173K，该温度远低于传统烧结温度（1873K），即实现低温烧结合成MgAl₂O₄。利用改进了的JMAK模型研究了ODS钢的奥氏体相变行为。基于拟合结果认为，在相同加热速率下，三种材料的奥氏体晶粒生长激活能值几乎没有差别，而Fe-9Cr-0.2Ti-0.3Y₂O₃奥氏体晶粒生长时界面迁移速率相比Fe-9Cr-0.3Y₂O₃和Fe-9Cr较小。这主要是由加热过程中奥氏体晶粒生长受到分布于基体中的粒子阻碍作用造成的。纳米级氧化物粒子通过钉扎阻力阻碍了作为新相的奥氏体界面向母相铁素体的推移，利用Zener公式定量地计算了钉扎阻力，计算结果表明：对于Fe-9Cr合金，基体中不存在纳米级氧化物颗粒，奥氏体界面的推进速度较快，奥氏体化过程在较窄的温度范围内完成。而对于Fe-9Cr-0.3Y₂O₃和Fe-9Cr-0.2Ti-0.3Y₂O₃ODS钢来说，基体中存在着纳米级氧化物颗粒，它们阻碍着奥氏体界面向铁素体界面的推进，钉扎阻力分别为Ppin YO0.78MJ/m³，23Ppin YTiO3.0MJ/m³，这与动力学拟合结果中指前因子V2270的减小是相符合的。在Fe-Cr-W-Ti-Y的基础上，以Fe₃Al作为Al载体添加Al组元合成Fe-Cr-W-Ti-Y-Al系ODS合金。研究表明：在热压烧结过程中，Al组元从基体中析出与Y₂O₃发生反应生成了Y-Al-O复合氧化物。Al组元与Ti组元存在竞争关系，在复合氧化物形成过程中，Al组元取代了Ti组元与Y₂O₃发生反应，这主要是由于Al组元相比Ti组元有较高的氧亲和力，因而Al组元更容易与Y₂O₃发生反应。屈服强度计算结果表明计算值和实验值是比较吻合的，并且主要是在0.5和1时的取值是接近的，良好的屈服强度主要源于细晶强化和弥散强化。更多还原

【Abstract】 Oxide dispersion strengthened （ODS） ferritic steels are believed to be the mostpromising candidates of structural materials for advanced nuclear systems for theirsuperior radiation resistance, excellent high temperature creep and tensile properties.A key strategy for designing high-performance materials is based on the introductionof dispersed nanoscaled oxide particles which are achieved by the new technology ofmechanical milling （MM） and subsequent hot pressing （HP）.In this thesis, fundamental issues such as elaboration, mechanism and phasetransformation of ODS steel were investigated. The main content of the thesis:formation mechanism of the nanoscaled oxide particles, microstructural evolution ofODS steels during elaboration processes, the elaboration of MgAl₂O₄ODS steel andformation of nanoscaled MgAl₂O₄particles at lower sintering temperature, kinetics ofaustenitisation process in steels and the elaboration of Fe-Cr-W-Ti-Al-Y ODS alloyby the addtion of Fe₃Al as aluminum carrier on the basis of Fe-Cr-W alloy powders..The research provides a base for exploration and application of ODS steel based onexperimental and theoretical investigations.Evolution of Y₂O₃during mechanical milling and subsequent annealing wasinvestigated. The particle size and the grain size of the mixed powder decrease in themechanical milling process, they almost do not decrease along with the formation ofdynamic equilibrium between cold-welding and repeated fracture after longer millingtime.The Y₂O₃is gradually fractured and nanocrystals are formed after MM. Growthof Y₂O₃nanocrystals takes place during subsequent annealing due to Ostwaldripening. The formation processes of Y₂O₃nanocrystalline may follow the sequence:ordered phase disordered phase （loss of long-range order） fine-grained（nanocrystalline） phaseThe formation mechanism of nanoscaled Ti-Y bioxide particles was investigated.Three types of nanoscaled oxide particles were observed in HPpedFe-12Cr-0.2Ti-0.3Y₂O₃ODS steel: fully crystallized particles, partially crystallizedparticles and particles with a structure characterized by crystalline domain whereamorphous domains are distributed inside. Nanoscaled Y₂O₃fragments andamorphous phase of[YMO]amorphousare formed after MM. The formation process of mixing of nanoscale fragments Y₂O₃and amorphous phase can be illustrated as:.The formation of Y₂Ti₂O₇crystallineduring subsequent HP process involves reactions between Y₂O₃fragments andtitanium （Ti） and crystallization of[YMO]_amorphous and the formation process can beillustrated as:Microstructural evolution of ODS steels during elaboration processes wasinvestigated. The HPped steels exibit a full ferritic microstructure and the grains areequiaxed with a micrometric size. The nanoscaled oxide particles are dispersed in thegrains of the HPped steels. Some unreinforced domains without the nanoscale oxideparticles indicate that there still exist inhomogeneous areas through the size of thoseoxide particles reaches nanoscaled. Calculated threshold stress varies at differentdomains randomly selected due to various dispersion states of the nanoscale oxideparticles such as the size and relatively homogeneousness. That may be the reasonwhy the threshold stress cannot be clearly achieved by the results of creep tests. Thecalculated results of threshold stress （360MPa） indicate that the size and dispersion ofthe nanoscaled oxide particles are beneficial to excellent properties of materials.MgAl₂O₄ODS steel was successfully elaborated by MM and HP. The MgO andAl₂O₃do not dissolve in Fe-Cr binary system during MM. The combination betweenmagnesium （and aluminum） atoms and oxygen atoms isn’t fractured by severe plasticdeformation.The MgAl₂O₄nanoparticles are formed at relatively lower temperature.The increase in interfacial energy due to the size decrease of the oxides and thechange of volume fraction of grain boundary of Fe crystals are considered to be thedriving force for the amorphization of the oxides during MM. The strengtheningphase of MgAl₂O₄formed due to crystallization of the amorphous phase during HPprocess.Phase transformation kinetics from ferrite to austenite in Fe-Cr ODS ferrite steelswas investigated. The kinetics analysis indicates that the nanoscaled oxide particles inreinforced alloys exhibit great influence on the growth process of austenite throughtheir pinning effect on the movement of interface. This pinning effect against themotion of/interface leads to the decrease of the values of pre-exponential factorV0with increase of additive. Moreover, the values of the pinning force exerted by those nanoscaled oxide particles were quantitatively evaluated, which agrees with theretardation phenomenon of austenitisation process according to the modified JMAKmodel. The values of the pinning forcep pinare0.78MJ/m³and2.99MJ/m³forFe-9Cr-0.3Y₂O₃alloy and Fe-9Cr-0.2Ti-0.3Y₂O₃alloy, respectively. Compared withunreinforced alloy Fe-9Cr, the value ofp pinincreases with the addition of yttria andtitanium in Fe-9Cr-0.3Y₂O₃alloy and Fe-9Cr-0.2Ti-0.3Y₂O₃alloy. Finer dispersionof nanocaled oxide particles （Y₂Ti₂O₇） in Fe-9Cr-0.2Ti-0.3Y₂O₃than that （Y₂O₃） ofFe-9Cr-0.3Y₂O₃is beneficial to provide more efficient blocks to the motion ofinterface, which is reasonably consistent with the fitted results of the values ofpre-exponential factorV0for the movement of interface/phase by phasetransformation model.Fe-Cr-W-Ti-Al-Y ODS alloy was elaborated by the addtion of Fe₃Al as aluminumcarrier on the basis of Fe-Cr-W alloy powders. The formation of Y-Al-O bioxideparticles are composed of two stages:（a） the aluminum component precipitated fromthe matrix and （b） the reaction between the aluminum component and yttria. Thealuminum component substituted the titanium component during the formationprocess of Y-Al-O bioxide particles due to its higher oxygen affinity. Thetransformation of the phase of the dispersion after apperence of the aluminumcomponent probably comes from a competition between the aluminum componentand titanium component. The calculated yield strengths results are in good agree withthe experimental ones when0.5and1, the better results of yield strengthscome from grain refinement strengthening and dispersion strengthening.更多还原