基于纳米片状粉末烧结特性的氧化物弥散增强(ODS)铁素体钢的可控制备与表征

Oxide dispersion strengthened (ODS) ferritic steel is one of the most promising candidates for the first wall cladding materials in the fourth-generation nuclear reactor system. For this kind of steel, the key is to control both the size distribution and the localization distribution of the Y2O3 particles in the ferrite matrix. There are some inherent weaknesses of the existing traditional mechanical alloying (MA) method, such as high risk of decomposition reaction of Y2O3 particles, poor oxygen content control and complicated microstructure. Powder metallurgy (PM) method can avoid the above shortcomings, but the mechanical performances of the ODS steel prepared by PM method are unsatisfactory. Based on our previous work (Scripta Mater, 78–79: 1-4, 2014), nano flake powders have special sintering coarsening behaviors. The coarsening rate through thickness direction of the flake powder is faster than that of through width direction during sintering. Thus, oxide particle is enveloped by the growth of the two neighboring flakes and has intragranular distribution in the ferrite matrix finally, which results in higher dispersion strengthening contributions due to the additional Orowan strengthening effect. In view of the above, the applicant proposed here to use the low energy ball milling (LEBM) to activate the pre-alloyed ferritic steel powders, and use the nano flake powders with high specific surface area as building blocks to assemble into ODS ferritic steel via PM method. The platform for ODS ferritic steels prepared by PM technique can be established through this project. And some constructive suggestions for fabricating high performance ODS ferritic steels will be put out.

氧化物弥散增强（ODS）铁素体钢是第四代核能系统反应堆第一壁包层最具潜力的候选材料之一。对于该材料而言，关键是控制Y2O3颗粒在铁素体内的尺度分布和位置分布。传统机械合金化（MA）法存在Y2O3易分解反应、含氧量不易控制以及微观组织复杂等缺点。粉末冶金（PM）方法可以规避上述缺点，但制备的ODS钢性能较差。申请人前期研究发现（Scripta Mater, 78–79: 1-4, 2014）纳米片状粉末具有特殊的烧结粗化行为，烧结后期粉末厚度方向生长速率高于宽度方向上的生长速率，使得氧化物颗粒被包覆在金属基体内而呈晶内弥散分布，导致材料由于具备额外的Orowan强化，弥散强化效果更显著。据此提出采用低能球磨活化铁素体预合金粉末，以高比表面积纳米片状粉末为组装单元通过PM技术制备ODS铁素体钢的工艺。本项目实施有望建立PM制备ODS铁素体钢的技术平台，为开发高性能ODS铁素体钢提供技术支撑。

本项目主要研究内容分为两项：一是ODS铁素体钢成型工艺，二是ODS铁素体钢微观组织特征和强韧化机制。针对ODS铁素体钢成型工艺的研究，本项目开发了一种ODS铁素体不锈钢的制备方法，包括：制备铁素体不锈钢预合金粉末，所述铁素体不锈钢中含有Ti或W；将所述铁素体不锈钢预合金粉末进行低能球磨，得到片状铁素体不锈钢预合金粉末；将所述片状铁素体不锈钢预合金粉末进行过滤，烘干后与Y2O3粉末进行混粉处理，得到复合粉末；将所述复合粉末进行冷压，再进行粉末烧结，然后将烧结后的材料进行热挤压，得到ODS铁素体不锈钢。本方法采用低能球磨处理获得了片状铁素体不锈钢预合金粉末，然后以其作为结构单元进行组装，有效控制了Y2O3颗粒相在铁素体不锈钢基体内的尺寸分布和空间分布，实现了Y2O3颗粒相在晶内的均匀弥散分布，获得了力学性能较好的铁素体不锈钢。该制备方法申请并授权国家发明专利一项。针对ODS铁素体钢微观组织特征和强韧化机制研究，本项目研究了具有双晶结构的高铝ODS钢微观组织特征和强韧化机制。研究结果表明，随着铝含量的增加，ODS钢基体形成由被细晶粒紧密包围的粗晶粒组成的双晶结构，并随着Al添加的增加，这种双晶结构特征更显著。此外，随着铝含量的增加，纳米级氧化铝颗粒在双晶铁素体晶粒内部沉淀析出。双晶结构材料由非均匀屈服产生的高背应力加强，也即所谓的背应力强化。由细晶粒包围的粗晶粒通过这种背应力有效地硬化，增加了粗晶粒中的位错滑移的难度。随着Al添加量从2.5增加到4.5，应变硬化缓慢改善。这很可能是因为背应力不仅提高了屈服强度，而且增强了应变硬化，从而提高了延展性。于此同时，纳米氧化铝颗粒的沉淀析出，提供额外的Orowan强化，因此通过背应力强化加上额外的Orowan强化来能够增强高Al含量ODS钢强度和韧性。