奥氏体钢中不同尺寸纳米析出相对晶界的钉扎机制

Introducing second-phase precipitates is one of the most commonly used methods to inhibit grain growth and achieve better mechanical properties in polycrystalline materials. With decreasing of the precipitate size, the impact of grain boundary-precipitate triple line is pronounced and can change the interaction between precipitates and grain boundaries. However, the pinning mechanism of nanoscale precipitates on the grain boundaries is not clear so far. This project will choose a new class of alumina-forming austenitic (AFA) heat-resistant steel as the model material, to study the interaction of grain boundaries with precipitates smaller than 50 nm. Nanoscale precipitates of different sizes will be achieved by different recrystallization and creep testing conditions. The specific areas containing grain boundaries will be prepared by focused ion beam (FIB) technique. With TEM characterization, this project will clarify the size-dependent pinning mechanism of MC-type and Cu-containing L12-type precipitates, distinguish different pinning mechanisms under the effect of grain boundary-precipitate triple line, and statistically analysis the size and distribution of nanoscale precipitates on the grain boundaries and within the grains. According to the experimental results, an analytical equation of the pinning force will be derived, and it is helpful for controlling the microstructure and improving mechanical properties by nanoscale precipitates.

析出相对晶界的钉扎作用可以抑制晶粒长大，进而提高材料的力学性能。随着析出相尺寸达到纳米级，晶界析出相三叉线的影响变得显著，会改变析出相与晶界的作用方式。但迄今，纳米析出相对晶界的钉扎机制尚不清晰。本项目选择新型氧化铝保护层自发形成的奥氏体(Alumina-Forming Austenitic)耐热钢为模型材料，针对其中两种小于50纳米的析出相进行研究。通过不同温度下再结晶处理和蠕变测试，获得不同尺寸的纳米析出相，采用聚焦离子束定点加工的技术，制备含有所需晶界区域的样品，利用TEM显微表征，揭示MC析出相对晶界的钉扎方式和具有L12结构的含铜析出相在晶界上的形貌随尺寸变化的关系，区分三叉线影响下纳米析出相对晶界的不同钉扎机制，统计分析纳米析出相在晶界上的尺寸和间距与晶粒中尺寸和体积分数的关系，从而建立符合实际情况的钉扎力解析式，为利用纳米析出相调控材料微观结构及其力学性能提供科学依据。

第二相颗粒对晶界的钉扎作用可以调控材料的微观结构，进而提高金属材料的力学性能，对此有各种钉扎力模型来解析再结晶以及晶粒长大等现象。当颗粒尺寸达到纳米级，析出相对晶界钉扎的作用机制会发生改变。本项目对纳米析出相对晶界的钉扎机制从实验和模拟两方面展开了研究。选择新型氧化铝保护层自发形成的奥氏体(Alumina-Forming Austenitic)耐热钢为模型材料，针对其中与基体共格的纳米级L12有序相和MC碳化物进行研究。研究了不同再结晶温度对AFA耐热钢组织、性能和纳米析出相的影响，优化热机械处理工艺，得到纳米级共格析出相，提高了该合金的抗蠕变性能。在700℃/150MPa应力条件下，AFA耐热钢持久蠕变寿命提高到2047小时，优于相同应力条件下的商用Super304H耐热钢。利用聚焦离子束在实际耐热钢合金的晶界上定点取样，电镜表征显示，在同一样品中不同晶界上的析出相分布截然不同：普通晶界两侧有纳米共格析出相的无析出区；孪晶晶界附近纳米共格析出相分布均匀。同时，利用分子动力学模拟，研究发现在纳米尺度下，存在迁移晶界切过和越过颗粒的两种机制，析出相与基体的共格关系可以通过多种回转方式实现，对不同位向差晶界的钉扎效率（对晶界迁移的减慢效果）会有数量级上的差别。这两种机制通过TEM微观表征可以实验确定，依据相应建立的钉扎力解析式，能够为利用纳米析出相调控微观结构及力学性能提供理论支持。