多级纳米孪晶奥氏体不锈钢的微区析出机制及高温增强增韧特性研究

Precipitation strengthening is the most effective method to improve the high-temperature mechanical properties of austenitic stainless steel. However, the coarsening of carbides precipitated along the grain boundaries leads to brittle fracture, and the nucleation interior the grains is hardly controlled. How to restrict the intergranular precipitation and control the nucleation probability are the challenge problems in the high-temperature strengthening. In our previous study, we found that the low-energy coherent nanotwin boundaries (TB) can interrupt the continuity of the intergranular precipitates and enrich the alloy elements in the local zone close to the TB. The local enrichment of alloy elements is very important for the precipitation strengthening, but precipitation controlled by pre-set TBs is still not clear. Additionally, microstructure of the nanotwinned (NT) austenitic steels, characterized by the multiscale (several nanometers to hundreds of nanometers) and multiform (primary NT, secondary NT, and intersected NT systems), which forms a hierarchical mesh structure. The NTed austenitic stainless steels exhibit super-high strength and good ductility, but the deformation mechanisms of the hierarchical NTs are not well understood. . In this work, we study the scaling effect of the deformation mechanisms of NTs, the anti-corrosion performance and high-temperature precipitation of the NTed austenitic stainless steel with a mesh structure. Firstly, a full scale deformation map of NTs is established by combining in-situ TEM tensile experiments, with dislocation-based mechanics model and molecular dynamics simulation to reveal the intrinsic relationship of twin scale (twin lamellar spacing) and deformation mode. Secondly, the effect of the mesh structure on the precipitation sequence, distribution and size of the second-phase nanoparticles are investigated in order to achieve better high-temperature mechanical properties. The nucleation and growth of the second-phase nanoparticles are observed by in-situ SEM heating technology. Thirdly, the relationship of precipitation with the high-temperature mechanical properties is investigated according to the different pre-set TB microstructure. The precipitation behaviors of the grain boundary, incoherent TB, coherent TB and dislocations interior the grain are analyzed. The strengthening and toughening mechanism of the precipitation strengthening steels are explored. The study of the precipitation kinetics of the second-phase controlled by the local zone can not only fulfill the theory of nanoparticle reinforcement, but also provide a new way to develop advanced austenitic stainless steel with high-temperature mechanical properties.

析出强化是改善奥氏体不锈钢高温力学性能最有效的方法。但，沿晶界析出的碳化物易于长大而导致脆性断裂，而晶粒内析出成核难于控制。如何抑制沿晶析出并控制晶内成核概率一直是高温强化的难题。我们在前期研究中发现低能共格纳米孪晶界(TB)可打断沿晶析出相的连续性，并在局域范围内富集合金元素。这对材料的析出强化至关重要。但对于预置TB微区调控析出机制尚缺乏深入研究。此外，奥氏体钢中纳米孪晶以多尺度、多形态(一级、二级/多级、交叉系)为特征，表现出良好的强韧性，但其变形机制仍存在若干空白之处。针对上述两个重要问题，本项目拟研究：(1)采用In-situ TEM拉伸，结合力学理论和分子动力学模拟建全多级纳米孪晶变形机制模型；(2)利用多级TB实现微区调控第二相粒子析出机制；(3)研究高温力学性能与析出相的本构关系，解析高温强塑性机制。本项目将进一步完善纳米析出增强理论，为发展高温奥氏体钢提供新的研究思路。

超级奥氏体不锈钢在高温服役环境下，要求材料保持良好的高温强度和塑性。纳米孪晶（TB）具有独特的耦合界面结构，可有效控制析出相的大小、分布、以及共格界面特征，实现微区调控析出相结构。进而可同时获得高温高强高塑性。本项目采用表面机械研磨（SMAT）技术处理超级奥氏体不锈钢，制备出多尺度梯度孪晶结构材料。研究多级纳米孪晶结构变形机制、共格析出机制及其高温力学性能。取得的成果如下：.(1)建立多级纳米孪晶变形机制图谱: 采用In-situ TEM拉伸，结合位错塑性力学模型和分子动力学模拟，在奥氏体不锈钢中发现了孪晶变形的分级尺度效应。在此基础上建立了从几纳米到几百纳米范围内孪晶的变形机制图谱。孪晶的变形临界尺度效应和变形机制图谱的建立不仅对微纳尺度金属材料强韧化机制的理解具有重要科学意义，同时也为设计高性能金属工程材料提供了全新思路。.(2)多级纳米孪晶微区调控第二相析出机制:由于孪晶界具有特殊的偶合对称结构，其界面能低于晶界，对合金元素的扩散起到调控作用。通过不同时效工艺（600 ℃～800 ℃）处理，观察到纳米孪晶结构析出相的析出顺序为：晶界＞晶界与孪晶界交叉处＞大尺寸孪晶界＞小尺寸孪晶界。在500 ℃时，大尺寸孪晶界有少量析出物。650 ℃时，大尺寸孪晶界开始有大量粒状析出物。纳米孪晶可稳定至800 ℃，之后发生再结晶，热稳定性开始出现下降。.(3)高温力学性能与析出相的本构关系：在500 ℃保温3 h时效工艺处理下，样品显微硬度值达到峰值340 HV，较原始样品的表面硬度值提高了约140 HV。700 ℃高温拉伸结果显示，屈服强度高达525 MPa，延伸率为26.7%。屈服强度较原始样品在同等试验条件下提升了约1.5倍。TEM表征发现退火后的纳米孪晶界处出现了弥散分布的纳米级析出相。纳米析出相与纳米孪晶强化协同提高了材料的高温力学性能。本项目完善了纳米析出增强理论，为发展高温奥氏体钢提供新的研究思路。