超高热稳定性纳米结构Fe基高温合金的制备及其高温力学性能研究

The poor thermal stability of nanostructured metals is a major concern for extending their high-temperature applications, and for the development of nanostructured superalloy. Inspired by previous work in pure copper and nickle where the nanograins produced from plastic deformation exhibit ultrahard and notable thermal stability below a critical size, we aim to introduce this new-type nanostructure with marked thermal stability in a solutionized iron-based superalloy, together with the pinning and stabilizing effect of nanosized γ´-Ni3(Ti,Al) particles on grain boundaries, achieving ultrahigh strength under both room- and high-temperatures. In this study, we will explore the universality of those ultrastable nanograins in alloy system for the first time, and systematically investigate their structural evolution and formation mechanisms during plastic deformation. Based on the quantitative characterization of the ultrastable nanograins during heat treatment, their structural coarsening, recrystallization and precipitation behaviors will be studied and the thermal stability will be evaluated; the way to combine the nanostructured matrix and γ´-Ni3(Ti,Al) particles together will be also discussed. Finally, the high-temperature hardness and tension tests were carried out to evaluate the effectiveness of grain boundary strengthening in those ultrastable nanograins under high temperatures for the first time, and the pinning effect of γ´ particles should be explored. In contrast to the traditional strengthening method used in superalloy (i.e. solid-solution strengthening and precipitation hardening due to high-alloying), we propose that a better understanding of strengthening based on the theory of “making materials plain”, using the abnormal thermal stability of nanograins below a critical size and the pinning effect of γ´ particles, which may extend the application of nanostructures to high-temperatures and provide a new strategy for the design of high strength superalloys.

结构热稳定性差是当前限制纳米金属材料高温应用所面临的主要挑战。基于前期在纯金属Cu及Ni中发现塑性变形方法制备的纳米晶，当晶粒细化至一定尺寸后兼具高硬度和高热稳定性的现象，本项目拟选用一种固溶态Fe基高温合金为研究对象，通过在结构中引入大量反常热稳定性纳米晶，并结合低能有序γ´相对界面的钉扎、稳定作用，探索室温及高温下，高温合金材料显著强化的可行性。本项目将首次探索合金中，塑性变形方法制备反常热稳定性纳米晶的普适性及其形成机制；研究反常热稳定性纳米晶加热过程中的结构粗化与时效析出行为，评价其结构热稳定性；通过硬度或拉伸实验，首次评价高温下，反常热稳定性纳米晶界面强化的有效性及其影响因素。不同于高温合金中常见的合金化手段，本项目基于“材料素化”思想，利用纳米晶的反常热稳定性与组织调控，提出一种全新的设计理念，相关研究结果对于拓展纳米材料的高温应用，以及新一代高温合金设计具有重要的指导意义。

在常温下，增加晶界是强化金属材料的一个重要手段，但在高温下，晶界迁移、晶界滑动、晶界扩散等失稳机制会导致晶界软化，晶界强化效应消失。此外，增加晶界密度会加剧晶界扩散（Coble）蠕变，合金晶粒尺寸越小，抗蠕变性能越差。如何有效提升热-力-时间耦合作用下晶界的结构稳定性，进而抑制晶界高温软化和扩散蠕变成为长期以来材料领域的一个重大科学难题，也是发展高性能高温合金的主要瓶颈之一。 .在本项目支持下，研究人员在这一科学难题研究上取得重要突破。研究团队利用自主研发的特种塑性变形技术，在一种商用单相高温合金Ni-Co-Cr-Mo（MP35N）中将晶粒细化至9 nm，晶界结构发生明显弛豫。研究发现，弛豫态晶界在热及热/力耦合下均保持稳定，大幅提升了高温合金的高温强度、高温蠕变等关键力学性能。该结构在700摄氏度、1GPa应力下的蠕变速率可低至10-7s-1，显著优于目前常用多晶高温合金性能。这是由于弛豫晶界可有效抑制晶界扩散，阻碍了高温下晶界迁移、晶界滑动、晶界扩散蠕变等失稳机制的启动，从而保持了晶界的强化作用。晶界一直被普遍认为在高温下是合金抗蠕变的“短板”，这一结果系统演示了通过结构弛豫，晶界可以大幅度提升高温合金的抗蠕变性能。此外这种晶界弛豫纳米晶高温合金可大幅降低对合金元素的依赖，为高性能高温合金的可持续发展开辟了一条新路。相关研究结果于11月11日发表在《科学》（Science）周刊上。