Karanth Yashaswini, Sharma Saurabh, Darling Kris, El Kadiri Haitham, Solanki Kiran
School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, AZ 85287, USA.
Army Research Directorate, DEVCOM Army Research Laboratory, Aberdeen Proving Ground, MD 21005, USA.
Materials (Basel). 2024 Nov 28;17(23):5842. doi: 10.3390/ma17235842.
Thermo-mechanically stabilized nanocrystalline (NC) alloys are increasingly valued for their enhanced mechanical strength and high-temperature stability, achieved through thermodynamic and kinetic stabilization methods. However, their fine-grained structure also increases susceptibility to internal oxidation due to higher atomic diffusivity associated with a greater volume fraction of grain boundaries (GBs). By incorporating solutes that form protective oxides, or the so-called thermally growing oxides (TGO), this vulnerability can be mitigated. The TGO scale acts as a diffusion barrier for oxygen that slows down the oxidation kinetics and prevents internal oxidation that impairs the structural integrity of the metal. This review examines advancements in oxidation-resistant NC alloys, focusing on the interplay between grain size and alloy chemistry. We explore how grain refinement influences diffusion coefficients, particularly the enhanced GB diffusion of Ni and Cr in Ni-Cr-based alloys, which improves oxidation resistance in NC variants like Ni-Cr and Cu-Cr compared to coarse-grained counterparts. We also analyze the role of third elements as oxygen scavengers and the impact of reactive elements such as Hf, Zr, and Y in NiAl alloys, which can slow down diffusion through early establishment of protective TGO layers and enhance oxidation resistance. The concomitant effect of grain size refinement, modifications in alloy stoichiometry, and enhanced atomic diffusion is shown to manifest via drastic reductions in oxidative mass gain, and visualization of the stable, protective oxide scales is delivered through characterization techniques such as TEM, SEM, and EDS. A brief overview is provided regarding stress effects and the role of induced stress in driving oxide scale spallation, which can negatively impact oxidation kinetics. Lastly, we propose future research directions aimed at developing micro-structurally stable NC alloys through multi-solute strategies and surface modification techniques, targeting robust materials for high-stress applications with improved oxidation resistance.
热机械稳定的纳米晶（NC）合金因其通过热力学和动力学稳定方法实现的增强机械强度和高温稳定性而越来越受到重视。然而，由于与更大体积分数的晶界（GBs）相关的更高原子扩散率，其细晶结构也增加了对内氧化的敏感性。通过掺入形成保护性氧化物（即所谓的热生长氧化物，TGO）的溶质，可以减轻这种脆弱性。TGO 层充当氧气的扩散屏障，减缓氧化动力学并防止损害金属结构完整性的内氧化。本综述研究了抗氧化 NC 合金的进展，重点关注晶粒尺寸与合金化学之间的相互作用。我们探讨了晶粒细化如何影响扩散系数，特别是 Ni 和 Cr 在 Ni-Cr 基合金中增强的晶界扩散，这与粗晶对应物相比提高了 Ni-Cr 和 Cu-Cr 等 NC 变体的抗氧化性。我们还分析了第三元素作为氧清除剂的作用以及 Hf、Zr 和 Y 等活性元素在 NiAl 合金中的影响，这些元素可以通过早期建立保护性 TGO 层来减缓扩散并增强抗氧化性。晶粒尺寸细化、合金化学计量的改变和增强的原子扩散的协同效应通过氧化质量增益的大幅降低得以体现，并且通过 TEM、SEM 和 EDS 等表征技术实现了稳定的保护性氧化膜的可视化。简要概述了应力效应以及诱导应力在驱动氧化膜剥落中的作用，这可能对氧化动力学产生负面影响。最后，我们提出了未来的研究方向，旨在通过多溶质策略和表面改性技术开发微观结构稳定的 NC 合金，目标是开发用于高应力应用且具有改进抗氧化性的坚固材料。
