Thermal Stabilization and Mechanical Properties of Nanocrystalline Fe-Cr-Ni Alloys

技术综述：已经提出，少量添加适当的合金元素(掺杂剂)可以通过热力学机制将纳米晶的晶粒度稳定在高温下。该概念基于溶质向晶界偏聚可以使有效晶界能降为零的思想。从而消除了晶粒生长的驱动力。在Fe-Ni-Cr合金中添加精心挑选的偏析到晶界的溶质，将与球磨一起使用，以生产粉末形式的纳米晶合金。采用适当的显微组织表征进行短期退火，以确定合金粉末可以通过热压实而不长大的温度范围。随后将对强度和延性性能进行初步评估。对控制长期热稳定性的颗粒生长机制的研究将通过全面和根本的方法来完成。将通过等温退火实验确定退火动力学和微观结构演变，并使用分辨率可达原子尺度的微观结构表征技术，以便识别机制并开发基于模型的外推来验证长期稳定性。模拟结果将被用于定制Fe-Ni-Cr纳米晶合金，以实现热稳定性、高强度和良好的拉伸延展性的最佳组合。非技术摘要：如果晶粒尺寸减小到纳米，金属的强度可以提高到非常高的水平。然而，这些纳米晶金属在高温下通常是不稳定的，因为颗粒会长大，强度会下降。拟议中的研究将生产出新的纳米晶钢和不锈钢(Fe-Ni-Cr合金)，它们具有非常高的强度，并且在高温下晶粒度稳定。这些材料可用于提高发电系统、发动机和其他需要高强度重量比和高温性能的应用的效率。这项研究的另一个同样重要的方面是以理工科毕业生的形式开发智力资源，他们可以开发新技术，并将知识转移到美国工业。

TECHNICAL SUMMARY: It has been proposed that small additions of appropriate alloying elements (dopants) can stabilize a nanocrystalline grain size to high temperatures by means of a thermodynamic mechanism. The concept is based on the idea that solute segregation to grain boundaries can reduce the effective grain boundary energy to zero. Hence the driving force for grain growth is eliminated. Alloy additions of carefully selected solutes that segregate to grain boundaries in Fe-Ni-Cr alloys will be used in conjunction with ball milling to produce nanocrystalline alloys in powder form. Short term annealing with suitable microstructure characterization will be done to identify a regime of temperatures where the alloy powders can be consolidated by hot compaction without grain growth. An initial evaluation of the strength and ductility properties will follow. Investigation of the grain growth mechanisms that govern long-term thermal stability will be accomplished by a comprehensive and fundamental approach. The annealing kinetics and microstructure evolution will be determined using isothermal annealing experiments, with microstructure characterization techniques that have resolution down to the atomic scale, so that mechanisms can be identified and model-based extrapolations can be developed to verify long-term stability. The modeling results will be used to tailor Fe-Ni-Cr nanocrystalline alloys for optimum combinations of thermal stability, high strength and good tensile ductility.NON-TECHNICAL SUMMARY: The strength of metals can be increased to very high levels if the grain size is reduced to nanometers. However, these nanocrystalline metals are normally unstable at elevated temperature because the grains grow in size and the strength is lost. The proposed research will produce new nanocrystalline steels and stainless steels (Fe-Ni-Cr alloys) that have very high strength and the grain size is stable at high temperatures. These materials can be used to improve the efficiency of power generation systems, engines, and other applications where high strength-to-weight ratios and elevated temperature performance are required. An equally important aspect of the research is the development of intellectual resources in the form of science and engineering graduates who can develop new technology and transfer that knowledge to US industry.