Gas-Phase Nitridation under Kinetic Control -- A New Concept for Surface Hardening of Ti-Base Alloys

技术：钛基合金具有很高的比强度和在氧化环境中的耐腐蚀性，因此在技术上具有重要意义。然而，在许多应用中(航空航天工程、生物医学、体育、珠宝、建筑)，这类合金的性能受到了严重限制，因为它们的表面硬度和耐刮擦性很差，摩擦系数很高，磨损速度很快，在还原介质中的耐腐蚀性也很差。克服这些缺陷的一种有效方法是在钛合金零件成形成最终形状后优先对其表面进行硬化。这种“表面”硬化最有效的方法是通过间隙溶质的向内扩散，例如氮气。然而，钛与所有相关的间隙溶质形成非常稳定的化合物。因此，传统的渗层硬化方法不可避免地会产生氧化物、碳化物或氮化物等析出物，严重降低合金的耐磨性、耐疲劳性和耐腐蚀性。PI团队的开创性实验产生了一种新概念，使氮能够从气相向内扩散到钛基合金中，而不会析出有害的氮化物颗粒。这一新概念被表示为“动力学控制下的氮化”，并已在两种不同的处理方案中成功实施，产生了氮浓度高达20at%的无沉淀情况，并极大地提高了表面硬度。在这项调查中，PI建议研究这些新的和不寻常的材料的表面、微结构和基本性质。非技术性：这项工作的更广泛影响包括钛基合金零部件的新工程应用，与这些新应用相关的新工作，在其中一些新应用中因钛基合金的高比强度而节省的一次能源，通过为医疗植入物提供更好的材料来对抗疾病的进展，以及飞机安全性的改善-例如通过减少直升机上钛合金液压管路的灾难性故障。拟议项目的结果也将深刻影响CWRU的材料科学课程。这两家私人投资机构都有支持代表性不足的少数群体的记录，并且在其研究小组中有相当一部分女性学生。Co-Pi一直致力于向中学生介绍材料科学和显微镜。

Technical: Ti-base alloys are technologically important because of their high specific strength and resistance against corrosion in oxidizing environments. However, in many applications (aerospace engineering, biomedicine, sports, jewelry, architecture) the performance of such alloys is severely limited by their notoriously poor surface hardness and scratch resistance, very high friction coefficients and rapid wear, as well as poor corrosion resistance in reducing media. A powerful approach to counter these deficiencies is to harden the surface of Ti-alloy parts preferentially after they have been formed into their final shape. Such "case" hardening is most effectively achieved by Inward diffusion of interstitial solutes, for example nitrogen. However, titanium forms very stable compounds with all relevant interstitial solutes. Consequently, conventional methods to case-harden Ti-base alloys with interstitial solutes inevitably produce precipitates (oxides, carbides, or nitrides), which severely degrade the wear-, fatigue-, and corrosion resistance. Pioneering experiments by the PI's group have resulted in a new concept enabling inward diffusion of nitrogen into titanium base alloys from a gas phase without precipitating detrimental nitride particles. This new concept is denoted as "Nitridation under kinetic control" and has been successfully implemented in two different processing schemes, yielding precipitation-free cases with nitrogen concentrations of up to 20 at% and a tremendous increase in surface hardness. In this investigation, the PIs propose to study the surface, microstructure, and fundamental properties of these new and unusual materials. Non-Technical: The broader impacts of this work include new engineering applications of parts and components made from Ti-base alloys, new jobs related to these new applications, savings of primary energy related to the high specific strength of Ti-base alloys in some of these new applications, advances in fighting diseases by making available better materials for medicalimplants, and improvements in aircraft safety - e. g. by reducing catastrophic failureof Ti-base alloy hydraulic lines in helicopters. The results of the proposed project will also profoundly impact materials science courses at CWRU. Both PIs have a record of supporting underrepresented minorities and have had a substantial fraction of women students in their research groups. The Co-PI has a continuing effort of introducing middle school students to materials science and microscopy.