Materials World Network: Stability of Colossally Supersaturated Structural Alloys

凯斯西储大学将与英国伯明翰大学合作，研究结构合金表面工程的新方法，包括奥氏体不锈钢、镍铬和钴铬合金。这两个小组已经进行了大量的先前工作，其揭示了在低温下从合金表面注入间隙溶质-碳或氮-有效地抑制了第二相的沉淀，并且能够形成约25微米厚的均匀的、无沉淀物的硬壳（“壳”），其中碳或氮浓度可以超过平衡溶解度极限约100，000次（！).这种具有间隙的巨大过饱和使表面硬度高达5倍，耐磨性和疲劳寿命增加约100倍，并将耐腐蚀性提高到最耐腐蚀的合金。新项目将专注于这些不寻常的非平衡材料在高温和机械应力下的可靠性和寿命。该项目的第二个目标是探索实际使用受控热后处理的可能性，以进一步改善合金表面性能和耐用性。在沉淀的高驱动力下，应该可以产生第二相颗粒的超细分散，这有望进一步提高硬度而不损害耐腐蚀性。非技术总结奥氏体不锈钢和相关结构合金在各种技术应用中的实用性可以通过将这些材料的成品部件暴露于将碳或氮原子注入其表面的工艺来显著提高。这种“个案硬化”在中世纪就已经应用于武器。然而，在提高表面硬度的同时，其进行的常规方式损害了其他性能，例如抵抗来自重复加载或来自环境的化学影响的损坏的能力。最近，已经开发了克服这些限制的新概念。已经证明，在低温下适当地进行碳或氮的表面注入实际上显著地改善了所有合金性能，因为通过提供非常高浓度的碳/氮，同时抑制碳化物或氮化物颗粒的沉淀。然而，这使得合金处于非平衡状态，并且迄今为止几乎没有任何关于以这种方式处理的合金的耐久性，寿命和温度稳定性的信息。此外，通过在注入碳或氮之后对材料进行回火，似乎存在将非平衡状态转变为提供进一步改进的合金性能的状态的巨大潜力。凯斯西储大学将与英国伯明翰大学这一领域的另一个领导小组合作，对这些高度技术相关的方面进行基本的科学理解。这对表面工程结构合金的许多应用，如表面处理、表面处理和表面处理等都具有重要意义。G.用于轴承、食品加工刀片、阀门、模具、核反应堆部件、医疗植入物以及外科和牙科器械。该项目得到了金属和金属纳米结构项目以及材料研究部特别项目办公室的支持。

TECHNICAL SUMMARYCase Western Reserve University will collaborate with the University of Birmingham, UK, on novel methods of engineering the surface of structural alloys, including austenitic stainless steel, Ni-Cr, and Co-Cr alloys. Both groups have carried out extensive prior work, which revealed that infusing interstitial solutes - carbon or nitrogen - from the alloy surface at low temperature effectively suppresses the precipitation of second phases and enables the formation of an approximately 25 micrometer thick homogeneous, precipitate-free hard shell ("case") with carbon or nitrogen concentrations that can exceed the equilibrium solubility limit by about 100,000 times (!). Such colossal supersaturation with interstitial makes the surface up to 5 times harder, increases the wear resistance and the fatigue life by about 100 times, and elevates the corrosion resistance to that of the most corrosion-resistant alloys. The new project will focus on the hitherto unexplored reliability and lifetime of these unusual non-equilibrium materials at elevated temperature and under applied mechanical stress. A second thrust the project is to explore the possibility actually using controlled thermal post-processing for further improving the alloy surface properties and durability. Under the high driving force for precipitation it should be possible to generate an ultrafine dispersion of second-phase particles, which is expected to further improve hardness without compromising corrosion resistance. NON-TECHNICAL SUMMARYThe usefulness of austenitic stainless steel and related structural alloys for a broad variety of technical applications can be dramatically improved by exposing finished parts of these materials to a process that infuses carbon or nitrogen atoms into their surface. Such "case hardening" was already applied to arms in the middle age. However, while improving the surface hardness, the conventional way in which it was carried out it compromises other properties, e.g. the resistance against damage from repeated loading or chemical impact from the environment. Recently, new concepts have been developed that overcome these limitations. It has been demonstrated that appropriately conducted surface infusion of carbon or nitrogen at low temperature actually dramatically improves all alloy properties because by providing very high concentrations of carbon/nitrogen while suppressing the precipitation of carbide or nitride particles. However, this leaves the alloy in a non-equilibrium state, and hardly anything is known to date about the durability, lifetime, and temperature stability of alloys treated in this way. Moreover, a great potential seems to exist for transforming the non-equilibrium state into one providing further improved alloy properties by tempering the material after the infusion of carbon or nitrogen. Case Western Reserve University will collaborate with another leading group in this field at the University of Birmingham, UK, to develop a fundamental scientific understanding of these highly technologically relevant aspects. This is of great importance for many applications of surface-engineered structural alloys, e. g. for bearings, food processing blades, valves, dies, nuclear reactor components, medical implants, and surgical and dental instruments.The project is supported by the Metals and Metallic Nanostructures program and the Office of Special Programs, Division of Materials Research.