Materials World Network: Stability of Colossally Supersaturated Structural Alloys

西储科技大学将与英国伯明翰大学合作，研究设计结构合金表面的新方法，包括奥氏体不锈钢、镍铬合金和钴铬合金。这两个小组都进行了广泛的前期工作，结果表明，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析出，并能够形成大约25微米厚的均匀、无沉淀物的硬壳(CASE)，其碳或氮浓度可超过平衡溶解极限约100,000倍(！)。这种含有间隙的巨大过饱和度使表面硬度增加5倍，耐磨性和疲劳寿命提高约100倍，耐腐蚀性提高到最耐腐蚀的合金的水平。新项目将重点研究这些不寻常的非平衡材料在高温和施加机械应力的情况下迄今尚未探索的可靠性和寿命。该项目的第二个推动力是探索实际使用受控热后处理的可能性，以进一步改善合金的表面性能和耐用性。在高的析出驱动力下，应该有可能产生第二相粒子的超细弥散，这有望在不影响耐腐蚀性的情况下进一步提高硬度。非技术总结通过将奥氏体不锈钢和相关结构合金的成品部分暴露在向其表面注入碳或氮原子的过程中，可以极大地提高这些材料在各种技术应用中的有用性。这种“案例硬化”在中世纪就已经应用于武器了。然而，在提高表面硬度的同时，传统的方法会损害其他性能，例如，抵抗重复加载或环境化学冲击造成的损坏。最近，新的概念已经被开发出来，以克服这些限制。已经证明，在低温下进行适当的表面碳或氮注入实际上显著改善了所有合金的性能，因为它提供了非常高的碳/氮浓度，同时抑制了碳化物或氮化物颗粒的析出。然而，这使得合金处于非平衡状态，而且到目前为止，关于以这种方式处理的合金的耐久性、寿命和温度稳定性几乎一无所知。此外，在注入碳或氮之后，通过回火材料，将非平衡状态转变为提供进一步改善合金性能的状态似乎存在巨大的潜力。凯斯西储大学将与英国伯明翰大学在这一领域的另一个领导小组合作，以发展对这些高度技术相关方面的基本科学理解。这对表面工程结构合金的许多应用非常重要，例如用于轴承、食品加工叶片、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计划以及材料研究部特殊计划办公室的支持。

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.