Materials World Network: Stability of Colossally Supersaturated Structural Alloys

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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.