Stability of Colossally Supersaturated Alloys

The work our groups have carried out on alloys exposed to recently developed processes for low-temperature carburising and nitriding revealed that extraordinarily high concentrations of carbon and nitrogen can be dissolved in Fe-Cr-Ni alloys (austenitic stainless steel). In this process, carbon or nitrogen diffuses into the alloy from the surface at a temperature sufficiently low to suppress the precipitation of carbides or nitrides. This enables the formation of a homogeneous, precipitate-free, superhard " S-phase" consisting with carbon or nitrogen concentrations that exceed the equilibrium solubility limit by about one-hundred-thousand times!

To date, our work has focused on the underlying physical principles of this "colossal supersaturation" (CSS) and the mechanical properties and corrosion resistance of the as-processed 25-50 micron -thick hardened layer provided by the interstitial solute ("case") below the surface. The work we propose here constitutes a first effort to investigate the reliability and lifetime of this unusual material at elevated temperature and under applied mechanical stress and to further process it by tempering - i. e. exposure to heat treatments. Moreover, we propose to extend the research to Ni-Cr and Co-Cr alloys, for which hardening by colossal supersaturation has great technological importance.

Alloys with a colossal supersaturation of interstitial solute (carbon, nitrogen) constitute unique model systems for fundamental studies of decomposition and precipitation phenomena in metals under unusual conditions. In particular, it will enable understanding the physical principles of carbide and nitride nucleation, growth, and ripening under extremely high supersaturation and how the decomposition affects the remarkable mechanical properties and outstanding corrosion resistance initially provided by CSS. This provides a high potential for "transformative" discoveries. The largely complementary expertise, experience, and instrumentation of the two involved research group will enable significant progress in this important field of surface engineering.

This project will lay the foundation for long-term cooperation of two leading groups in the field of surface engineering. The physical understanding of the decomposition of CSS-hardened alloys under applied temperature and stress we intend to obtain is of great importance for many applications of structural alloys, e. g. bearings, food processing blades, valves, bushings, dies, nuclear reactor components, medical implants and surgical and dental instruments. The increase of reliability and lifetime we hope to enable by understanding their physical foundations will lead to major savings of primary energy resources and conservation of strategic raw materials by avoiding unnecessary production and recycling of alloy parts. The availability of super-hard wear-and corrosion-resistant alloys will strengthen US and UK industry by enabling the design of new products, which can contribute to the creation of new jobs. The long-life body implants can improve the quality of life of patients and reduce UK/US health service costs. The results of the proposed research are highly likely to generate new intellectual property.

The project will have significant impact on student education and training at CWRU and UoB. The results will directly impact the theoretical and practical parts of materials science courses at both universities. Further, the project will enable significant practical participation of undergraduate students and will therefore help to attract excellent undergraduate students to our institutions. The PIs have a record in engaging undergraduate students and women students and in exchanging research students and postdoctoral research associates in their research activities.

The proposed collaborative research will systematically investigate the stability of S-phase formed in para-equilibrium hardened corrosion-resistant alloys under thermal (temperature), mechanical (stress) and both thermal and mechanical conditions via both theoretical and experimental approaches, thus advancing scientific understanding of S-phase and facilitating the safe use of the S-phase surface engineering technology. The impacts forecasted from the project are four-fold:

(1) Scientific impact - The materials research community
This US/UK collaborative research is expected to produce significant scientific impacts mainly through greatly advancing scientific understanding of the metastability of S-phase, which will significantly enrich materials science knowledge and physical metallurgy and surface engineering in particular. Other researchers will benefit from better understanding of the metastability of S-phase through developing new alloys and/or new para-equilibrium hardening processes to generate more stable S-phase. PhD students and post-doctorial research fellows will benefit from being engaged in world-leading S-phase research.

(2) Technological benefits - Engineering component and medical device designers
The stability maps and modelling tools developed from this research can provide engineering components and medical device designers with essential tools for the design and manufacture of reliable, high-performance and long-life engineering components (such as for nuclear reactors, for the chemical and petrochemical processing and for food equipment) as well as medical devices (such as joint prostheses and bone fixation devices).

(3) Economic benefits - UK P.L.C. & NHS
UK surface engineering and many other many industrial sectors (especially the nuclear reactor, chemical, petrochemical and food equipment, and medical device sectors) will benefit from the reliable S-phase treated components with high performance and prolonged life-span through developing product market, enhancing global competitiveness and increasing wealth creation. The NHS will also benefit greatly from high performance, long-life medical devices (cost-saving).

(4) Societal & ecological impact - Aging population and everyone
The aging population will benefit most from the provision of joint replacements with improved durability and reduced risk of corrosion-induced premature failure of S-phase surface engineered joints prostheses. Equally, reliable surface treated components for nuclear reactors and for chemical and petrochemical processing equipment can avoid potential life and environment disaster. In addition, prolonged life of corrosion resistant components can reduce the consumption of strategic elements (such as Cr and Ni) and energy.