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Periodic Report Summary 1 - CO-SUPERALLOYS APT (Development of Next Generation Cobalt Superalloys - Understanding and Limiting Microstructure Degradation and Creep)

The original project had two primary objectives: a) To contribute to the scientific knowledge of the recently discovered Co-based superalloy system. These alloys are a potential successor to the now mature Ni-based superalloys that are used in gas-turbine engine hot-sections. A successful alloy development that can run at higher operating temperatures within a gas-turbine is of particular importance to the European Union, as it reduces the emission of environmentally harmful gases such as CO2 and NOx. Secondly it will improve the competitiveness of the European based aerospace industry, allowing for industry growth and creation of further employment; b) The second project objective is to further develop the abilities of the primary researcher, Dr. James Coakley, to the point where he can establish a European based world-leading research group. Beyond the increased responsibility of project management, this focused on training the researcher in atom-probe tomography (APT) at the world leading Northwestern University center of atom-probe tomography (NUCAPT). It is clear the European Union will require more highly trained APT scientists as the technique continues to experience rapid growth, with the author being aware of three new APT instruments recently or currently being installed at European Institutions (Oxford University, FAU Erlangen Nurnberg, and the Max-Planck-Institut).

The cobalt superalloys are precipitate strengthened by the same microstructure as the nickel superalloys, with a coherent ordered L12 (γ’) phase within a fcc (γ) matrix. One significant difference is that the cobalt superalloys are of positive lattice parameter misfit, meaning the γ’ phase lattice parameter is larger than the γ phase. Typically, the opposite is true of the nickel superalloys. During high temperature deformation, as is experienced under gas-turbine operating conditions, the precipitates become aligned and coalesce, and the direction of coalescence is dependent on both the stress applied (compressive vs. tensile), and whether the misfit is positive or negative. The researcher contributed to the development of a high-solvus Co-based superalloy, and illustrated that the coalescence forms parallel to the tensile loading direction. This is significant, as it results in a microstructure analogous to a fiber reinforced composite. The study of coallesence is currently under review at Acta Materialia. It is predicted that this will improve mechanical properties due to load shedding from the matrix to the precipitate. In-situ neutron diffraction experiments have been performed at Oak Ridge National Laboratory on Co-based superalloy single crystals to investigate this, and the data is currently under analysis.

A shortcoming to date of the Co-based alloys is the high level of W content within the alloy, typically about 20 wt.%. This results in a very high density alloy, which is of significant concern for a high stress rotating within a gas-turbine. The researcher illustrated that W-free alloys have similar creep resistance to a high W containing alloy, however removing the W from the alloy significantly decreased the alloy solvus temperature. These findings are currently under review with Metallurgical and Materials Transactions A.

In order to learn atom probe tomography, the author performed APT measurements on beta-Ti alloys, as part of an ongoing collaboration with Northwestern University and Imperial College London. On quenching, and subsequent low temperature ageing, the beta-Ti alloys form metastable phases that are detrimental to mechanical properties. This limit the alloy applications in industry. Understanding of these phase transformations, and subsequent alloy development to limit precipitate formation, would result in alloys that are particularly desirable to the gas-turbine industry, due to unique properties such as energy absorption in the elastic loading unloading cycle. The precipitates that form are about 5nm in diameter, and thus are ideal for atom probe tomography investigations. With this alloy system, the researcher was taught how to optimise experimental conditions for APT, perform the mass-spectra analysis, and extract the quantitative data of interest from the sample reconstruction. Four APT papers have been published thus far in Acta Materialia, Scripta Materialia, Journal of Alloys and Compounds, and Materials Science & Engineering A, focused on Ti alloys, with three more under review at Microscopy & Microanalysis, Philosophical Magazine Letters, and Scripta Materialia. Two important findings were illustrating that isothermal omega phase forms within one hour of heat-treatment at 300oC in the commercially used Ti-5Al-5Mo-5V-3Cr wt.% alloy that are Ti rich and depleted in all solute additions. This results in the alloy becoming inherently brittle. A second finding was that heat-treatment removes Nb rich embryos, corresponding to a loss of superelasticity and energy absorption in the alloy Ti-24Nb-4Zr-8Sn wt.%. These findings are of particular benefit to European industry, as Ti-5Al-5Mo-5V-3Cr wt.% alloy is used in aircraft forgings, and Ti-24Nb-4Zr-8Sn wt.% is being developed for biomedical applications, with the gas-turbine seeking to find an alloy with the same mechanical properties, but the alloy must be stable.

In the final year, the researcher will continue to study directional coallesence of γ’ precipitates in Co-based superalloys. The research will illustrate if the microstructure that is analogous to fiber-reinforced composities is superior to the currently employed uniformly distributed precipitates, which is of particular relevance to Rolls-Royce UK, amongst others. An improved microstructure may allow for lower mass in aircraft gas-turbine engines, lowering the weight of the aircraft and thereby the amount of fuel used per flight, thereby reducing undesirable greenhouse gas emissions.

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