CO-SUPERALLOYS APT · Development of Next Generation Cobalt Superalloys - Understanding and Limiting Microstructure Degradation and Creep

Development of Next Generation Cobalt Superalloys - Understanding and Limiting Microstructure Degradation and Creep

MAIN RESULTS: The two primary goals of this project were to simultaneously (a) train the researcher in atom probe tomography (APT), and (b) develop the Co-based superalloys. ATOM PROBE TOMOGRAPHY: APT is the only technique offering extensive capabilities for both three-dimensional imaging and chemical composition measurements at the atomic scale. The use of APT has grown rapidly since the commercialisation of the manufacturer CAMECA's local electrode atom probe (LEAP) instruments, and there is now a shortage of well trained instrument scientists to work at APT facilities. The following two paragraphs illustrate that the researcher advanced the understanding of phase transformations in beta-Ti alloys while learning APT methodology, application and analysis. Firstly, the researched examined the microstructural evolution of the alloy Ti-24Nb-4Zr-8Sn wt.% during low-temperature ageing by APT and X-ray diffraction (XRD). The ageing was deleterious to the desirable mechanical properties, such as ultra-low elastic modulus and superelasticity. Initially, the cold-rolled alloy possessed a martensitic alpha"-precipitate/beta-matrix microstructure. On ageing, Ti-rich/solute-lean precipitates grew in linear arrangements, which were likely associated with dislocations. Additionally, the composition and number density of Nb-rich domains (which are associated with superelasticity) were quantified for the first time. The domains were unstable, decreasing in number density during ageing, causing the deterioration in mechanical properties. Secondly, APT was also performed on the metastable beta-Ti alloy Ti-5Al-5Mo-5V-3Cr wt.% (Ti-5553), aged at 300 °C for 0 – 8 h, to precipitate the isothermal omega phase. A dependency of precipitate quantification with instrument ion detection efficiency and laser pulse energy was found by comparing measurements from different generations of CAMECA LEAP instruments. Ultraviolet laser pulse energies above about 30 pJ resulted in significant complex molecular ion formation during field-evaporation, causing mass spectral peak overlaps that inherently complicate data analyses. Observation and accurate quantification of omega-phase under such conditions were difficult. The effect is minimized or eliminated by using smaller laser pulse energies. With a small laser pulse energy and high detection efficiency of the LEAP 5000 XS (77%), Ti-rich and solute depleted precipitates of isothermal omega phase with an oxygen enriched interface were observed as early as after 1 h aging time. This result corrected the archival literature, and the discrepancies were rationalized. The Al concentration in the matrix/precipitate interfacial region increased during aging. Nucleation of the alpha-phase at longer aging times may be facilitated by the O and Al enrichment at the matrix/precipitate interface (both strong alpha-stabilisers). The kinetics and compositional trajectory of omega-phase with aging time were quantified. CO BASED SUPERALLOYS: While the demand for increased engine efficiency has only increased, the development of Ni-based superalloys has stagnated lately. It is clear that their temperature capability is the fundamental limitation with regard to improving engine efficiency and reducing CO2 emissions. Co-based superalloys that possess gamma/gamma-prime precipitate-strengthened microstructures are the subject of extensive research and development as potential successors to the Ni-based superalloys used in the hot-sections of gas-turbine engines. A [h00] oriented Co-based superalloy single crystal was produced by the Bridgman furnace method. The alloy had a low W composition and high gamma-prime solvus temperature, marking progress in alloy development. Following heat-treatment, samples were crept under tension at 940 C/100 MPa, resulting in a P-type raft morphology with extensive particle coalescence along the [h00] loading direction. However, particle coalescence was also observed in two perpendicular directions on the (h00) plane, normal to the loading axis. Tensile creep experiments were performed with in-situ neutron diffraction at 800 C/500 MPa on this initially rafted gamma-prime microstructure, and for comparison at (i) 900 C/260 MPa, and at (ii) 750 C/875 MPa, both with initially cuboidal gamma-prime microstructures. The alloy was shown to exhibit a positive lattice parameter misfit, and during the first hour of creep at 900 C/260 MPa, the lattice parameter evolution indicated changes in phase composition associated with gamma-prime dissolution as the alloy achieved phase equilibrium at 900 C. For all three in-situ creep measurements, there was a significant divergence of the gamma-prime and gamma lattice parameters as creep proceeded. The lattice parameter misfit values between the precipitates and the matrix approached their unconstrained values during creep, and were notably large compared to those of Ni-based superalloys. This is indicative of a loss of coherency at the precipitate/matrix interfaces. Such a loss of coherency at the precipitate/matrix interfaces will likely degrade certain mechanical properties such as fatigue resistance, as has been shown for the Ni-based superalloys. CONCLUSIONS: The researcher was trained to such a high-level in APT during his fellowship that he could correct experimental and analysis errors in the metallurgy research community. In his work on beta-titanium alloys, he correlated the deterioration of mechanical properties with changes in microstructure. He contributed to the development of a Co-based superalloy with low tungsten additions. This gives a low density, a requirement of gas-turbine materials. With advanced diffraction analysis, he also identified that the precipitate and matrix of the alloy were losing coherency at their interface during gas-turbine conditions. This is a consequence of large crystal lattice mismatch between the two phases, and this alloy design flaw must be addressed in the next generation of alloy development. SOCIO-ECONOMIC IMPACT AND GROUPS WITH WHOM THIS RESEARCH MAY BE OF INTEREST: This research is of direct interest to those associated with the gas-turbine industry. In Europe, this is primarily Rolls-Royce plc. The driving force behind the research is to develop alloys that improve gas-turbine efficiency, which in turn will help European countries achieve their Paris agreement obligations to cut back CO2 emissions. The researcher identified the cause of the deterioration of beta-titanium alloys at elevated temperatures, which may guide the development of new alloys. He also contributed to reducing the density of Co-based superalloys, and identified that the mismatch in crystal lattice parameter size between precipitate and matrix needs to be reduced if these alloys are to find a place in gas-turbine engines. The research is on-going, but the potential of successful development of these alloys would be worth billions of euro to the aerospace industry. The publications of Dr. James Coakley can be found on google scholar: https://scholar.google.com/citations?user=Vg2xWVsAAAAJ&hl=en

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