In 2006, a new A1-L12 Co-Al-W ternary system, where the L12 - γ′ phase Co3(Al,W) coexists with the FCC Co matrix, was discovered. This new L12 phase has been found to exhibit the flow stress anomaly and is coherent with the matrix. It has been proposed that it can form the basis of a new generation of high temperature alloys superior to nickel base superalloys, the performance of which are reaching a plateau. The operating efficiency of jet engines ultimately depends on the temperature capability of the materials used in the turbine. Therefore, this possibility is of great interest to the gas turbine industry.
In this project, the influence of different alloying additions to Co-Al-W base alloys on the phase metallurgy has been investigated. Alloys were fabricated by vacuum arc melting and hot rolling. Mo, V and Fe lower the γ′ solvus temperature, whereas Ti and Ta are beneficial. Addition of Cr improves the oxidation behaviour. However, too much Cr destabilises the γ/γ′ microstructure. When a continuous phase field was established between Ni3Al and Co3(Al,W), it was found that increasing the Ni content in the Co-Al-W-Cr base alloy increases the γ′ solvus temperature and stabilises the γ/γ′ microstructure.
Alloying effects on oxidation behaviour were also studied. The oxide scale in the non-Cr containing alloys was not found to be protective and the thickness was in most cases greater than 100μm after 196 hours at 800◦C. It was found that a 10 at.% Cr addition is sufficient to modify the oxidation behaviour such that a three-layer oxide scale is formed. The outer layer is an Al2CoO4, spinel, the middle layer contains both CrO2 and Cr2O3, and the inner layer is Al2O3. These three layers with low oxygen diffusivity adhere well to both the substrate and each other and protect the alloy against high temperature oxidation.
Synchrotron and neutron diffraction experiments were performed on Co-7Al-5W-2Ta and Co- 6Al-6W-2Ti (at. %) alloys. The HRPD instrument at the ISIS time of flight neutron source was found to produce the most satisfactory results, with the fundamental and superlattice peaks being easily separable. The new I12 diffractometer at the Diamond synchrotron X-ray source had poorer resolution. Thus, whilst more data was obtained, the results were less reliable. The new Vulcan diffractometer at the Spallation Neutron Source at the Oak Ridge National Laboratory also suffered from poor resolution, despite the high neutron flux that was available. The latter two instruments, however, had the benefit of being able to perform in-situ loading experiments. Hence, it was possible to assess the elastic behaviour of the two phases at temperature. It was found that the -Ti containing alloy has a lower lattice misfit than the -Ta alloy, and that the misfit values increase with temperature. The room temperature misfit is large and positive (∼ 0.5 %), in contrast to the small negative misfit observed in nickel superalloys. It was found that the Co3(Al,W) phase is less stiff than the Co matrix in all orientations, this is an undesired feature which means that load shedding will occur to the γ phase.