Effect of systematic variation in Co:Ni ratio on elemental partitioning, lattice misfit and oxidation behaviour of γ'-strengthened (Ni,Co)75Al5Ti5Cr15 (at.%) alloys
Ni-based superalloys currently in-service as aero-engine turbine discs are operating at the limit of their high-temperature capability. Next-generation turbine disc superalloys must be capable of sustaining the higher operating temperatures (approaching 800°C) and stresses required for greater thermal efficiency, whilst maintaining mechanical integrity and excellent oxidation resistance. Studies on the emerging γ'-strengthened Co-based superalloys based on the Co-Al-W and Co-Al-Mo-Nb/Ta systems have been successful in mitigating the respective drawbacks of high mass density and low γ' solvus temperature by incorporation of Ni, Ti or Cr. However, systematic investigation of the fundamental quinary system, Ni-Co-Al-Ti-Cr, is essential to provide a baseline against which to compare the influence of higher-order alloying with elements, such as W, that potentially confer superior high-temperature properties. To this end, seven γ'-strengthened model superalloys of composition (Ni,Co)75Al5Ti5Cr15 (at.%) were fabricated, aged at 800°C for 1000 hours, and the effect of increasing Co:Ni ratio on their resultant microstructural characteristics and critical properties was determined. For alloys of Co content 0, 9, 19, 28, 38, 47 and 56 at.%, γ' solvus temperature decreased with increasing Co:Ni ratio, whilst alloy mass density remained fairly constant across the alloy series at ~8.1 g.cm-3. Atom probe tomography (APT) was used to determine the elemental partitioning behaviour and γ' volume fraction for each of the seven alloys, denoted 0Co, 9Co, 19Co, 28Co, 38Co, 47Co and 56Co. From APT, a non-monotonic trend in Ti, Al and Cr partitioning was observed as Co:Ni ratio increased, with a transition at ~19 at.% Co. From neutron diffraction analysis, the γ-γ' lattice misfits of alloys 0-47Co were found to be positive at all test temperatures (ambient, 400, 600, 700 and 800°C) and the magnitude of lattice misfit increased as Co content increased from 0-38 at.%. Lattice misfit values decreased with increasing temperature, the lowest values being obtained for 800°C. Crucially, this study also investigated the effect of a systematic increase in Co:Ni ratio on oxidation performance of γ-γʹ alloys of composition (Ni,Co)75Al5Ti5Cr15 (at.%). The seven model superalloys (0Co, 9Co, 19Co, 28Co, 38Co, 47Co and 56Co) underwent isothermal oxidation in air at 800°C using both thermogravimetric analysis (100 hours) and box-furnace exposure (1000 hours). X-ray diffraction was carried out to identify surface oxide phases present. Scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS) and secondary ion mass spectrometry were performed to map elemental distribution and to provide precise chemical analysis of any thin surface oxide layers present. Following 1000 hours oxidation at 800°C, the 28Co, 38Co and 47Co alloys were observed to exhibit a flat, compact, well-defined external scale and Cr2O3 layer with minimal oxygen ingress, together with a distinct Al2O3 sub-scale. This work represents the first detailed systematic study on the fundamental quinary Ni-Co-Al-Ti-Cr alloy system describing the effect of variation in Co:Ni ratio on the evolution of surface oxides and on the partitioning of elements to γ and γ' phases and associated lattice misfit.
Nickel, Cobalt, Superalloys, Systematic, Elemental partitioning, Atom probe tomography, Lattice misfit, Neutron diffraction, Oxidation
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