Final Report Summary - LEAP TOMOGRAPHY (Exploring the three-dimensional nanoscale space around defects in Ni-based superalloys for aircraft applications employing atom-probe tomography) Project context and conclusionsOne of the most efficient energy conversion devices is the turbine engine, which is utilised in either aeronautical jet engines or natural-gas fired land-based electrical power generators; a single unit can produce up to 500 megawatts. Owing to their excellent high-temperature strength as well as creep and oxidation resistance, nickel-based superalloys are the ideal materials for turbine blade applications. In order to increase the thermodynamic efficiency of the turbines, i.e. to obtain a high ratio of energy yield to fuel consumption, higher working temperatures are required (>1 200 ºC). Elevating the service temperature of a turbine engine implies improving the high-temperature properties of these superalloys. A major factor limiting the operating temperatures is, however, the formation of chains of misorientated grains, called 'freckles', on the surface of the single-crystal turbine blades during their solidification. Freckles introduce internal interfaces and serve as nucleation sites for micro-cracks as well as short-circuit diffusion paths, thereby reducing creep resistance. Therefore, the prime technological challenge is to eliminate the formation of freckles. This goal can only be achieved by completely characterising the alloy's crystallography, morphology, and composition at the micrometre to nanometre length scales.Work performedRecent technological instrumentation developments enabled us to propose an innovative and novel approach to investigate this problem. To directly access tiny (10-100 µm) regions of interest (ROIs) in the freckles vicinity we utilised dual-beam focused ion-beam (FIB) microscopy, and prepared specimens for further analysis. To quantify the concentration of elements and their phase partitioning in these ROIs, thereby to determine their role in freckle formation, we employed 3-D local-electrode atom-probe tomography (APT), which provided us with both superior mass-resolution and detectability in the level of <100 atomic parts per million (ppm) and excellent spatial resolution (<0.5 nanometre or nm). APT is based on field-evaporation of individual atoms, using picosecond laser pulses, from a sharply pointed specimen, which has a 30-50 nm radius of curvature. This enabled us to collect and identify the evaporating ions with time-of-flight mass spectrometry, together with their individual x-, y- and z- coordinates in 3-D direct space, thus producing a 3-D tomographic display of both morphology and composition with subnanometre spatial resolution.We investigated a multicomponent Ni-based alloy, ME-15, by employing atom-probe tomography (APT) combined with energy dispersive spectroscopy (EDS) to track the solid/liquid and the ?(f.c.c.)/?'(L12) partitioning behaviours of its different elements. The directionally-solidified ME-15 alloy exhibits a two-phase (? + ?'), single-crystalline (SC) microstructure with freckles. We performed EDS composition analyses in the following four ROIs: the SC dendritic cores (SC-DC); inter-dendritic regions (SC-ID); the freckle dendritic cores (F-DC); and inter-dendritic regions (F-ID).Main resultsThe hierarchical microstructure of the ME-15 alloy consists of: a. misorientated freckles with a length scale of 0.1-1 mm diam. embedded in the SC; b, dendritic cores and inter-dendritic regions, having a mean periodicity of about 50 µm, are present in both the SC and freckles, and c. these four ROIs contain ?-channels among ?'-precipitates with an average diameter 0.1-1 nm.We utilised a unique application of the lever-rule methodology for quantifying the elemental concentrations from relatively large regions with length scales of 10 µm to 1 mm, which is typical for energy dispersive X-ray spectroscopy (EDS), together with the superior mass-resolving power and detectability of APT. For the first time, we quantified the concentrations of the elements Ni, Al, Cr, Co, Ta, Mo, W, Re, Hf, Fe and B in the four ROIs.We found that the solid/liquid re-distribution of elements correlates with their ?/?' partitioning. The elements that prefer the solid-phase are the same ones that partition to the ?-phase. The inverse is true for the elements partitioning to the liquid phase as they partition to the ?'-phase.Based on the calculated compositions of the four ROIs, we performed computational thermodynamic simulations at these ROIs using Thermo-Calc. The calculated liquidus temperatures for the four ROIs are 1 663, 1 631, 1 681, and 1 638 K.We utilised the compositions measured for the four ROIs and the ROIs' calculated liquidus temperatures to estimate the densities of the multicomponent liquids that have the same compositions as the four ROIs. The densities calculated for ROIs SC-DC, SC-ID, F-DC and F-ID are 7.98±0.20 6.86±0.15 9.01±0.49 and 6.39±0.18 g/cm3, respectively; for the nominal composition we obtained an average density of 6.83 g/cm3. This yields a relative difference of ??/?o = 16.55 % between the densities of the ROIs SC-DC and SC-ID, which serves as the driving force for freckle formation, implying a high probability of freckle formation.In this research we applied, for the first time, a combination of highly-sophisticated techniques to quantify elemental concentrations in freckles and their immediate vicinity with great accuracy. The results obtained in this research promote our knowledge of how to precisely predict and control the formation of freckles in nickel-based superalloys. This will help the aeronautical industry to increase the efficiency of turbine engines and also save energy and production costs. Related documents Final Report - LEAP TOMOGRAPHY (Exploring the three-dimensional nanoscale space around defects in Ni-based superalloys for aircraft applications employing atom-probe tomography)
