Kinetics of precipitation in new generation of cobalt‐based superalloys

Abstract

Superalloys are key materials in aircraft engines and power generation systems. They are subjected to very high stress under temperatures in the range of 700‐1000°C. Currently Ni‐based superalloys are the most widely used materials for high temperature applications. These alloys owe their excellent mechanical proprieties to their microstructure characterized by the presence of a high volume fraction (up to 70%) of thermodynamically stable, coherent L1 2 ordered γ' precipitates (Ni 3 Al or Ni 3 Nb type) embedded in a disordered fcc γ matrix. Compared to Ni based superalloys, conventional Co based alloys exhibit hot corrosion, oxidation and wear resistance but their applications are restricted to temperatures below 750°C due to their instability at high temperatures. In 2006 J. Sato et al. [1] discovered a new stable L1 2 ordered, Co 3 (Al,W) phase embedded in the disordered γ‐Co solid‐solution matrix. Mechanical properties (creep and plastic deformation, elastic property, structural stability) of Co‐based alloy have been widely investigated but the precipitation process have been the subject of very few studies [2,3]. The aim of this work is to study precipitation kinetics in model CoAlW superalloys at the atomic scale. This work is focused on the kinetics transformation paths during precipitation. The temporal evolution of average size, volume fraction and number density of γ' precipitates as well as that of phase composition has been studied as a function of aging time at 900°C employing three dimensional Atom Probe Tomography (APT). In addition Transmission and Scanning Electron Microscopy (TEM, SEM) have been used to complement APT studies. Different alloy compositions will be studied in this work, figures 1 and 2 show respectively bright field transmission electron micrographs of Co‐9.7Al‐10.8W (atomic %) alloy and dark field image of Co‐9.1Al‐7W alloy after annealing at 900°C for 10h. The microstructures reveal precipitates of cuboidal shape, respectively 100 nm (Fig 1) and 50 nm (Fig 2) in size, homogeneously distributed in the parent γ phase. The γ and γ' phase compositions are major parameters controlling the properties of superalloys, that have been measured by APT. Figure 2 shows 3D reconstruction in the Co‐7Al‐9W aged at 900°C for 10h, showing γ' precipitates delineated by a 12 at.% W isoconcentration surface. Composition profile across γ/γ' interface reveals that W shows a very strong tendency to partition to the γ' phase unlike Al that exhibit a very weak tendency to partition between γ and γ' phases. The summation of Al content and W content in the γ' phase is close to 25 atomic % that is to the expected stoichiometry of the γ' Co 3 (Al,W) ordered phase. It can this suggests that Al and W occupy preferentially the corner sites of the same sub‐lattice in the ordered L1 2 structure. This presentation will come in more details on the temporal evolution of both the microstructure and phase composition.