Precipitation Kinetics In Ni-Al Alloys Using Phase Field Simulation

Nickel-based superalloys are the important structural material for the hottest parts of aeroplane engines and industrial gas turbine blade because of their excellent fatigue resistance, toughness, high temperature creep strength, surface stability, oxidation and thermal corrosion resistance. The performances of nickel-based superalloys depend on the volume fraction, particle size and spatial distribution of the γ’ phase. Therefore, it’s crucial to study the precipitation kinetics of γ’ phase in nickel-based superalloys.Based on the CALPHAD thermodynamic database and sublattice model, this paper is focus on the precipitation process in Ni-Al alloys by using the Cahn-Hilliard equation and the Ginzburg-Landau equation, known as phase field simulation. The precipitation kinetics rules of Ni-Al alloy under specific temperature and composition, the effects of temperature and composition on precipitation kinetics rules of Ni-Al alloys and the precipitation kinetics of inverse Ni-Al alloys are investigated.The average particle radius of γ’ phase and the aging time shows an exponent relationship<r>∝(t*)n for the Ni-17.2 at.% Al alloy aged at 1073 K, with three obvious stages for n=1.35,0.42 and 0.22. Relatively, the average aspect ratio stays at about 1.03 at the initial stage, then it increases rapidly and keeps at 1.59 at last. The peak of the PSD decreases and the width of PSD increases as the aging progresses.With the increase of the aging temperature, the volume fraction of γ’ phase decreases, the coarsening rate increases, the peak value of PSD is shifted from r/<r> large than 1.0 to less than 1.0, the width of PSD increases, the peak of the PSD becomes low and the interfacial width increases; while as the Al composition increases, the volume fraction of γ’ phase increases, the coarsening rate decreases, the peak value of PSD moves from r/<r> less than 1.0 to large than 1.0 and it is close to the prediction of BW (Brailsford-Wynblatt) theory.The coarsening mechanisms, Ostwald ripening and coalescence of neighbouring precipitates, are also presented in the inverse Ni-Al alloys. For the inverse Ni-Al alloys aged at 1200 K for t*=1000 with Al composition cAl=20.2,20.4,20.6 and 20.8 at.%, the time exponent n for the four alloys are all interested equal to 0.21, namely<r>∝(t*)0.21, while the relationships between number density and time are (t*)-0.42, (t*)-0.35, (t*)-0.23 and (t*)-0.13, respectively. In addition, Al atom is easier to diffuse in y precipitates than in γ’ matrix.