Recent solidification experiments identified an oscillatory growth
instability during directional solidification of Ni-based superalloy CMSX4
under a given range of cooling rates. From a modeling perspective, the
quantitative simulation of dendritic growth under convective conditions remains
challenging, due to the multiple length scales involved. Using the dendritic
needle network (DNN) model, coupled with an efficient Navier-Stokes solver, we
reproduced the buoyancy-induced growth oscillations observed in CMSX4
directional solidification. These previous results have shown that, for a given
alloy and temperature gradient, oscillations occur in a narrow range of cooling
rates (or pulling velocity, Vpβ) and that the selected primary dendrite arm
spacing (Ξ) plays a crucial role in the activation of the flow leading
to oscillations. Here, we show that the oscillatory behavior may be generalized
to other binary alloys within an appropriate range of (Vpβ,Ξ) by
reproducing it for an Al-4at.%Cu alloy. We perform a mapping of oscillatory
states as a function of Vpβ and Ξ, and identify the regions of
occurrence of different behaviors (e.g., sustained or damped oscillations) and
their effect on the oscillation characteristics. Our results suggest a minimum
of Vpβ for the occurrence of oscillations and confirm the correlation between
the oscillation type (namely: damped, sustained, or noisy) with the ratio of
average fluid velocity V over Vpβ. We describe the different
observed growth regimes and highlight similarities and contrasts with our
previous results for a CMSX4 alloy