We perform molecular dynamics simulations of directional growth of a binary alloy. We fix the temperature gradient, pulling speed, impurity concentration and only vary the impurity segregation coefficient. By changing the range of the Lennard Jones potential of impurity atoms as compared to the range of the potential of solvent atoms, the elastic energy cost causes a decreases of solubility of the impurity atoms into the matrix (solvent) solid phase and consequently a decrease in the impurity segregation coefficient. Within certain range of segregation coefficients, the growing interface is planar; below it, the interface becomes unstable and a cellular structure emerges.