Nucleation in a Concentration Gradient

Predicting the Rate of Homogeneous Intermetallic Nucleation within Steep Composition Gradients

“…While the absence of a nucleation event could arise simply because of the limited time of the observation window, we believe it arises because of the existence of a limiting compositional gradient above which nucleation is not possible, as we will explain next. Therefore, we suggest that the critical gradients (∇c*) are 0.0375 Å −1 for 900 K and 0.025 Å −1 for 1200 K, and the critical widths (d react * ) are 8 Å for 900 K and 12 Å for 1200 K. Based on the studies of Desréand co-workers, 16,17 the suppression of nucleation is associated with an increase of the nucleation barrier. With a nonzero gradient, ∇c, the nucleus will adopt an approximately spheroidal shape with a rotational symmetry about the gradient direction, as shown in Figure 1.…”

“…Nucleation occurs near the composition Ni50. The composition profile within the crystal phase is statistically the same as that in the amorphous phase, indicating that the nucleation proceeds in polymorphous mode, 17 which is a result of the structural transformation proceeding faster than the long-range diffusion. At low temperatures, for example, 900 K, the structural transformation is expected to be faster because of the larger phase transition driving force and the decrease in the interdiffusion rate compared to 1200 K. We conclude that the nucleation mode is polymorphous for the two temperatures studied in this work.…”

“…Two different mechanisms have been proposed to explain the above-mentioned phase formation behavior at interfaces, focusing respectively on the growth and nucleation of the new phase. − The growth-controlled mechanism , is likely more suitable for interfaces where nucleation occurs heterogeneously, as also demonstrated in our prior MD simulation . The nucleation-controlled mechanism , is more applicable for systems with amorphous interfaces where the rate-limiting step of the reactions may be a homogeneous nucleation process.…”

“…14−19 The growth-controlled mechanism 14,15 is likely more suitable for interfaces where nucleation occurs heterogeneously, as also demonstrated in our prior MD simulation. 20 The nucleation-controlled mechanism 16,17 is more applicable for systems with amorphous interfaces where the rate-limiting step of the reactions may be a homogeneous nucleation process.…”

“…According to Desré and co-workers, , the composition gradient affects the homogeneous nucleation rate by altering the stability of the crystal nuclei. Their theory predicts that nucleation could be completely suppressed above a certain critical gradient.…”