Barrier-Free Nucleation at Grain-Boundary Triple Junctions During Solid-State Phase Transformations

Molecular dynamics simulations are used to provide strong evidence for barrier-free nucleation events in a heterogeneous solid-solid system. The barrier-free events are characterized by an absence of an incubation time and a growth rate of the emerging phase that is independent of the system size. Furthermore, an analysis of the size and shape of the critical nucleus using the Winterbottom construction indicates that no solution exists for these barrier-free cases. We propose that barrier-free nucleation, which will have a profound effect on phase transformation kinetics, may be a general phenomenon for any polycrystalline material.

Figure 1

Barrier-free nucleation at a triple GB junction. (a)–(e) Progress of nucleus $B_1$, atoms distinguished by CN, green ($\gamma$), orange ($\alpha$), yellow (GB), and blue (defect). (f) bcc atomic ratio ($f_{\alpha }$) vs time for nucleus $B_1$ for an $NVT$ ensemble (blue line with circles), an $NPT$ ensemble (black), in a fully 3D environment (magenta), and isolated in the dynamic region (green).

Figure 2

Classical nucleation processes at triple GB junctions. (a) A quasi-2D system ($z=1.2\mathrm{nm}$) of nucleus $B_4$. (b) Image of a critical nucleus. (c) bcc atomic ratio of nucleus $B_4$ ($Z\times 1$) and twice the $z$ thickness ($z=2.4\mathrm{nm}$) ($Z\times 2$). (d) and (e) show the nucleus $B_5$ that forms in a 3D system ($z=9\mathrm{nm}$). (f) bcc atomic ratio of nucleus $B_5$ run in an $NVT$ ensemble (green lines) and in an $NPT$ ensemble (magenta lines).

Figure 3

(a) Winterbottom construction for nucleus $B_1$. A perfect wetting condition occurs at the GB junctions (inset). (b) Analysis of the uncertainty in the nucleation energies (see the text).

Figure 4

(a) The free energy analysis of nucleus $B_4$, with a calculated nucleation energy barrier 3.6 eV and critical size $3.0{\mathrm{nm}}^2$. (b) Comparison of the model results (solid and dashed lines) with MD observation (colored points, distinguished by potential energy). The critical size measured from the MD simulation was $2.9\pm 0.4{\mathrm{nm}}^2$.