I'm modelling bainite sheaf growth in a simple Fe-C system using coupling with ThermoCalc. The simulations begin with finite radii (see images below). I have two questions relating to my outputs and the underlying phase-field approach. My driving file is attached for reference. You'll see that I have removed driving force averaging in the interface; I did this to compare interface equilibrium concentrations of carbon in austenite (c1pha1) with my calculated common-tangent amount from ThermoCalc, and compare maximum chemical driving forces with my calculated amount based on parallel tangent displacement at the bulk steel composition (294 J/cm3 at 0.4 wt% carbon). Since averaging is removed, these maximum driving forces are observed, in my simulations, at the outer edge of the interface, where carbon is the bulk amount (0.4 wt%). My two questions:
(1) Why are equilibrium carbon contents not always produced in the interface? I've found that this depends largely on C diffusivity in austenite and the capillarity (so interfacial energy and tip radius). I'm guessing this is due to the quasi-equilibrium constraint, which only imposes equal diffusion potentials and mass balance across the interface.
(2) Why does the chemical driving force as outputted depend on capillarity? The attached image shows two simulations that differ only in terms of initial ferrite radius (0.4 um and 0.2 um); the driving file is for the 0.2 um run. In the right panels, both the curvature and chemical driving force are higher (more blue cells). As a result, the smaller ferrite grows more quickly (since V = MG) and eventually propagates with a steady-state radius (see the other simulation snapshot). The larger ferrite slows to a stop before the steady-state tip forms; in this simple diffusional scenario, nothing should stop growth as long as there is austenite ahead of the tip with 0.4 wt% carbon.
Thanks in advance for your help
,Nick