Dislocation assisted phase separation: A phase field study

In this paper, we show the influence of dislocations on phase separation in alloys with a miscibility gap. Specifically, solute segregation to dislocations leads to spinodal phase separation even in systems with nominal compositions outside the spinodal limit. Further, when spinodal decomposition occurs along the dislocation cores, geometrically, the evolution is like that of a perturbed cylinder. Thus, unless the faster atomic mobility along the dislocation overcomes the bulk diffusion flux, the spinodal is transient and the fluctuations die down as time proceeds due to the bulk flux. We have summarised this interaction between the chemical instability and the cylindrical geometry of the instability in the form of mechanism maps. Our simulations also show the surprising phenomena of phase separation through both (i) nucleation and growth, and, (ii) spinodal decomposition, at the same time – for the case of intersecting dislocations. Finally, the prominent role played by dislocation and solute strain fields in influencing the morphology of precipitates is explored. We show that the range of parameter values chosen by us are of relevance to real alloys, and, some of the microstrucural features and morphologies agree qualitatively with recent Atom Probe Tomography (APT) results in iron based systems.