Deviations from cooperative growth mode during eutectoid transformation: insights from phase-field approach

TL;DR

This study uses a phase-field model to simulate and analyze the transition from lamellar to divorced eutectoid morphology in Fe-C alloys, revealing the influence of inter-particle spacing and undercooling on transformation mechanisms.

Contribution

It provides a detailed numerical simulation of non-cooperative eutectoid transformation, incorporating thermodynamic data and constructing a comprehensive LDC map.

Findings

Transition from lamellar to divorced eutectoid depends on inter-particle spacing and undercooling.

Identifies a concurrent growth and coarsening regime at small spacing and low undercooling.

Provides insights into the physics of complex transformation pathways.

Abstract

The non-cooperative eutectoid transformation relies on the presence of pre-existing cementite particles in the parent austenitic phase and yields a product, popularly known as the divorced eutectoid. In isothermal conditions, two of the important parameters, which influence the transformation mechanism and determine the final morphology are undercooling (below A1 temperature) and inter-particle spacing. Although, the criteria which governs the morphological transition from lamellar to divorced is experimentally well established, numerical studies that give a detailed exposition of the non-cooperative transformation mechanism, have not been reported extensively. In the present work, we employ a multiphase-field model, that uses the thermodynamic information from the CALPHAD database, to numerically simulate the pulling-away of the advancing ferrite-austenite interface from cementite, which results in a transition from lamellar to divorced eutectoid morphology in Fe-C alloy. We also identify the onset of a concurrent growth and coarsening regime at small inter-particle spacing and low undercooling. We analyze the simulation results to unravel the essential physics behind this complex spacial and temporal evolution pathway and amend the existing criteria by constructing a Lamellar-Divorced-Coarsening (LDC) map.