Deviation From Local Equilibrium During the Austenite to Ferrite Transformation in Steel-A Modelling Approach

University dissertation from Stockholm : Materialvetenskap

Abstract: This thesis highlights the role of phase interfaces on phasetransformations in metallic materials. The deviation from localequilibrium at the moving phase interface has been analysed interms of solute drag theory and finite interface mobility. Inparticular the planar growth of proeutectoid ferrite fromaustenite in steel has been studied. The deviation from localequilibrium is caused by dissipation of Gibbs energy bydiffusion inside the phase interface and interface friction. Inthe analysis the interface is divided into three zones and thethermodynamic and kinetic properties are assumed to varycontinuously across the interface. A new model suitable formulticomponent alloys is developed. The model reduces to thefamiliar solute drag model by Cahn under simplifyingconditions. It was demonstrated how the interface model couldbe combined with a method for calculating the volume diffusionin both the growing and parent phases. With this combination ofprocedures the changes in local conditions at the interface, asthe growth rate changes due to long-range diffusion, could bedemonstrated for the case of continuous cooling in an Fe-Nialloy.The critical limit for massive transformation in the Fe-Niand Fe-C systems was calculated and found to lie well below theT0 line for both systems. The calculated limit for Fe-Ni wascompared with a recent experimental study and reasonableagreement was found. For the Fe-C system the limit calculatedwith the present model was compared with a phase-field model.The two approaches showed qualitatively the same behaviour andthe quantitative difference was due to different assumptions onhow properties vary across the interface.Finally, an attempt to simulate the partitionless growth offerrite in austenite in the Fe-Ni-C system was performed. Inthe applied model the dissipation of Gibbs energy inside theinterface and in the nickel spike ahead of the migratinginterface were accounted for. The long-range diffusion ofcarbon in austenite was treated with an approximate analyticalgrowth equation. A continuous change from paraequilibriumconditions and quasi-paraconditions could be shown in anisothermal section of the Fe-Ni-C phase diagram. Partitionlessgrowth starts in a parabolic fashion but slows down. For alloysoutside the limit for quasiparaconditions partitionless growthis predicted to stop abruptly while for alloys inside thatlimit growth approaches a second parabolic growth law. However,the latter case should not be expected in practise because ofimpingement effects.