An Atomistic Simulation Of The Interphase Boundary Migration And The Dislocation-interphase Boundary Interaction In FCC/BCC Systems

Many metallic structural materials are strengthened by the multi-phase microstructure through phase transformation.Therefore,the comprehensive theory of the phase transformation microstructure evolution and the interlink between phase transformation microstructure and materials' mechanical properties is a fundamental basis for a knowledge-based material design,which requires a deep understanding of interphase boundary(IPB)migration and interactions between dislocation and IPB.However,up to now the related knowledge regarding the above two topics is still rather limited.In the present work,interphase boundary migration and dislocation-IPB interaction in FCC/BCC systems will be systematically investigated via the molecular dynamics simulations.The migration of individual IPBs in a pure iron system are simulated.It was found that the IPB migration proceeds by the gliding of interfacial dislocations on their own slip planes.There are two different modes for IPB migration,i.e.,uniform migration and non-uniform migration,which depends on whether strong interactions of interfacial dislocation(e.g.,reactions or tangling)occurs.If there are strong interactions between interfacial dislocations,then the IPB migrate non-uniformly,otherwise it can uniformly migrate.Shear-coupled interface migration was observed and quantitatively analyzed,which can provide insights for the interpretation of the surface relief effect.Meanwhile an analytic expression for the atomic displacements during IPB migration was deduced and an important formula quantitatively connecting the atomic displacements,dislocation motion and macroscopic deformation was derived,which hierarchically deepens our understanding on the mechanism of IPB migration.Based on the simulation results,the dependence of interface velocity on the angle between the slip plane and interface,dislocation interactions and dislocation core structure was carefully analyzed.Additionally,the simulation result shows that as the temperature increases,the interface velocity experiences a rise and fall before an increase in the reversed direction.This trend is originated from the influence of temperature on the driving force and the mobility of atoms.By calculating the dynamical data for IPB migration such as the interface mobility and activation energy,the Arrhenius relationship between the mobility and temperature was observed,indicating a thermally-activated process of the IPB migration.The three-dimensional growth of the product phase was simulated.An obvious anisotropic growth of the coherent product phase was observed.The Burgers vector content(BVC)method was applied to analyze the misfit between the two phases.It was found that the fastest growth direction and the broadest facet of the coherent product phase are the best-matching direction and plane calculated from the BVC method respectively,and are also consistent with the strain energy calculation under the framework of Eshelby's inclusion theory.The present simulation also reveals the rule of variant selection during the growth of a semi-coherent product phase,with the selection and formation sequence of variants are determined by the combination of both strain energy and interfacial energy.Specifically,the strain energy determines whether or not a variant can form,and then variants which can form low-energy interfaces(low-angle grain boundary or coherent twin boundary)with the existing variants nucleate prior to others.Additionally,the IPBs in a cross-sectional sample of a duplex stainless steel was found to have low migration velocities,which is consistent with the experimental observation.The formation of spike is associated with the growth of the product phase in the cross-sectional sample due to the incompatible migration of IPBs with different intrinsic velocities.With systematic simulations,four dislocation-IPB interaction modes were observed:(1)Dislocation does not move;(2)Dislocation stops at the interface;(3)Dislocation directly pass through the interface;(4)Dislocation motion is hindered by the interface,but new partial slip systems are initiated in FCC phase.Based on the rich simulation results,a detailed analysis was performed to explore how the influencing factors,including the resolved shear stress,the continuity of slip systems and the interactions between matrix/interfacial dislocations,work on the processes and results of the dislocation-IPB interactions.