Dynamic Accommodation of Internal Stress and Selection of Crystallographic Orientation Relationship in Pearlite

Abstract

For deeper understanding of a dynamic accommodation mechanism of internal stress in pearlite originated from the lattice misfit between ferrite and cementite phases, the lattice parameter ratios of cementite, b_θ/a_θ and c_θ/a_θ, were locally analyzed in detail by using the electron backscatter diffraction (EBSD) technique. The EBSD analysis has revealed that lattice parameter ratios of cementite lamellae obviously differ from those of spheroidized cementite particles, which demonstrates that pearlite has a certain amount of internal stress as long as it maintains lamellar structure. The internal stress in pearlite gradually decreased during isothermal holding at 923 K after pearlitic transformation due to interfacial atomic diffusion of iron atoms. However, comparing with theoretical values under Pitsch-Petch orientation relationship, it was understood that large amount of internal stress had been already accommodated upon pearlitic transformation by introduction of misfit dislocations and structural ledges on ferrite/cementite lamellar interfaces. That is, the internal stress of pearlite is dynamically reduced by two different processes; built-in accommodation upon pearlitic transformation and additional time-dependent relaxation after pearlitic transformation. On the other hand, EBSD analysis and neutron diffraction technique gave remarkably different lattice parameters of cementite. From this result, it is concluded that various crystallographic orientation relationships between ferrite and cementite coexist in pearlite. Furthermore, elastic strain energy analysis suggests that the invariant-line criterion on ferrite/cementite interface plays an important role for the selection of orientation relationships in pearlite.