Heterogeneous Deformation Behavior Studied by in Situ Neutron Diffraction during Tensile Deformation for Ferrite, Martensite and Pearlite Steels

Abstract

Tensile behavior was investigated by using in situ TOF neutron diffraction comparatively for ferritic steels; ferrite (F), as-quenched martensite (QM), tempered martensite (TM) and pearlite (P) steels. Changes in lattice spacing, diffraction intensity and FWHM with increasing of the applied stress were measured. Preferential plastic flow takes place depending on crystal orientation, so that intergranular stresses are yielded due to the misfit plastic strains in differently oriented [hkl] family grains for steel F, in blocks for the other steels. Because of the existence of cementite in steels TM and P, phase stresses are superposed upon the intergranular stresses. When the averaged phase strain is subtracted from the measured lattice strain, the trend in generation of intergranular strain in steel TM is similar to that observed in steels F and QM, while that in steel P differs from the other steels. The changes in [hkl] diffraction intensity and FWHM with tensile deformation are similar in steels F, TM and P, while those in steel QM are different from the others. FWHM decreases with tensile deformation suggesting the decrease in dislocation density in steel QM. That is, dislocations induced during martensitic transformation move preferentially and are annihilated by coalescence of dislocations with different signs in the beginning of deformation and hence transformation induced dislocation structure changes to deformation induced one that shows lower dislocation density but higher resistance to tensile flow. The preferential movement of transformation induced dislocations in steel QM leads to a different texture evolution which is recognized from the change in diffraction intensity with tensile deformation.