On the Strain Hardening Behavior of a Twinning Deficient Fe–26mn–1al–0.14c Steel

Abstract

The uniaxial tensile strain hardening behavior of an Fe–26Mn–1Al–0.14C steel with a medium grain size of ~ 5 µm has been investigated using X-ray diffraction and transmission electron microscopy and compared to that in the same steel treated to give a grain size of ~ 35 µm. The steel with the medium grain size exhibited no significant deformation twinning, until failure at ~ 50% true strain, yet it showed a stable high strain hardening rate i.e . ~ 2 GPa, comparable to that of the steel with the coarser grain size. The twin deficiency is explained by dearth of dislocation pile-ups and stacking faults. Grain refinement and interacting dislocations in the medium grain size steel increased its effective stacking fault energy, as determined using X-ray diffraction: it was ~ 60 mJ/m 2 compared to ~ 21 mJ/m 2 in the coarse grained steel. The dominant deformation mechanisms under these twinning deficient conditions are the formation of Taylor lattices at small strains (~ 2%) and dislocation cross-slip. The observed flow stress of the steel beyond 2% strain was interpreted as being due to lattice friction, dislocation loops and newly created mobile dislocations obeying Taylor’s hardening model. The contribution of dislocation loops to flow stress was estimated to be only ~ 33 MPa and assumed to remain constant throughout the deformation strain range. The nearly stable work hardening observed until higher strains could be explained through the cessation of cross-slip at high strains and the newly created mobile dislocations breaking away from the pre-existing dislocation obstacle configurations. The implications of this novel strain hardening mechanism based on the cross-slip assisted formation of organized dislocation patterns proposed by Kubin and Devincre is discussed.