Experimental analysis of bifurcation and post bifurcation in iron single crystals

Localization of the plastic deformation during metal forming processes depends upon various characteristics of the material, such as texture, hardening constitutive law and boundary conditions. The elementary mechanisms leading to such heterogeneous mode of deformation are analyzed in iron single crystals using recent techniques: Tensile tests inside a scanning electron microscope (SEM), microextensometry by microgrids, stereography and electron backscattering diffraction (EBSD) were carried out to determine in 3D, the components of the deformation gradient tensor F and the orientation of the lattice R, at different stages of the plastic deformation, in each element of the microgrid. A kink band was observed in the necking area of the samples. The localization is analyzed in terms of velocity strain (D) and spin fields (W) and lattice spin field We in the band and in the matrix. The evolution of D, W and We fields, were computed at different stages of propagation of this instability. Large gradients of lattice rotation field and of plastic strain field were pointed out within the observed band. Propagation mechanism of the band is analyzed.