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 m echanisms leading to such heterogeneous mode of deformation are analyz ed in iron single crystals using recent techniques: Tensile tests insi de a scanning electron microscope (SEM), microextensometry by microgri ds, stereography and electron backscattering diffraction (EBSD) were c arried out to determine in 3D, the components of the deformation gradi ent 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 l attice spin field W-e in the band and in the matrix. The evolution of D, W and W-e 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. Propag ation mechanism of the band is analyzed. (C) 1997 Elsevier Science S.A .