EVOLUTION OF DISLOCATION-STRUCTURES AND DEFORMATION-BEHAVIOR OF IRON AT DIFFERENT TEMPERATURES .1. STRAIN-HARDENING CURVES AND CELLULAR STRUCTURE

The deformation behavior of iron has been investigated at different temperatures by means of tension tests. There exist two temperature ranges for deformation. In the low-temperature range (T < 293 K), the flow stress sigma, the work-hardening rate THETA at epsilon = 0.06, and the yield stress-sigma(y) decrease with increasing temperature, but in the higher temperature range (T greater-than-or-equal-to 293 K), sigma and THETA at the same strain increase while sigma(y) decreases more slowly. The change of dislocation density, with temperature, at epsilon = 0.06 exhibits the same tendency as that of the flow stress. The strain-hardening rates decrease almost linearly with increasing stress up to necking in the low-temperature range, except the initial strain range. At the higher temperature range, the hardening rates decrease linearly with stress only at the early stage of deformation, but above certain strains, the decreases become more gradual; that is, the THETA-sigma curves deviate from the linear region. The evolution of dislocation structure has also been observed by transmission electron microscopy (TEM). The results show that a substructural transition takes place in the nonlinear range of THETA-sigma curves. In the linear decreasing region of strain-hardening curves, the deformation is controlled by the uniformly distributed dislocations or cell multiplication prevails. However, in the nonlinear region of THETA-sigma curves, cell multiplication seems to be balanced by cell annihilation.