Abstract

A microstructural analysis of plastic deformation processes, based on transmission electron microscopy observations, is performed for polycrystalline copper and steel. In the particular case of copper, the stress-strain curves obtained in tension, present different levels depending on the angle between the tensile axis and the rolling direction. This behaviour is not the result of an anisotropic flow behaviour but is connected with the different values of the Taylor factor. There is a coupling effect between the grain size and the loading conditions in the microstructural behaviour of metals. In the tension of small-grained metals, internal stresses, due to the accommodation process between adjacent grains, act on the development of dislocation substructure; in large-grained metals, the strain accommodation complexity is not important and the dislocation structure is similar to that of single crystals. In rolling, at low strains, the effect of grain size on the microstructural evolution is not relevant. Whatever the grain size, the imposed strain state leads to a high degree of constraint in each grain and
consequently to the activation of a high number of slip systems. With increasing strain, the grain size starts to influence the character of the plastic deformation and microbands with or without associated shear develop.


During complex strain paths, the amplitude of the strain path change is the most sensitive parameter controlling the plastic instability phenomenon. For each amplitude, the mechanical the behaviour of metals is ' controlled by microstructural events such as similar sets of slip systems, latent hardening and the Bauschinger effect.  The effect of grain size on the microstructural evolution of metals deformed under complex strain, paths are shown in a way which is different from that observed during monotonic tests.


Depending on the grain size, severe changes in strain, a path can produce the development of microinstabilities (microbands) or contribute only towards the destruction of the previous dislocation cells. The stability of grains during deformation is an essential factor contributing to the persistence of the microinstabilities just at high strain levels. On the contrary, when grains rotate the slip rapidly saturates in the microbands. A thermal recovery performed after prestrain does not permit the development of microbands after reloading and accelerates the dynamic recovery processes during the initial stages of the subsequent deformation.