Flow Stress Evolution Research Articles

Evolution of flow stress in aluminium during ultra-high straining at elevated temperatures. Part II

During torsional deformation of A1 (purity 99·7%) with 0·1 or 2mm grains at 400°C and 0·2s-1, the flow curve rises to a steady-state plateau of 18 MPa for ∊ = 0·5-3·0 and then gradually decreases b...

Low energy dislocation structures in highly deformed materials

Gross low temperature plastic deformation of metals results from the movement of large numbers of dislocations. This movement is characterized by dislocation-dislocation interaction events statistically distributed in time and space and by the continuing trend of the dislocation density to rearrange into low energy configurations. The various approaches proposed to link flow stress and strain hardening with the evolving substructure may be grouped into families, emphasizing one or the other of those aspects. The first examines possible low energy dislocation configurations and derives the observed flow stress and strain hardening from the characteristics of the proposed dislocation architecture. The second phase relates flow stress evolution to the kinetics of dislocation movement, i.e. the build-up of the substructure as resulting from successive interaction events. The two approaches have been developed independently. It is the aim of the present paper to examine to what extent experimental observations fit into the framework of the existing models (the “kinetic” model being somewhat modified by an addition proposed in this paper) and to what extent the models are complementary in covering different aspects of the one same truth or are mutually exclusive.

Stable and unstable modes in tensile deformation

Unstable plastic flow is often due to a deficiency of mobile dislocations. In order to describe this phenomenon an equation of state is proposed with the densities of mobile and immobile dislocations as structure parameters. Consequently the evolution of dislocation structure is given by a set of coupled rate equations. Constitutive functions which describe the evolution of flow stress, strain rate and structure are introduced. On the basis of these concepts, a distinction is made between the ideal and effective stability of plastic deformation. The effective stability limit is identical with the necking condition in tensile deformation. It is rederived here by considering spatial fluctuations of the state parameters. A mathematically simple model is then used to compare the stable and unstable modes of deformation.