Work hardening in heterogeneous alloys - a microstructural approach based on three internal state variables

Abstract

A new work-hardening model for homogeneous and heterogeneous cell-forming alloys is introduced. It distinguishes three internal state variables in terms of three categories of dislocations: mobile dislocations, immobile dislocations in the cell interiors and immobile dislocations in the cell walls. For each dislocation population an evolution law is derived taking into account dislocation generation, annihilation and storage by dipole and lock formation. In particular, these rate equations take into account the number of active glide systems and, thus, introduce texture in the model in addition to the Taylor factor. Microstructure is represented by the dislocation cell structure as well as second-phase particles, which may undergo changes by precipitation and Ostwald ripening. Interaction of mobile dislocations with the microstructure is taken into account through an effective slip length of the mobile dislocations. For the same set of parameters, the predictions are in excellent agreement with measured stress-strain curves of both a precipitation-hardened aluminum alloy (Al-4.16 wt% Cu-1.37 wt% Mg, AlCuMg2) and a precipitation-free model alloy (Al-0.35 wt% Cu-0.25 wt% Mg), the composition of which corresponds to the matrix of the two-phase alloy.