A phase mixture model for anisotropic creep of forged Al-Cu-Mg-Si alloy

Aluminium wrought alloys are frequently applied to manufacture structural components of complex shape and used in a temperature range where creep phenomena take place. The aim of this paper is to analyze creep behavior of forged AA2014 alloy and to develop a constitutive model to describe inelastic response under multi-axial stress state. Experimental data show that creep rates depend essentially on the loading direction within the power law creep regime, while in the power law breakdown range creep anisotropy is not essential and can be neglected. Microstructural observations suggest that the anisotropy of creep is primarily caused by elongated grains and grain boundaries as a result of material processing. Assuming the inhomogeneous deformation in different microstructural zones including interiors of elongated grains and grain boundary regions, a phase mixture model of inelastic deformation is developed. The model includes constitutive equations for individual phases and an interaction rule to capture the direction dependent creep. Additional state variables including the normalized dislocation density and the normalized particle size as well as corresponding evolution equations are introduced to describe hardening/recovery and overageing processes. Through a change of variables the model is reduced to a set of kinetic equations such that the material parameters can be identified from families of creep curves for two loading directions. Results of identification are presented for the temperature of 150 °C and several stress levels.