Quasi-static and Dynamic Constitutive Behavior of Precipitate Strengthened Al and Mg Alloys: An Experimental and Crystal Plasticity Modeling Investigation

A present challenge in material science lies in applying the knowledge of single crystal level deformation behavior to predict the polycrystalline response. Mean field polycrystal plasticity models such as Elasto-Plastic Self-Consistent (EPSC) and Visco-Plastic Self-Consistent (VPSC) models provide computationally efficient tools to link the single crystal response to the polycrystal aggregate.
Many commercial alloys are precipitation hardenable, and the introduction of second phase particles in the matrix leads to an increase in strength and changes the work hardening behavior. Besides affecting the uniaxial flow behavior, the precipitates also alter the anisotropy of the material. The flow response and anisotropy of two commercial precipitation-hardenable alloys were measured experimentally, as well as modeled and predicted using the crystal plasticity approach.
In the first part of the dissertation, EPSC modeling was employed to describe the flow behavior, anisotropy and texture evolution of Mg alloy WE43 in the T5 (hot worked and artificially peak aged) temper, which has recently been proposed as a potential lightweight metallic armor. The independent contribution of texture, grain size and various strengthening mechanisms were determined and their impact on individual slip modes was quantified. Additionally, the strain rate sensitivities of each individual mode were determined. The results show that deformation twinning and basal slip can be treated as rate insensitive at least up to the strain rate regime investigated here. The modeling efforts led to the development of a set of parameters which were then validated using data obtained from T3 and T6 tempered samples which possessed microstructures which were far from that of the material used to develop the model. Furthermore, the aging response and the fracture behavior of this alloy was explored and it was established that the T5 temper has the optimum combination of strength and ductility. The results confirm the notion that the poor ageing hardening characteristics of WE43, as compared to Al alloys, is due to a low number density/volume fraction of the strengthening phases.
The second part of the dissertation focuses on modeling the homogenous deformation response of Al-7085 alloy, a new high strength precipitation hardenable alloy which is used for aerospace applications and has also been considered as a lightweight metallic armor. First, the texture evolution and large strain behavior was measured and then modeled using the VPSC code, which permitted the role of latent hardening and grain shape effects to be explored. These initial experimental modeling results highlighted the potential importance of ‘backstresses’ generated by non-shearable precipitates. To account for this, a micromechanical model of kinematic hardening was implemented within the EPSC framework. This modified EPSC model was then used to predict the flow stress and strain anisotropy of this alloy in various tempers. The results demonstrate that incorporation of precipitate-induced backstresses do result in strength and strain anisotropy trends which qualitatively match the trends observed experimentally. However, to quantitatively predict all of the generalized Bauschinger (or backstress) effects, it is suggested that a dislocation-density based intragranular approach must also be taken into consideration.