A modified two-state-variable unified constitutive model is presented to model the high-temperature stress-strain behavior of a 319 cast aluminum alloy with a T7 heat treatment. A systematic method is outlined, with which one can determine the material parameters used in the experimentally based model. The microstructural processes affecting the material behavior were identified using transmission electron microscopy and were consequently correlated to the model parameters. The stress-strain behavior was found to be dominated by the decomposition of the metastable ' precipitates within the dendrites and the subsequent coarsening of the phase, which was manifested through remarkable softening with cycling and time. The model was found to accurately simulate experimental stressstrain behavior such as strain-rate sensitivity, cyclic softening, aging effects, transient material behavior, and stress relaxation, in addition to capturing the main deformation mechanisms and microstructural changes as a function of temperature and inelastic strain rate.
The stress-strain behavior of cast 319-T6 aluminum-copper alloys with three different secondary dendrite arm spacings (SDASs) was studied at high temperatures and under thermomechanical deformation, exposing marked cyclic softening. A two state-variable unified inelastic constitutive model proposed earlier was modified to describe the stress-strain responses of these alloys by considering the variation of hardening and recovery functions of back-stress and drag stress. The SDAS was incorporated in the model as a length-scale parameter, and the material constants were determined systematically from experiments on a cast 319-T6 aluminum with small and large SDASs. The capabilities of the constitutive model were checked by the comparisons of simulations to experiments in the small-strain regime (Ͻ0.005). The results show that the model provides successful simulations for material response after thermal exposure at high temperature and cyclic transient stress-strain behavior. The causes of mechanical behaviors at the macro scale are discussed based on microstructural changes during thermal exposure.
Thermomechanical fatigue and isothermal deformation experiments were conducted on cast Al 319 alloys with small secondary arm spacings (SDAS) in the range of 25 to 35μm. The alloy was studied in the overaged state designated as T7B. In the case of the T7B treatment the material possesses dimensional stability, but incurs considerable loss of strength with time and cyclic deformation at temperatures exceeding 250°C. A two-state variable unified constitutive model was developed to characterize the stress-strain response for the material. The model handles temperature and strain rate effects and captures the microstructurally induced changes on the stress-strain response. The thermomechanical fatigue response under in-phase (TMF IP) and out-of-phase (TMF OP) conditions was simulated and the material exhibited a decrease in the stress range by as much as 50% with continued cycling. The decrease in strength was attributed to the significant coarsening of the precipitates at high temperatures and was confirmed by transmission electron microscopy.
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