Stress-level-dependency and bimodal precipitation behaviors during creep ageing of Al-Cu alloy: Experiments and modeling

Understanding the deformation and strengthening mechanism at different stress levels is crucial for accurate prediction of creep ageing behaviors, especially for alloys subjected to a complex stress-state. A comprehensive investigation has been performed on the creep deformation and age hardening during creep ageing of an aluminum-copper alloy AA2219-T4 under various stress levels that lead to elastic or initial plastic deformation. Microstructural variables including dislocations and precipitates have been characterized in detail by X-ray diffractometer, scanning/transmission electron microscopy and differential scanning calorimetry. The creep mechanism is found to be dominated by diffusion creep under small stress level and then by dislocation climb with increasing stress. The high-stress-induced dislocations raise the creep strain considerably and cause a notable extension of the primary creep stage. The existence of preferential alignment of theta '' precipitates (stress-orienting effect) in creep-aged samples results in an evident decline in the age hardening potential. A typical bimodal distribution of precipitates comprised of theta '' homogeneously dispersed in the matrix and relatively large stable theta' heterogeneously nucleated at dislocations forms during high-stress ageing. The theta' precipitates exhibit no stress-orienting effect and lead to a considerable enhancement of hardening response. Based on detailed microscopic observations, a unified constitutive model is developed and calibrated by experimental data to predict the observed stress-level dependency of creep ageing behaviors. The dislocation evolution during both the loading stage and the creep stage as well as the bimodal precipitation of shearable and nonshearable phases are quantified and incorporated in a dislocation-based internal variable creep model. The proposed model is able to well describe the concurrent evolution of dislocations and precipitation, whose interplay gives rise to a characteristic upturn behavior of stress effects on creep strain and age hardening response. The model can be applied to simulate the creep age forming of complex aluminum alloy component structures.