Evolution of strengthening phases under in-service stresses and temperatures: phase-field and experimental study

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Precipitation is the most important hardening mechanism in Al alloys. The size and spacing of the precipitates, however, are subject to change during the phase separation. During precipitation, chemical variations in the system largely overlap with internal stresses around the precipitate. Thus, a coupled interaction between the diffusion and mechanical relaxation is expected within the system. In the first period of this project, a strong chemo-mechanical cross-coupling due to composition-dependent elastic constants has been investigated on binary (Al-Li, Al-Cu) and ternary (Al-Cu-Li) alloys and supported by mechanical and microstructural experiments. A paradigm-changing mechanism of inverse ripening and rearrangement of precipitates was uncovered under this chemo-mechanical cross-coupling. Furthermore, theoretical and experimental work on the effect of external load on the microstructure evolution are in progress.In the second period of this project, we will focus on the microstructure evolution under creep conditions. A model for diffusion of vacancies in multicomponent system will be developed. Our studies will focus on the significance of point defects (solute atoms and vacancies). The chemomechanical coupling model will be extended to include the composition-dependent strains (Vegard's law) that will be added to the existing model for composition-dependent elastic constants. We will investigate the redistribution of defects in different stressed environments, i.e. next to (i) precipitates (to understand the effect of vacancies on the inverse ripening), (ii) grain boundaries (to understand the criteria for void nucleation) and (iii) next to the dislocations (to understand the mechanism of climb). The insight gained by these investigations will be combined with the new model of surface/interface diffusion (mentioned-above) to address formation of voids in the later stages of creep.As in the previous project, the simulations will be complemented by experiments on our high purity Al-4Cu-1Li-0.25Mn alloy for validation. Thermomechanical treatments will be applied to vary the vacancy and the dislocation density. The resulting effect on the nucleation and coarsening processes of the precipitates will be studied and quantified by microstructural investigations involving TEM, HRTEM and SAXS measurements. The formation and evolution of voids during creep will be studied by performing creep experiments to different stages of secondary and tertiary creep followed by optical microstructural analysis and micro computer tomography (CT) experiments.