Influence of precipitate free zones in relation to other microstructural parameters on the short crack growth in the high strength aluminum alloy AA7075

• Applicants: Dr. Carla Beckmann; Dr.-Ing. Sebastian Fritsch
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 561091004

High-strength aluminum alloys exhibit a wide range of properties. High static strength is, however, often accompanied by relatively low fatigue strength. This is typically attributed to the heterogeneous spatial distribution, size distribution and morphology of strengthening precipitates. In particular during conventional thermo-mechanical treatments, precipitate free zones (PFZs) are formed. Because of the locally reduced strength, PFZs promote crack initiation and growth. This research project considers the effect of different microstructural parameters on the mechanical properties, especially on the fatigue behavior, of the high-strength aluminum alloy AA7075. Special emphasis is placed on the relations between grain size (ultra-fine grained to coarse grained), size and distribution of precipitates, and width and morphology of PFZs. By combining (in some cases severe) plastic deformation with relatively low ageing temperatures, short ageing times and low heating rates, we systematically study the range of possible microstructures that can be obtained by targeted PFZ design. Subsequent mechanical characterization is focused on growth of short cracks and fatigue damage in the different microstructures, and on the detailed analysis of the effect of PFZ on the relative contributions of inter- and trans-crystalline crack growth and on possible changes of the dominant damage mechanism. The experimental results form a basis for probabilistic modelling of crack growth in the framework of micromechanical models. The numerical approaches, to be in part developed further in this project, allow to systematically model and evaluate the effect of different microstructural parameters. One scientific challenge is the numerical simulation of the different crack growth mechanisms in (ultra-)fine grained microstructures. Comparing the numerical results with in-situ experiments on crack growth in micro-scale samples allows for further validation. Combining the experimental and numerical results, the research project will lead to a more general understanding of the fatigue behavior of high-strength aluminum alloys. Moreover, it will lead to the identification of suitable heat treatment strategies that combine high static strength with improved fatigue resistance.

• DFG Programme: Research Grants