Influence of trace elements with high vacancy binding energy on the precipitation hardening of Al-Cu-alloys

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AlCu-based alloys have important applications as high-strength light-weight materials for the fuselage in aviation (AlCuMg, AlCuLi) and as cast-alloys (AlSiCu) for engine blocks in the transport sector. However, adding trace elements to these alloys can significantly improve their strength, while the underlying atomic mechanisms of nucleation are so far only superficially understood. Increasing strength (age-hardening) in aluminium based materials is based on the vacancy-diffusion-controlled formation of precipitates during or immediately after the heat treatment of aluminium alloys. Hence, a high vacancy-binding energy of trace elements has a decisive influence on the precipitation process. In this project we are going to study the influence of trace elements (< 0.1 at% of e.g. In, Sn, Sb, Pb, Bi) on the nucleation and, thus, on the age-hardening behaviour resp. strength of AlCu-based alloys from pure elements. The aim of this project - concerning trace elements - is to understand basic aspects of their vacancy-binding phenomenon as a function of their concentration and their binding strength in aluminium alloys. To reveal the complex interplay between alloying as well as trace elements with quenched-in vacancies from the heat treatment, we will employ several complementary methods including simulation tools. For this purpose, the vacancy-binding behaviour of selected trace elements - in a first stage for unalloyed pure aluminium - will be examined by positron annihilation spectroscopy (PAS) while X-ray absorption spectroscopy (XAFS) reveals for the very early stages of decomposition the short-range order near copper atoms and atoms of trace elements. The spatial distribution of trace elements in aluminium will be investigated by electron probe micro analysis (EPMA) to estimate the respective maximum solubility in aluminium solid solution. Precipitates forming and growing during aging are examined in more detail in order to determine the total precipitation sequence of the Al-Cu-X alloy. Small-angle X-ray scattering (SAXS) will be employed as a particularly effective method for the quantitative description of the precipitation kinetics (coarsening) in the nanometer range. Since size and volume fraction of precipitates are responsible for the strength / hardness of the alloys, observing simultaneously the hardness development during natural and artificial aging contributes significantly to the understanding of the influence of trace elements on the age hardening. Based on the knowledge obtained on the role of trace elements during nucleation and growth of precipitates, we expect an important contribution to improve existing and to the development of new age-hardened wrought and cast aluminium alloys. Additionally, a significantly enhanced understanding of trace elements lead to a much better usability of secondary aluminium in recycling processes.