The development of a novel crossover aluminum alloy has led to an improved combination of strength and ductility compared to conventionally available Al alloys. Precipitation hardening using a complex intermetallic hardening phase - the T-phase Mg32(Zn,Al)49 - enhances strength in this new class of alloys. A high radiation resistance of crossover alloys has been observed in further experiments, primarily attributed to the precipitation of the T-phase. This may allow for potential use in environments with elevated particle radiation, as required for space applications. It has been shown that the T-phase remains stable up to 1.0 displacement-per-atom (dpa), while conventional hardening phases - such as the Mg2Si-phase in Al-Mg-Si-alloys - dissolve at 0.2 dpa. The stability of the microstructure and hardening phases is crucial for the material. For example, upon dissolution of the hardening phase, the material loses its strength, compromising the properties for which it was initially designed. Furthermore, point defects are introduced by irradiation, which can aggregate into voids or dislocations, contributing to embrittlement. Previous research has also observed this adverse phenomenon in crossover alloys, where dislocation loops form in the Al-matrix, ultimately leading to premature failure. To mitigate radiation-induced defects, increasing the number of interfaces, such as grain and phase boundaries, has been proposed in the literature. While the crossover alloy contains 10 vol.-% of the radiation-resistant T-phase, these interfaces alone have proven insufficient to significantly reduce defect formation. Therefore, grain size reduction was necessary. Through the severe plastic deformation method High-Pressure Torsion (HPT), the grain size of the crossover alloy, was successfully refined, producing a stable ultrafine-grained microstructure. In-situ irradiation experiments revealed that the alloy remained free of radiation-induced defects, even at an extreme dose level of 24 dpa. This reduction in grain size, while effective in defect mitigation, also influenced the precipitation sequence. Consequently, the precipitation behavior in both coarse- and ultrafine-grained regimes was examined and compared. Given the potential influence of the electron microscope environment (low vacuum and thin film conditions) on these experiments, the precipitation behavior was also assessed using various microstructural characterisation techniques. The novel ultrafine-grained crossover alloy demonstrated significant resistance to radiation-induced defects at high dose levels. The T-phase¿s stability was enhanced by increased chemical complexity, and the microstructure remained intact. Additionally, the reduction in grain size accelerated the kinetics of T-phase precipitation, shortening the time required to reach quasi-equilibrium conditions. However, it was also observed that the precipitation behaviour is also influenced by the type of experimental technique used for examination.