Evaluation of the Micro-/Nano-mechanical Properties of a Novel Al-Cu-Li Alloy Throughout Heat Treatment Processes: A Nanoindentation Study

The multi-phase microstructure is a characteristic of high-strength Al-Li alloys, with each phase contributing to the mechanical properties to varying degrees. To date, there have been no studies characterizing the specific contribution of individual phases to mechanical properties at the micrometer scale. Additionally, the impact of secondary phases on mechanical properties under different heat-treatment conditions remains largely unexplored. To evaluate the micro-/nano-mechanical properties of a novel Al-Cu-Li alloy throughout heat-treatment processes, we have performed nanoindentation tests on the eutectic phases, primary dispersoids, and α-Al matrix at various states. It has been found that the significant disparity in crystal structure between the eutectic phase, primary dispersoids, and the matrix leads to pronounced dislocation activities and evident “pop-in” events during the nanoindentation loading process. The three-stage solution annealing not only reduces the residual eutectic phase content but also prevents it from re-melting. Meanwhile, orientation-dependent mechanical behaviors at the sub-micron scale have also been identified as a function of deformation characteristics. The size distribution of nano-scale phases has a significant impact on the creep behavior, which is relevant to the δ′-PFZs (precipitation-free zones) inside the local deformation region. These insights clarify the correlation between the microstructure and mechanical properties of high-strength Al-Li alloys at microscale levels.