Optimizing sample preparation for In situ TEM heating studies of aluminum alloys

Aluminum-copper-lithium (Al-Cu-Li) alloy is a key structural material in the aerospace field. Its mechanical properties are closely related to the dynamic evolution of T₁ phase during the aging process. To accurately capture this dynamic process, the in situ transmission electron microscope (TEM) combined with the microelectromechanical system (MEMS) heating chip was used to observe the T₁ precipitation behavior in an Al-Cu-Li alloy in real time. All experimental samples were solution treated at 510 °C for 30 min and water quenched externally, and then heated to 180 °C on a MEMS heater chip at a heating rate of 1 °C/s. This study presents an improved sample preparation and transfer protocol to mitigate Ga infiltration. High-angle annular dark-field scanning TEM (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) were used to evaluate the effects of preparation methods, transfer techniques, and sample thickness (80-300 nm) on gallium (Ga) and platinum (Pt) contamination and precipitation behavior. The results show that Ga segregation, in the form of ∼10 nm intragranular particles and grain boundary enrichment, significantly distorts the intrinsic precipitation of T₁ phases. However, combining an external transfer method with low-energy ion milling at an accelerating voltage of 3 kV effectively suppresses Ga/Pt contamination. Furthermore, sample thickness critically influences precipitation kinetics in Al-Cu-Li alloys: sub-100 nm samples exhibit surface-driven abnormal coarsening of T₁ precipitates, while samples exceeding 250 nm suffer from reduced imaging resolution due to limited electron transparency. A thickness range of 150-200 nm optimally balances resolution fidelity with representative precipitation dynamics.