In Situ Observation of High-Temperature Microstructure Evolution and Phase Transformation of 2195 Al-Li Alloy

By using the high-temperature metallography in situ observation system (HTM IOS), the high-temperature microstructure evolution and phase transformation are investigated as well as the surface oxidation dynamics of 2195 Al-Li alloy in T8 temper. The results indicate that the main strengthening phase of the T8 raw material is T₁ (Al₂CuLi) phase, which gradually dissolves into the matrix at 300 °C to 400 °C, causing a drastic drop in micro-hardness. The T_B phase is distributed on the grain boundaries (GBs), resulting from Cu enrichment in the thin liquid film trapped between solidified α phases. The polygonal τ₂(Al₇Cu₂Fe) phase, on the other hand, cannot be eliminated in the heating process even if the liquidus temperature is reached. In the continuous heating scenario, the GBs start to show themselves at 300 °C to 400 °C, then coarsens and become more obvious at 400 °C to 500 °C, accompanied by the color change of the surface, which is mainly due to the surface oxidation; at last, pond-like melt appear at the triangle area among the grains, or the spots where inclusions are present when the temperature reaches 500 °C to 600 °C. Finally, the oxidation level of the 2195 Al-Li alloy’s surface is found to be dependent on the holding temperature and holding time based on the Arrhenius formula.