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

Abstract

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 TB 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.