Comparison of hot deformation mechanisms in overaged and solution treated Al-Li-Cu-Mg-Zr alloys

Studies using TEM have been performed with the aim of understanding the hot deformation mechanisms of overaged 8090 and 8091 AI-Li based alloys in comparison with the solution treated condition. Al-Li-Cu-Mg-Zr alloys were overaged at 400 degrees C for 7.5 h and deformed in the temperature range 300-500 degrees C at strain rates 0.1-4 s(-1). Solution treated alloys deformed under the same conditions showed slightly higher values of maximum flow stress. Overaging at 400 degrees C caused precipitation of quasicrystlline, icosahedral T-2 phase (Al6CuLi3) which, together with Al3Zr affected the hot deformation behaviour at 500, 400 and 300 degrees C. During deformation at 500 degrees C the T2 phase partly dissolved, depending on preheating time. Electron microscopic analysis of solution treated alloys revealed different deformation mechanisms at 500, 400 and 300 degrees C. There were a large number of helical dislocations, prismatic loops and some Orowan loops after deformation at 500 degrees C. At 300 degrees C dynamic precipitation of T-2 and T-1(Al2CuLi) strongly affected the hot deformation behaviour. The change in average subgrain size with the conditions of deformation, i.e. Zener-Hollomon parameter (Z) and steady state flow stress (sigma(s)) was quantitatively characterized and compared.