Stability and formation of Al-Cu-(Li,Mg) icosahedral phases

Single icosahedral (i ) phase samples can be obtained by rapid solidification at the composition ${\mathrm{Al}}_{51}$ ${\mathrm{Cu}}_{12.5}$ ${\mathrm{Li}}_{\mathit{x}}$ ${\mathrm{Mg}}_{36.5\mathrm{-}\mathit{x}}$ (x=0.0, 16.5, 25.9) and ${\mathrm{Al}}_{5.5}$ ${\mathrm{CuLi}}_{3.4}$. Differential-scanning-calorimetry (DSC) measurements of ${\mathrm{Al}}_{51}$ ${\mathrm{Cu}}_{12.5}$ ${\mathrm{Li}}_{\mathit{x}}$ ${\mathrm{Mg}}_{36.5\mathrm{-}\mathit{x}}$ i phases show that they transform to single body-centered-cubic (bcc) crystalline phase via an exothermic process, suggesting that they are a metastable phase. The heat released in the transformation (ΔH), which is much smaller than that in other icosahedral alloys, decreases from 0.36 KJ/mol to less than 0.02 KJ/mol as Li content increases from x=0 to x=25.9. The ${\mathrm{Al}}_{5.5}$ ${\mathrm{CuLi}}_{3.4}$ i phase can be retained up to the melting point; the enthalpy changes ΔH are nearly the same for i→liquid and bcc→liquid transformations. A large enthalpy relaxation effect is observed in the DSC measurement of Al-Cu-Li alloys upon annealing.