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

Abstract

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