Positron-annihilation studies of rhombic triacontahedral-type icosahedral quasicrystals and their 1/1 and 2/1 approximants in the Al-Mg-Zn alloy system

Abstract

Rhombic triacontahedral-type icosahedral quasicrystal and its 1/1 and 2/1 approximants in Al-Mg-Zn alloy system were systematically studied by means of positron-annihilation lifetime measurements and positron-annihilation lifetime and Doppler-broadening measurements using the slow-positron-beam technique. We could successfully derive the structural vacancy density in this alloy system. The structural vacancy densities were $\ensuremath{\sim}4.56\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ for the 1/1 approximants, $4.82\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ for the 1/2 approximant, and $5.05\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ for the icosahedral quasicrystal. In addition, the positron motions were investigated. The average positron-diffusion length, average positron-diffusion length in matrix, positron-diffusion coefficient, and positron mobility in this alloy system including the icosahedral quasicrystal were estimated. The positron-diffusion coefficients for the 1/1 approximants were almost identical $(\ensuremath{\sim}1.79 {\mathrm{cm}}^{2}{\mathrm{s}}^{\ensuremath{-}1})$ to each other, but decrease down to $1.61\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}1} {\mathrm{cm}}^{2}{\mathrm{s}}^{\ensuremath{-}1}$ for the 2/1 approximant and $1.44\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}1} {\mathrm{cm}}^{2}{\mathrm{s}}^{\ensuremath{-}1}$ for the icosahedral quasicrystal. This first experimental estimation was performed by coupling the positron-annihilation lifetime measurements to the positron-annihilation Doppler-broadening measurements using the slow-positron beam excellently. The results indicated that the thermarized positrons in matrix tend to localize as the order of the approximant increases.