Effect of matrix precipitation on cellular growth kinetics in an Al-28 at. % Zn alloy

The reported existence of two metastable miscibility gaps in the aluminium-zinc (Al-Zn) system is confirmed by an electron microscopic study of precipitate reversion in an Al-28 at. % Zn alloy. The metastable Guinier-Preston (G.P.) zones are replaced above ～125° C within minutes by a metastable rhombohedrally distorted f c c phase (R-phase). Consequently, the recently published calculations for the growth rate of the cellular reaction in the 100 to 250° C temperature range are revised in accordance with the nature of the continuous precipitate ahead of the advancing cell boundary. The observed growth rates are now shown to agree most satisfactorily with current theories of cellular decomposition as controlled by cell boundary diffusion and influenced by continuous precipitation in the matrix.