Kinetics of unmixing in AlZn single crystals studied by neutron small angle scattering

Abstract

The unmixing of Al5.3 at.% Zn and Al6.8 at.% Zn single crystals quenched below the miscibility gap and aged at various temperatures between 20°C and 110°C has been followed in situ by neutron small angle scattering. The scattering patterns are first analyzed through a two-phase model which leads to the time evolution of the shape, sizes and density of the Guinier-Preston zones. The averaged scattering cross-section is next compared with recent computer simulations of the Ising model and it is shown, in agreement with the simulations, that the structure function can be scaled with time after a transient period; the scaling appears also practically independent of the Zn concentration and the aging temperatures. The fact that the relevant length scale is the Guinier-Preston zone size allows to show that the two kinetics data analysis, in the direct and reciprocal spaces, are consistent. The results are then discussed in terms of nucleation and growth theories. The late time aging appears to follow at low temperature a coarsening process of the Lifshiftz-Slyozov type. When the temperature is raised, the decrease of the exponents of the power laws to which satisfy the zone growing, could possibly be attributed to the intervening of a coagulation process.