NEUTRON SMALL ANGLE SCATTERING FROM THE ALLOY Al-Zn ABOVE THE CRITICAL POINT

The coherent composition fluctuation in the system Al-Zn was measured by neutron small angle scattering in the temperature range between 435 and 324°C. No critical scattering could be detected at the incoherent critical point (Tinc = 357.5°C). The intensity of the scattered neutrons increases continuously when the temperature decreases from 435 to 324°C. The scattering curves below T = 324°C differ from the others essentially by their high intensity for small κ- and their sharper decrease with increasing κ-values. These results agree with Cahn's theory which predicts a decrease of the critical point due to the elastic energy built up by composition fluctuation. The measured shift of ΔT = 28° is in good agreement with the theoretical estimate. The strong increase of intensity below T = 324°C is due to the rapid decomposition behaviour in the unstable state of the system.