Structural study of eutectic Mg 0.72Zn 0.28 alloy II. Crystallization processes in the amorphous, liquid and undercooled states of the eutectic alloy Mg 0.72Zn 0.28

Abstract

The structural changes which occur in the disordered eutectic alloy Mg 0.72Zn 0.28 during the crystallization of the amorphous state and the solidification of the liquid and undercooled melt, are observed in situ by neutron diffraction experiments. The structure factors and the radial distribution functions which have been obtained allow us to show that the amorphous material, obtained by rapid liquid quench, has an instantaneous structure different from that of the liquid or undercooled melt. This structural difference is discussed and a comparison with the previously published experiments on the amorphous alloy is made. Our results are also discussed in the light of the theoretical model taking defect relaxation as a basis for structural evolution. Motion and annihilation of structural defects are not sufficient to explain the local atomic rearrangement. Various atomic movements are necessary which concern essentially the sites with peculiar coordination. Whatever may be the initial disordered state, the final atomic arrangement is always the Mg 51Zn 20 crystal; but the approach to the crystalline order is achieved in different ways depending on the initial state and also on the cooling rate (in the case of the liquid).