The relationship between the chemical and topological disorder in the intermetallic compound Cu 4Ti 3

Abstract

Using in situ measurements of the Bragg-Williams long-range order parameter, S, in a HVEM, the relationship between electron irradiation induced chemical disordering and the crystalline to amorphous (C-A) transition is studied in the line compound Cu 4Ti 3. The existence of critical temperatures above which the attainable chemical disordering is critically reduced and above which the C-A transition does not occur is shown, and it is found that the two critical temperatures are coincident. Below the critical temperature, significant chemical disordering is produced and the C-A transition always occurs; above the critical temperature, the attainable degree of chemical disordering is severely limited, the C-A transition does not occur, and secondary defects are formed. Thermodynamical consideration is developed in which chemical disordering raises the energy of the crystal lattice until it becomes unstable with respect to the amorphous state. In Cu 4Ti 3, the necessary degree of chemical disordering corresponds to an S value of approx. 0.3. The thermodynamical consideration is supported by a HREM study of the crystalline-amorphous interface, the observation of a preferred C-A transition at antiphase domain boundaries and energy comparisons. Finally, the role of point defects in the C-A transition is discussed in terms of the temperature dependence of the chemical disordering rate and the existence of the critical temperature.