Different behaviours of amorphization induced by ion mixing

Abstract

Ion mixing of multiple metal layers was conducted at 77 K in five selected binary metal systems, i.e. the Ag-Cr, Co-Cu, Cu-Fe, Fe-Mo and Fe-Nd systems, which all have a nearly zero or positive heat of formation (Δ H For ). TEM examination showed a common microscopic feature of localized amorphization after high dose ion irradiation, i.e. the amorphous phase was frequently interspersed in a crystalline matrix. EDS analysis was therefore carried out to determine the actual compositions of the phases formed. It revealed that all local amorphous regions resided within rather narrow composition ranges. The narrow range found here for amorphization is probably the reason that amorphous alloy formation was difficult in these systems. After comparing these observations with those reported earlier for the some 30 systems so far studied, it is found that the behaviour of ion mixing induced amorphization, and hence the binary metal systems, can be classified into two categories. The first category contains the readily amorphous phase forming systems and usually features very limited terminal solid solubilities along with large negative heat of formation. Typical examples are the Au-Ti, Ni-Mo, Ni-Nb and Ru-Zr systems. In these systems amorphization can be achieved over a wide composition range once a minimum dose is reached. For systems in the second category, amorphization is only allowed in a rather narrow composition range, as in the systems studied here. This is also the case when a system has a large solid solubility, even though its Δ H For is rather negative, e.g. the Mo-Ru and Au-V systems. In such situations, the results of phase formation may be sensitive to the composition as well as to the ion irradiation dose. The possible mechanism responsible for the different behaviors of phase formation by ion mixing are also discussed.