Stability of amorphous Cu/Ta and Cu/W alloys

Although both the Cu/W and Cu/Ta alloy systems are known to be immiscible, the response of coevaporated amorphous films of a-CuxW1−x (x=0.45–0.72) and a-CuxTa1−x (x=0.10–0.55) alloys to thermal annealing and ion irradiation is very different. Alloys of a-CuxTa1−x crystallize between 600 and 800 °C, with the crystallization temperature increasing with Ta concentration, while a-CuxW1−x alloys crystallize below 200 °C. Likewise in Xe- and Ne-ion irradiation studies, a-Cu/Ta can still be observed after ∼13 displacements per atom (DPA), whereas a-Cu/W alloys completely transform to solid solutions after ∼0.08 DPA. The results of this study, in combination with previous investigations of amorphous alloys of immiscible systems, show that the crystallization temperature Tc is not predicted equally well for systems with a positive or negative heat of formation. It appears that the kinetic model of crystallization only applies if the transformation proceeds through long-range diffusion. Amorphous alloys which have a positive heat of mixing are predicted to undergo a polymorphic transformation at lower temperatures.