Amorphization by mechanical alloying and by solid-state reaction: Similarities and differences

Abstract

Amorphization of AB multilayers by solid-state reaction is usually obtained when the heat of AB mixing, ΔH Mix « 0 and the constituent diffusivities are very different with D B a ̊ D A . Experimentally it is found that compound nucleation and growth take over when the amorphous layer thickness grows beyond a critical thickness x c. We review expressions derived for x c based on various kinetic arguments and show that they scale with the asymmetry of the diffusivities ( x c ⪡ D B/ D A). A new expression for x c based on the fact that the thermodynamic driving-force for crystallization disappears with large concentration gradients is shown to depend on the heat of mixing ΔH mix. Neither ΔH mix « 0 nor D B a ̊ D A are required for amorphization by mechanical alloying. The critical thickness x c is shown to be ≈2 nm for alloys with ΔH mix = 0 such that amorphization by solid-state reaction remains insignificant. It is shown, however, that in mechanical alloying, amorphous layer growth is accompanied by simultaneous thickness shrinkage due to mechanical deformation. This allows the amorphous interlayer thickness to remain below the critical value x c while it volume fraction inccreaases.