AMORPHIZATION BY MECHANICAL ALLOYING AND BY SOLID-STATE REACTION - SIMILARITIES AND DIFFERENCES

Amorphization of AB multilayers by solid-state reaction is usually obtained when the heat of AB mixing, DELTA-H(mix) << 0 and the constituent diffusivities are very different with D(B) >> 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) infinity 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 DELTA-H(mix). Neither DELTA-H(mix) << 0 nor D(B) >> D(A) are required for amorphization by mechanical alloying. The critical thickness X(c) is shown to be almost-equal-to 2 nm for alloys with DELTA-H(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 a simultaneous thickness shrinkage due to mechanical deformation. This allows the amorphous interlayer thickness to remain below the critical value X(c) while its volume fraction increases.