NUCLEATION OF CRYSTALLINE PHASES IN PLASTICALLY DEFORMED AMORPHOUS NIZR

The thermodynamic and kinetic aspects of the nucleation of crystalline phases in plastically deformed a-NiZr layers have been examined considering the effect of a concentration gradient and of an advancing interface on the suppression of both the homogeneous and heterogeneous nucleation process. In establishing the effect of a concentration gradient, the possibility of limited atomic mobility has been explicitly taken into account. An extension to the case of plastic deformation of the concept of a critical velocity of the advancing interface has allowed us to derive a simple expression for the critical thickness of the amorphous layer which can be tested experimentally. Contrary to the case of thermally induced growth of the amorphous layer it is shown that in the case of plastic deformation, because of the combined effect of thermodynamic and kinetic constraints, there is only a finite range of thickness of the amorphous phase which allows heterogeneous nucleation of a crystalline phase. The theoretical predictions on the critical thickness of the amorphous layer and on the critical size of the crystalline nuclei have been compared with the experimental observation that, during plastic deformation, homogeneous nucleation of a metastable crystalline phase is followed at a slightly higher thickness of the amorphous phase by heterogeneous nucleation of a different crystalline compound at the a-NiZr/Zr interface. The theoretical predictions are compatible with the experimental observations and allow us to interpret both the observed sequence of nucleation events and the different growth behavior of the crystalline phases formed by homogeneous and heterogeneous nucleation. © 1995 American Institute of Physics.