TEM STUDIES OF THE MICROMECHANISMS OF SOLID STATE REACTIONS IN Ni-Zr BULK DIFFUSION COUPLES

Abstract
Solid state amorphisation reactions in Ni-Zr bulk diffusion couples prepared by cold-rolling of sandwiches of elemental foils were investigated by cross sectional transmission electron microscopy in the temperature range between 210 and 380°C. After nucleation of the amorphous phase at intersections of Zr grain boundaries with the Ni/Zr interface very fast lateral growth has been observed leading to the early formation of a continous amorphous layer. As soon as the amorphous layer exceeds a critical thickness, crystallization of the orthorhombic NiZr compound can occur either by backward growth into the amorphous layer or by growth into the crystalline Zr foil. By comparing the reaction in these coarse-grained bulk diffusion couples with the behaviour in thin film diffusion couples with much smaller grain sizes the influence of Zr grain goundaries on the nucleation processes of the amorphous phase as well as of the second, crystalline compound has been eludicated. In addition, evidence has been found for an strong influence of texture and orientation relationship. In conclusion a more detailed model of the solid state amorphisation can be given.