INTERDIFFUSION REACTION, PHASE SEQUENCE, AND GLASS-FORMATION IN NI-ZR COMPOSITES

The progress of solid-state reaction in Ni-Zr composite wires with different elemental layer thicknesses has been studied in detail. Besides x-ray diffraction and differential scanning calorimetry, dilatometric measurements, magnetization and resistivity measurements, and cross-sectional transmission electron microscopy were used to monitor the reaction during constant-rate heating and to characterize the various reaction products. An amorphous phase initially forms at the interface between the elemental layers. As soon as the layer thickness exceeds a critical value, the intermetallic NiZr phase appears at the interface between the amorphous phase and pure Zr, as shown by TEM investigations. This is due to a reduced velocity of the reaction front caused by the longer diffusion path enabling the intermetallic phase to become stable. As shown in experiments at a constant heating rate, a second intermetallic phase forms at higher temperatures at the interface between Zr and crystalline NiZr. The amorphous phase remains unchanged up to crystallization at about 520-degrees-C. To obtain fully amorphous material, the interdiffusion reaction must be completed (or especially the Zr layers must be completely reacted) before the intermetallic NiZr phase starts to form. A criterion for achieving completely amorphous bulk material is derived.