Interdiffusion reaction, phase sequence, and glass formation in Ni-Zr composites
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K. Urban KFAJulich, Institut fur Festkorperforschung, D-5170 Jiilich, Germany (Received 20 March 1991; accepted 8 May 1991)
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 °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.
I. INTRODUCTION The formation of amorphous metals by a solid-state amorphizing transformation was first demonstrated by Schwarz and Johnson1 for Au-La thin film multilayers. Subsequently, this amorphization reaction was found for several other systems both for evaporated thin film diffusion couples2'3 and ultrafine layered bulk metallic composites prepared by mechanical deformation4"6 (for a recent review see Ref. 7). Usually the amorphization reaction is performed by an isothermal annealing at temperatures well below the crystallization temperature of the amorphous phase. These experiments give information on the reaction kinetics of the amorphous phase formation5'8'9 and on the phase equilibria.10'11 However, constant-heating-rate experiments also provide quick information about the reaction process. Differential scanning calorimetry (DSC) first used for metallic composites5 is used to measure both the enthalpy of the interdiffusion reaction and of the subsequent crystallization during the same run.12^14 Electrical resistivity versus temperature measurements provides a quick check to see if an alloy system is a candidate for solid-state amorphization.15 In this contribution, we present elabo-
rate results on DSC and dilatometric investigations and on measurements of resistivity and saturation magnetization versus temperature, showing that the formation of the amorphous phase is kinetically favored. Furthermore, the different reaction products were characterized by cross-sectional transmission electron microscopy (CSTEM) investigations. Finally, a model based on nucleat
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