Anisotropic grain growth of δ-NiMo compound by exothermic solid state diffusional intermixing

Abstract

Solid state interdiffusion in Ni-Mo, based on a thin film diffusion couple geometry, was studied at temperatures between 500 and 900 °C. Anomalous, anisotropic grain growth of δ-NiMo intermetallic compound was observed in samples annealed between 700 and 900 °C. At 700 and 800 °C, two distinct reaction regions, showing small equiaxed grains and large columnar grains, were detected. The columnar grains were single crystals, and their growth direction was parallel to the direction of the nickel diffusion flux. At 900 °C, an agglomerated, spider-web-like structure was generated as the result of nickel diffusion through, and complete reaction with, the molybdenum films. Apparent interdiffusion coefficients were estimated to be four to five orders of magnitude greater than literature data. The enhanced diffusion kinetics and anomalous, anisotropic grain growth of δ-NiMo are discussed on the basis of exothermic reactions between nickel and molybdenum during diffusional intermixing. The enthalpy of formation of δ-NiMo was calculated and demonstrated to be sufficient to give rise to the interfacial melting- resolidification of the compound. Experimental evidence substantiating the occurrence of melting and solidification is presented. Comparisons are made between the present thin film reaction geometry and multilayer and consolidated powder compact reaction geometries used in self-sustaining high-temperature synthesis.