Thin intergranular films and solid-state activated sintering in nickel-doped tungsten

Nickel-doped tungsten specimens were prepared with high purity chemicals and sintered. Although activated sintering starts more than 400 °C below the bulk eutectic temperature, the nickel-rich crystalline secondary phase does not wet the tungsten grain boundaries in the solid state. These results contrast with the classical activated sintering model whereby the secondary crystalline phase was presumed to wet grain boundaries completely. High resolution transmission electron microscopy and Auger electron spectroscopy revealed the presence of nanometer-thick, nickel-enriched, disordered films at grain boundaries well below the bulk eutectic temperature. These interfacial films can be regarded as metallic counterparts to widely observed equilibrium-thickness intergranular films in ceramics. Assuming they form at a true thermodynamic equilibrium, these films can alternatively be understood as a class of combined grain boundary disordering and adsorption structures resulting from coupled premelting and prewetting transitions. It is concluded that enhanced diffusion in these thin intergranular films is responsible for solid-state activated sintering.