Preliminary results of activated sintering mechanism and grain boundary prewetting/premelting in nickel-doped tungsten

Supported by prior lattice-gas and phase-field simulations, we proposed that nanoscale intergranular and surficial amorphous films in multicomponent ceramic materials can be treated as a case of combined interfacial prewetting and premelting. Consequently, a class of parallel interfacial phenomena, i.e., coupled interfacial adsorption and disordering, is anticipated to occur in multicomponent metallic alloys. An exploratory study was carried out wherein grain boundary segregation in a model binary metallic alloy (Ni-doped W) was characterized as a function of temperature and dopant concentration. Doped specimens were prepared using high purity chemicals, sintered in flowing H ₂/N ₂ mixture, and examined using Auger spectroscopy and electron microscopy. Preliminary results are presented and discussed with respect to a prewetting/premelting model versus the classical Langmuir-McLean and BET models. An additional goal of this study is to resolve the long-standing mystery of solid-state activated sintering mechanism for nickel-doped tungsten. Use of ultra-pure materials confirmed the occurrence of nickel activated sintering of tungsten in the solid-state. We demonstrated that, contrary to the previous belief, Ni-rich secondary bulk phase does not penetrate along GBs and the solid-state activator should be a nanoscale interfacial phase that does not appear in the bulk phase diagram. The solid-state activated sintering in the model metallic system Ni-doped W is therefore attributed to the enhanced diffusion in a coupled grain boundary disordering and adsorption region, analogous to activated sintering via accelerated mass transport in nanoscale intergranular and surficial amorphous film in the model oxide system Bi ₂O ₃-doped ZnO.