Enhancing stability of powder-route nanocrystalline tungsten-titanium via alloy thermodynamics

Abstract

Improvement in material properties as a result of grain size refinement to the nanoscale is often limited by an inherent tendency of these nanostructured materials to coarsen especially at the high temperatures required for processing. The structural instability stems from a large volume fraction of grain boundaries that carry an intrinsic energy penalty, but can be overcome by a consideration of the thermodynamics-based mechanism of interface energy relief via alloying. Suitable alloying conditions can provide a solute segregated grain boundary configuration that enables a nanostructured alloy to become the system's most energetically preferable state. A thermodynamics-based Monte Carlo method that captures the physics of regular solution mixing and grain boundary segregation in nanostructured alloys is developed and used to study the energetics and equilibrium structures of binary alloys. Our simulation is used to identify the alloying elements with preferable interface stabilizing capability appropriate for the high-temperature sintering requirement for powder-route nanocrystalline tungsten. Based on both alloy simulation and consideration of material properties, titanium is selected as a suitable alloying element. Nanocrystalline tungsten alloys with 0-20 atomic percent titanium content are produced by high-energy ball milling and tested at the expected sintering temperature of 1100°C. With an addition of 20 atomic percent titanium, nanocrystalline tungsten shows retention of nanoscale grain size after a one-week equilibration at 1100°C. Scanning transmission electron microscopy and atom probe tomography techniques reveal a heterogeneous distribution of titanium in the alloy with enhanced grain stability, which contradicts the expectation of a uniform solid solution by conventional bulk thermodynamics but is explicitly predicted by the alloy simulation when grain boundaries are included as possible equilibrium states. The segregation profiles from the experimental characterizations and simulated results show depletion of titanium from tungsten grain centers and enrichment of titanium well above the nominal concentration in the grain boundary vicinity in a form of complex segregation state.