Abstract
Exploiting grain boundary engineering in the design of alloys for extreme environments provides a promising pathway for enhancing performance relative to coarse-grained counterparts. Due to its attractive properties as a plasma facing material for fusion devices, tungsten presents an opportunity to exploit this approach in addressing the significant materials challenges imposed by the fusion environment. Here, we employ a ternary alloy design approach for stabilizing W against recrystallization and grain growth while simultaneously enhancing its manufacturability through powder metallurgical processing. Mechanical alloying and grain refinement in W-10 at.% Ti-(10,20) at.% Cr alloys are accomplished through high-energy ball milling with transitions in the microstructure mapped as a function of milling time. We demonstrate the multi-modal nature of the resulting nanocrystalline grain structure and its stability up to 1300 °C with the coarser grain size population correlated to transitions in crystallographic texture that result from the preferred slip systems in BCC W. Field-assisted sintering is employed to consolidate the alloy powders into bulk samples, which, due to the deliberately designed compositional features, are shown to retain ultrafine grain structures despite the presence of minor carbides formed during sintering due to carbon impurities in the ball-milled powders.
Highlights
Grain size refinement and the introduction of compositional complexities are two key tools in designing unique microstructures with nanoscale features that can enhance material properties relative to classical engineering materials
We focused our high-energy ball milling and field-assisted sintering technology (FAST) investigation on two W-rich alloy concentrations selected based on prior studies of the binary W-Ti and W-Cr systems
The effect of sintering on the nanostructures achieved during milling is mapped at the same temperatures employed for annealing of the ternary alloy powders and benchmarked against pure coarse-grained W sintered under identical conditions
Summary
Grain size refinement and the introduction of compositional complexities are two key tools in designing unique microstructures with nanoscale features that can enhance material properties relative to classical engineering materials. The deliberate introduction of solute that has an energetic preference to segregate to grain boundaries—often referred to as grain boundary doping (including deliberate alloying and impurity segregation)—has emerged as an effective means of stabilizing nanocrystalline materials against thermal [1,2,3,4,5], mechanical [6,7,8], and irradiation instabilities [9,10,11] Because these materials contain a high-volume fraction of grain boundaries with excess free energy, they are not processed via traditional thermomechanical processing routes and instead rely on far-from-equilibrium processes such as physical vapor deposition (PVD), electrodeposition, high-energy ball milling, and severe plastic deformation (SPD) [12]. High-energy ball milling represents a class of powder metallurgical processing that is inherently scalable and, due to the high deformation energy, can mechanically alloy combinations of powders while simultaneously refining the grain size to produce single-phase nanocrystalline alloys [13].
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