Nanoscale segregation behavior and high-temperature stability of nanocrystalline W–20at.% Ti

Abstract

Abstract:Nanocrystalline W powders with ~20 nm average grain size are produced by high-energy ball milling and exposed to a target consolidation temperature of 1100°C. After one week, unalloyed W exhibits substantial grain growth where a W alloy with 20 at.% Ti retains its nanoscale structure. A heterogeneous distribution of Ti is observed by independent characterization methods including scanning transmission electron microscopy, energy dispersive spectroscopy, and atom probe tomography. This heterogeneous solute distribution is different from the expected homogeneous solid solution based on bulk W-Ti phase diagrams. Using a Monte Carlo simulation that includes the possibility of grain boundary segregation and allows grain boundaries as potential equilibrium states, a complex nanoscale structure of Ti around W-rich crystallites is explicitly reproduced. This simulated structure has both grain size and extrema in local Ti content in line with the experimental observations.

Phase separation of a supersaturated nanocrystalline Cu–Co alloy and its influence on thermal stability

“…As a consequence, no stable nc state is expected in the alloy system studied in this paper by thermodynamic considerations [25][26][27]. Therefore, an ideal nanostructure as recently reported for a W-20at.% Ti alloy is not attained during annealing in our alloy system [70]. Nonetheless, it is shown in this study that the phase decomposition of the metastable Cu-Co solid solution has a significant influence on the thermal stability, which can be linked to the underlying microstructure that forms during annealing.…”

Phase transitions in stable nanocrystalline alloys

“…12,[28][29][30][31] At the same time, the method has a limited temperature window in which it operates well. This is because the grain swaps are highly localized movements in configuration space; the simulation is not necessarily able to sample the grain boundary network configuration space ergodically and can become trapped in metastable states.…”

“…The latter mechanism is particularly promising, as substantial decreases to the grain boundary energy from grain boundary segregation could stabilize nanoscale grain sizes to higher temperatures and for longer times. [8][9][10][11][12][13] There is an even more enticing prospect when alloying to reduce the grain boundary energy: if the excess energy of the grain boundary is eliminated by grain boundary segregation, grain growth can be entirely avoided and a thermodynamically stable grain size in the nanocrystalline regime could exist. This concept was put forth by Weissmüller,14,15 with an accompanying analytical model that revealed that a system with an enthalpy of grain boundary segregation large enough to offset the pure grain boundary energy should have such a stable nanocrystalline state.…”

“…Several of these alloy systems have shown considerable nanometer-scale grain size stability in experiments, including Ni-P, Ni-W, Fe-Zr, and W-Ti. [8][9][10][11][12][13] While these models are able to identify systems with potential stability through energetic considerations of grain boundary segregation, the accurate accounting of entropic effects has been limited by assumptions needed to make the analytical models tractable, i.e., dilute limit or regular solution assumptions. A number of analytical [20][21][22][23] and atomistic models 24,25 have been developed for determining the change in free energy associated with grain boundary segregation under a variety of conditions, but typically do not allow the grain size to change during disordering.…”

“…Pure nanocrystalline materials undergo rapid grain growth into micron-scale grain sizes at relatively low homologous temperatures, [1][2][3][4] but with the addition of alloying elements grain growth can be greatly hindered. [5][6][7][8][9][10][11][12][13] Two mechanisms for suppressing grain growth by alloying have been proposed: alloying elements can either kinetically impede grain growth by increasing the activation energy for grain boundary motion (effectively decreasing grain boundary mobility) or decrease the grain boundary energy thermodynamically through grain boundary segregation (or both). The latter mechanism is particularly promising, as substantial decreases to the grain boundary energy from grain boundary segregation could stabilize nanoscale grain sizes to higher temperatures and for longer times.…”