Duplex nanocrystalline alloys: Entropic nanostructure stabilization and a case study on W-Cr

Grain boundary (GB) segregation can markedly improve the stability of nanostructured alloys, where the fraction of GB sites is inherently large. Here, we explore the concept of entropically supported GB segregation in alloys with a tendency to phase-separate and its role in stabilizing nanostructures therein. These duplex nanocrystalline alloys are notably different, both in a structural and thermodynamic sense, from the previously studied "classical" nanocrystalline alloys, which are solid solutions with GB segregation of solute. Experiments are conducted on the W-Cr system, in which nanoduplex structures are expected. Upon heating ball-milled W-15 at.% Cr up to 950 degrees C, a nanoscale Cr-rich phase was found along the GBs. These precipitates mostly dissolved into the W-rich grains leaving behind Cr-enriched GBs upon further heating to 1400 degrees C. The presence of Cr-rich nanoprecipitates and GB segregation of Cr is in line with prediction from our Monte Carlo simulation when GB states are incorporated into the alloy thermodynamics.