Control of discontinuous and continuous precipitation of γʹ-strengthened high-entropy alloys through nanoscale Nb segregation and partitioning

Control of discontinuous and continuous precipitation of L1₂-ordered γʹ precipitates is crucial for tailoring microstructure and mechanical properties of coherent precipitation-strengthened high-entropy alloys (HEAs). In this study, we show that the appropriate addition of Nb not only suppresses discontinuous γʹ precipitation through grain boundary segregation but also promotes continuous γʹ precipitation through nanoscale solute partitioning, leading to substantially improved mechanical properties of the HEAs. Specifically, we explore the effects of Nb on the discontinuous and continuous precipitation microstructures, grain boundary segregation, and mechanical properties of (CoCrFeNi)_94-xAl₃Ti₃Nb_x (x = 0, 0.4, 0.8, 1.6, and 2.3 at.%) HEAs. Atom probe tomography reveals that Nb exhibits preferential segregation at grain boundaries of HEAs, which substantially inhibits the grain boundary precipitation and migration due to the reduction of grain boundary energy and solute drag effect, thereby suppressing discontinuous γʹ precipitation at the grain boundaries. Moreover, Nb partitions to the continuous γʹ nanoparticles in grain interiors, which leads to a high chemical driving force for continuous γʹ precipitation. Because of these beneficial effects, Nb-modified HEAs with a uniform distribution of γʹ nanoparticles throughout the matrix were developed, and the correlation between the precipitate microstructure and mechanical properties of these HEAs is discussed.