Enhancement of strength-ductility balance of heavy Ti and Al alloyed FeCoNiCr high-entropy alloys via boron doping

As one of the most effective mechanisms, precipitation-hardening is widely used to strengthen high-entropy alloys. Yet, heavy precipitation-hardened high-entropy alloys usually exhibit serious embrittlement. How to effectively achieve ultra-high strength and maintain reliable ductility remains a challenge. Here, we report a study of doping extremely little boron to meet this target. We found that adding of 30 ppm boron into the heavy Ti and Al alloyed FCC FeCoNiCr high-entropy, (FeCoNiCr)₈₈Ti₆Al₆ HEA (at.%) which is strengthened mainly by both coarse BCC-based (Ni, Co)₂TiAl Heusler and fine L1₂-type FCC-based (Ni, Co)₃TiAl precipitates and shows ultrahigh strength but poor ductility, could significantly change the original microstructure and consequently improve mechanical performance, owing to the well-known effect of boron on reducing the energy of grain boundaries. The boron addition can (1) eliminate microcavities formed at Heusler precipitate-matrix interfaces; (2) suppress the formation and segregation of coarse BCC Heusler precipitates; (3) promote the formation of L1₂ nanoparticles. This changes of microstructure substantially improve the tensile ductility more than by ∼86 % and retain comparable or even better ultimate tensile strength. These findings may provide a simple and costless solution to produce heavy precipitation-strengthened HEAs with ultrahigh strength and prevent accidental brittleness.