Superior tensile properties of Al_0.3CoCrFeNi high entropy alloys with B2 precipitated phases at room and cryogenic temperatures

Al_0.3CoCrFeNi high-entropy alloys (HEAs) with B2 ordered phase mainly precipitated at grain boundaries are obtained by cold rolling, annealing and aging treatment. Tensile experiments show that both CRSA (annealed at 1100 °C for 1 h) and CRSA-600-24 (annealed at 1100 °C for 1 h and subsequently aging treatment at 600 °C for 24 h) samples have excellent combinations of yield strength, tensile strength and tensile elongation at room temperature, and more importantly, simultaneous enhancements in strength and ductility at liquid nitrogen (77 K) are attained for the latter condition. Analyses reveal that after aging treatment, the precipitation of B2 phase with ultrafine-grained or even nanoscale sizes, as well as moderate grain refinement, yields the significant increase in tensile strength and fascinating tensile plasticity. By transmission electron microscopy (TEM) characterization, it is found that the deformation mode dominated by dislocation glide at room temperature is transformed to that combined with dislocation glide plus nanoscale twinning at 77 K. Moreover, the twinning-induced work hardening renders the onset of necking instability to a higher strain/stress value, which therefore improves the strength and ductility simultaneously.