High Strength Refractory AlHfNbTiV B2 High Entropy Alloys with High Fracture Strains

Abstract: We demonstrate the development of a series of refractory high-entropy alloys containing aluminum AlRHEAs in the ordered BCC-B2 phase by varying the aluminum content within 10 to 25 atomic percent, with the goal of high strength and good ductility synergy. The AlRHEAs obtained are found to show promising potential for high-temperature applications. The incorporation of Al lowers the density and promotes the long-range atomic ordering, which in turn stabilizes the B2 formation, and strengthens the material but usually deteriorates ductility. Several B2 AlRHEAs that contain a combination of Ti, Hf, Nb, and V with moderate to high Poisson ratios are investigated for high strength and ductility. Furthermore, through statistical analysis, we identify a valley around the valence electron concentration VEC of 6 where low ductility is prominently observed. Machine-learning models are employed to screen the vast compositional space of AlRHEA alloys to predict B2 formation and toughness indicated by the yield strength and fracture strain. High prediction accuracies are achieved. As the Al content decreases, the B2 atomic ordering decreases, compression yield strengths decrease from 1500 MPa to 1200 MPa, and compression fracture strains increase from 0.06 to over 0.5. Notably, Al10Hf20Nb22Ti33V15 retains a compression yield strength exceeding 800 MPa up to 700 C, tensile yield strength of 1100 MPa, and fracture strain of 0.083. Our findings on enhancing ductility in pure B2 alloys pave the way for further research on Al-RHEA superalloys, striving to achieve high strength and ductility, reduced density, and improved oxidation resistance.