The effect of Ti/Al ratios on the evolution of precipitates and their effects on tensile properties for Fe3.75Cr1.25NiAl0.6-xTix high entropy alloys

The work developed a series of novel Co-free high-entropy alloys (HEAs), i.e. Fe_3.75Cr_1.25NiAl_0.6-xTi_x (x = 0, 0.15, 0.30 and 0.40, molar ratio) HEAs abbreviated as Ti0, Ti0.15, Ti0.30 and Ti0.40, respectively. The effects of Ti/Al ratio (Al partially replaced by Ti) on the microstructure and tensile properties were investigated systematically. The Ti0 HEA contains the FCC phase in dendritic regions and the BCC and B2 phases (BCC/B2) in interdendritic regions. Moreover, the spherical ordered B2 nanoparticles are embedded in the BCC matrix. With increasing the Ti/Al ratio from 0 to 1/3, there is a phase transition from B2-NiAl to L2₁-Ni₂AlTi. Furthermore, the morphology of the nanoparticles evolves from spherical to cuboidal. With further increasing the Ti/Al ratio from 1/3 to 1 and 2, the morphology of nanoparticles evolves from cuboidal back to spherical. In addition, the Ti0.40 HEA has σ phases besides FCC, BCC, and L2₁ phases. The analysis of tensile properties shows that properly adjusting Ti/Al ratios sharply improves the plasticity and slightly improves the ultimate strength, but only causes a limited decrease in the yield strength. Specifically, the Ti0.15 HEA has a superior combination of strength and plasticity, exhibiting a yield strength of 819 MPa, an ultimate strength of 1113 MPa, and a fracture strain of 13.4 %. The SRO, the solid-solution strengthening, the phase transformation, and the shape of nanoparticles were discussed in detail, which reveals the origins of the excellent tensile properties. In addition, the excessive addition of Ti forms the σ phase, leading to a serious brittleness.

These findings are believed to promote the development of low-cost HEAs with BCC/L2₁ for practical applications.