Effects of Cr/Fe Ratio on Structural Evolution and Tensile Properties of the Fe75.7-xCrxNi15.1Al4.6Ti4.6 High-Entropy Alloys

According to the effect of valence electron concentration (VEC) on the phase evolution of high-entropy alloys (HEAs), Fe75.7-xCrxNi15.1Al4.6Ti4.6 (at%, x = 17.1, 18.9, 20.6, 22.3, and 25.3) HEAs are designed. Effects of the Cr addition on the microstructure and tensile properties are investigated systemically. As the Cr/Fe ratio increases (i.e., decreasing VEC), there is a phase transition from face-centered-cubic (FCC) to body-centered-cubic (BCC) and Heusler-type beta ' -Ni2TiAl (L2(1)). When x is between 17.1 and 22.3, the alloys, with the mixed structure of the FCC and BCC/L2(1) phases, behave like composites with a sharp increase in strength but reduced ductility. When x = 25.3, the alloy, with only BCC/L2(1) phases, becomes extremely brittle. Specifically, Fe56.8Cr18.9Ni15.1Al4.6Ti4.6 exhibits a good combination of a high strength of 1142 MPa and an elongation strain of 9.5%. The excellent tensile properties of this alloy are attributed to the synergetic effects of a soft FCC phase, a hard BCC phase, and the uniform distribution of high-density L2(1) nanoparticles in BCC matrix. The phase-evolution principle is delineated by parameters of the VEC and the atomic size difference (delta), and the strengthening mechanism is well discussed. These findings offer a guidance for the development of BCC/L2(1)-containing HEAs for practical applications.