Heterogeneous-structure-induced ultrahigh strength and ductility in a metastable dual-phase Fe60Cr15Ni16Al9 medium entropy alloy

The emergence of medium-entropy alloys (MEAs) provides new insights for the design of the next generation structural alloys. In the present work, Fe60Cr15Ni15Co10-xAlx (x = 0, 3, 5 and 7 at.%) and Fe60Cr15Ni16Al9 MEAs were designed and prepared by vacuum arc melting. The microstructure was characterized by X-ray diffrac-tometer, electron backscatter diffraction and transmission electron microscope. The results show that the volume fraction of body-centered-cubic (BCC) phase increase with the increase of Al content. The dual-phase Fe60Cr15Ni16Al9 MEA with heterogeneous grain size (from-200 nm to-3 & mu;m) is consisting of Fe-Cr-Ni-rich face-centered-cubic (FCC) and Ni-Al-rich BCC phases. The uniaxial tensile tests were conducted at 298 K and 77 K. The results show that the strength of the MEAs increases with the increase of Al content, and replacing Co with Al significantly promotes deformation-induced martensitic transformation from the FCC to BCC in these MEAs. Such unique dual-phase structure results in excellent mechanical properties of the Fe60Cr15Ni16Al9 MEA (yield strength, ultimate tensile strengths and ductility at 298 K and 77 K are 900 and 1300 MPa, 1100 and 1680 MPa, 26% and 35%, respectively), which are mainly attributed to deformation-induced martensitic transformation and hetero-deformation-induced hardening. The alloy-design principles in this study pave the way to design low-cost MEAs with outstanding mechanical properties at room and cryogenic temperatures.