Heterogeneous precipitation strengthened non-equiatomic NiCoFeAlTi medium entropy alloy with excellent mechanical properties

The outstanding strength-ductility combination has always been a challenging problem in the development of metal materials, high entropy alloys/medium entropy alloys (HEAs/MEAs) are considered to be the key material to solve this problem. Here a novel non-equiatomic Ni46Co24Fe24Al3Ti3 MEA with equiaxed grains was prepared by vacuum suspension melting, which consists of FCC matrix and heterogeneous L1(2) precipitate. In the current work, the different states of the Ni46Co24Fe24Al3Ti3 MEA from as-cast to after thermo-mechanical processing (TMP) were systematically researched. The result shows that the heterogeneous L1(2) precipitate of the as-cast alloy is composed of cubic precipitates at the grain boundary and spherical precipitates in the grain interior. After TMP, a short-rod L1(2) precipitated by discontinuous precipitation reaction (DP) at the grain boundary, and a spherical L1(2) precipitated by continuous precipitation reaction (CP) in the grain interior. Meanwhile, the alloy shows outstanding mechanical properties in the tensile test at room temperature. The ultimate tensile strength of the as-cast alloy is 935 MPa, with an elongation of 32%. The strength of the alloy is further improved after TMP, the yield strength and ultimate tensile strength are respectively 820 MPa and 1236 MPa, with an elongation of 23% retained. Compared with the single-phase NiCoFe MEA, the yield strength and ultimate tensile strength are significantly increased by 310% and 147%. The excellent mechanical properties of the alloy are attributed to the combination of the soft FCC matrix and the strengthening of the L1(2) precipitate.