Design and coherent strengthening of ultra-high strength refractory high entropy alloys based on laser additive manufacturing

The WNbMoTa-based refractory high entropy alloy (RHEA) is expected to become a new generation of high-temperature materials due to its high thermal stability, which can be used in aerospace, nuclear energy and other fields. In this paper, TN10 (TN is the abbreviation of Ti and Ni elements), TN15, TN20 and TN25 non-equimolar WNbMoTa-based RHEAs were designed based on phase engineering and formed by laser powder bed fusion (LPBF) technology. The solidification thermodynamics, microstructure and mechanical properties (compressive strength at 25 °C) of the four alloys were analyzed, and the strengthening mechanism of TN20 alloy was studied. The compressive yield strength of the four alloys exceeds 2050 MPa, the ultimate compressive strength exceeds 2550 MPa, and the elongation exceeds 9%. The compressive yield strength of TN20 alloy is 2513 MPa, the ultimate strength is 3127 MPa, and the elongation is 11.38%. The room temperature (25 °C) compressive yield strength of TN20 alloy is the highest among the high entropy alloys studied so far. TN20 alloy is composed of BCC matrix phase and grain boundary phase containing three different structures, which are B2 TiNi phase, Ni55Nb25Ti15Ta5 phase generating stacking faults and Ni70Ti20Nb10 secondary precipitated phase. The micrometer matrix phase and nanometer grain boundary phase of TN20 alloy show a network distribution in fine crystal region. After the matrix phase cracks due to high thermal stress, the flow TiNi-rich grain boundary phases precipitate to fill the cracks. The grain boundary phase with high proportion in this region is enhanced by the secondary phase and its nanoprecipitated phase. In other regions, stacking faults are generated by the Ni55Nb25Ti15Ta5 phases, which convert the thermal stress that induces crack into lattice defects of the crystal. On the other hand, the content of B2 phase is the highest in grain boundary phase, and the grain boundary between B2 phase and matrix phase is a coherent grain boundary, which realizes the high quality combination of grain boundary phase and matrix phase. The design and strengthening of WNbMoTa-based non-equimolar RHEAs are studied in this paper, which is expected to provide a reference for the design of a new generation of high entropy alloy.