Achieving superior high-temperature strength in an additively manufactured high-entropy alloy by controlled heat treatment

High-entropy alloys (HEAs) usually exhibit high strengths at ambient and low temperatures but rapidly degraded tensile properties with increased temperature. In this study, a high-strength HEA, (CoCrNi)₉₄(TiAl)₆, is selected and subjected to laser powder bed fusion (L-PBF) and ageing treatment. The microstructural evolution and mechanical property development of the material over a wide range of temperatures are thoroughly investigated. It is found that the as-printed microstructure is dominated by numerous ultrafine cellular structures (∼1 μm) with cell boundaries decorated by Al₂O₃ nanoparticles, leading to high 0.2% yield strength (YS = 725∼750 MPa) and excellent elongation (>28%) at room temperature. The cellular structures remain up to 700 °C but disappear at or above 800 °C. Ageing at or above 600 °C leads to significant γ′ precipitation with the particle size increasing constantly with increased temperature. The samples containing both cellular structures and coarsened γ′ precipitates (aged at 700 °C) exhibit the highest YS (∼1227 MPa) and ultimate tensile strength (UTS∼1539 MPa) at room temperature and display unprecedented YS at high temperatures, i.e., 949 MPa at 600 °C and 728 MPa at 700 °C, respectively. The exceptional tensile strengths are mainly due to the γ′ precipitates and cell boundaries decorated by Al₂O₃ nanoparticles which may have acted as strong barriers for dislocation motion. At room temperature, the sample deforms mainly by dislocation slip and formation of stacking faults while at elevated temperatures, deformation becomes increasingly planar as characterized by the formation of increased number of stacking faults and the activation of twinning.