Achieving strength-ductility synergy of spark plasma sintered (CoCrNi)94Al3Ti3 medium-entropy alloy via post-sintering in-situ precipitation treatment

The single-phase face-centered cubic medium-entropy alloys (MEAs) normally have coarse grains in as-cast state, which exhibit insufficient strength for engineering applications. Here, a superior tensile strength-ductility synergy in a fine grained (CoCrNi)₉₄Al₃Ti₃ MEA hardened by nanoscale L1₂ precipitates was fabricated by spark plasma sintering (SPS) and post-sintering in-situ precipitation treatment. The SPSed MEAs have a fine grain size of ⁓ 5 μm, and a high number density of L1₂ precipitates form after in-situ annealing within the SPS machine. A high tensile yield strength of 1141 MPa with an adequate elongation to fracture of 25.8% was achieved in (CoCrNi)₉₄Al₃Ti₃ MEA after annealing at 700 °C for 4 h. Electron backscattered diffraction and transmission electron microscopy characterizations indicate that the superior mechanical properties mainly originate from fine grains and the coherent spherical L1₂ precipitates. The dislocation slips and stacking faults prevail in all SPSed MEAs during tensile deformation, while extra Lomer-Cottrell locks are observed in annealed MEAs. The deformation twinning is absent in these precipitation-hardened MEAs with a low stacking fault energy, which may be attributed to the fine grains and numerous nanoscale L1₂ precipitates. This study not only confirms the effectiveness of powder metallurgy when sintering and precipitation are combined in-situ during the SPS cycle, but also provide guidance for the microstructure regulation process and practical applications of SPSed HEAs/MEAs.