Synergistic improvement of strength and ductility by nano-lamellar L12 precipitates in Co-free NiFeCrAlTi medium-entropy alloy

Abstract

High/medium-entropy alloys (H/MEAs) with L1₂ precipitates show a considerable strength-ductility combination. These alloys often contain the cobalt element that contributes to the high-temperature stability of L1₂ precipitates, while the high price of cobalt perplexes the balance of cost and performance. In this work, a novel Co-free Ni₄₆Fe₃₃Cr₁₃Al₅Ti₃ MEA with the unique coherent nano-lamellar Ni₃(Al,Ti) - L1₂ precipitates was processed by generating discontinuous precipitation through 600 °C direct aging after cold rolling (A600), achieving the high strength-ductility synergy with a yield strength of 1518 MPa and uniform elongation of 14.1 %. The high strength originates from the high-density FCC/L1₂ nano-lamellar boundaries, where more dislocations are accumulated in the A600 MEA than that with near spherical L1₂ precipitates aging at 900 °C (A900). In addition, the coherent nano-lamellar allows mobile dislocations to shear through it, reducing the stress concentration at the lamellar phase boundary. A synergistic improvement of strength and ductility achieves in this optimized microstructure for A600 MEA. The Co-free coherent nano-lamellar MEA shows the substantial potential in designing and application of advanced H/MEAs.