Coherent precipitation and strengthening in a dual-phase AlNi2CO2Fe1.5Cr1.5 high-entropy alloy

The present work studied the coherent precipitation and strengthening in a dual-phase AlNi2Co2Fe1.5Cr1.5 high entropy alloy (HEA) systematically. It was constituted of a primary FCC phase and a small amount of BCC/B2 phase in the inter-dendrites at the as-cast state. After a series of thermo-mechanical processing, including cold-rolling, homogenizing at 1573 K for 2 h followed by water-quenching, and aging at 923 K for 4 h followed by water-quenching, nano-sized ellipsoidal ordered L1(2) particles with a diameter of 8 nm are formed in the FCC dendrites, and the BCC particle size in B2 inter-dendrites reduces to about 10 nm. Both the tensile strength and microhardness of aged HEA will increase with the aging time first, and then reach a maximum, and decrease finally, which is attributed to the coarsening of these coherent nanoprecipitates. The ultimate tensile strength of the aged HEA can be enhanced up to 1240 MPa, twice as high as that of the homogenized state (637 MPa). The variation of strengths at different states is discussed via the precipitation strengthening mechanisms, which is dominated by the size of L1(2) and BCC nanoparticles. This work will provide a new approach to enhance the strength and ductility in dual-phase alloys via coherent precipitation, i.e., coherent nanoparticles are precipitated on their respective parent phase matrix simultaneously.