Microstructure and mechanical behavior of a novel Co20Ni20Fe20Al20Ti20 alloy fabricated by mechanical alloying and spark plasma sintering

A novel equiatomic Co20Ni20Fe20Al20Ti20 (at%) alloy was designed and synthesized to study the effect of high atomic concentrations of Al and Ti elements on the microstructure, phase composition and mechanical behavior of high-entropy alloys (HEAs) fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). Following the MA process, the Co20Ni20Fe20Al20Ti20 alloy was composed of a primary body-centered cubic (BCC) supersaturated solid solution and a face-centered cubic (FCC) supersaturated solid solution. However, following SPS, a primary FCC solid-solution phase, a BCC solid-solution phase and a trace amount of Al3Ti intermetallics were observed. Transmission electron microscopy (TEM) results confirmed the presence of the FCC solid-solution phase, the BCC (B2-type) solid-solution phase and Al3Ti intermetallics in the bulk alloy. The FCC and B2-type phases are ultrafine-grained, and Al3Ti intermetallics is nano/ultrafine-grained. Our results suggest that consideration of a single existing empirical design criterion is inadequate to explain phase formation in the Co20Ni20Fe20Al20Ti20 alloy. Solid-solution strengthening, grain-boundary strengthening, twin-boundary strengthening, the presence of the strong B2-type BCC phase, and precipitate strengthening due to the presence of a trace amount of Al3Ti are responsible for the ultra-high compressive strength of ~2988 MPa and hardness of ~704 Hv. The strain-to-failure of ~5.8% with visible ductility is dominated by the FCC solid-solution phase.