The effect of cooling rate on the precipitation behavior and fracture toughness of the η phase in LDED CoCrFeNiTi high-entropy alloy

Precipitation strengthening is recognized as an effective method for strengthening high-entropy alloys (HEAs) or medium-entropy alloys (MEAs), especially the recently developed CoCrFeNiTi HEA. The eta phase plays a crucial role in strengthening CoCrFeNiTix HEAs, though it remains relatively understudied. In this study, CoCrFeNiTi HEA was fabricated by laser directed energy deposition (LDED). The research investigated the effect of cooling rate on the precipitation behavior and fracture toughness of the eta phase in non-equilibrium solidified CoCrFeNiTi HEA. The findings revealed that the precipitation of the eta phase in the as-deposited LDED sample was restrained, resulting in a very low volume fraction. However, a high-volume-fraction eta phase was achieved through annealing at 900 degrees C for 6 h, followed by air cooling. This approach not only maintained the desired similar to 6.2 % volume fraction of the eta phase but also prevented its growth and coarsening (similar to 403 nm), resulting in an optimal combination of microstructure and mechanical properties. The eta phase exhibited the Blackburn type orientation relationship with the FCC phase: <11<(2)over bar>0>(eta)//<1<(1)over bar>0>FCC and {0001}(eta)//{111}(FCC). The L interface of the eta phase demonstrated a coherent relationship with the FCC phase ({0001}(eta)//{111}(FCC)), while the W interface was incoherent with the FCC phase ({11 (2) over bar0}(eta)//{1 (1) over bar0}(FCC)). The unique interface characteristics promoted a higher growth rate at the W interface compared to the L interface, leading to the preferred growth of the eta phase. The chemical compositions of both the FCC phase and eta phase were accurately predicted by experiments and simulations, elucidating the precipitation kinetics of the eta phase. When v >= v(WC), the nucleation and precipitation of the eta phase were inhibited. Conversely, when v <= v(FC), significant coarsening of the eta phase occurred. However, when v approximate to v(AC), the AC sample exhibited superior mechanical properties, with a microhardness of similar to 860 HV and excellent fracture toughness. These findings provide valuable insights and guidance for further enhancement and optimization of HEAs/MEAs strengthened by the eta phase.