Ni_43.5Co₁₉Cr₁₀Fe₁₀Al₁₅Ti₂Mo_0.5共晶高熵合金快速凝固行为及组织调控

Eutectic high-entropy alloys (EHEAs) have exhibited excellent mechanical properties. However, their rapid solidification behaviors during additive manufacturing processes still require further investigation. In the present study, Ni_43.5Co₁₉Cr₁₀Fe₁₀Al₁₅Ti₂Mo_0.5 EHEA via copper mold casting and selective laser melting (SLM) were produced to study the influence of solidification conditions, such as cooling rate, on the phase area fraction, phase composition, grain morphology, and mechanical properties of the alloy. SEM, EBSD, TEM, and tensile testing were used to systematically analyze the solidification behaviors and mechanical properties of this EHEA. The center of the as-cast specimen has the lowest cooling rate, a large amount of fcc primary phase appears and forms equiaxed crystals, with a B2 phase area fraction of 16.9%. As the cooling rate increases, the amount of fcc primary phase decreases, and the area fraction of the B2 phase increases to 23.0% on the surface of the as-cast specimen. Meanwhile, the concentration of Al in the B2 phase of each region in the as-cast specimen is 23.2% (atomic fraction, the same below). However, with the continuous increase in the cooling rate, the area fraction of B2 phase tends to reach its lowest value in the SLM specimen, with only 15.8% in edge-on orientation, and the concentration of Al in the B2 phase decreases to 16.5%. The decrease in the area fraction of the B2 phase in SLM samples is due to solute trapping caused by the high cooling rate, resulting in the formation of a supersaturated solid solution and a reduction in the amount of liquid phase available for forming eutectic-phase. In addition, during the SLM process, a high scanning rate results in a large temperature gradient, which promotes the formation of columnar crystals. Reducing the scanning rate to 500 mm/s causes a columnar-to-equiaxed transition due to the decrease in temperature gradient. The mechanical properties of the SLM specimens are superior to those of the as-cast specimens, with a room-temperature ultimate tensile strength of (1320.5 ± 4.5) MPa and a fracture elongation of (25.8 ± 2.2)%.