Laser-directed energy deposition of a high performance additively manufactured (CoCrNi)94(TiAl)6 medium-entropy alloy with a novel core-shell structured strengthening phase

Additively manufactured (CoCrNi)94(TiAl)6 medium-entropy alloys (MEAs) were fabricated by laser-directed energy deposition. The strengthening induction mechanism in the additively manufactured (CoCrNi)94(TiAl)6 MEAs was investigated using electron backscatter diffraction and transmission electron microscopy. The results showed that the addition of TiAl powder led to increases of 29.6% and 43.5% in the ultimate tensile strength (UTS) and yield strength (YS), respectively, at 298 K, and a decrease of 17.1% in the elongation. For the samples tested at 77 k after the addition of the TiAl powder, the UTS and YS improved by 26.6% and 28.0%, respectively, and the elongation increased by 26.3%. The microstructural observation results indicated that the matrix grains changed from initial columnar grains with a uniform growth direction to fine dendrites. Numerous TiO clad Al2O3 strengthening nano-precipitates with a novel core–shell structure were found in the additively manufactured (CoCrNi)94(TiAl)6 MEAs, and both featured a face-centered cubic structure. No crystallographic orientation was observed between the nano-precipitates and the matrix. At 298 K, the dislocation line bypassed the nano-precipitates during the plastic deformation period and Lomer–Cottrell locks appeared near the nano-precipitates, thereby strengthening the additively manufactured (CoCrNi)94(TiAl)6 MEAs. At 77 K, plastic deformation resulted in the synergistic deformation of the nano-precipitates and matrix, the formation of a large number of dislocations, and a twinning induced plasticity effect, which further increased the strength and plasticity of the additively manufactured (CoCrNi)94(TiAl)6 MEAs.