Nano-scale Heterogeneous Medium-entropy Alloy with High Yield Strength Fabricated by Laser-Powder Bed Fusion Additive Manufacturing

The laser-powder bed fusion (L-PBF) method has the advantage of making complex shapes with excellent quality. Alloys made by L-PBF process share a characteristic fine cellular microstructure, which is originated by extremely fast heating and cooling rates during processing. The L-PBF also provides unique cellular microstructures, including a heterogeneity of composition when solute segregates toward the cell wall. It has been recently reported that the addition of Si into conventional CrCoNi medium-entropy alloy reduces stacking fault energy (SFE), resulting in enhancement of both strength and ductility. Here, we fabricated CrCoNiSi alloy via L-PBF method and the optimized mechanical property was obtained through precise control of processing parameters. The optimal alloy has a high yield strength of 929 MPa and a moderate elongation of 14% with sufficient strain hardening, leading to an ultimate tensile strength of 1264 MPa. High yield strength is attributed to the synergetic effect between finer cell size and nano-dispersed inclusions. Finer cell structures by extreme cooling rate successfully impede dislocation movements and hence have a significant effect on the high yield strength. Also, there are fairly large numbers of fine-dispersed nano-inclusions in the cell walls. These inclusions were found to be mostly oxides and sulfides, in diameter of less than 50nm, which gives additional strength increase based on the Orowan strengthening mechanism. As well as those conventional strengthening contributions, a strong segregation of Si and Cr toward cell wall was observed. Cr-Si rich sigma phase is stable at high temperatures over 800 celsius in this alloy system, confirmed by the phase diagram calculated by CALPHAD approach. During processing, subsequent heat is repeatedly applied to previously solidified melt pool during deposition of adjacent layers, which is similar to the formation mechanism of the heat-affected zone (HAZ) in welding. Repeated heating of solidified melt pool promotes segregation of Si and Cr to form stable sigma phase in cell wall. This sigma phase is vulnerable to cracks and provides crack propagation paths. The formation of nano-twinning was found in as-built state with a thickness of ~1.5 nm, resulted from the low stacking fault energy by the addition of Si into CrCoNi alloyAs well as the initial twins, the deformation twins from during tensile deformation, which contributes to additional strain hardening. Therefore, this work presents a new microstructural design strategy using the formation of high-temperature precipitation during additive manufacturing of MEAs, with according strategies for customizing mechanical property.