Laser additive manufacturing of a CoCr-based medium-entropy alloy: Impressive stacking fault-induced nanotwinning strengthening through rapid solidification

摘要

The directed energy deposition (DED) technique was used to deposit a CoCrNiWC medium entropy alloy (MEA) through a robotic-assisted laser additive manufacturing (LAM) machine. The microstructural features, including solidification grains, structural defects, and phase transformations, along with uniaxial tensile behavior in different orientations, were comprehensively studied. Furthermore, a quantitative assessment was carried out to evaluate the contribution of strengthening mechanisms through apprising stacking fault constant of 11362 MPa nm for the developed MEA, enrolling the impact of FCC to HCP crystal structure transition for the cobalt matrix after rapid cooling LAM solidification in the solid-state board. In addition, the presence of numerous ordinary screw dislocations interacting with each other, stacking faults (SFs), nanotwins, and refined grains led to their hindering as the critical factor for strengthening. The SF intersections resulted in the formation of effective Lomer-Cottrell (LC) locks and nanotwins, which were barriers to slip and attributed to dislocation interactions. Accordingly, the rapid cooling solidification of the LAM process contributed to impressive mechanical properties of deposited CoCr-based MEA with remarkable yield and tensile strengths of ∼895 and 1410 MPa, confirming the direct contributions of statistically stored dislocations upon impact SF and nanotwining strengthening.