Nano to macro-mechanical properties of laser directed energy deposited CoCrNi medium entropy alloy

Abstract

The CoCrNi Medium Entropy Alloy is a popular subset of the Cantor alloy due to its superior cryogenic and elevated temperature properties. This study reports the successful deployment of a laser-based near-net shaping technique known as laser directed energy deposition (LDED) to fabricate the CoCrNi MEA. The effect of laser power and scan speed on track geometry is studied in detail. The bulk deposition is performed using a parametric combination yielding regular defect-free tracks with an aspect ratio greater than five and a maximum deposition rate. The EBSD analysis revealed a strong < 001 > cubic texture along the build direction, due to the thermal gradient in that direction. Across both the scan and build directions, the cellular and columnar substructure is observed. Solid solution strengthening and dislocation strengthening contribute 39 % and 42 % to yield strength. The nanoindentation analysis revealed hardness, reduced modulus, and elastic modulus of 6.32 ± 0.32 GPa, 222.08 ± 5.72 GPa, and 250.62 ± 8.25 GPa, respectively. The contact stiffness, reduced modulus, and hardness showed a positive linear relationship with strain rate. The strain rate sensitivity of MEA is in the range of 0.030–0.042, which is higher than that of conventional fcc metals. It is attributed to the higher lattice frictional stresses and different atomic-level structures of MEA than conventional fcc metals. Furthermore, chemical short-range ordering and a stronger peierls barrier resulted in a lower activation volume of the order of ∼100b3. The present study provides a detailed understanding of the effect of process parameters on melt-pool geometry, microstructure evolution, as well as the influence of strain rate on nanomechanical properties.