Effect of strain rate on compressive behavior and deformation mechanism of CoCrNi medium entropy alloy fabricated via cold spray additive manufacturing

The equiatomic CoCrNi medium entropy alloy (MEA) has shown better mechanical properties than many MEAs and high entropy alloys (HEAs) at room temperatures. However, the mechanical behaviors and deformation mechanisms of CoCrNi MEAs have yet to be studied, especially at different strain rates. In this study, cold spray additive manufacturing (CSAM) was applied to fabricate freestanding CoCrNi deposits. The investigation of compressive behaviors and deformation mechanisms were studied at a low-strain rate of 0.001 s⁻¹ and at a high strain rate ranging from 2300 s⁻¹to 4200 s⁻¹. The as-fabricated sample exhibited refined grains, unfused powders, and large porosity, resulting in limited metallurgical bonding within the bulk CoCrNi deposit and deformed particle interface. Therefore, post-heat treatment at 1350 °C was applied to improve the interparticle bonding and strengthen the as-fabricated CoCrNi deposits. As expected, the annealed deposit significantly improved the ultimate compressive strength (UCS) and strain elongation of the as-fabricated deposits from ∼90 MPa and ∼5% to ∼1400 MPa, and ∼70% at a strain rate of 0.001 s⁻¹. Besides, the annealed samples showed improvement twofold in UCS and ductility of the as-fabricated deposits from ∼67 MPa and ∼8.6% to ∼1266 MPa and ∼34% at a strain rate of 4200 s⁻¹ and 3800 s⁻¹ respectively. The TEM image of the annealed CoCrNi sample showed deformation twining, nano twins, planar slips, and microbands, which are the dominant deformation mechanisms that help to promote the plasticity and the work hardening behavior of this alloy. This work validates the compressive behavior and deformation mechanism of cold-sprayed CoCrNi MEA associated with post-heat treatment at different strain rates.