Microstructure and fracture behavior of multi-elements strengthened CoCrNi alloy produced by laser-directed energy deposition

Abstract

Addition of single non-constituent element to strengthen equiatomic CoCrNi alloy has proven effective but faces limitations due to the solubility constraints of these additives. Excessive addition of single element causes the precipitation of brittle phases and deteriorates the mechanical properties of CoCrNi alloy. This study investigates the phase composition and mechanical properties of CoCrNi alloy strengthened by more than two elements simultaneously. Using laser-directed energy deposition (LDED), a series of CoCrNi(Al0.6TiFe)x alloys were rapidly fabricated by feeding both CoCrNi and CoCrNiAl0.6TiFe powders simultaneously. Microstructural analysis revealed cellular structures in all alloys, with coarsening observed at higher additive contents. Besides, increasing Al, Ti, Fe content in the alloy led to the precipitation of a BCC phase at cellular boundaries, causing stress concentration and resulting in a transition from ductile fracture to cleavage fracture. Among these alloys, one deposited with 70% CoCrNi and 30% CoCrNiAl0.6TiFe powders, exhibited remarkable mechanical properties, with a yield strength of 689.3 MPa, ultimate tensile strength of 1004.3 MPa, and elongation of 30.2%, representing a 41.8% increase in yield strength compared to the CoCrNi alloy. The improved properties are attributed to a high initial dislocation density of 2.50 × 10¹⁵ m^-2 caused by the high solubility of Al, Ti, and Fe elements about 14.82 at.%. These findings highlight a novel strategy for developing high-performance alloys by incorporating multiple non-constituent elements via LDED.