Additive Manufacturing of Interstitial Nitrogen-Strengthened CoCrNi Medium Entropy Alloy

Equiatomic CoCrNi medium entropy alloy (MEA) is known for its excellent mechanical properties, oxidation resistance, and hydrogen embrittlement resistance. Interstitial elements like carbon and nitrogen further enhance these properties. However, there is limited research on interstitial solid solution-strengthened alloys produced via powder bed fusion-laser beam (PBF-LB). This study investigates the role of nitrogen as an interstitial alloying element in CoCrNi MEA using PBF-LB. Two alloy variants, one without nitrogen and one prealloyed with nitrogen, are analyzed through thermodynamic calculations and experimental validations. A minor nitrogen loss occurs in as-printed samples compared to prealloyed powders, but the remaining nitrogen stays as an interstitial solid solution without forming detrimental secondary phases. This leads to improved mechanical properties, with yield and ultimate tensile strengths increasing from about 690 and 900 MPa in nitrogen-free samples to over 750 and 1000 MPa in nitrogen-containing samples while maintaining the high ductility of the nitrogen-free counterpart in the as-printed materials. Additionally, the nitrogen-containing CoCrNi-N MEA exhibits approximate to 35HV higher hardness than the nitrogen-free variant in both as-printed and heat-treated states. These findings highlight the benefits of nitrogen addition in enhancing the mechanical properties of CoCrNi MEA produced by PBF-LB. The epitaxially grown columnar grains and their internal cellular solidification structure in the as-printed CoCrNi and CoCrNi-N samples.image (c) 2024 WILEY-VCH GmbH