Enhancing mechanical properties and electrochemical behavior of equiatomic FeNiCoCr high-entropy alloy through sintering and hot isostatic pressing for binder jet 3D printing

This study investigates the influence of sintering temperature and hot isostatic pressing (HIP) on the microstructure, mechanical properties, and electrochemical behavior of equiatomic FeNiCoCr high-entropy alloy (HEA) samples produced via binder jet 3D printing (BJ3DP). Green samples with a relative density of approximately 59.3 ± 0.4% were initially printed using pre-alloyed powders. Subsequently, sintering was performed at temperatures ranging from 1410 to 1440 °C for 4 h with an interval of 10 °C. The sintered samples exhibited a single face-centered cubic phase, and their relative densities increased with higher sintering temperatures, reaching values of approximately 78.1%, 88.3%, 91.0%, and 98.1% for temperatures of 1410, 1420, 1430, and 1440 °C, respectively. The effect of hot isostatic pressing (HIP) at 1200 °C under a pressure of 150 MPa for 2 h in an argon atmosphere was found to be minimal for samples with relative densities below 91.0%. However, HIP substantially enhanced both the densities and mechanical properties for samples with relative densities of 91.0% or higher. The samples sintered at 1440 °C (#S1440) displayed the optimal mechanical properties, i.e., the values of yield strength (σy), ultimate tensile strength (σUTS), and total elongation (εt) are ∼173 MPa, ∼476 MPa, and ∼41%, respectively. Further processing #S1440 sample using HIP at 1200 °C (noted as #S1440H) led to an improved relative density to ∼99.4%, along with enhanced strength and ductility. Electrochemical testing in a 3.5 wt% NaCl solution demonstrated superior performance of the #S1440H sample compared to wrought 316 L stainless steel (SS), although it exhibited slightly lower corrosion resistance compared to #S1440 due to coarser grains. X-ray photoelectron spectroscopy (XPS) analyses identified the formation of Co(Fe,Cr)2O4 in the passive film, contributing to the improved corrosion resistance of FeCoNiCr.