Influence of B4C alloying on the phase stability of Fe-Mn-Co-Cr-Si high entropy alloys fabricated using laser powder bed fusion

Abstract

This study investigates the impact of 0.25 wt% B4C addition on the microstructure, phase stability, and mechanical properties of metastable Fe40Mn20Co20Cr15Si5 (at.%) high-entropy alloys (HEAs) fabricated via laser powder bed fusion (LPBF). A comparative analysis between B4C-containing (BC) and B4C-free (CS) alloys explored the influence of varying LPBF parameters. Electron backscattered diffraction (EBSD) revealed a strong dependence of microstructure on laser power and scanning speed in CS alloys, with significant variations in grain size and morphology. Conversely, BC alloys exhibited enhanced microstructural stability, indicating a more robust grain growth mechanism due to the influence of B4C. B4C addition also promoted grain refinement and stabilized the γ-f.c.c. phase. Mechanical testing showed a substantial increase in yield strength (YS) from 508 MPa (CS) to 670 MPa (BC) and a moderate increase in ultimate tensile strength (UTS) from 843 MPa (CS) to 854 MPa (BC). However, ductility decreased from 25 % to 5 %. Critically, synchrotron X-ray diffraction revealed deviations from the ideal c/a ratio (1.633) for both alloys. CS alloys showed an increase in c/a ratio after tensile deformation, indicative of a deformation-induced phase transformation, while BC alloys exhibited a decrease, suggesting a distinct deformation mechanism. This novel observation provides key insights into the role of B4C in controlling the deformation behavior of this HEA.