In situ B₄C decomposition enables strength–ductility synergy in additively manufactured Ni-based superalloy at intermediate temperature

Additively manufactured superalloys suffer from insufficient ductility at intermediate temperatures, leading to high cracking susceptibility and compromised structural reliability. In this study, a crack-free multiphase TMS3 superalloy was fabricated by laser-directed energy deposition (L-DED) through the addition of B₄C nanoparticles. During processing, B₄C first decomposes and subsequently reacts with matrix elements to form MC carbides and M₂B borides. Precipitates formed during printing induce segmented grain boundary migration, leading to pronounced grain boundary serration significantly exceeding that achieved by conventional processing. The coupled effect of precipitates and serrated grain boundaries enhances the stability of the weakest regions of the alloy at high temperatures. The addition of 1.5 wt% B₄C increases the yield strength and elongation of the TMS3 alloy at 800 °C by 7.3% and 206.7%, respectively, thereby achieving a strength–ductility synergy. During deformation, precipitates and elemental segregation intensify lattice distortion in grain boundaries and interdendritic regions, promoting the formation of high-density low-angle grain boundaries (LAGBs), while curvature variations in serrated grain boundaries drive their propagation into the grain interior, dispersing stress and enabling more uniform deformation. During large deformation, the intrinsically low stacking fault energy (<10 mJ/m²) of the as-printed TMS3 alloy promotes stacking faults and deformation twins, enhancing strain accommodation and reducing stress concentration. These findings provide insights into the role of B₄C particles in tailoring microstructure and offer a viable strategy for achieving crack-free additively manufactured superalloys with enhanced performance.